The cell is the basic structural and functional unit of
life forms. Every cell consists of a
cytoplasm enclosed within a
membrane, and contains many
biomolecule
A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include large ...
s such as
proteins,
DNA and
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, as well as many small molecules of nutrients and
metabolites.
[Cell Movements and the Shaping of the Vertebrate Body](_blank)
in Chapter 21 of
Molecular Biology of the Cell
'' fourth edition, edited by Bruce Alberts (2002) published by Garland Science. The Alberts text discusses how the "cellular building blocks" move to shape developing embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s. It is also common to describe small molecules such as amino acids as
molecular building blocks
". The term comes from the
Latin word meaning 'small room'.
Cells can acquire specified function and carry out various tasks within the cell such as replication,
DNA repair, protein synthesis, and motility. Cells are capable of specialization and mobility within the cell. Most cells are measured in micrometers due to their small size.
Most plant and animal cells are only visible under a
light microscope
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...
, with dimensions between 1 and 100
micrometres.
Electron microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
gives a much higher resolution showing greatly detailed cell structure. Organisms can be classified as
unicellular (consisting of a single cell such as
bacteria) or
multicellular (including plants and animals).
Most
unicellular organisms are classed as
microorganisms. The number of cells in plants and animals varies from species to species; it has been approximated that the human body contains an estimated 37 trillion (3.72×10
13) cells. The brain accounts for around 80 billion of these cells.
The study of cells and how they work has led to many other studies in related areas of biology, including:
discovery of DNA The history of molecular biology begins in the 1930s with the convergence of various, previously distinct biological and physical disciplines: biochemistry, genetics, microbiology, virology and physics. With the hope of understanding life at its mo ...
,
cancer systems biology Cancer systems biology encompasses the application of systems biology approaches to cancer research, in order to study the disease as a complex adaptive system with emerging properties at multiple biological scales.Wang, Edwin. ''Cancer Systems Bi ...
,
aging
Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...
and
developmental biology
Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of Regeneration (biology), regeneration, asexual reproduction, metamorphosis, and the growth and di ...
.
Cell biology
Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and ...
is the study of cells, which were discovered by
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
in 1665, who named them for their resemblance to
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
inhabited by
Christian monks in a monastery.
Cell theory, first developed in 1839 by
Matthias Jakob Schleiden and
Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells. Cells emerged on Earth about 4 billion years ago.
Cell types
Cells are of two types:
eukaryotic, which contain a
nucleus, and
prokaryotic cells, which do not have a nucleus, but a nucleoid region is still present. Prokaryotes are
single-celled organisms, while eukaryotes may be either single-celled or
multicellular.
Prokaryotic cells
Prokaryotes include
bacteria and
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
, two of the
three domains of life
In Biology, biological Taxonomy (biology), taxonomy, a domain ( or ) (Latin: ''regio''), also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of ...
. Prokaryotic cells were the first form of
life on Earth, characterized by having vital
biological process
Biological processes are those processes that are vital for an organism to live, and that shape its capacities for interacting with its environment. Biological processes are made of many chemical reactions or other events that are involved in the ...
es including
cell signaling. They are simpler and smaller than eukaryotic cells, and lack a
nucleus, and other membrane-bound
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s. The
DNA of a prokaryotic cell consists of a single
circular chromosome that is in direct contact with the
cytoplasm. The nuclear region in the cytoplasm is called the
nucleoid
The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome of a prokaryote is circular, and its length is very large compared to the cell dim ...
. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter.
A prokaryotic cell has three regions:
* Enclosing the cell is the
cell envelope – generally consisting of a
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 that separates and protects the interior of all cells from the outside environment (t ...
covered by a
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
which, for some bacteria, may be further covered by a third layer called a
capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as ''
Mycoplasma
''Mycoplasma'' is a genus of bacteria that, like the other members of the class ''Mollicutes'', lack a cell wall around their cell membranes. Peptidoglycan (murein) is absent. This characteristic makes them naturally resistant to antibiotics ...
'' (bacteria) and ''
Thermoplasma'' (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of
peptidoglycan in bacteria and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (
cytolysis) from
osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure ...
due to a
hypotonic environment. Some eukaryotic cells (
plant cell
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capabi ...
s and
fungal cells) also have a cell wall.
* Inside the cell is the
cytoplasmic region that contains the
genome (DNA), ribosomes and various sorts of inclusions.
The genetic material is freely found in the cytoplasm. Prokaryotes can carry
extrachromosomal DNA
Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosomes, either inside or outside the nucleus of a cell. Most DNA in an individual genome is found in chromosomes contained in the nucleus. Multiple forms of extrachromo ...
elements called
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...
s, which are usually circular. Linear bacterial plasmids have been identified in several species of
spirochete bacteria, including members of the genus ''
Borrelia'' notably ''
Borrelia burgdorferi'', which causes Lyme disease. Though not forming a nucleus, the
DNA is condensed in a
nucleoid
The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome of a prokaryote is circular, and its length is very large compared to the cell dim ...
. Plasmids encode additional genes, such as
antibiotic resistance
Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...
genes.
* On the outside,
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
and
pili Pili may refer to:
Common names of plants
* ''Canarium ovatum'', a Philippine tree that is a source of the pili nut
* ''Heteropogon contortus'', a Hawaiian grass used to thatch structures
Places
* Pili, Camarines Sur, is a municipality in the ...
project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.
Eukaryotic cells
Plants,
animals,
fungi,
slime moulds,
protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, and
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
are all
eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is
compartmentalization: the presence of membrane-bound
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s (compartments) in which specific activities take place. Most important among these is a
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells ...
,
an organelle that houses the cell's
DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Some of the other differences are:
* The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
* The eukaryotic DNA is organized in one or more linear molecules, called
chromosomes, which are associated with
histone proteins. All chromosomal DNA is stored in the
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells ...
, separated from the cytoplasm by a membrane.
Some eukaryotic organelles such as
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
also contain some DNA.
* Many eukaryotic cells are
ciliated with
primary cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
. Primary cilia play important roles in chemosensation,
mechanosensation, and
thermosensation. Each cilium may thus be "viewed as a sensory cellular
antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
* Motile eukaryotes can move using
motile cilia or
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
. Motile cells are absent in
conifer
Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
s and
flowering plants.
[PH Raven, Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York] Eukaryotic flagella are more complex than those of prokaryotes.
Cell shapes
Cell shape, also called cell morphology, has been hypothesized to form from the arrangement and movement of the cytoskeleton. Many advancements in the study of cell morphology come from studying simple bacteria such as ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...
'', ''
E. coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'', and
''B. subtilis''. Different cell shapes have been found and described, but how and why cells form different shapes is still widely unknown.
Some cell shapes that have been identified include rods, cocci and spirochaetes. Cocci have a circular shape, bacilli have an elongated rod-like shape, and spirochaetes have a spiral shape. Many other shapes have also been determined.
Subcellular components
All cells, whether
prokaryotic or
eukaryotic, have a
membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the
electric potential of the cell. Inside the membrane, the
cytoplasm takes up most of the cell's volume. Except
red blood cells, which lack a cell nucleus and most organelles to accommodate maximum space for
hemoglobin, all cells possess
DNA, the hereditary material of
genes, and
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, containing the information necessary to
build
Build may refer to:
* Engineering something
* Construction
* Physical body stature, especially muscle size; usually of the human body
* Build (game engine), a 1995 first-person shooter engine
* "Build" (song), a 1987 song by The Housemartins
* ...
various
proteins such as
enzymes, the cell's primary machinery. There are also other kinds of
biomolecule
A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include large ...
s in cells. This article lists these primary
cellular components, then briefly describes their function.
Cell membrane
The
cell membrane, or plasma membrane, is a selectively permeable
biological membrane
A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the ce ...
that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a
double layer of phospholipids, which are
amphiphilic (partly
hydrophobic and partly
hydrophilic). Hence, the layer is called a
phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the
porosome
440px
280px Porosomes are cup-shaped supramolecular structures in the cell membranes of eukaryotic cells where secretory vesicles transiently dock in the process of vesicle fusion and secretion. The transient fusion of secretory vesicle membr ...
the universal secretory portal in cells and a variety of
protein molecules that act as channels and pumps that move different molecules into and out of the cell.
The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (
molecule or
ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain
receptor proteins that allow cells to detect external signaling molecules such as
hormones.
Cytoskeleton
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during
endocytosis
Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...
, the uptake of external materials by a cell, and
cytokinesis, the separation of daughter cells after
cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
s,
intermediate filament
Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate ''Branchiostoma''.
Intermedia ...
s and
microfilaments. In the cytoskeleton of a
neuron the intermediate filaments are known as
neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.
The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape,
polarity
Polarity may refer to:
Science
* Electrical polarity, direction of electrical current
* Polarity (mutual inductance), the relationship between components such as transformer windings
* Polarity (projective geometry), in mathematics, a duality of o ...
and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called
actin. The subunit of microtubules is a dimeric molecule called
tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments include
vimentin,
desmin,
lamin (lamins A, B and C),
keratin (multiple acidic and basic keratins), and neurofilament proteins (NF–L, NF–M).
Genetic material
Two different kinds of genetic material exist:
deoxyribonucleic acid (DNA) and
ribonucleic acid
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
(RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is
encoded in its DNA sequence.
RNA is used for information transport (e.g.,
mRNA) and
enzymatic functions (e.g.,
ribosomal RNA).
Transfer RNA
Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
(tRNA) molecules are used to add amino acids during protein
translation.
Prokaryotic genetic material is organized in a simple
circular bacterial chromosome in the
nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different,
linear molecules called
chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
and
chloroplasts (see
endosymbiotic theory
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...
).
A
human cell has genetic material contained in the
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells ...
(the
nuclear genome) and in the mitochondria (the
mitochondrial genome). In humans, the nuclear genome is divided into 46 linear DNA molecules called
chromosomes, including 22
homologous chromosome pairs and a pair of
sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the
mitochondrial DNA
Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
is very small compared to nuclear chromosomes,
it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called
transfection. This can be transient, if the DNA is not inserted into the cell's
genome, or stable, if it is. Certain
viruses also insert their genetic material into the genome.
Organelles
Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to the
organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function).
Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.
There are several types of organelles in a cell. Some (such as the
nucleus and
Golgi apparatus) are typically solitary, while others (such as
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
,
chloroplasts,
peroxisomes and
lysosomes) can be numerous (hundreds to thousands). The
cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.
Eukaryotic
* Cell nucleus: A cell's information center, the
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells ...
is the most conspicuous organelle found in a
eukaryotic cell. It houses the cell's
chromosomes, and is the place where almost all
DNA replication and
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
synthesis (
transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the
nuclear envelope
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membrane ...
, space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing,
DNA is
transcribed, or copied into a special
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, called
messenger RNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the p ...
(mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The
nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the
cytoplasm.
* Mitochondria and chloroplasts: generate energy for the cell.
Mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.
Respiration occurs in the cell mitochondria, which generate the cell's energy by
oxidative phosphorylation
Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine tri ...
, using
oxygen to release energy stored in cellular nutrients (typically pertaining to
glucose) to generate
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
(
aerobic respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
). Mitochondria multiply by
binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through
photosynthesis.
* Endoplasmic reticulum: The
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
(ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes.
The smooth ER plays a role in calcium sequestration and release and also helps in synthesis of
lipid.
* Golgi apparatus: The primary function of the Golgi apparatus is to process and package the
macromolecule
A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
s such as
proteins and
lipids that are synthesized by the cell.
* Lysosomes and peroxisomes:
Lysosomes contain
digestive enzymes (acid
hydrolases). They digest excess or worn-out
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s, food particles, and engulfed
viruses or
bacteria.
Peroxisome
A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen pero ...
s have enzymes that rid the cell of toxic
peroxides, Lysosomes are optimally active at
acidic pH. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.
* Centrosome: the cytoskeleton organizer: The
centrosome produces the
microtubules of a cell – a key component of the
cytoskeleton. It directs the transport through the
ER and the
Golgi apparatus. Centrosomes are composed of two
centrioles which lie perpendicular to each other in which each has an organization like a
cartwheel, which separate during
cell division and help in the formation of the
mitotic spindle. A single centrosome is present in the
animal cells. They are also found in some fungi and algae cells.
* Vacuoles:
Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled spaces and are surrounded by a membrane. Some cells, most notably ''
Amoeba'', have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. Vacuoles of plant cells are surrounded by
tonoplast which helps in transport of ions and other substances against concentration gradients.
Eukaryotic and prokaryotic
* Ribosomes: The
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
is a large complex of
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
and
protein molecules.
They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).
* Plastids:
Plastid
The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosy ...
are membrane-bound organelle generally found in plant cells and
euglenoids
Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typica ...
and contain specific ''pigments'', thus affecting the colour of the plant and organism. And these pigments also helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments.
Chloroplasts(contains
chlorophyll
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
and some carotenoid pigments which helps in the tapping of light energy during photosynthesis),
Chromoplasts
Chromoplasts are plastids, heterogeneous organelles responsible for pigment synthesis and storage in specific photosynthetic eukaryotes. It is thought that like all other plastids including chloroplasts and leucoplasts they are descended from sym ...
(contains fat-soluble
carotenoid
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage),
Leucoplasts
Leucoplasts (λευκός leukós "white", πλαστός plastós "formed, molded") are a category of plastid and as such are organelles found in plant cells. They are non-pigmented, in contrast to other plastids such as the chloroplast.
Lacki ...
(are non-pigmented plastids and helps in storage of nutrients).
Structures outside the cell membrane
Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the
semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.
Cell wall
Many types of prokaryotic and eukaryotic cells have a
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of
cellulose, fungi cell walls are made up of
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
and bacteria cell walls are made up of
peptidoglycan.
Prokaryotic
Capsule
A gelatinous
capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
as in
pneumococci,
meningococci or
polypeptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
as ''
Bacillus anthracis
''Bacillus anthracis'' is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent ( obligate) pathogen within the genus ''Bacillus''. Its infection is a ...
'' or
hyaluronic acid
Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...
as in
streptococci
''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...
.
Capsules are not marked by normal staining protocols and can be detected by
India ink or
methyl blue
Methyl blue is a chemical compound with the molecular formula C37H27N3Na2O9S3. It is used as a stain in histology, and stains collagen blue in tissue sections. It can be used in some differential staining techniques such as Mallory's connec ...
; which allows for higher contrast between the cells for observation.
Flagella
Flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.
Fimbriae
A
fimbria (plural fimbriae also known as a
pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called
pilin (
antigenic
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
) and are responsible for the attachment of bacteria to specific receptors on human cells (
cell adhesion
Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as cell junctions or indir ...
). There are special types of pili involved in
bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus. It is a parasexual mode of reproduction in bacteri ...
.
Cellular processes
Replication
Cell division involves a single cell (called a ''mother cell'') dividing into two daughter cells. This leads to growth in
multicellular organism
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
s (the growth of
tissue) and to procreation (
vegetative reproduction) in
unicellular organisms.
Prokaryotic cells divide by
binary fission, while
eukaryotic cells usually undergo a process of nuclear division, called
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
, followed by division of the cell, called
cytokinesis. A
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
cell may also undergo
meiosis to produce haploid cells, usually four.
Haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
cells serve as
gametes in multicellular organisms, fusing to form new diploid cells.
DNA replication, or the process of duplicating a cell's genome,
always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the
cell cycle.
In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before
meiosis I. DNA replication does not occur when the cells divide the second time, in
meiosis II. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
DNA repair
In general, cells of all organisms contain enzyme systems that scan their DNA for
DNA damage and carry out
repair processes when damage is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to
mutation.
''E. coli'' bacteria are a well-studied example of a cellular organism with diverse well-defined
DNA repair processes. These include: (1)
nucleotide excision repair
Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucle ...
, (2)
DNA mismatch repair, (3)
non-homologous end joining of double-strand breaks, (4)
recombinational repair and (5) light-dependent repair (
photoreactivation
Photolyases () are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. These enzymes require visible light (from the violet/blue end of the spectrum) both for their own activation and for the actual DNA repair. The DN ...
).
Growth and metabolism
Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions:
catabolism
Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...
, in which the cell breaks down complex molecules to produce energy and
reducing power, and
anabolism
Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking-do ...
, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.
Complex sugars consumed by the organism can be broken down into simpler sugar molecules called
monosaccharides such as
glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
),
a molecule that possesses readily available energy, through two different pathways.
Protein synthesis
Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from
amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps:
transcription and
translation.
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give
messenger RNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the p ...
(mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
s located in the
cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to
transfer RNA
Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
(tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.
Motility
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
and
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
.
In multicellular organisms, cells can move during processes such as wound healing, the immune response and
cancer metastasis
Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, the ...
. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.
Navigation, control and communication
In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to
navigate
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused
chemoattractant
Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemical ...
s which enable them to sense upcoming maze junctions before reaching them, including around corners.
Multicellularity
Cell specialization/differentiation
Multicellular organisms are
organisms that consist of more than one cell, in contrast to
single-celled organisms.
In complex multicellular organisms, cells specialize into different
cell types that are adapted to particular functions. In mammals, major cell types include
skin cells,
muscle cells,
neurons,
blood cells,
fibroblast
A fibroblast is a type of cell (biology), biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and plays a critical role in wound ...
s,
stem cells, and others. Cell types differ both in appearance and function, yet are
genetically identical. Cells are able to be of the same
genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
but of different cell type due to the differential
expression of the
genes they contain.
Most distinct cell types arise from a single
totipotent cell, called a
zygote, that
differentiates into hundreds of different cell types during the course of
development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of
molecules during
division).
Origin of multicellularity
Multicellularity has evolved independently at least 25 times,
including in some prokaryotes, like
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
,
myxobacteria,
actinomycetes, ''
Magnetoglobus multicellularis'', or ''
Methanosarcina''. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.
It evolved repeatedly for plants (
Chloroplastida), once or twice for
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s, once for
brown algae, and perhaps several times for
fungi,
slime molds, and
red algae
Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
. Multicellularity may have evolved from
colonies of interdependent organisms, from
cellularization, or from organisms in
symbiotic relationships
Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
.
The first evidence of multicellularity is from
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
-like organisms that lived between 3 and 3.5 billion years ago.
Other early fossils of multicellular organisms include the contested
Grypania spiralis and the fossils of the black shales of the
Palaeoproterozoic Francevillian Group Fossil B Formation in
Gabon.
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in
evolution experiments using predation as the selective pressure.
Origins
The origin of cells has to do with the Abiogenesis, origin of life, which began the timeline of evolution, history of life on Earth.
Origin of the first cell
There are several theories about the origin of small molecules that led to life on the early Earth. They may have been carried to Earth on meteorites (see Murchison meteorite), created at Hydrothermal vent, deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment). There is little experimental data defining what the first self-replicating forms were.
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as Abiogenesis#Clay hypothesis, clay or peptide nucleic acid.
Cells emerged at least 3.5 billion years ago.
The current belief is that these cells were heterotrophs. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered Vesicle (biology and chemistry), vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiosis, symbiotic community of prokaryotic cells. DNA-bearing organelles like the
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
and the
chloroplasts are descended from ancient symbiotic oxygen-breathing Alphaproteobacteria and "Cyanobacteria", respectively, which were Endosymbiotic theory, endosymbiosed by an ancestral
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
n prokaryote.
There is still considerable debate about whether organelles like the hydrogenosome predated the origin of
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.
History of research
* 1632–1723: Antonie van Leeuwenhoek taught himself to make Lens (optics), lenses, constructed basic optical microscopes and drew protozoa, such as ''Vorticella'' from rain water, and Bacterium, bacteria from his own mouth.
* 1665:
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
discovered cells in Cork (material), cork, then in living plant tissue using an early compound microscope. He coined the term ''cell'' (from
Latin ''cellula'', meaning "small room"
) in his book ''Micrographia'' (1665).
[ – Hooke describing his observations on a thin slice of cork. See also]
Robert Hooke
* 1839:
Theodor Schwann and
Matthias Jakob Schleiden elucidated the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
* 1855: Rudolf Virchow stated that new cells come from pre-existing cells by
cell division (''omnis cellula ex cellula'').
* 1859: The belief that life forms can occur spontaneously (''Abiogenesis, generatio spontanea'') was contradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
* 1931: Ernst Ruska built the first transmission electron microscope (TEM) at the University of Berlin. By 1935, he had built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
* 1953: Based on Rosalind Franklin's work, James D. Watson, Watson and Francis Crick, Crick made their first announcement on the double helix structure of DNA.
* 1981: Lynn Margulis published ''Symbiosis in Cell Evolution'' detailing the
endosymbiotic theory
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...
.
See also
* Cell cortex
* Cell culture
* Cellular model
* Cytorrhysis
* Cytoneme
* Cytotoxicity
* Human cell
* Lipid raft
* Outline of cell biology
* ''Parakaryon myojinensis''
* Plasmolysis
* Syncytium
* Tunneling nanotube
* Vault (organelle)
References
Further reading
*
* ; Th
fourth edition is freely available from National Center for Biotechnology Information Bookshelf.
*
*
External links
MBInfo – Descriptions on Cellular Functions and ProcessesMBInfo – Cellular OrganizationInside the Cell – a science education booklet by National Institutes of Health, in PDF and ePub.
Cells Alive!in "The Biology Project" of University of Arizona.
Centre of the Cell online!-- Partly by Queen Mary University. -->
The Image & Video Library of The American Society for Cell Biology, a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
HighMag Blog still images of cells from recent research articles.
March 4, 2011 – Howard Hughes Medical Institute.
WormWeb.org: Interactive Visualization of the ''C. elegans'' Cell lineage– Visualize the entire cell lineage tree of the nematode ''Caenorhabditis elegans, C. elegans''
Cell Photomicrographs
{{DEFAULTSORT:Cell (Biology)
Cell biology,
Cell anatomy,
1665 in science