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A chloroplast is a type of
membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
known as a
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
that conducts
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
mostly in
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
and
algal cells
algal cells
. The photosynthetic pigment
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
captures the
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

energy
from
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
, converts it, and stores it in the energy-storage molecules
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
and
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
while freeing
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
from water in the cells. The ATP and NADPH is then used to make organic molecules from
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
in a process known as the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
. Chloroplasts carry out a number of other functions, including
fatty acid synthesis Fatty acid synthesis is the creation of fatty acid fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration. In chemistry Chemistry is the scientific discipline invo ...
, much
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
synthesis, and the
immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, Parasitism, parasites, and Fungus ...

immune response
in plants. The number of chloroplasts per cell varies from one, in unicellular algae, up to 100 in plants like ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae Brassicaceae () or Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the c ...

Arabidopsis
'' and
wheat Wheat is a grass widely cultivated for its seed, a cereal grain which is a worldwide staple food. The many species of wheat together make up the genus ''Triticum''; the most widely grown is common wheat Common wheat (''Triticum aestivum'' ...

wheat
. A chloroplast is characterized by its two membranes and a high concentration of
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
. Other plastid types, such as the
leucoplast s Leucoplasts (λευκός leukós "white", πλαστός plastós "formed, molded") are a category of plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells o ...
and the
chromoplast is controlled by a specialized organelle in plant cells called a chromoplast. Chromoplasts are plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants ...
, contain little chlorophyll and do not carry out photosynthesis. Chloroplasts are highly dynamic—they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce. Their behavior is strongly influenced by environmental factors like light color and intensity. Chloroplasts, like
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
, contain their own
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
, which is thought to be inherited from their ancestor—a photosynthetic
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
that was
engulfed
engulfed
by an early
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
cell. Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division. With one exception (the
amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora''), all chloroplasts can probably be traced back to a single
endosymbiotic event An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...
, when a cyanobacterium was engulfed by the eukaryote. Despite this, chloroplasts can be found in an extremely wide set of organisms, some not even directly related to each other—a consequence of many
secondary Secondary is an adjective meaning "second" or "second hand". It may refer to: * Secondary (chemistry), term used in organic chemistry to classify various types of compounds * The group of (usually at least four) defensive backs in gridiron football ...
and even
tertiary endosymbiotic events An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...
. The word ''chloroplast'' is derived from the Greek words ''chloros'' (χλωρός), which means green, and ''plastes'' (πλάστης), which means "the one who forms".


Discovery

The first definitive description of a chloroplast (''Chlorophyllkörnen'', "grain of chlorophyll") was given by
Hugo von Mohl Hugo von Mohl FFRS HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged to be "eminently distin ...

Hugo von Mohl
in 1837 as discrete bodies within the green plant cell. In 1883,
Andreas Franz Wilhelm Schimper Andreas Franz Wilhelm Schimper (12 May 1856 – 9 September 1901) was a German botany, botanist and phytogeography, phytogeographer who made major contributions in the fields of histology, ecology and plant geography. He travelled to South Eas ...
would name these bodies as "chloroplastids" (''Chloroplastiden''). In 1884,
Eduard Strasburger Eduard Adolf Strasburger (1 February 1844 – 18 May 1912) was a Polish- German professor and one of the most famous botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', ...
adopted the term "chloroplasts" (''Chloroplasten'').


Lineages and evolution

Chloroplasts are one of many types of organelles in the plant cell. They are considered to have evolved from
endosymbiotic An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
.
Mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

Mitochondria
are thought to have come from a similar endosymbiosis event, where an
aerobic Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
was engulfed. This origin of chloroplasts was first suggested by the Russian biologist
Konstantin Mereschkowski Konstantin Sergeevich Mereschkowski ( rus, Константи́н Серге́евич Мережко́вский, p=mʲɪrʲɪˈʂkofskʲɪj; – 9 January 1921) was a prominent Russian biologist Francesco Redi, the founder of biology, is re ...

Konstantin Mereschkowski
in 1905 after
Andreas Franz Wilhelm Schimper Andreas Franz Wilhelm Schimper (12 May 1856 – 9 September 1901) was a German botany, botanist and phytogeography, phytogeographer who made major contributions in the fields of histology, ecology and plant geography. He travelled to South Eas ...
observed in 1883 that chloroplasts closely resemble
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
. Chloroplasts are only found in
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, and three species of
amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Cl ...

amoeba
– ''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora'', ''P. micropora'', and marine ''P. longichromatophora''.


Parent group: Cyanobacteria

Chloroplasts are considered endosymbiotic Cyanobacteria. Cyanobacteria are sometimes called blue-green
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
even though they are
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s. They are a diverse
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
of
gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few microm ...
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
capable of carrying out
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. Cyanobacteria also contain a peptidoglycan cell wall, which is thicker than in other gram-negative bacteria, and which is located between their two cell membranes. Like chloroplasts, they have
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s within them On the thylakoid membranes are photosynthetic pigments, including . Phycobilins are also common cyanobacterial pigments, usually organized into hemispherical
phycobilisomes Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. General structure Phycobilisomes are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes. They are made of stac ...
attached to the outside of the thylakoid membranes (phycobilins are not shared with all chloroplasts though).


Primary endosymbiosis

Somewhere between 1 and 2 billion years ago, a free-living
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
entered an early
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
cell, either as food or as an internal
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
, but managed to escape the phagocytic vacuole it was contained in. The two innermost that surround all chloroplasts correspond to the outer and inner
membranes Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membr ...

membranes
of the ancestral cyanobacterium's
gram negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan c ...
cell wall, and not the phagosomal membrane from the host, which was probably lost. The new cellular resident quickly became an advantage, providing food for the eukaryotic host, which allowed it to live within it. Over time, the cyanobacterium was assimilated, and many of its genes were lost or transferred to the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
of the host. From genomes that probably originally contained over 3000 genes only about 130 genes remain in the chloroplasts of contemporary plants. Some of its proteins were then synthesized in the cytoplasm of the host cell, and imported back into the chloroplast (formerly the cyanobacterium). Separately, somewhere about 90–140 million years ago, it happened again and led to the
amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora''. This event is called ''
endosymbiosis An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualism (biology), mutualistic relationship. (The term endosymbiosis is from the Greek language, Greek: ...

endosymbiosis
'', or "cell living inside another cell with a mutual benefit for both". The external cell is commonly referred to as the ''host'' while the internal cell is called the ''endosymbiont''. Chloroplasts are believed to have arisen after
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
, since all
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
contain mitochondria, but not all have chloroplasts. This is called ''''—an early eukaryote engulfing the
mitochondrion A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondrion
ancestor, and some descendants of it then engulfing the chloroplast ancestor, creating a cell with both chloroplasts and mitochondria. Whether or not primary chloroplasts came from a single endosymbiotic event, or many independent engulfments across various eukaryotic lineages, has long been debated. It is now generally held that organisms with primary chloroplasts share a single ancestor that took in a
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
600–2000 million years ago. It has been proposed this the closest living relative of this bacterium is ''
Gloeomargarita lithophora ''Gloeomargarita lithophora'' is a cyanobacterium, and is the proposed sister of the endosymbiotic plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of p ...
.'' The exception is the
amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora'', which descends from an ancestor that took in a ''
Prochlorococcus ''Prochlorococcus'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also ref ...
''
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
90–500 million years ago. These chloroplasts, which can be traced back directly to a cyanobacterial ancestor, are known as ''primary
plastids The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a Biological membrane, membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be ...
'' (''"
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
"'' in this context means almost the same thing as chloroplast). All primary chloroplasts belong to one of four chloroplast lineages—the
glaucophyte The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater unicellular algae, less common today than they were during the Proterozoic. Only 15 species have been described, but more species are likely to exi ...

glaucophyte
chloroplast lineage, the
amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora'' lineage, the rhodophyte (red algal) chloroplast lineage, or the chloroplastidan (green) chloroplast lineage. The rhodophyte and chloroplastidan lineages are the largest, with chloroplastidan (green) being the one that contains the
land plants The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...
.


Glaucophyta

Usually the endosymbiosis event is considered to have occurred in the
Archaeplastida The Archaeplastida (or kingdom ') are a major group of s, comprising the (Rhodophyta), the , and the s, and some smaller groups such as the s. All of the lineages of Archaeplastida have become ic, except for the lineage , sister to the Rhodoph ...
, within which the
glaucophyta The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater unicellular algae, less common today than they were during the Proterozoic. Only 15 species have been described, but more species are likely to e ...

glaucophyta
being the possible earliest diverging lineage. The glaucophyte chloroplast group is the smallest of the three primary chloroplast lineages, being found in only 13 species, and is thought to be the one that branched off the earliest. Glaucophytes have chloroplasts that retain a peptidoglycan wall between their double membranes, like their
cyanobacterial Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterial
parent. For this reason, glaucophyte chloroplasts are also known as 'muroplasts' (besides 'cyanoplasts' or 'cyanelles'). Glaucophyte chloroplasts also contain
concentric In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

concentric
unstacked
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s, which surround a
carboxysome Image:Carboxysomes EM.jpg, 460px, Electron micrographs showing alpha-carboxysomes from the chemoautotrophic bacterium ''Halothiobacillus, Halothiobacillus neapolitanus'': (A) arranged within the cell, and (B) intact upon isolation. Scale bars indica ...

carboxysome
– an
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non-similarity (geometry), similar shapes of icosahedra, some of them ...
structure that glaucophyte chloroplasts and cyanobacteria keep their
carbon fixation Carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon ...
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
in. The starch that they synthesize collects outside the chloroplast. Like cyanobacteria, glaucophyte and rhodophyte chloroplast thylakoids are studded with light collecting structures called
phycobilisomes Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. General structure Phycobilisomes are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes. They are made of stac ...
. For these reasons, glaucophyte chloroplasts are considered a primitive intermediate between cyanobacteria and the more evolved chloroplasts in
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
and
plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Docto ...

plants
.


Rhodophyceae (red algae)

The rhodophyte, or
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
chloroplast group is another large and diverse chloroplast lineage. Rhodophyte chloroplasts are also called ''rhodoplasts'', literally "red chloroplasts". Rhodoplasts have a double membrane with an intermembrane space and
phycobilinPhycobilins (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 millio ...
pigments organized into phycobilisomes on the thylakoid membranes, preventing their thylakoids from stacking. Some contain
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
s. Rhodoplasts have and
phycobilinPhycobilins (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 millio ...
s for photosynthetic pigments; the phycobilin
phycoerythrin Phycoerythrin (PE) is a red protein-pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compoun ...
is responsible for giving many red algae their distinctive
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength Image:dominant wavelength.png, frame, Dominant/complementary wavelength example on the CIE color ...

red
color. However, since they also contain the blue-green and other pigments, many are reddish to purple from the combination. The red phycoerytherin pigment is an adaptation to help red algae catch more sunlight in deep water—as such, some red algae that live in shallow water have less phycoerythrin in their rhodoplasts, and can appear more greenish. Rhodoplasts synthesize a form of starch called floridean starch, which collects into granules outside the rhodoplast, in the cytoplasm of the red alga.


Chloroplastida (green algae and plants)

The
chloroplastida Viridiplantae (literally "green plants") are a of organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the , which are primarily aquatic, and t ...
chloroplasts, or
green Green is the color between blue and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 Nanometre, nm. In subtractive color systems, used in painting and color printing, it is created by ...

green
chloroplasts, are another large, highly diverse primary chloroplast lineage. Their host organisms are commonly known as the
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
and
land plants The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...
. They differ from glaucophyte and red algal chloroplasts in that they have lost their
phycobilisomes Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. General structure Phycobilisomes are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes. They are made of stac ...
, and contain instead. Most green chloroplasts are (obviously)
green Green is the color between blue and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 Nanometre, nm. In subtractive color systems, used in painting and color printing, it is created by ...

green
, though some aren't, like some forms of '' Hæmatococcus pluvialis'', due to accessory pigments that override the chlorophylls' green colors. Chloroplastida chloroplasts have lost the peptidoglycan wall between their double membrane, leaving an intermembrane space. Some
plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Docto ...

plants
seem to have kept the
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
for the synthesis of the
peptidoglycan layer Peptidoglycan or murein is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pent ...
, though they've been repurposed for use in chloroplast division instead. Most of the chloroplasts depicted in this article are green chloroplasts. Green algae and plants keep their
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
''inside'' their chloroplasts, and in plants and some algae, the chloroplast thylakoids are arranged in grana stacks. Some green algal chloroplasts contain a structure called a
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
, which is functionally similar to the glaucophyte
carboxysome Image:Carboxysomes EM.jpg, 460px, Electron micrographs showing alpha-carboxysomes from the chemoautotrophic bacterium ''Halothiobacillus, Halothiobacillus neapolitanus'': (A) arranged within the cell, and (B) intact upon isolation. Scale bars indica ...

carboxysome
in that it is where
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
and CO are concentrated in the chloroplast. '' Helicosporidium'' is a genus of nonphotosynthetic parasitic green algae that is thought to contain a vestigial chloroplast. Genes from a chloroplast and nuclear genes indicating the presence of a chloroplast have been found in ''Helicosporidium'' even if nobody's seen the chloroplast itself.


''Paulinella chromatophora''

While most chloroplasts originate from that first set of endosymbiotic events, ''
Paulinella ''Paulinella'' is a genus of about nine species of freshwater amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first two being freshwater forms and ...
chromatophora'' is an exception that acquired a photosynthetic cyanobacterial endosymbiont more recently. It is not clear whether that symbiont is closely related to the ancestral chloroplast of other eukaryotes. Being in the early stages of endosymbiosis, ''Paulinella chromatophora'' can offer some insights into how chloroplasts evolved. ''Paulinella'' cells contain one or two sausage shaped blue-green photosynthesizing structures called chromatophores, descended from the cyanobacterium ''
Synechococcus ''Synechococcus'' (from the Greek ''synechos'', in succession, and the Greek ''kokkos'', granule) is a unicellular cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (somet ...
''. Chromatophores cannot survive outside their host. Chromatophore DNA is about a million
base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
long, containing around 850 protein encoding
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
—far less than the three million base pair ''Synechococcus'' genome, but much larger than the approximately 150,000 base pair genome of the more assimilated chloroplast. Chromatophores have transferred much less of their DNA to the nucleus of their host. About 0.3–0.8% of the nuclear DNA in ''Paulinella'' is from the chromatophore, compared with 11–14% from the chloroplast in plants.


Secondary and tertiary endosymbiosis

Many other organisms obtained chloroplasts from the primary chloroplast lineages through secondary endosymbiosis—engulfing a red or green alga that contained a chloroplast. These chloroplasts are known as secondary plastids. While primary chloroplasts have a double membrane from their
cyanobacterial Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...
ancestor, secondary chloroplasts have additional membranes outside of the original two, as a result of the secondary endosymbiotic event, when a nonphotosynthetic
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
engulfed a chloroplast-containing alga but failed to digest it—much like the cyanobacterium at the beginning of this story. The engulfed alga was broken down, leaving only its chloroplast, and sometimes its
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
and
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
, forming a chloroplast with three or four membranes—the two cyanobacterial membranes, sometimes the eaten alga's cell membrane, and the phagosomal vacuole from the host's cell membrane. The genes in the phagocytosed eukaryote's nucleus are often transferred to the secondary host's nucleus. Cryptomonads and chlorarachniophytes retain the phagocytosed eukaryote's nucleus, an object called a nucleomorph, located between the second and third membranes of the chloroplast. All secondary chloroplasts come from green algae, green and
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
—no secondary chloroplasts from glaucophytes have been observed, probably because glaucophytes are relatively rare in nature, making them less likely to have been taken up by another eukaryote.


Green algal derived chloroplasts

Green algae have been taken up by the euglenids, chlorarachniophytes, a lineage of dinoflagellates, and possibly the ancestor of the CASH lineage (cryptomonads, Alveolata, alveolates, stramenopiles and haptophytes) in three or four separate engulfments. Many green algal derived chloroplasts contain
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
s, but unlike chloroplasts in their green algal ancestors, storage product collects in granules outside the chloroplast.


= Euglenophytes

= Euglenophytes are a group of common flagellated protists that contain chloroplasts derived from a green alga. Euglenophyte chloroplasts have three membranes—it is thought that the membrane of the primary endosymbiont was lost, leaving the cyanobacterial membranes, and the secondary host's phagosomal membrane. Euglenophyte chloroplasts have a
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
and
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s stacked in groups of three. Photosynthetic product is stored in the form of paramylon, which is contained in membrane-bound granules in the cytoplasm of the euglenophyte.


= Chlorarachniophytes

= Chlorarachniophytes are a rare group of organisms that also contain chloroplasts derived from green algae, though their story is more complicated than that of the euglenophytes. The ancestor of chlorarachniophytes is thought to have been a eukaryote with a ''red'' algal derived chloroplast. It is then thought to have lost its first red algal chloroplast, and later engulfed a green alga, giving it its second, green algal derived chloroplast. Chlorarachniophyte chloroplasts are bounded by four membranes, except near the cell membrane, where the chloroplast membranes fuse into a double membrane. Their thylakoids are arranged in loose stacks of three. Chlorarachniophytes have a form of polysaccharide called chrysolaminarin, which they store in the cytoplasm, often collected around the chloroplast
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
, which bulges into the cytoplasm. Chlorarachniophyte chloroplasts are notable because the green alga they are derived from has not been completely broken down—its nucleus still persists as a nucleomorph found between the second and third chloroplast membranes—the periplastid space, which corresponds to the green alga's cytoplasm.


= Prasinophyte-derived dinophyte chloroplast

= ''Lepidodinium viride'' and its close relatives are dinophytes (see below) that lost their original peridinin chloroplast and replaced it with a green algal derived chloroplast (more specifically, a prasinophyte). ''Lepidodinium'' is the only dinophyte that has a chloroplast that's not from the rhodoplast lineage. The chloroplast is surrounded by two membranes and has no nucleomorph—all the nucleomorph genes have been transferred to the dinophyte cell nucleus, nucleus. The endosymbiotic event that led to this chloroplast was serial secondary endosymbiosis rather than tertiary endosymbiosis—the endosymbiont was a green alga containing a primary chloroplast (making a secondary chloroplast).


Red algal derived chloroplasts


= Cryptophytes

= Cryptomonad, Cryptophytes, or cryptomonads are a group of algae that contain a red-algal derived chloroplast. Cryptophyte chloroplasts contain a nucleomorph that superficially resembles that of the chlorarachniophytes. Cryptophyte chloroplasts have four membranes, the outermost of which is continuous with the rough endoplasmic reticulum. They synthesize ordinary
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
, which is stored in granules found in the periplastid space—outside the original double membrane, in the place that corresponds to the red alga's cytoplasm. Inside cryptophyte chloroplasts is a
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
and
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s in stacks of two. Their chloroplasts do not have
phycobilisomes Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. General structure Phycobilisomes are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes. They are made of stac ...
, but they do have phycobilin pigments which they keep in their thylakoid space, rather than anchored on the outside of their thylakoid membranes. Cryptophytes may have played a key role in the spreading of red algal based chloroplasts.


= Haptophytes

= Haptophytes are similar and closely related to cryptophytes or heterokontophytes. Their chloroplasts lack a nucleomorph, their thylakoids are in stacks of three, and they synthesize chrysolaminarin sugar, which they store completely outside of the chloroplast, in the cytoplasm of the haptophyte.


= Heterokontophytes (stramenopiles)

= The heterokontophytes, also known as the stramenopiles, are a very large and diverse group of eukaryotes. The photoautotrophic lineage, Ochrophyta, including the diatoms and the brown algae, golden algae, and yellow-green algae, also contains red algal derived chloroplasts. Heterokont chloroplasts are very similar to haptophyte chloroplasts, containing a
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
, triplet thylakoids, and with some exceptions, having four layer plastidic envelope, the outermost epiplastid membrane connected to the endoplasmic reticulum. Like haptophytes, heterokontophytes store sugar in chrysolaminarin granules in the cytoplasm. Heterokontophyte chloroplasts contain and with a few exceptions Chlorophyll c, chlorophyll ''c'', but also have carotenoids which give them their many colors.


= Apicomplexans, chromerids, and dinophytes

= The alveolates are a major clade of unicellular eukaryotes of both autotrophic and heterotrophic members. The most notable shared characteristic is the presence of cortical (outer-region) alveoli (sacs). These are flattened vesicles (sacs) packed into a continuous layer just under the membrane and supporting it, typically forming a flexible pellicle (thin skin). In dinoflagellates they often form armor plates. Many members contain a red-algal derived plastid. One notable characteristic of this diverse group is the frequent loss of photosynthesis. However, a majority of these heterotrophs continue to process a non-photosynthetic plastid. ;Apicomplexans Apicomplexans are a group of alveolates. Like the helicosproidia, they're parasitic, and have a nonphotosynthetic chloroplast. They were once thought to be related to the helicosproidia, but it is now known that the helicosproida are green algae rather than part of the CASH lineage. The apicomplexans include ''Plasmodium'', the malaria parasite. Many apicomplexans keep a vestigial red algal derived chloroplast called an apicoplast, which they inherited from their ancestors. Other apicomplexans like ''Cryptosporidium'' have lost the chloroplast completely. Apicomplexans store their energy in amylopectin granules that are located in their cytoplasm, even though they are nonphotosynthetic. Apicoplasts have lost all photosynthetic function, and contain no photosynthetic pigments or true thylakoids. They are bounded by four membranes, but the membranes are not connected to the endoplasmic reticulum. The fact that apicomplexans still keep their nonphotosynthetic chloroplast around demonstrates how the chloroplast carries out important functions other than
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. Plant chloroplasts provide plant cells with many important things besides sugar, and apicoplasts are no different—they synthesize fatty acids, isopentenyl pyrophosphate, iron-sulfur clusters, and carry out part of the heme pathway. This makes the apicoplast an attractive target for drugs to cure apicomplexan-related diseases. The most important apicoplast function is isopentenyl pyrophosphate synthesis—in fact, apicomplexans die when something interferes with this apicoplast function, and when apicomplexans are grown in an isopentenyl pyrophosphate-rich medium, they dump the organelle. ;Chromerids The Chromerida is a newly discovered group of algae from Australian corals which comprises some close photosynthetic relatives of the apicomplexans. The first member, ''Chromera velia'', was discovered and first isolated in 2001. The discovery of ''Chromera velia'' with similar structure to the apicomplexanss, provides an important link in the evolutionary history of the apicomplexans and dinophytes. Their plastids have four membranes, lack chlorophyll c and use the type II form of
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
obtained from a horizontal transfer event. ;Dinophytes The dinoflagellates are yet another very large and diverse group of protists, around half of which are (at least mixotrophy, partially) photosynthetic. Most dinophyte chloroplasts are secondary red algae, red algal derived chloroplasts. Many other dinophytes have lost the chloroplast (becoming the nonphotosynthetic kind of dinoflagellate), or replaced it though ''tertiary'' endosymbiosis—the engulfment of another eukaryotic algae containing a red algal derived chloroplast. Others replaced their original chloroplast with a green algae, green algal derived one. Most dinophyte chloroplasts contain form II RuBisCO, at least the photosynthetic pigments , chlorophyll c2, chlorophyll ''c2'', beta-carotene, ''beta''-carotene, and at least one dinophyte-unique xanthophyll (peridinin, dinoxanthin, or diadinoxanthin), giving many a golden-brown color. All dinophytes store starch in their cytoplasm, and most have chloroplasts with thylakoids arranged in stacks of three. The most common dinophyte chloroplast is the peridinin-type chloroplast, characterized by the carotenoid pigment peridinin in their chloroplasts, along with and chlorophyll c2, chlorophyll ''c''2. Peridinin is not found in any other group of chloroplasts. The peridinin chloroplast is bounded by three membranes (occasionally two), having lost the red algal endosymbiont's original cell membrane. The outermost membrane is not connected to the endoplasmic reticulum. They contain a
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
, and have triplet-stacked thylakoids. Starch is found outside the chloroplast. An important feature of these chloroplasts is that their chloroplast DNA is highly genome reduction, reduced and fragmented into many small circles. Most of the genome has migrated to the nucleus, and only critical photosynthesis-related genes remain in the chloroplast. The peridinin chloroplast is thought to be the dinophytes' "original" chloroplast, which has been lost, reduced, replaced, or has company in several other dinophyte lineages.


= Fucoxanthin-containing (haptophyte-derived) dinophyte chloroplasts

= The fucoxanthin dinophyte lineages (including ''Karlodinium'' and ''Karenia (dinoflagellate), Karenia'') lost their original red algal derived chloroplast, and replaced it with a new chloroplast derived from a haptophyte endosymbiont. ''Karlodinium'' and ''Karenia (dinoflagellate), Karenia'' probably took up different heterokontophytes. Because the haptophyte chloroplast has four membranes, tertiary endosymbiosis would be expected to create a six membraned chloroplast, adding the haptophyte's
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
and the dinophyte's phagosomal vacuole. However, the haptophyte was heavily reduced, stripped of a few membranes and its nucleus, leaving only its chloroplast (with its original double membrane), and possibly one or two additional membranes around it. Fucoxanthin-containing chloroplasts are characterized by having the pigment fucoxanthin (actually 19′-hexanoyloxy-fucoxanthin and/or 19′-butanoyloxy-fucoxanthin) and no peridinin. Fucoxanthin is also found in haptophyte chloroplasts, providing evidence of ancestry.


= Diatom-derived dinophyte chloroplasts

= Some dinophytes, like ''Kryptoperidinium'' and ''Durinskia'' have a diatom (heterokontophyte) derived chloroplast. These chloroplasts are bounded by up to ''five'' membranes, (depending on whether the entire diatom endosymbiont is counted as the chloroplast, or just the red algal derived chloroplast inside it). The diatom endosymbiont has been reduced relatively little—it still retains its original
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
, and has endoplasmic reticulum, ribosomes, a cell nucleus, nucleus, and of course, red algal derived chloroplasts—practically a complete cell (biology), cell, all inside the host's endoplasmic reticulum lumen. However the diatom endosymbiont can't store its own food—its storage polysaccharide is found in granules in the dinophyte host's cytoplasm instead. The diatom endosymbiont's nucleus is present, but it probably can't be called a nucleomorph because it shows no sign of genome reduction, and might have even been ''expanded''. Diatoms have been engulfed by dinoflagellates at least three times. The diatom endosymbiont is bounded by a single membrane, inside it are chloroplasts with four membranes. Like the diatom endosymbiont's diatom ancestor, the chloroplasts have triplet thylakoids and
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
s. In some of these genera, the diatom endosymbiont's chloroplasts aren't the only chloroplasts in the dinophyte. The original three-membraned peridinin chloroplast is still around, converted to an Eyespot apparatus, eyespot.


Kleptoplastidy

In some groups of mixotrophic protists, like some dinoflagellates (e.g. ''Dinophysis''), chloroplasts are separated from a captured alga and used temporarily. These kleptoplasty, klepto chloroplasts may only have a lifetime of a few days and are then replaced.


Cryptophyte-derived dinophyte chloroplast

Members of the genus ''Dinophysis'' have a
phycobilinPhycobilins (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 millio ...
-containing chloroplast taken from a cryptomonad, cryptophyte. However, the cryptophyte is not an endosymbiont—only the chloroplast seems to have been taken, and the chloroplast has been stripped of its nucleomorph and outermost two membranes, leaving just a two-membraned chloroplast. Cryptophyte chloroplasts require their nucleomorph to maintain themselves, and ''Dinophysis'' species grown in cell culture alone cannot survive, so it is possible (but not confirmed) that the ''Dinophysis'' chloroplast is a kleptoplast—if so, ''Dinophysis'' chloroplasts wear out and ''Dinophysis'' species must continually engulf cryptophytes to obtain new chloroplasts to replace the old ones.


Chloroplast DNA

Chloroplasts, like other types of
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
, contain a genome separate from that in the cell
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
. The existence of chloroplast DNA (cpDNA) was identified biochemically in 1959, and confirmed by electron microscopy in 1962. The discoveries that the chloroplast contains ribosomes and performs protein synthesis revealed that the chloroplast is genetically semi-autonomous. Chloroplast DNA was first sequenced in 1986. Since then, hundreds of chloroplast DNAs from various species have been DNA sequencing, sequenced, but they are mostly those of
land plants The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...
and
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
—glaucophytes,
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
, and other algal groups are extremely underrepresented, potentially introducing some Selection bias, bias in views of "typical" chloroplast DNA structure and content.


Molecular structure

With few exceptions, most chloroplasts have their entire chloroplast genome combined into a single large circular DNA molecule, typically 120,000–170,000
base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
long. They can have a contour length of around 30–60 micrometers, and have a mass of about 80–130 million dalton (unit), daltons. While usually thought of as a circular molecule, there is some evidence that chloroplast DNA molecules more often take on a linear shape.


Inverted repeats

Many chloroplast DNAs contain two ''inverted repeats'', which separate a long single copy section (LSC) from a short single copy section (SSC). While a given pair of inverted repeats are rarely completely identical, they are always very similar to each other, apparently resulting from concerted evolution. The inverted repeats vary wildly in length, ranging from 4,000 to 25,000
base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
long each and containing as few as four or as many as over 150 genes. Inverted repeats in plants tend to be at the upper end of this range, each being 20,000–25,000 base pairs long. The inverted repeat regions are highly Conserved sequence, conserved among land plants, and accumulate few mutations. Similar inverted repeats exist in the genomes of cyanobacteria and the other two chloroplast lineages (glaucophyta and rhodophyceae), suggesting that they predate the chloroplast, though some chloroplast DNAs have since lost or flipped the inverted repeats (making them direct repeats). It is possible that the inverted repeats help stabilize the rest of the chloroplast genome, as chloroplast DNAs which have lost some of the inverted repeat segments tend to get rearranged more.


Nucleoids

New chloroplasts may contain up to 100 copies of their DNA, though the number of chloroplast DNA copies decreases to about 15–20 as the chloroplasts age. They are usually packed into nucleoids, which can contain several identical chloroplast DNA rings. Many nucleoids can be found in each chloroplast. In primitive
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
, the chloroplast DNA nucleoids are clustered in the center of the chloroplast, while in green plants and
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
, the nucleoids are dispersed throughout the stroma (fluid), stroma. Though chloroplast DNA is not associated with true histones, in
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
, similar proteins that tightly pack each chloroplast DNA ring into a nucleoid have been found.


DNA repair

In chloroplasts of the moss ''Physcomitrella patens'', the DNA mismatch repair protein Msh1 interacts with the homologous recombination, recombinational DNA repair, repair proteins RecA and RecG to maintain chloroplast genome stability. In chloroplasts of the plant ''Arabidopsis thaliana'' the RecA protein maintains the integrity of the chloroplast's DNA by a process that likely involves the recombinational repair of DNA damage (naturally occurring), DNA damage.


DNA replication

The mechanism for chloroplast DNA (cpDNA) replication has not been conclusively determined, but two main models have been proposed. Scientists have attempted to observe chloroplast replication via electron microscopy since the 1970s. The results of the microscopy experiments led to the idea that chloroplast DNA replicates using a double displacement loop (D-loop). As the D-loop moves through the circular DNA, it adopts a theta intermediary form, also known as a Cairns replication intermediate, and completes replication with a rolling circle mechanism. Transcription starts at specific points of origin. Multiple replication forks open up, allowing replication machinery to transcribe the DNA. As replication continues, the forks grow and eventually converge. The new cpDNA structures separate, creating daughter cpDNA chromosomes. In addition to the early microscopy experiments, this model is also supported by the amounts of deamination seen in cpDNA. Deamination occurs when an amino group is lost and is a mutation that often results in base changes. When adenine is deaminated, it becomes hypoxanthine. Hypoxanthine can bind to cytosine, and when the XC base pair is replicated, it becomes a GC (thus, an A → G base change). In cpDNA, there are several A → G deamination gradients. DNA becomes susceptible to deamination events when it is single stranded. When replication forks form, the strand not being copied is single stranded, and thus at risk for A → G deamination. Therefore, gradients in deamination indicate that replication forks were most likely present and the direction that they initially opened (the highest gradient is most likely nearest the start site because it was single stranded for the longest amount of time). This mechanism is still the leading theory today; however, a second theory suggests that most cpDNA is actually linear and replicates through homologous recombination. It further contends that only a minority of the genetic material is kept in circular chromosomes while the rest is in branched, linear, or other complex structures. One of competing model for cpDNA replication asserts that most cpDNA is linear and participates in homologous recombination and replication structures similar to the linear and circular DNA structures of bacteriophage T4. It has been established that some plants have linear cpDNA, such as maize, and that more species still contain complex structures that scientists do not yet understand. When the original experiments on cpDNA were performed, scientists did notice linear structures; however, they attributed these linear forms to broken circles. If the branched and complex structures seen in cpDNA experiments are real and not artifacts of concatenated circular DNA or broken circles, then a D-loop mechanism of replication is insufficient to explain how those structures would replicate. At the same time, homologous recombination does not expand the multiple A --> G gradients seen in plastomes. Because of the failure to explain the deamination gradient as well as the numerous plant species that have been shown to have circular cpDNA, the predominant theory continues to hold that most cpDNA is circular and most likely replicates via a D loop mechanism.


Gene content and protein synthesis

The chloroplast genome most commonly includes around 100 genes that code for a variety of things, mostly to do with the protein synthesis, protein pipeline and
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. As in prokaryotes, genes in chloroplast DNA are organized into operons. Unlike prokaryotic DNA molecules, chloroplast DNA molecules contain introns (plant mitochondrial DNAs do too, but not human mtDNAs). Among land plants, the contents of the chloroplast genome are fairly similar.


Chloroplast genome reduction and gene transfer

Over time, many parts of the chloroplast genome were transferred to the nuclear genome of the host, a process called ''endosymbiotic gene transfer''. As a result, the chloroplast genome is heavily genome reduction, reduced compared to that of free-living cyanobacteria. Chloroplasts may contain 60–100 genes whereas cyanobacteria often have more than 1500 genes in their genome. Recently, a plastid without a genome was found, demonstrating chloroplasts can lose their genome during endosymbiotic the gene transfer process. Endosymbiotic gene transfer is how we know about the #Secondary and tertiary endosymbiosis, lost chloroplasts in many CASH lineages. Even if a chloroplast is eventually lost, the genes it donated to the former host's nucleus persist, providing evidence for the lost chloroplast's existence. For example, while diatoms (a heterokontophyte) now have a red algal derived chloroplast, the presence of many green algal genes in the diatom nucleus provide evidence that the diatom ancestor had a green algal derived chloroplast at some point, which was subsequently replaced by the red chloroplast. In land plants, some 11–14% of the DNA in their nuclei can be traced back to the chloroplast, up to 18% in ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae Brassicaceae () or Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the c ...

Arabidopsis
'', corresponding to about 4,500 protein-coding genes. There have been a few recent transfers of genes from the chloroplast DNA to the nuclear genome in land plants. Of the approximately 3000 proteins found in chloroplasts, some 95% of them are encoded by nuclear genes. Many of the chloroplast's protein complexes consist of subunits from both the chloroplast genome and the host's nuclear genome. As a result, protein synthesis must be coordinated between the chloroplast and the nucleus. The chloroplast is mostly under nuclear control, though chloroplasts can also give out signals regulating gene expression in the nucleus, called ''retrograde signaling (cell biology), retrograde signaling''.


Protein synthesis

Protein synthesis within chloroplasts relies on two RNA polymerases. One is coded by the chloroplast DNA, the other is of cell nucleus, nuclear origin. The two RNA polymerases may recognize and bind to different kinds of promoter (genetics), promoters within the chloroplast genome. The ribosomes in chloroplasts are similar to bacterial ribosomes.


Protein targeting and import

Because so many chloroplast genes have been moved to the nucleus, many proteins that would originally have been Translation (biology), translated in the chloroplast are now synthesized in the cytoplasm of the plant cell. These proteins must be directed back to the chloroplast, and imported through at least two chloroplast membranes. Curiously, around half of the protein products of transferred genes aren't even targeted back to the chloroplast. Many became exaptations, taking on new functions like participating in cell division, protein routing, and even disease resistance. A few chloroplast genes found new homes in the mitochondrial genome—most became nonfunctional pseudogenes, though a few tRNA genes still work in the
mitochondrion A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondrion
. Some transferred chloroplast DNA protein products get directed to the secretory pathway, though many #Secondary and tertiary endosymbiosis, secondary plastids are bounded by an outermost membrane derived from the host's
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
, and therefore topologically outside of the cell because to reach the chloroplast from the cytosol, the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
must be crossed, which signifies entrance into the extracellular space. In those cases, chloroplast-targeted proteins do initially travel along the secretory pathway. Because the cell acquiring a chloroplast #Primary endosymbiosis, already had
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
(and peroxisomes, and a
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
for secretion), the new chloroplast host had to develop a unique protein targeting system to avoid having chloroplast proteins being sent to the wrong organelle. In most, but not all cases, nuclear-encoded chloroplast proteins are Translation (biology), translated with a ''cleavable transit peptide'' that's added to the N-terminus of the protein precursor. Sometimes the transit sequence is found on the C-terminus of the protein, or within the functional part of the protein.


Transport proteins and membrane translocons

After a chloroplast polypeptide is synthesized on a ribosome in the cytosol, an enzyme enzyme specificity, specific to chloroplast proteins phosphorylates, or adds a phosphate group to many (but not all) of them in their transit sequences. Phosphorylation helps many proteins bind the polypeptide, keeping it from protein folding, folding prematurely. This is important because it prevents chloroplast proteins from assuming their active form and carrying out their chloroplast functions in the wrong place—the cytosol. At the same time, they have to keep just enough shape so that they can be recognized by the chloroplast. These proteins also help the polypeptide get imported into the chloroplast. From here, chloroplast proteins bound for the stroma must pass through two protein complexes—the TOC complex, or ''translocon, translocon on the outer chloroplast membrane'', and the TIC translocon, or ''translocon on the inner chloroplast membrane translocon''. Chloroplast polypeptide chains probably often travel through the two complexes at the same time, but the TIC complex can also retrieve preproteins lost in the chloroplast intermembrane space, intermembrane space.


Structure

In Embryophyte, land plants, chloroplasts are generally lens-shaped, 3–10 μm in diameter and 1–3 μm thick. Corn seedling chloroplasts are ≈20 µm3 in volume. Greater diversity in chloroplast shapes exists among the
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, which often contain a single chloroplast that can be shaped like a net (e.g., ''Oedogonium''), a cup (e.g., ''Chlamydomonas''), a ribbon-like spiral around the edges of the cell (e.g., ''Spirogyra''), or slightly twisted bands at the cell edges (e.g., ''Sirogonium''). Some algae have two chloroplasts in each cell; they are star-shaped in ''Zygnema'', or may follow the shape of half the cell in order (biology), order Desmidiales. In some algae, the chloroplast takes up most of the cell, with pockets for the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
and other organelles, for example, some species of ''Chlorella'' have a cup-shaped chloroplast that occupies much of the cell. All chloroplasts have at least three membrane systems—the outer chloroplast membrane, the inner chloroplast membrane, and the
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
system. Chloroplasts that are the product of #Endosymbiosis, secondary endosymbiosis may have additional membranes surrounding these three. Inside the outer and inner chloroplast membranes is the chloroplast stroma (fluid), stroma, a semi-gel-like fluid that makes up much of a chloroplast's volume, and in which the thylakoid system floats. There are some common misconceptions about the outer and inner chloroplast membranes. The fact that chloroplasts are surrounded by a double membrane is often cited as evidence that they are the descendants of endosymbiotic
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
. This is often interpreted as meaning the outer chloroplast membrane is the product of the host's
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
infolding to form a vesicle to surround the ancestral
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
—which is not true—both chloroplast membranes are Homology (biology), homologous to the cyanobacterium's original double membranes. The chloroplast double membrane is also often compared to the Mitochondrion, mitochondrial double membrane. This is not a valid comparison—the inner mitochondria membrane is used to run proton pumps and carry out oxidative phosphorylation across to generate
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
energy. The only chloroplast structure that can considered analogy (biology), analogous to it is the internal thylakoid system. Even so, in terms of "in-out", the direction of chloroplast hydronium, H ion flow is in the opposite direction compared to oxidative phosphorylation in mitochondria. In addition, in terms of function, the inner chloroplast membrane, which regulates metabolite passage and synthesizes some materials, has no counterpart in the mitochondrion.


Outer chloroplast membrane

The outer chloroplast membrane is a semi-porous membrane that small molecules and ions can easily diffuse across. However, it is not permeable to larger proteins, so chloroplast polypeptides being synthesized in the cell cytoplasm must be transported across the outer chloroplast membrane by the TOC complex, or ''translocon, translocon on the outer chloroplast'' membrane. The chloroplast membranes sometimes protrude out into the cytoplasm, forming a stromule, or stroma (fluid), stroma-containing tubule. Stromules are very rare in chloroplasts, and are much more common in other
plastids The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a Biological membrane, membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be ...
like chromoplasts and amyloplasts in petals and roots, respectively. They may exist to increase the chloroplast's surface area to volume ratio, surface area for cross-membrane transport, because they are often branched and tangled with the endoplasmic reticulum. When they were first observed in 1962, some plant biologists dismissed the structures as artifactual, claiming that stromules were just oddly shaped chloroplasts with constricted regions or dividing chloroplasts. However, there is a growing body of evidence that stromules are functional, integral features of plant cell plastids, not merely artifacts.


Intermembrane space and peptidoglycan wall

Usually, a thin intermembrane space about 10–20 nanometers thick exists between the outer and inner chloroplast membranes. Glaucophyte algal chloroplasts have a peptidoglycan layer between the chloroplast membranes. It corresponds to the peptidoglycan cell wall of their
cyanobacterial Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...
ancestors, which is located between their two cell membranes. These chloroplasts are called ''muroplasts'' (from Latin ''"mura"'', meaning "wall"). Other chloroplasts have lost the cyanobacterial wall, leaving an intermembrane space between the two chloroplast envelope membranes.


Inner chloroplast membrane

The inner chloroplast membrane borders the stroma and regulates passage of materials in and out of the chloroplast. After passing through the TOC complex in the outer chloroplast membrane, polypeptides must pass through the TIC complex ''(translocon, translocon on the inner chloroplast membrane)'' which is located in the inner chloroplast membrane. In addition to regulating the passage of materials, the inner chloroplast membrane is where fatty acids, lipids, and carotenoids are synthesized.


Peripheral reticulum

Some chloroplasts contain a structure called the chloroplast peripheral reticulum. It is often found in the chloroplasts of C4 plant, plants, though it has also been found in some angiosperms, and even some gymnosperms. The chloroplast peripheral reticulum consists of a maze of membranous tubes and vesicles continuous with the inner chloroplast membrane that extends into the internal stroma (fluid), stromal fluid of the chloroplast. Its purpose is thought to be to increase the chloroplast's surface area to volume ratio, surface area for cross-membrane transport between its stroma and the cell cytoplasm. The small vesicles sometimes observed may serve as transport vesicles to shuttle stuff between the
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s and intermembrane space.


Stroma

The protein-rich, alkaline, aqueous fluid within the inner chloroplast membrane and outside of the thylakoid space is called the stroma, which corresponds to the cytosol of the original
cyanobacterium Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...

cyanobacterium
. Nucleoids of chloroplast DNA, chloroplast ribosomes, the thylakoid system with plastoglobuli,
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
granules, and many proteins can be found floating around in it. The
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
, which fixes CO2, CO into Glyceraldehyde 3-phosphate, G3P takes place in the stroma.


Chloroplast ribosomes

Chloroplasts have their own ribosomes, which they use to synthesize a small fraction of their proteins. Chloroplast ribosomes are about two-thirds the size of Eukaryotic Ribosome (80S), cytoplasmic ribosomes (around 17 Nanometre, nm vs 25 Nanometre, nm). They take mRNAs transcribed from the chloroplast DNA and translation (biology), translate them into protein. While similar to bacterial ribosomes, chloroplast translation is more complex than in bacteria, so chloroplast ribosomes include some chloroplast-unique features. Small subunit ribosomal RNAs in several Chlorophyta and euglenid chloroplasts lack motifs for shine-dalgarno sequence recognition, which is considered essential for Translation (biology), translation initiation in most chloroplasts and
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s. Such loss is also rarely observed in other
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
s and prokaryotes. An additional 4.5S rRNA with homology to the 3' tail of 23S is found in "higher" plants.


Plastoglobuli

Plastoglobuli (singular ''plastoglobulus'', sometimes spelled ''plastoglobule(s)''), are spherical bubbles of lipids and proteins about 45–60 nanometers across. They are surrounded by a lipid monolayer. Plastoglobuli are found in all chloroplasts, but become more common when the chloroplast is under oxidative stress, or when it ages and transitions into a gerontoplast. Plastoglobuli also exhibit a greater size variation under these conditions. They are also common in etioplasts, but decrease in number as the etioplasts mature into chloroplasts. Plastoglubuli contain both structural proteins and enzymes involved in lipid synthesis and metabolism. They contain many types of lipids including plastoquinone, vitamin E, carotenoids and chlorophylls. Plastoglobuli were once thought to be free-floating in the stroma (fluid), stroma, but it is now thought that they are permanently attached either to a
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
or to another plastoglobulus attached to a thylakoid, a configuration that allows a plastoglobulus to exchange its contents with the thylakoid network. In normal green chloroplasts, the vast majority of plastoglobuli occur singularly, attached directly to their parent thylakoid. In old or stressed chloroplasts, plastoglobuli tend to occur in linked groups or chains, still always anchored to a thylakoid. Plastoglobuli form when a bubble appears between the layers of the lipid bilayer of the thylakoid membrane, or bud from existing plastoglubuli—though they never detach and float off into the stroma. Practically all plastoglobuli form on or near the highly curved edges of the
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
disks or sheets. They are also more common on stromal thylakoids than on granum, granal ones.


Starch granules

Starch granules are very common in chloroplasts, typically taking up 15% of the organelle's volume, though in some other plastids like amyloplasts, they can be big enough to distort the shape of the organelle. Starch granules are simply accumulations of starch in the stroma, and are not bounded by a membrane. Starch granules appear and grow throughout the day, as the chloroplast synthesizes sugars, and are consumed at night to fuel Cellular respiration, respiration and continue sugar export into the phloem, though in mature chloroplasts, it is rare for a starch granule to be completely consumed or for a new granule to accumulate. Starch granules vary in composition and location across different chloroplast lineages. In
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
, starch granules are found in the cytoplasm rather than in the chloroplast. In C4 plant, plants, mesophyll tissue, mesophyll chloroplasts, which do not synthesize sugars, lack starch granules.


RuBisCO

The chloroplast stroma contains many proteins, though the most common and important is
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
, which is probably also the most abundant protein on the planet.
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
is the enzyme that fixes CO2, CO into sugar molecules. In C3 plant, plants, RuBisCO is abundant in all chloroplasts, though in C4 plant, plants, it is confined to the bundle sheath chloroplasts, where the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
is carried out in plants.


Pyrenoids

The chloroplasts of some hornworts and algae contain structures called
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
s. They are not found in higher plants. Pyrenoids are roughly spherical and highly refractive bodies which are a site of starch accumulation in plants that contain them. They consist of a matrix opaque to electrons, surrounded by two hemispherical starch plates. The starch is accumulated as the pyrenoids mature. In algae with Photosynthesis#Carbon concentrating mechanisms, carbon concentrating mechanisms, the enzyme
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
is found in the pyrenoids. Starch can also accumulate around the pyrenoids when CO2 is scarce. Pyrenoids can divide to form new pyrenoids, or be produced De novo synthesis, "de novo".


Thylakoid system

Thylakoids (sometimes spelled ''thylakoïds''), are small interconnected sacks which contain the membranes that the light reactions of photosynthesis take place on. The word ''thylakoid'' comes from the Greek word ''thylakos'' which means "sack". Suspended within the chloroplast stroma is the
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
system, a highly dynamic collection of membranous sacks called
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s where
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
is found and the light reactions of
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
happen. In most vascular plant chloroplasts, the thylakoids are arranged in stacks called grana, though in certain C4 plant, plant chloroplasts and some algal chloroplasts, the thylakoids are free floating.


Thylakoid structure

Using a light microscope, it is just barely possible to see tiny green granules—which were named Thylakoid, grana. With electron microscopy, it became possible to see the thylakoid system in more detail, revealing it to consist of stacks of flat
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s which made up the grana, and long interconnecting stromal thylakoids which linked different grana. In the transmission electron microscope, thylakoid membranes appear as alternating light-and-dark bands, 8.5 nanometers thick. For a long time, the three-dimensional structure of the thylakoid membrane system had been unknown or disputed. Many models have been proposed, the most prevalent being the Helix, helical model, in which granum stacks of thylakoids are wrapped by helical stromal thylakoids. Another model known as the 'bifurcation model', which was based on the first electron tomography study of plant thylakoid membranes, depicts the stromal membranes as wide lamellar sheets perpendicular to the grana columns which bifurcates into multiple parallel discs forming the granum-stroma assembly. The helical model was supported by several additional works, but ultimately it was determined in 2019 that features from both the helical and bifurcation models are consolidated by newly discovered left-handed helical membrane junctions. Likely for ease, the thylakoid system is still commonly depicted by older "hub and spoke" models where the grana are connected to each other by tubes of stromal thylakoids. Grana consist of a stacks of flattened circular granal thylakoids that resemble pancakes. Each granum can contain anywhere from two to a hundred thylakoids, though grana with 10–20 thylakoids are most common. Wrapped around the grana are multiple parallel right-handed helical stromal thylakoids, also known as frets or lamellar thylakoids. The helices ascend at an angle of ~20°, connecting to each granal thylakoid at a bridge-like slit junction. The stroma lamellae extend as large sheets perpendicular to the grana columns. These sheets are connected to the right-handed helices either directly or through bifurcations that form left-handed helical membrane surfaces. The left-handed helical surfaces have a similar tilt angle to the right-handed helices (~20°), but ¼ the pitch. Approximately 4 left-handed helical junctions are present per granum, resulting in a pitch-balanced array of right- and left-handed helical membrane surfaces of different radii and pitch that consolidate the network with minimal surface and bending energies. While different parts of the thylakoid system contain different membrane proteins, the thylakoid membranes are continuous and the thylakoid space they enclose form a single continuous labyrinth.


Thylakoid composition

Embedded in the thylakoid membranes are important protein complexes which carry out the light reactions of
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. Photosystem II and photosystem I contain light-harvesting complexes with
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
and carotenoids that absorb light energy and use it to energize electrons. Molecules in the thylakoid membrane use the energized electrons to pump hydrogen ions into the thylakoid space, decreasing the pH and turning it acidic. ATP synthase is a large protein complex that harnesses the concentration gradient of the hydrogen ions in the thylakoid space to generate
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
energy as the hydrogen ions flow back out into the stroma—much like a dam turbine. There are two types of thylakoids—granal thylakoids, which are arranged in grana, and stromal thylakoids, which are in contact with the stroma (fluid), stroma. Granal thylakoids are pancake-shaped circular disks about 300–600 nanometers in diameter. Stromal thylakoids are helicoid sheets that spiral around grana. The flat tops and bottoms of granal thylakoids contain only the relatively flat photosystem II protein complex. This allows them to stack tightly, forming grana with many layers of tightly appressed membrane, called granal membrane, increasing stability and surface area for light capture. In contrast, photosystem I and ATP synthase are large protein complexes which jut out into the stroma. They can't fit in the appressed granal membranes, and so are found in the stromal thylakoid membrane—the edges of the granal thylakoid disks and the stromal thylakoids. These large protein complexes may act as spacers between the sheets of stromal thylakoids. The number of thylakoids and the total thylakoid area of a chloroplast is influenced by light exposure. Shaded chloroplasts contain larger and more Thylakoid, grana with more thylakoid membrane area than chloroplasts exposed to bright light, which have smaller and fewer grana and less thylakoid area. Thylakoid extent can change within minutes of light exposure or removal.


Pigments and chloroplast colors

Inside the photosystems embedded in chloroplast thylakoid membranes are various photosynthetic pigments, which absorb and transfer light energy. The types of pigments found are different in various groups of chloroplasts, and are responsible for a wide variety of chloroplast colorations.
Paper chromatography, Paper chroma-tography of some spinach leaf extract shows the various pigments present in their chloroplasts.
Xanthophylls, Xanthophylls
Chlorophyll a, Chlorophyll ''a''
Chlorophyll b, Chlorophyll ''b''


= Chlorophylls

= Chlorophyll a, Chlorophyll ''a'' is found in all chloroplasts, as well as their
cyanobacterial Cyanobacteria , also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ca ...
ancestors. Chlorophyll ''a'' is a blue-green pigment partially responsible for giving most cyanobacteria and chloroplasts their color. Other forms of chlorophyll exist, such as the accessory pigments , Chlorophyll c, chlorophyll ''c'', chlorophyll d, chlorophyll ''d'', and chlorophyll f, chlorophyll ''f''. Chlorophyll b, Chlorophyll ''b'' is an olive green pigment found only in the chloroplasts of
plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Docto ...

plants
,
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
, any secondary chloroplasts obtained through the secondary endosymbiosis of a green alga, and a few
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
. It is the chlorophylls ''a'' and ''b'' together that make most plant and green algal chloroplasts green. Chlorophyll c, Chlorophyll ''c'' is mainly found in secondary endosymbiotic chloroplasts that originated from a red alga, although it is not found in chloroplasts of red algae themselves. Chlorophyll ''c'' is also found in some
green algae The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate Division (botany), divisions, together with the more basal Mesostigmatoph ...

green algae
and
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
. Chlorophylls chlorophyll d, ''d'' and chlorophyll f, ''f'' are pigments found only in some cyanobacteria.


= Carotenoids

= In addition to chlorophylls, another group of yellow–orange (colour), orange pigments called carotenoids are also found in the photosystems. There are about thirty photosynthetic carotenoids. They help transfer and dissipate excess energy, and their bright colors sometimes override the chlorophyll green, like during the autumn, fall, when the leaves of Deciduous tree, some land plants change color. β-carotene is a bright red-orange carotenoid found in nearly all chloroplasts, like . Xanthophylls, especially the orange-red zeaxanthin, are also common. Many other forms of carotenoids exist that are only found in certain groups of chloroplasts.


= Phycobilins

= Phycobilins are a third group of pigments found in
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
, and
glaucophyte The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater unicellular algae, less common today than they were during the Proterozoic. Only 15 species have been described, but more species are likely to exi ...

glaucophyte
, red algal, and Cryptomonad, cryptophyte chloroplasts. Phycobilins come in all colors, though phycoerytherin is one of the pigments that makes many red algae red. Phycobilins often organize into relatively large protein complexes about 40 nanometers across called
phycobilisomes Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. General structure Phycobilisomes are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes. They are made of stac ...
. Like photosystem I and ATP synthase, phycobilisomes jut into the stroma, preventing thylakoid stacking in red algal chloroplasts. Cryptomonad, Cryptophyte chloroplasts and some cyanobacteria don't have their phycobilin pigments organized into phycobilisomes, and keep them in their thylakoid space instead.


Specialized chloroplasts in plants

To fix
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
into sugar molecules in the process of
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
, chloroplasts use an enzyme called
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
. RuBisCO has trouble distinguishing between
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, so at high oxygen concentrations, RuBisCO starts accidentally adding oxygen to sugar precursors. This has the result of
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ATP
energy being wasted and being released, all with no sugar being produced. This is a big problem, since O is produced by the initial light reactions of photosynthesis, causing issues down the line in the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
which uses RuBisCO. C4 plants, plants evolved a way to solve this—by spatially separating the light reactions and the Calvin cycle. The light reactions, which store light energy in
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
and
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
, are done in the mesophyll tissue, mesophyll cells of a leaf. The Calvin cycle, which uses the stored energy to make sugar using RuBisCO, is done in the bundle sheath cells, a layer of cells surrounding a vein (botany), vein in a leaf. As a result, chloroplasts in mesophyll cells and bundle sheath cells are specialized for each stage of photosynthesis. In mesophyll cells, chloroplasts are specialized for the light reactions, so they lack
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
, and have normal Thylakoid, grana and
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s, which they use to make ATP and NADPH, as well as oxygen. They store in a four-carbon compound, which is why the process is called C4 carbon fixation, '' photosynthesis''. The four-carbon compound is then transported to the bundle sheath chloroplasts, where it drops off and returns to the mesophyll. Bundle sheath chloroplasts do not carry out the light reactions, preventing oxygen from building up in them and disrupting RuBisCO activity. Because of this, they lack thylakoids organized into Thylakoid, grana stacks—though bundle sheath chloroplasts still have free-floating thylakoids in the stroma where they still carry out cyclic electron flow, a light-driven method of synthesizing
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ATP
to power the Calvin cycle without generating oxygen. They lack photosystem II, and only have photosystem I—the only protein complex needed for cyclic electron flow. Because the job of bundle sheath chloroplasts is to carry out the Calvin cycle and make sugar, they often contain large
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
grains. Both types of chloroplast contain large amounts of chloroplast peripheral reticulum, which they use to get more surface area to volume ratio, surface area to transport stuff in and out of them. Mesophyll chloroplasts have a little more peripheral reticulum than bundle sheath chloroplasts.


Location


Distribution in a plant

Not all cells in a multicellular plant contain chloroplasts. All green parts of a plant contain chloroplasts—the chloroplasts, or more specifically, the
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
in them are what make the photosynthetic parts of a plant green. The plant cells which contain chloroplasts are usually parenchyma cells, though chloroplasts can also be found in collenchyma tissue. A plant cell which contains chloroplasts is known as a chlorenchyma cell. A typical chlorenchyma cell of a land plant contains about 10 to 100 chloroplasts. In some plants such as cacti, chloroplasts are found in the Plant stem, stems, though in most plants, chloroplasts are concentrated in the leaves. One square millimeter of leaf tissue can contain half a million chloroplasts. Within a leaf, chloroplasts are mainly found in the Mesophyll cell, mesophyll layers of a leaf, and the guard cells of stomata. Palisade mesophyll cells can contain 30–70 chloroplasts per cell, while stomatal guard cells contain only around 8–15 per cell, as well as much less
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
. Chloroplasts can also be found in the bundle sheath cells of a leaf, especially in C4 plants, C plants, which carry out the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
in their bundle sheath cells. They are often absent from the Epidermis (botany), epidermis of a leaf.Lawson T. and J. I. L. Morison. Essay 10.1 Guard Cell Photosynthesis. Plant Physiology and Development, Sixth Editio

/ref>


Cellular location


Chloroplast movement

The chloroplasts of plant and algal cells can orient themselves to best suit the available light. In low-light conditions, they will spread out in a sheet—maximizing the surface area to absorb light. Under intense light, they will seek shelter by aligning in vertical columns along the plant cell's cell wall or turning sideways so that light strikes them edge-on. This reduces exposure and protects them from photooxidative damage. This ability to distribute chloroplasts so that they can take shelter behind each other or spread out may be the reason why land plants evolved to have many small chloroplasts instead of a few big ones. Chloroplast movement is considered one of the most closely regulated stimulus-response systems that can be found in plants.
Mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

Mitochondria
have also been observed to follow chloroplasts as they move. In higher plants, chloroplast movement is run by phototropins, blue light Photoreceptor protein, photoreceptors also responsible for plant phototropism. In some algae, mosses, ferns, and flowering plants, chloroplast movement is influenced by red light in addition to blue light, though very long red wavelengths inhibit movement rather than speeding it up. Blue light generally causes chloroplasts to seek shelter, while red light draws them out to maximize light absorption. Studies of ''Vallisneria gigantea'', an aquatic flowering plant, have shown that chloroplasts can get moving within five minutes of light exposure, though they don't initially show any net directionality. They may move along microfilament tracks, and the fact that the microfilament mesh changes shape to form a honeycomb structure surrounding the chloroplasts after they have moved suggests that microfilaments may help to anchor chloroplasts in place.


Function and chemistry


Guard cell chloroplasts

Unlike most epidermal cells, the guard cells of plant stomata contain relatively well-developed chloroplasts. However, exactly what they do is controversial.


Plant innate immunity

Plants lack specialized immune cells—all plant cells participate in the plant plant innate immunity, immune response. Chloroplasts, along with the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
,
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
, and endoplasmic reticulum, are key players in pathogen defense. Due to its role in a plant cell's immune response, pathogens frequently target the chloroplast. Plants have two main immune responses—the hypersensitive response, in which infected cells seal themselves off and undergo programmed cell death, and systemic acquired resistance, where infected cells release signals warning the rest of the plant of a pathogen's presence. Chloroplasts stimulate both responses by purposely damaging their photosynthetic system, producing reactive oxygen species. High levels of reactive oxygen species will cause the hypersensitive response. The reactive oxygen species also directly kill any pathogens within the cell. Lower levels of reactive oxygen species initiate systemic acquired resistance, triggering defense-molecule production in the rest of the plant. In some plants, chloroplasts are known to move closer to the infection site and the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
during an infection. Chloroplasts can serve as cellular sensors. After detecting stress in a cell, which might be due to a pathogen, chloroplasts begin producing molecules like salicylic acid, jasmonic acid, nitric oxide and reactive oxygen species which can serve as defense-signals. As cellular signals, reactive oxygen species are unstable molecules, so they probably don't leave the chloroplast, but instead pass on their signal to an unknown second messenger molecule. All these molecules initiate retrograde signaling (cell biology), retrograde signaling—signals from the chloroplast that regulate gene expression in the nucleus. In addition to defense signaling, chloroplasts, with the help of the peroxisomes, help synthesize an important defense molecule, jasmonate. Chloroplasts synthesize all the fatty acids in a plant cell—linoleic acid, a fatty acid, is a precursor to jasmonate.


Photosynthesis

One of the main functions of the chloroplast is its role in
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
, the process by which light is transformed into chemical energy, to subsequently produce food in the form of sugars. Water (H2O) and
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
(CO2) are used in photosynthesis, and sugar and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
(O2) is made, using light energy. Photosynthesis is divided into two stages—the light reactions, where water is split to produce oxygen, and the dark reactions, or
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
, which builds sugar molecules from carbon dioxide. The two phases are linked by the energy carriers adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADP+).


Light reactions

The light reactions take place on the thylakoid membranes. They take light energy and store it in
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
, a form of NADP+, and
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ATP
to fuel the dark reactions.


= Energy carriers

= ATP is the phosphorylated version of adenosine diphosphate (ADP), which stores energy in a cell and powers most cellular activities. ATP is the energized form, while ADP is the (partially) depleted form. NADP+ is an electron carrier which ferries high energy electrons. In the light reactions, it gets redox reaction, reduced, meaning it picks up electrons, becoming
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
.


= Photophosphorylation

= Like mitochondria, chloroplasts use the potential energy stored in an Hydron (chemistry), H+, or hydrogen ion gradient to generate ATP energy. The two photosystems capture light energy to energize electrons taken from water, and release them down an electron transport chain. The Plastoquinone, molecules between the photosystems harness the electrons' energy to pump hydrogen ions into the thylakoid space, creating a concentration gradient, with more hydrogen ions (up to a thousand times as many) inside the thylakoid system than in the stroma. The hydrogen ions in the thylakoid space then diffuse back down their concentration gradient, flowing back out into the stroma through ATP synthase. ATP synthase uses the energy from the flowing hydrogen ions to phosphorylate adenosine diphosphate into adenosine triphosphate, or ATP. Because chloroplast ATP synthase projects out into the stroma, the ATP is synthesized there, in position to be used in the dark reactions.


= NADP+ reduction

= Electrons are often removed from the electron transport chains to charge NADP+, NADP+ with electrons, reduction reaction, reducing it to
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
. Like ATP synthase, ferredoxin—NADP+ reductase, ferredoxin-NADP+ reductase, the enzyme that reduces NADP+, releases the NADPH it makes into the stroma, right where it is needed for the dark reactions. Because NADP+ reduction removes electrons from the electron transport chains, they must be replaced—the job of photosystem II, which splits water molecules (H2O) to obtain the electrons from its hydrogen atoms.


= Cyclic photophosphorylation

= While photosystem II photolyzes water to obtain and energize new electrons, photosystem I simply reenergizes depleted electrons at the end of an electron transport chain. Normally, the reenergized electrons are taken by NADP+, though sometimes they can flow back down more H+-pumping electron transport chains to transport more hydrogen ions into the thylakoid space to generate more ATP. This is termed cyclic photophosphorylation because the electrons are recycled. Cyclic photophosphorylation is common in C4 plants, plants, which need more
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ATP
than
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
.


Dark reactions

The
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. ...

Calvin cycle
, also known as the dark reactions, is a series of biochemical reactions that fixes CO2, CO2 into Glyceraldehyde 3-phosphate, G3P sugar molecules and uses the energy and electrons from the
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
and
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
made in the light reactions. The Calvin cycle takes place in the stroma of the chloroplast. While named ''"the dark reactions"'', in most plants, they take place in the light, since the dark reactions are dependent on the products of the light reactions.


= Carbon fixation and G3P synthesis

= The Calvin cycle starts by using the enzyme
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is convert ...
to fix CO2 into five-carbon Ribulose bisphosphate (RuBP) molecules. The result is unstable six-carbon molecules that immediately break down into three-carbon molecules called 3-phosphoglyceric acid, or 3-PGA. The
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ATP
and
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
made in the light reactions is used to convert the 3-PGA into glyceraldehyde-3-phosphate, or G3P sugar molecules. Most of the G3P molecules are recycled back into RuBP using energy from more ATP, but one out of every six produced leaves the cycle—the end product of the dark reactions.


= Sugars and starches

= Glyceraldehyde-3-phosphate can double up to form larger sugar molecules like glucose and fructose. These molecules are processed, and from them, the still larger sucrose, a disaccharide commonly known as table sugar, is made, though this process takes place outside of the chloroplast, in the cytoplasm. Alternatively, glucose monomers in the chloroplast can be linked together to make
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
, which accumulates into the chloroplast starch granule, starch grains found in the chloroplast. Under conditions such as high atmospheric CO2 concentrations, these starch grains may grow very large, distorting the grana and thylakoids. The starch granules displace the thylakoids, but leave them intact. Waterlogged roots can also cause
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
buildup in the chloroplasts, possibly due to less sucrose being exported out of the chloroplast (or more accurately, the plant cell). This depletes a plant's free phosphate supply, which indirectly stimulates chloroplast starch synthesis. While linked to low photosynthesis rates, the starch grains themselves may not necessarily interfere significantly with the efficiency of photosynthesis, and might simply be a side effect of another photosynthesis-depressing factor.


= Photorespiration

= Photorespiration can occur when the oxygen concentration is too high. RuBisCO cannot distinguish between oxygen and carbon dioxide very well, so it can accidentally add O2 instead of CO2 to RuBP. This process reduces the efficiency of photosynthesis—it consumes ATP and oxygen, releases CO2, and produces no sugar. It can waste up to half the carbon fixed by the Calvin cycle. Several mechanisms have evolved in different lineages that raise the carbon dioxide concentration relative to oxygen within the chloroplast, increasing the efficiency of photosynthesis. These mechanisms are called carbon dioxide concentrating mechanisms, or CCMs. These include Crassulacean acid metabolism, C4 carbon fixation, carbon fixation, and
pyrenoid Pyrenoids are sub-cellular micro-compartments found in chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the en ...
s. Chloroplasts in plants are notable as they exhibit a distinct #Specialized chloroplasts in C4 plants, chloroplast dimorphism.


pH

Because of the Hydron (chemistry), H+ gradient across the thylakoid membrane, the interior of the thylakoid is acidic, with a pH around 4, while the stroma is slightly basic, with a pH of around 8. The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3. CO2, CO2 in water can form carbonic acid, which can disturb the pH of isolated chloroplasts, interfering with photosynthesis, even though CO2 is Enzyme substrate (biology), used in photosynthesis. However, chloroplasts in living plant cells are not affected by this as much. Chloroplasts can pump Potassium, K+ and H+ ions in and out of themselves using a poorly understood light-driven transport system. In the presence of light, the pH of the thylakoid lumen can drop up to 1.5 pH units, while the pH of the stroma can rise by nearly one pH unit.


Amino acid synthesis

Chloroplasts alone make almost all of a plant cell's amino acids in their stroma (fluid), stroma except the sulfur-containing amino acids, sulfur-containing ones like cysteine and methionine. Cysteine is made in the chloroplast (the proplastid too) but it is also synthesized in the cytosol and
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
, probably because it has trouble crossing membranes to get to where it is needed. The chloroplast is known to make the precursors to methionine but it is unclear whether the organelle carries out the last leg of the pathway or if it happens in the cytosol.


Other nitrogen compounds

Chloroplasts make all of a cell's purines and pyrimidines—the nitrogenous bases found in
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
and RNA. They also convert nitrite (NO2) into ammonia (NH3) which supplies the plant with nitrogen to make its
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s and nucleotides.


Other chemical products

The plastid is the site of diverse and complex lipid synthesis in plants.Buchanan BB, Gruissem W, Jones RL (Eds.). 2015. Biochemistry & Molecular Biology of Plants. Wiley Blackwell. The carbon used to form the majority of the lipid is from acetyl-CoA, which is the decarboxylation product of pyruvate. Pyruvate may enter the plastid from the cytosol by passive diffusion through the membrane after production in glycolysis. Pyruvate is also made in the plastid from phosphoenolpyruvate, a metabolite made in the cytosol from pyruvate or 3-Phosphoglyceric acid, PGA. Acetate in the cytosol is unavailable for lipid biosynthesis in the plastid.Bao X, Focke M, Pollard M, Ohlrogge J. 2000. Understanding in vivo carbon precursor supply for fatty acid synthesis in leaf tissue. Plant Journal 22, 39–50. The typical length of fatty acids produced in the plastid are 16 or 18 carbons, with 0-3 Cis–trans isomerism, cis double bonds. The biosynthesis of fatty acids from acetyl-CoA primarily requires two enzymes. Acetyl-CoA carboxylase creates malonyl-CoA, used in both the first step and the extension steps of synthesis. Fatty acid synthase (FAS) is a large complex of enzymes and cofactors including acyl carrier protein (ACP) which holds the acyl chain as it is synthesized. The initiation of synthesis begins with the condensation of malonyl-ACP with acetyl-CoA to produce ketobutyryl-ACP. 2 reductions involving the use of
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle The Calvin cycle, light-independent reactions, bio syn ...

NADPH
and one dehydration creates butyryl-ACP. Extension of the fatty acid comes from repeated cycles of malonyl-ACP condensation, reduction, and dehydration. Other lipids are derived from the Non-mevalonate pathway, methyl-erythritol phosphate (MEP) pathway and consist of Gibberellin, gibberelins, sterols, abscisic acid, phytol, and innumerable secondary metabolites.


Differentiation, replication, and inheritance

Chloroplasts are a special type of a plant cell organelle called a plastid, though the two terms are sometimes used interchangeably. There are many other types of plastids, which carry out various functions. All chloroplasts in a plant are descended from undifferentiated proplastids found in the zygote, or fertilized egg. Proplastids are commonly found in an adult plant's apical meristems. Chloroplasts do not normally develop from proplastids in root tip meristems—instead, the formation of starch-storing amyloplasts is more common. In shoots, proplastids from shoot apical meristems can gradually develop into chloroplasts in photosynthetic leaf tissues as the leaf matures, if exposed to the required light. This process involves invaginations of the inner plastid membrane, forming sheets of membrane that project into the internal Stroma (fluid), stroma. These membrane sheets then fold to form
thylakoid Thylakoids are membrane-bound compartments inside chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy ...

thylakoid
s and Thylakoid, grana. If angiosperm shoots are not exposed to the required light for chloroplast formation, proplastids may develop into an etioplast stage before becoming chloroplasts. An etioplast is a plastid that lacks
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
, and has inner membrane invaginations that form a lattice of tubes in their stroma, called a prolamellar body. While etioplasts lack chlorophyll, they have a yellow chlorophyll precursor (chemistry), precursor stocked. Within a few minutes of light exposure, the prolamellar body begins to reorganize into stacks of thylakoids, and chlorophyll starts to be produced. This process, where the etioplast becomes a chloroplast, takes several hours. Gymnosperms do not require light to form chloroplasts. Light, however, does not guarantee that a proplastid will develop into a chloroplast. Whether a proplastid develops into a chloroplast some other kind of plastid is mostly controlled by the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
and is largely influenced by the kind of cell it resides in.


Plastid interconversion

Plastid differentiation is not permanent, in fact many interconversions are possible. Chloroplasts may be converted to
chromoplast is controlled by a specialized organelle in plant cells called a chromoplast. Chromoplasts are plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants ...
s, which are pigment-filled plastids responsible for the bright colors seen in flowers and ripe fruit. Starch storing amyloplasts can also be converted to chromoplasts, and it is possible for proplastids to develop straight into chromoplasts. Chromoplasts and amyloplasts can also become chloroplasts, like what happens when a carrot or a potato is illuminated. If a plant is injured, or something else causes a plant cell to revert to a meristematic state, chloroplasts and other plastids can turn back into proplastids. Chloroplast, amyloplast, chromoplast, proplast, etc., are not absolute states—intermediate forms are common.


Division

Most chloroplasts in a photosynthetic cell do not develop directly from proplastids or etioplasts. In fact, a typical shoot meristematic plant cell contains only 7–20 proplastids. These proplastids differentiate into chloroplasts, which divide to create the 30–70 chloroplasts found in a mature photosynthetic plant cell. If the cell Mitosis, divides, chloroplast division provides the additional chloroplasts to partition between the two daughter cells. In single-celled
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, chloroplast division is the only way new chloroplasts are formed. There is no proplastid differentiation—when an algal cell divides, its chloroplast divides along with it, and each daughter cell receives a mature chloroplast. Almost all chloroplasts in a cell divide, rather than a small group of rapidly dividing chloroplasts. Chloroplasts have no definite S-phase—their DNA replication is not synchronized or limited to that of their host cells. Much of what we know about chloroplast division comes from studying organisms like ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae Brassicaceae () or Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the c ...

Arabidopsis
'' and the red alga ''Cyanidioschyzon merolæ''. The division process starts when the proteins FtsZ1 and FtsZ2 assemble into filaments, and with the help of a protein ARC6, form a structure called a Z-ring within the chloroplast's stroma. The Min system manages the placement of the Z-ring, ensuring that the chloroplast is cleaved more or less evenly. The protein MinD prevents FtsZ from linking up and forming filaments. Another protein ARC3 may also be involved, but it is not very well understood. These proteins are active at the poles of the chloroplast, preventing Z-ring formation there, but near the center of the chloroplast, MinE inhibits them, allowing the Z-ring to form. Next, the two plastid-dividing rings, or PD rings form. The inner plastid-dividing ring is located in the inner side of the chloroplast's inner membrane, and is formed first. The outer plastid-dividing ring is found wrapped around the outer chloroplast membrane. It consists of filaments about 5 nanometers across, arranged in rows 6.4 nanometers apart, and shrinks to squeeze the chloroplast. This is when chloroplast constriction begins.
In a few species like ''Cyanidioschyzon merolæ'', chloroplasts have a third plastid-dividing ring located in the chloroplast's intermembrane space. Late into the constriction phase, dynamin proteins assemble around the outer plastid-dividing ring, helping provide force to squeeze the chloroplast. Meanwhile, the Z-ring and the inner plastid-dividing ring break down. During this stage, the many chloroplast DNA plasmids floating around in the stroma are partitioned and distributed to the two forming daughter chloroplasts. Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane, to cleave the chloroplast in two daughter chloroplasts. A remnant of the outer plastid dividing ring remains floating between the two daughter chloroplasts, and a remnant of the dynamin ring remains attached to one of the daughter chloroplasts. Of the five or six rings involved in chloroplast division, only the outer plastid-dividing ring is present for the entire constriction and division phase—while the Z-ring forms first, constriction does not begin until the outer plastid-dividing ring forms.


Regulation

In species of
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
that contain a single chloroplast, regulation of chloroplast division is extremely important to ensure that each daughter cell receives a chloroplast—chloroplasts can't be made from scratch. In organisms like plants, whose cells contain multiple chloroplasts, coordination is looser and less important. It is likely that chloroplast and cell division are somewhat synchronized, though the mechanisms for it are mostly unknown. Light has been shown to be a requirement for chloroplast division. Chloroplasts can grow and progress through some of the constriction stages under Chlorophyll#Spectrophotometry, poor quality green light, but are slow to complete division—they require exposure to bright white light to complete division. Spinach leaves grown under green light have been observed to contain many large dumbbell-shaped chloroplasts. Exposure to white light can stimulate these chloroplasts to divide and reduce the population of dumbbell-shaped chloroplasts.


Chloroplast inheritance

Like Mitochondrion#Replication and inheritance, mitochondria, chloroplasts are usually inherited from a single parent. Biparental chloroplast inheritance—where plastid genes are inherited from both parent plants—occurs in very low levels in some flowering plants. Many mechanisms prevent biparental chloroplast DNA inheritance, including selective destruction of chloroplasts or their genes within the gamete or zygote, and chloroplasts from one parent being excluded from the embryo. Parental chloroplasts can be sorted so that only one type is present in each offspring. Gymnosperms, such as pine trees, mostly pass on chloroplasts paternally, while flowering plants often inherit chloroplasts maternally. Flowering plants were once thought to only inherit chloroplasts maternally. However, there are now many documented cases of angiosperms inheriting chloroplasts paternally. Flowering plant, Angiosperms, which pass on chloroplasts maternally, have many ways to prevent paternal inheritance. Most of them produce sperm cells that do not contain any plastids. There are many other documented mechanisms that prevent paternal inheritance in these flowering plants, such as different rates of chloroplast replication within the embryo. Among angiosperms, paternal chloroplast inheritance is observed more often in Hybrid (biology), hybrids than in offspring from parents of the same species. This suggests that incompatible hybrid genes might interfere with the mechanisms that prevent paternal inheritance.


Transplastomic plants

Recently, chloroplasts have caught attention by developers of genetically modified crops. Since, in most flowering plants, chloroplasts are not inherited from the male parent, transgenes in these plastids cannot be disseminated by pollen. This makes plastid transformation a valuable tool for the creation and cultivation of genetically modified plants that are biologically contained, thus posing significantly lower environmental risks. This biological containment strategy is therefore suitable for establishing the Co-existence of genetically modified and conventional crops and derived food and feed, coexistence of conventional and organic agriculture. While the reliability of this mechanism has not yet been studied for all relevant crop species, recent results in tobacco plants are promising, showing a failed containment rate of transplastomic plants at 3 in 1,000,000.


References


External links


Chloroplast – Cell Centered Database
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NCBI full chloroplast genome
{{Authority control Organelles Photosynthesis Endosymbiotic events