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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" (''Doctor Who'' audio), an episode of the audio drama ''Cyberman'' * Conversion (Stargate Atlantis), "Conversion" (''Stargate Atlantis ...
light energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...

light energy
into
chemical energy Chemical energy is the energy of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects th ...
that, through
cellular respiration Cellular respiration is a set of metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such a ...

cellular respiration
, can later be released to fuel the organism's activities. Some of this chemical energy is stored in
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
molecules, such as
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
s and
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 ...
es, which are synthesized 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
and
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
– hence the name ''photosynthesis'', from the
Greek#REDIRECT 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 million as of ...
''phōs'' (), "light", and ''sunthesis'' (), "putting together". In most cases,
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 ...
is also released as a waste product that stores three times more chemical energy than the carbohydrates. Most
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
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
perform photosynthesis; such organisms are called
photoautotrophs Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlorophyll ...
. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...

life
on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s called
reaction center A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors that together execute the primary energy conversion reactions of photosynthesis Photosynthesis is a process used by plants and other organisms to ...
s that contain green
chlorophyll Chlorophyll (also chlorophyl) is any of several related green 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 ...

chlorophyll
(and other colored) pigments/chromophores. In plants, these proteins are held inside
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s called
chloroplast 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 structure ...

chloroplast
s, which are most abundant in leaf cells, while in bacteria they are embedded in the
plasma 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 ...
. In these light-dependent reactions, some energy is used to strip
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
s from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced
nicotinamide adenine dinucleotide phosphate 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 sy ...
(NADPH) and
adenosine triphosphate Adenosine triphosphate (ATP) is an 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 properti ...

adenosine triphosphate
(ATP), the "energy currency" of cells. In plants, algae and cyanobacteria, sugars are synthesized by a subsequent sequence of reactions called the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can ...

Calvin cycle
. In the Calvin cycle, atmospheric carbon dioxide is
incorporated Incorporated may refer to: * Incorporated community * Incorporated (Grip Inc. album), ''Incorporated'' (Grip Inc. album), 2004, by Grip Inc. * Incorporated (Legion of Doom album), ''Incorporated'' (Legion of Doom album), 2006 * Incorporated (TV seri ...
into already existing organic carbon compounds, such as
ribulose bisphosphate Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal carbon fixation, acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose (ketone-containing sug ...
(RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
. In other bacteria, different mechanisms such as the
reverse Krebs cycle The reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle, or the reductive tricarboxylic acid cycle, or the reductive TCA cycle) is a sequence of chemical reactions A ...
are used to achieve the same end. The first photosynthetic organisms probably
evolved Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from parent to offs ...
early in the
evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water ...
and most likely used
reducing agent A reducing agent (also called a reductant, reducer, or electron donor) is an element or compound that loses or "donates" an electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one ele ...
s such as
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
or
hydrogen sulfide Hydrogen sulfide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...

hydrogen sulfide
, rather than water, as sources of electrons. Cyanobacteria appeared later; the
excess oxygen
excess oxygen
they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 
terawatts The watt (symbol: W) is a unit of Power (physics), power or radiant flux. In the International System of Units (SI), it is defined as a SI derived unit, derived unit of (in SI base units) 1 kg⋅m2⋅s−3 or, equivalently, 1 joule per second. I ...
, which is about eight times the current
power consumption of human civilization World energy supply and consumption is global production and preparation of fuel, generation of electricity, energy transport and energy consumption. It is a basic part of economic activity. It does not include energy from food. Many countries ...
. Photosynthetic organisms also convert around 100–115 billion
ton The ton is a unit of measure A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size ...
s (91–104 petagrams) of carbon into
biomass Biomass is plant or animal material used as fuel to produce electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position ...
per year. That plants receive some energy from light – in addition to air, soil, and water – was first discovered in 1779 by
Jan Ingenhousz Jan (or John) Ingenhousz or Ingen-Housz FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the Unite ...

Jan Ingenhousz
. Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds carbon in plants and further in soils and harvested products.
Cereal A cereal is any Poaceae, grass cultivated (grown) for the edible components of its grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, cereal germ, germ, and bran. The term may also refer to the resulting grain ...

Cereal
s alone are estimated to bind 3825 Tg (teragrams) of carbon dioxide every year, i.e. 3.825 billion tons.


Overview

Most photosynthetic organisms are
photoautotrophPhotoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs utilize absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlo ...
s, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis;
photoheterotrophPhotoheterotrophs ('' Gk'': ''photo'' = light, ''hetero'' = (an)other, ''troph'' = nourishment) are heterotrophic phototrophs – that is, they are organisms that use light for energy, but cannot use carbon dioxide Carbon dioxide (chemical form ...
s use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This oxygenic photosynthesis is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in
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
,
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
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
, the overall process is quite similar in these organisms. There are also many varieties of
anoxygenic photosynthesis Bacterial anoxygenic photosynthesis is differentiated from the more renowned terrestrial plant oxygenic photosynthesis by method of a terminal reductant that is (e.g. hydrogen sulfide Hydrogen sulfide is the chemical compound with the chemical f ...
, used mostly by bacteria, which consume carbon dioxide but do not release oxygen. Carbon dioxide is converted into sugars in a process called
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 ...
; photosynthesis captures energy from sunlight to convert carbon dioxide into
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
. Carbon fixation is an
endothermic In thermochemistry Thermochemistry is the study of the heat energy which is associated with chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substanc ...
redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

redox
reaction. In general outline, photosynthesis is the opposite of
cellular respiration Cellular respiration is a set of metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such a ...

cellular respiration
: while photosynthesis is a process of reduction of carbon dioxide to carbohydrate, cellular respiration is the oxidation of carbohydrate or other
nutrient A nutrient is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and t ...
s to carbon dioxide. Nutrients used in cellular respiration include carbohydrates, amino acids and fatty acids. These nutrients are oxidized to produce carbon dioxide and water, and to release chemical energy to drive the organism's
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a phys ...

metabolism
. Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different
cellular compartment Cellular compartments in cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemis ...
s. The general
equation In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...
for photosynthesis as first proposed by
Cornelis van Niel Cornelis Bernardus van Niel (November 4, 1897, Haarlem Haarlem (; predecessor of ''Harlem'' in ) is a and in the . It is the of the of . Haarlem is situated at the northern edge of the , one of the s in Europe; it is also part of the . H ...
is: : + + → + + Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is: : + + → + + This equation emphasizes that water is both a reactant in the
light-dependent reaction In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen (anatomy), lumen, and outside the thylakoid membrane is the stroma, where the light-independent react ...
and a product of the
light-independent reaction The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or conv ...
, but canceling ''n'' water molecules from each side gives the net equation: : + + → + Other processes substitute other compounds (such as
arsenite In chemistry, an arsenite is a chemical compound containing an arsenic oxoanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions. ...
) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to
arsenate The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), ...

arsenate
: The equation for this reaction is: : + + → + (used to build other compounds in subsequent reactions) Photosynthesis occurs in two stages. In the first stage, ''light-dependent reactions'' or ''light reactions'' capture the energy of light and use it to make the hydrogen carrier
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 the energy-storage molecule
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
. During the second stage, the ''light-independent reactions'' use these products to capture and reduce carbon dioxide. Most organisms that utilize oxygenic photosynthesis use
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
for the light-dependent reactions, although at least three use shortwave
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

infrared
or, more specifically, far-red radiation. Some organisms employ even more radical variants of photosynthesis. Some
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
use a simpler method that employs a pigment similar to those used for vision in animals. The
bacteriorhodopsin Bacteriorhodopsin is a protein used by Archaea, most notably by haloarchaea, a class (biology), class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. ...

bacteriorhodopsin
changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.


Photosynthetic membranes and organelles

In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in
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
s. In its simplest form, this involves the membrane surrounding the cell itself. However, the membrane may be tightly folded into cylindrical sheets called
thylakoid Thylakoids are membrane-bound compartments inside chloroplast 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 functi ...

thylakoid
s, or bunched up into round
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features of ...
called ''intracytoplasmic membranes''. These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb. In plants and algae, photosynthesis takes place in
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s called
chloroplast 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 structure ...

chloroplast
s. A typical
plant cell Plant cells are 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, Molec ...

plant cell
contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral and
peripheral membrane protein Peripheral membrane proteins are membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane ...
complexes of the photosynthetic system. Plants absorb light primarily using the
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 compound , CH4; is among the simplest organic compou ...
chlorophyll Chlorophyll (also chlorophyl) is any of several related green 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 ...

chlorophyll
. The green part of the light spectrum is not absorbed but is reflected which is the reason that most plants have a green color. Besides chlorophyll, plants also use pigments such as
carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the exce ...

carotene
s and
xanthophyll Xanthophylls (originally phylloxanthins) are yellow 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 ofte ...
s. Algae also use chlorophyll, but various other pigments are present, such as
phycocyanin Phycocyanin is a 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 compound , CH4; is among ...
,
carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the exce ...

carotene
s, and
xanthophyll Xanthophylls (originally phylloxanthins) are yellow 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 ofte ...
s in
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
,
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 ...
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
(rhodophytes) and
fucoxanthin Fucoxanthin is a xanthophyll indicates the presence of a xanthophyll pigment, and is the typical color of lutein or zeaxanthin of the xanthophylls, a division of the carotenoids group. Xanthophylls (originally phylloxanthins) are yellow pigments t ...

fucoxanthin
in
brown algae The brown algae (singular: alga), comprising the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
and
diatoms Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

diatoms
resulting in a wide variety of colors. These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together. Such a combination of proteins is also called a
light-harvesting complex A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a photosystem Photosystems are functional and structural units of protein complex A protein comple ...
. Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
. Certain species adapted to conditions of strong sunlight and
arid A region is arid when it is characterized by a severe lack of available water, to the extent of hindering or preventing the growth and development Development or developing may refer to: Arts *Development hell, when a project is stuck in d ...

arid
ity, such as many ''
Euphorbia ''Euphorbia'' is a very large and diverse genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscr ...

Euphorbia
'' and
cactus A cactus (plural cacti, cactuses, or less commonly, cactus) is a member of the plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light en ...

cactus
species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the
mesophyll A leaf (plural leaves) is the principal lateral appendage of the vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...
, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant
wax Waxes are a diverse class of organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prope ...
y
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
that protects the leaf from excessive
evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

evaporation
of water and decreases the absorption of
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
or
blue Blue is one of the three primary colours of pigments in painting and traditional colour theory In the visual arts The visual arts are art forms such as painting Painting is the practice of applying paint Paint is any pig ...

blue
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
to reduce
heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these ...

heat
ing. The transparent
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

epidermis
layer allows light to pass through to the
palisade A palisade, sometimes called a stakewall or a paling, is typically a fence A fence is a structure that encloses an area, typically outdoors, and is usually constructed from posts that are connected by boards, wire, rails or netting. A fen ...
mesophyll cells where most of the photosynthesis takes place.


Light-dependent reactions

In the
light-dependent reactions In photosynthesis 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" (''Doctor Who'' audio), an episode ...
, one molecule of the
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 compound , CH4; is among the simplest organic compou ...
chlorophyll Chlorophyll (also chlorophyl) is any of several related green 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 ...

chlorophyll
absorbs one
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
and loses one
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
. This electron is taken up by a modified form of chlorophyll called
pheophytin Pheophytin or phaeophytin is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held t ...
, which passes the electron to a
quinone The quinones are a class of organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties ...

quinone
molecule, starting the flow of electrons down an
electron transport chain An electron transport chain (ETC) is a series of protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Anc ...

electron transport chain
that leads to the ultimate reduction of
NADP 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 ...
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 ...
. In addition, this creates a
proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in Concentration, solute concentration across a me ...
(energy gradient) across the
chloroplast membrane Chloroplast A chloroplast is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, con ...
, which is used by
ATP synthase ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation of ...

ATP synthase
in the synthesis of
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
. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called
photolysis Photodissociation, photolysis, or photodecomposition is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classi ...
, which releases a
dioxygen There are several known allotropy, allotropes of oxygen. The most familiar is oxygen, molecular oxygen (O2), present at significant levels in Atmosphere of Earth, Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the high ...
(O2) molecule as a high-energy waste product. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is: Not all
wavelength 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 regular su ...

wavelength
s of light can support photosynthesis. The photosynthetic action spectrum depends on the type of
accessory pigmentAccessory pigments are pigment, light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll a, chlorophyll ''a''. They include other forms of this pigment, such as chlorophyll ''b'' in green algal and highe ...
s present. For example, in green plants, the action spectrum resembles the
absorption spectrum Absorption may refer to: Chemistry and biology *Absorption (chemistry) In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: th ...
for
chlorophyll Chlorophyll (also chlorophyl) is any of several related green 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 ...

chlorophyll
s and
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organ ...
s with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above-ground green plants. The non-absorbed part of the
light spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without steps, across a Continuum (theory), ...

light spectrum
is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.


Z scheme

In plants,
light-dependent reaction In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen (anatomy), lumen, and outside the thylakoid membrane is the stroma, where the light-independent react ...
s occur in the
thylakoid membrane Thylakoids are membrane-bound compartments inside chloroplast 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 functi ...

thylakoid membrane
s of the
chloroplast 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 structure ...

chloroplast
s where they drive the synthesis of ATP and NADPH. The light-dependent reactions are of two forms: cyclic and non-cyclic. In the non-cyclic reaction, the
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
s are captured in the light-harvesting
antenna complex The light-harvesting complex (or antenna complex; LH or LHC) is an array of protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallogr ...
es of
photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ...
by
chlorophyll Chlorophyll (also chlorophyl) is any of several related green 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 ...

chlorophyll
and other
accessory pigmentsAccessory pigments are light-absorbing compounds, found in 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 ...
(see diagram at right). The absorption of a photon by the antenna complex loosens an electron by a process called
photoinduced charge separation Photoinduced charge separation is the process of an electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physic ...
. The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That loosened electron is taken up by the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an
electron transport chain An electron transport chain (ETC) is a series of protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Anc ...
(the so-called ''Z-scheme'' shown in the diagram), a
chemiosmotic potential An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in Concentration, solute concentration across a me ...
is generated by pumping proton cations (H+) across the membrane and into the thylakoid space. An
ATP synthase ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation of ...

ATP synthase
enzyme uses that chemiosmotic potential to make ATP during
photophosphorylation In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of ...
, whereas
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
is a product of the terminal
redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

redox
reaction in the ''Z-scheme''. The electron enters a chlorophyll molecule in
Photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

Photosystem I
. There it is further excited by the light absorbed by that
photosystem Photosystems are functional and structural units of protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', A ...
. The electron is then passed along a chain of
electron acceptor An electron acceptor is a chemical entity that accepts electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle phy ...
s to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is eventually used to reduce the co-enzyme NADP with a H+ to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends. The cyclic reaction is similar to that of the non-cyclic but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name ''cyclic reaction''.


Water photolysis

Linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized. Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center (P700) of
photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

photosystem I
are replaced by transfer from
plastocyanin Plastocyanin is a copper-containing protein that mediates electron-transfer. It is found in a variety of plants, where it participates in photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or ...
, whose electrons come from electron transport through
photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ...

photosystem II
. Photosystem II, as the first step of the ''Z-scheme'', requires an external source of electrons to reduce its oxidized, high-energy chlorophyll ''a'' reaction center, called P680. The source of electrons for photosynthesis in green plants and cyanobacteria is water. Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule of diatomic
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
and four
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
ions. The electrons yielded are transferred to a redox-active
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemi ...

tyrosine
residue that is oxidized by the energy of P680. This resets the ability of P680 to absorb another photon and release another photo-dissociated electron. The oxidation of water is
catalyzed Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalyzed
in photosystem II by a redox-active structure that contains four
manganese Manganese is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical e ...

manganese
ions and a calcium ion; this
oxygen-evolving complex The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the cofactor of the photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic ...
binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Kok's S-state diagrams). Photosystem II is the only known biological
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
that carries out the oxidation of water. The hydrogen ions are released in the thylakoid lumen and therefore contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen and its energy for
cellular respiration Cellular respiration is a set of metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such a ...

cellular respiration
, including photosynthetic organisms.


Light-independent reactions


Calvin cycle

In the light-independent (or "dark") reactions, the
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 ...
captures from the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
and, in a process called the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can ...

Calvin cycle
, uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is
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 ...
produces the three-carbon sugar intermediate, which is then converted into the final carbohydrate products. The simple carbon sugars produced by photosynthesis are then used to form other organic compounds, such as the building material
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
, the precursors for
lipid 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 mechanis ...
and
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
biosynthesis, or as a fuel in
cellular respiration Cellular respiration is a set of metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such a ...

cellular respiration
. The latter occurs not only in plants but also in
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s when the carbon and energy from plants is passed through a
food chain A food chain is a linear network of links in a food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource sy ...

food chain
. The fixation or reduction of carbon dioxide is a process in which
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
combines with a five-carbon sugar,
ribulose 1,5-bisphosphate Ribulose 1,5-bisphosphate (RuBP) is an organic substance In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone o ...
, to yield two molecules of a three-carbon compound,
glycerate 3-phosphate 3-Phosphoglyceric acid (3PG) is the conjugate acid of glycerate 3-phosphate (GP). The glycerate is a biochemically significant metabolic intermediate in both glycolysis Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway ...

glycerate 3-phosphate
, also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of
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
produced during the light-dependent stages, is reduced to
glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is the metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical ...

glyceraldehyde 3-phosphate
. This product is also referred to as 3-phosphoglyceraldehyde ( PGAL) or, more generically, as
triose A triose is a monosaccharide Monosaccharides (from Greek#REDIRECT 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 Southe ...
phosphate. Most (5 out of 6 molecules) of the glyceraldehyde 3-phosphate produced are used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "recycled" often condense to form
hexose In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
phosphates, which ultimately yield
sucrose Sucrose is a type of sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosacchari ...

sucrose
,
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 ...
and
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
. The sugars produced during carbon
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a phys ...

metabolism
yield carbon skeletons that can be used for other metabolic reactions like the production of
amino acids 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 acids
and
lipids 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 mechanis ...

lipids
.


Carbon concentrating mechanisms


On land

In hot and dry conditions, plants close their
stomata In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...

stomata
to prevent water loss. Under these conditions, will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, causing an increase of
photorespiration Photorespiration (also known as the oxidative photosynthetic carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere ...

photorespiration
by the
oxygenase An oxygenase is any 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 ...
activity of
ribulose-1,5-bisphosphate carboxylase/oxygenase 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 decrease in carbon fixation. Some plants have
evolved Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from parent to offs ...

evolved
mechanisms to increase the concentration in the leaves under these conditions. Plants that use the C4 carbon fixation process chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule
phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the substi ...
(PEP), a reaction catalyzed by an enzyme called
PEP carboxylase PEP carboxylase may refer to: * Phosphoenolpyruvate carboxylase Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPCEC 4.1.1.31 PDB ID: 3ZGE) is an enzyme Enzymes () are proteins that act as biological catalysts ...
, creating the four-carbon organic acid
oxaloacetic acid Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical ...

oxaloacetic acid
. Oxaloacetic acid or
malate Malic acid is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavi ...
synthesized by this process is then translocated to specialized
bundle sheath A vascular bundle is a part of the transport system in vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'wind ...
cells where 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 ...
and other Calvin cycle enzymes are located, and where released by
decarboxylation Decarboxylation is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter having constant ...

decarboxylation
of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon
3-phosphoglyceric acid 3-Phosphoglyceric acid (3PG) is the conjugate acid of glycerate 3-phosphate (GP). The glycerate is a biochemically significant metabolic intermediate in both glycolysis Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway ...
s. The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases fixation and, thus, the
photosynthetic capacity Photosynthetic capacity (Amax) is a measure of the maximum rate at which leaves are able to carbon fixation, fix carbon during photosynthesis. It is typically measured as the amount of carbon dioxide that is fixed per metre squared per second, for ...
of the leaf. plants can produce more sugar than plants in conditions of high light and temperature. Many important crop plants are plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use carbon fixation, compared to 3% that use carbon fixation; however, the evolution of in over 60 plant lineages makes it a striking example of
convergent evolution Convergent evolution is the independent evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; eit ...
.
Xerophytes A xerophyte (from Greek#REDIRECT 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 approxim ...
, such as
cacti A cactus (plural cacti, cactuses, or less commonly, cactus) is a member of the plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light en ...

cacti
and most
succulents In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...

succulents
, also use PEP carboxylase to capture carbon dioxide in a process called
Crassulacean acid metabolism Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation Carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide Carbon dioxide (chemical form ...
(CAM). In contrast to metabolism, which ''spatially'' separates the fixation to PEP from the Calvin cycle, CAM ''temporally'' separates these two processes. CAM plants have a different leaf anatomy from plants, and fix the at night, when their stomata are open. CAM plants store the mostly in the form of
malic acid Malic acid is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavi ...
via carboxylation of
phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the substi ...
to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. CAM is used by 16,000 species of plants.
Calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium Calcium is a with the Ca and 20. As an , calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties ...

Calcium oxalate
accumulating plants, such as ''
Amaranthus hybridus ''Amaranthus hybridus'', commonly called green amaranth, slim amaranth, smooth amaranth, smooth pigweed, or red amaranth, is a species of annual flowering plant. It is a weedy species found now over much of North America and introduced into Europ ...

Amaranthus hybridus
'' and ''
Colobanthus quitensis ''Colobanthus quitensis'', the Antarctic pearlwort, is one of two native flowering plants found in the Antarctic region. It has yellow flowers and grows about 5 cm (two inches) tall, with a cushion plant, cushion-like growth habit that gives it a ...
,'' show a variation of photosynthesis where calcium oxalate
crystals A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinet ...
function as dynamic carbon pools, supplying carbon dioxide () to photosynthetic cells when
stoma In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...

stoma
ta are partially or totally closed. This process was named Alarm photosynthesis. Under stress conditions (e.g. water deficit) oxalate released from calcium oxalate crystals is converted to CO2 by an
oxalate oxidase In enzymology, an oxalate oxidase () is an enzyme that catalysis, catalyzes the chemical reaction :oxalate + O2 + 2 H+ \rightleftharpoons 2 CO2 + H2O2 The 3 substrate (biochemistry), substrates of this enzyme are oxalate, oxygen, O2, and hydrogen ...
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
and the produced CO2 can support the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can ...

Calvin cycle
reactions. Reactive hydrogen peroxide (), the byproduct of oxalate oxidase reaction, can be neutralized by
catalase Catalase is a common enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one loc ...

catalase
. Alarm photosynthesis represents a photosynthetic variant to be added to the well-known C4 and
CAM A cam is a rotating or sliding piece in a mechanical linkage A mechanical linkage is an assembly of systems connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to ...
pathways. However, alarm photosynthesis, in contrast to these pathways, operates as a biochemical pump that collects carbon from the organ interior (or from the soil) and not from the atmosphere.


In water

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
possess
carboxysome Carboxysomes are bacterial microcompartments (BMCs) consisting of polyhedral protein shells filled with the enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)—the predominant enzyme in carbon fixation and the Rate-determining ...

carboxysome
s, which increase the concentration of around RuBisCO to increase the rate of photosynthesis. An enzyme,
carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyst, catalyze the interconversion between carbon dioxide and water and the Dissociation (chemistry), dissociated ions of carbonic acid (i.e. bicarbonate and ...

carbonic anhydrase
, located within the carboxysome releases CO2 from dissolved hydrocarbonate ions (HCO). Before the CO2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO ions are made from CO2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase. This causes the HCO ions to accumulate within the cell from where they diffuse into the carboxysomes.
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
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
hornwort Hornworts are a group of bryophyte Bryophytes are an informal group consisting of three divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways th ...
s also act to concentrate around RuBisCO.


Order and kinetics

The overall process of photosynthesis takes place in four stages:


Efficiency

Plants usually convert light into
chemical energy Chemical energy is the energy of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects th ...
with a
photosynthetic efficiencyThe photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cell ...
of 3–6%. Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%) re-emitted as
chlorophyll fluorescence Chlorophyll fluorescence is light re-emitted by chlorophyll molecules during return from Excited state, excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in plants, algae and bacteria. Excited chlorophy ...
at longer (redder) wavelengths. This fact allows measurement of the light reaction of photosynthesis by using chlorophyll fluorometers. Actual plants' photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of carbon dioxide in the atmosphere, and can vary from 0.1% to 8%. By comparison,
solar panels A solar cell panel, solar electric panel, photo-voltaic (PV) module or just solar panel is an assembly of photo-voltaic cells mounted in a framework for installation. Solar panels use sunlight Sunlight is a portion of the given off by ...
convert light into
electric energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been converted ''from'' electric potential energy. This energy is supplied by the com ...
at an efficiency of approximately 6–20% for mass-produced panels, and above 40% in laboratory devices. Scientists are studying photosynthesis in hopes of developing plants with increased yield. The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex. For example, the ATP and NADPH energy molecules, created by the light reaction, can be used for carbon fixation or for photorespiration in C3 plants. Electrons may also flow to other electron sinks. For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions. Chlorophyll fluorescence of photosystem II can measure the light reaction, and infrared gas analyzers can measure the dark reaction. It is also possible to investigate both at the same time using an integrated chlorophyll fluorometer and gas exchange system, or by using two separate systems together. Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO2, and of ΔH2O using reliable methods CO2 is commonly measured in μmols/(m2/s), parts per million or volume per million and H2O is commonly measured in mmol/(m2/s) or in mbars. By measuring CO2 assimilation, ΔH2O, leaf temperature, barometric pressure, leaf area, and photosynthetically active radiation or PAR, it becomes possible to estimate, "A" or carbon assimilation, "E" or transpiration, "gs" or stomatal conductance, and Ci or intracellular CO2. However, it is more common to used chlorophyll fluorescence for plant stress measurement, where appropriate, because the most commonly used parameters FV/FM and Y(II) or F/FM' can be measured in a few seconds, allowing the investigation of larger plant populations. Gas exchange systems that offer control of CO2 levels, above and below ambient, allow the common practice of measurement of A/Ci curves, at different CO2 levels, to characterize a plant's photosynthetic response. Integrated chlorophyll fluorometer – gas exchange systems allow a more precise measure of photosynthetic response and mechanisms. While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of CC to replace Ci. The estimation of CO2 at the site of carboxylation in the chloroplast, or CC, becomes possible with the measurement of mesophyll conductance or gm using an integrated system. Photosynthesis measurement systems are not designed to directly measure the amount of light absorbed by the leaf. But analysis of chlorophyll-fluorescence, P700- and P515-absorbance and gas exchange measurements reveal detailed information about e.g. the photosystems, quantum efficiency and the CO2 assimilation rates. With some instruments, even wavelength-dependency of the photosynthetic efficiency can be analyzed. A phenomenon known as
quantum walk Quantum walks are quantum mechanics, quantum analogues of classical random walks. In contrast to the classical random walk, where the walker occupies definite states and the randomness arises due to stochastic Transition of state, transitions betwe ...
increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of an alga, bacterium, or plant, there are light-sensitive molecules called
chromophore A chromophore is the part of a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in posi ...

chromophore
s arranged in an antenna-shaped structure named a photocomplex. When a photon is absorbed by a chromophore, it is converted into a
quasiparticle 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 regular suc ...
referred to as an
exciton An exciton is a bound state In quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of ...

exciton
, which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form accessible to the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose" the most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time. Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances. Obstacles in the form of destructive interference cause the particle to lose its wave properties for an instant before it regains them once again after it is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks.


Evolution

Early photosynthetic systems, such as those in
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 ...
and purple sulfur and
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 ...
and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules than water as
electron donor An electron donor is a chemical entity that donates electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics ...
s. Green and purple sulfur bacteria are thought to have used
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
as electron donors. Green nonsulfur bacteria used various
amino In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...

amino
and other
organic acid An organic acid is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beha ...
s as an electron donor. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time. Fossils of what are thought to be Protein filament, filamentous photosynthetic organisms have been dated at 3.4 billion years old. More recent studies also suggest that photosynthesis may have begun about 3.4 billion years ago. The main source of
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
in the Earth's atmosphere derives from Oxygen evolution, oxygenic photosynthesis, and the first appearance of this high-energy molecule is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that 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
, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic, using water as an electron donor, which is oxidized to molecular oxygen in the photosynthetic reaction center.


Symbiosis and the origin of chloroplasts

Several groups of animals have formed symbiosis, symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes. In addition, a few marine mollusks ''Elysia viridis'' and ''Elysia chlorotica'' also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies (see Kleptoplasty). This allows the mollusks to survive solely by photosynthesis for several months at a time. Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive. An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosome, and similar proteins in the photosynthetic reaction center. The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those 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
. DNA in chloroplasts codes for
redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

redox
proteins such as those found in the photosynthetic reaction centers. The CoRR Hypothesis proposes that this co-location of genes with their gene products is required for redox regulation of gene expression, and accounts for the persistence of DNA in bioenergetic organelles.


Photosynthetic eukaryotic lineages

Symbiotic and Kleptoplasty, kleptoplastic organisms excluded: *The glaucophytes and the Red algae, red and Viridiplantae, green algae—clade Archaeplastida (uni- and multicellular) *The Cryptophyceae, cryptophytes—clade Cryptista (unicellular) *The haptophytes—clade Haptista (unicellular) *The dinoflagellates and Chromerida, chromerids in the superphylum Myzozoa—clade Alveolata (unicellular) *The Ochrophyta, ochrophytes—clade Heterokonta (uni- and multicellular) *The chlorarachniophytes and 3 species of Paulinella in the phylum Cercozoa—clade Rhizaria (unicellular) *The euglenids—clade Excavata (unicellular) Except for the euglenids, which are found within the Excavata, all of these belong to the Plants+HC+SAR megagroup, Diaphoretickes. Archaeplastida and the photosynthetic Paulinella got their plastids, which are surrounded by two membranes, through primary Endosymbiont, endosymbiosis in two separate events, by engulfing a cyanobacterium. The plastids in all the other groups have either a red or green algal origin, and are referred to as the "red lineages" and the "green lineages". In dinoflagellates and euglenids the plastids are surrounded by three membranes, and in the remaining lines by four. A nucleomorph, remnants of the original algal nucleus located between the inner and outer membranes of the plastid, is present in the cryptophytes (from a red alga) and chlorarachniophytes (from a green alga). Some dinoflaggelates that lost their photosyntethic ability later regained it again through new endosymbiotic events with different algae. While able to perform photosynthesis, many of these eukaryotic groups are mixotrophs and practice heterotrophy to various degrees.


Cyanobacteria and the evolution of photosynthesis

The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant
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
(formerly called blue-green algae), which are the only prokaryotes performing oxygenic photosynthesis. The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier. Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen. Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of cyanobacteria. Cyanobacteria remained the principal primary producers of oxygen throughout the Proterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation. Green algae joined cyanobacteria as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251–66 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the primary production of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as Oxygen cycle, primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.


Experimental history


Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century. Jan Baptist van Helmont, Jan van Helmont began the research of the process in the mid-17th century when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate – much of the gained mass comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's
biomass Biomass is plant or animal material used as fuel to produce electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position ...
comes from the inputs of photosynthesis, not the soil itself. Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar and burned a candle in it (which gave off CO2), the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly Breathing, "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1779,
Jan Ingenhousz Jan (or John) Ingenhousz or Ingen-Housz FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the Unite ...

Jan Ingenhousz
repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to revive a mouse in a matter of hours. In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterward, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2 but also to the incorporation of water. Thus, the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.


Refinements

Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen Reducing agent, reduces (donates its atoms as electrons and protons to) carbon dioxide. Robert Emerson (scientist), Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light. With the red alone, the light reactions were suppressed. When blue and red were combined, the output was much more substantial. Thus, there were two photosystems, one absorbing up to 600 nm wavelengths, the other up to 700 nm. The former is known as PSII, the latter is PSI. PSI contains only chlorophyll "a", PSII contains primarily chlorophyll "a" with most of the available chlorophyll "b", among other pigments. These include phycobilins, which are the red and blue pigments of red and blue algae, respectively, and fucoxanthol for brown algae and diatoms. The process is most productive when the absorption of quanta is equal in both PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII systems, which in turn powers the photochemistry. Robin Hill (biochemist), Robert Hill thought that a complex of reactions consisted of an intermediate to cytochrome b6 (now a plastoquinone), and that another was from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water were performed by Hill in 1937 and 1939. He showed that isolated
chloroplast 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 structure ...

chloroplast
s give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. In the Hill reaction: :2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2 A is the electron acceptor. Therefore, in light, the electron acceptor is reduced and oxygen is evolved. Sam Ruben, Samuel Ruben and Martin Kamen used Radionuclide, radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water. Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the
Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can ...

Calvin cycle
, but many scientists refer to it as the Calvin-Benson, Benson-Calvin, or even Calvin-Benson-Bassham (or CBB) Cycle. Nobel Prize-winning scientist Rudolph A. Marcus was later able to discover the function and significance of the electron transport chain. Otto Heinrich Warburg and Dean Burk discovered the I-quantum photosynthesis reaction that splits CO2, activated by the respiration. In 1950, first experimental evidence for the existence of
photophosphorylation In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of ...
''in vivo'' was presented by Otto Kandler using intact ''Chlorella'' cells and interpreting his findings as light-dependent
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
formation. In 1954, Daniel I. Arnon et al. discovered photophosphorylation ''in vitro'' in isolated
chloroplast 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 structure ...

chloroplast
s with the help of P32. Louis N.M. Duysens and Jan Amesz discovered that chlorophyll "a" will absorb one light, oxidize cytochrome f, while chlorophyll "a" (and other pigments) will absorb another light but will reduce this same oxidized cytochrome, stating the two light reactions are in series.


Development of the concept

In 1893, Charles Reid Barnes proposed two terms, ''photosyntax'' and ''photosynthesis'', for the biological process of ''synthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the influence of light''. Over time, the term ''photosynthesis'' came into common usage. Later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term.


C3 : C4 photosynthesis research

In the late 1940s at the University of California, Berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemists Melvin Calvin, Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques. The pathway of CO2 fixation by the algae ''Chlorella'' in a fraction of a second in light resulted in a 3 carbon molecule called phosphoglyceric acid (PGA). For that original and ground-breaking work, a Nobel Prize in Chemistry was awarded to Melvin Calvin in 1961. In parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 μmol CO2·m−2·s−1, with the conclusion that all terrestrial plants have the same photosynthetic capacities, that are light saturated at less than 50% of sunlight. Later in 1958–1963 at Cornell University, field grown maize was reported to have much greater leaf photosynthetic rates of 40 μmol CO2·m−2·s−1 and not be saturated at near full sunlight. This higher rate in maize was almost double of those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. At the University of Arizona, detailed gas exchange research on more than 15 species of Monocotyledon, monocot and Dicotyledon, dicot uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species. In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 μmol CO2·m−2·s−1, and the leaves have two types of green cells, i. e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. This type of anatomy was termed Kranz anatomy in the 19th century by the botanist Gottlieb Haberlandt while studying leaf anatomy of sugarcane. Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light. The research at Arizona was designated a Citation Classic in 1986. These species were later termed C4 plants as the first stable compound of CO2 fixation in light has 4 carbons as malate and aspartate. Other species that lack Kranz anatomy were termed C3 type such as cotton and sunflower, as the first stable carbon compound is the 3-carbon PGA. At 1000 ppm CO2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 μmol CO2·m−2·s−1 indicating the suppression of photorespiration in C3 plants.


Factors

There are three main factors affecting photosynthesis and several corollary factors. The three main are: * Light irradiance and
wavelength 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 regular su ...

wavelength
* Carbon dioxide concentration * Temperature. Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the
chloroplast 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 structure ...

chloroplast
s to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.


Light intensity (irradiance), wavelength and temperature

The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life. The radiation climate within plant communities is extremely variable, in both time and space. In the early 20th century, Frederick Blackman and Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation. * At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance. * At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased. These two experiments illustrate several important points: First, it is known that, in general, photochemical reactions are not affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are the light-dependent 'photochemical' temperature-independent stage, and the light-independent, temperature-dependent stage. Second, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, a series of proteins with different pigments surround the reaction center. This unit is called a phycobilisome.


Carbon dioxide levels and photorespiration

As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors.
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 ...
, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, called
photorespiration Photorespiration (also known as the oxidative photosynthetic carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere ...

photorespiration
, uses energy, but does not produce sugars. RuBisCO oxygenase activity is disadvantageous to plants for several reasons: # One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle. # Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis. # Salvaging glycolate is an energetically expensive process that uses the glycolate pathway, and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produce ammonia (NH3), which is able to Molecular diffusion, diffuse out of the plant, leading to a loss of nitrogen. ::A highly simplified summary is: :::2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP + NH3 The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.


See also

* Jan Anderson (scientist) * Artificial photosynthesis * Calvin-Benson cycle *
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 ...
* Cellular respiration * Chemosynthesis * Daily light integral * Hill reaction * Integrated fluorometer * Light-dependent reaction * Organic reaction * Photobiology * Photoinhibition * Photosynthetic reaction center * Photosynthetically active radiation * Photosystem *
Photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

Photosystem I
* Photosystem II * Quantum biology * Radiosynthesis (metabolism), Radiosynthesis * Red edge * Vitamin D


References


Further reading


Books

* * * *


Papers

* *


External links


A collection of photosynthesis pages for all levels from a renowned expert (Govindjee)


* [https://web.archive.org/web/20090428090455/http://scienceaid.co.uk/biology/biochemistry/photosynthesis.html Science Aid: Photosynthesis] Article appropriate for high school science
Metabolism, Cellular Respiration and Photosynthesis – The Virtual Library of Biochemistry and Cell Biology


* [http://www.life.uiuc.edu/govindjee/photosynBook.html Overall Energetics of Photosynthesis]
The source of oxygen produced by photosynthesis
Interactive animation, a textbook tutorial *
Photosynthesis – Light Dependent & Light Independent Stages

Khan Academy, video introduction
{{Authority control Photosynthesis, Agronomy Biological processes Botany Cellular respiration Ecosystems Metabolism Plant nutrition Plant physiology Quantum biology