Cell (biology)
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The cell is the basic structural and functional unit of
life form Life form (also spelled life-form or lifeform) is an wikt:entity, entity that is Life, living, such as plants (flora) and animals (fauna). It is estimated that more than 99% of all species that ever existed on Earth, amounting to over five billi ...
s. Every cell consists of a
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryote, eukaryotic Cell (biology), cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear envelope, nuc ...
enclosed within a
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
, 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 developmental biology, development. Biom ...
s such as
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s, DNA and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
, as well as many small molecules of nutrients and
metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
.Cell Movements and the Shaping of the Vertebrate Body
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 Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell ...
s. It is also common to describe small molecules such as
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s as
molecular building blocks
".
The term comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
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 spectrum, visible light and a system of lens (optics), lenses to generate magnified images of small objects. Optical microscopes ...
, with dimensions between 1 and 100 
micrometre The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
s.
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 A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms ...
(consisting of a single cell such as
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
) or
multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
(including plants and animals). Most
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell (biology), cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryoti ...
s are classed as
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s. 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 (3.72×1013) 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 emergence, emerging properties at multiple biological scales.Wang, Edwin. ''Cancer S ...
,
aging Ageing (British English, BE) or aging (American English, AE) is the process of becoming Old age, older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals a ...
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 Anatomy, structure, Physiology, function, and behavior of cell (biology), cells. All living organisms are made of cells. A cell is the basic unit of life th ...
is the study of cells, which were discovered by
Robert Hooke Robert Hooke Fellow of the Royal Society, 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 ...
in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery.
Cell theory In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of Cell (biology), cells, that they are the basic structural/organizational unit of all organisms, and that all cell ...
, first developed in 1839 by
Matthias Jakob Schleiden Matthias Jakob Schleiden (; 5 April 1804 – 23 June 1881) was a German Botany, botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow. Career Matthias Jakob Schleiden was born in Hamburg. on 5 April 1804. His fat ...
and
Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German physician and physiology, physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the d ...
, 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 Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
, which contain a
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, 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 A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
.


Prokaryotic cells

Prokaryote A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...
s include
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
, two of the three domains of life. Prokaryotic cells were the first form of
life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
on Earth, characterized by having vital
biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are cla ...
es including
cell signaling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a Cell (biology), cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property ...
. They are simpler and smaller than eukaryotic cells, and lack a
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, and other membrane-bound
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
s. The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryote, eukaryotic Cell (biology), cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear envelope, nuc ...
. The nuclear region in the cytoplasm is called the
nucleoid The nucleoid (meaning ''nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, c ...
. 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 The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In gram-negative bacteria an bacterial outer membrane, outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is abs ...
– 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 cytoplasm, interior of all Cell (biology), cells from th ...
covered by a
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
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 Peptidoglycan or murein is a unique large macromolecule, a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units ...
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 (chemistry), 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 soluti ...
due to a
hypotonic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two Solution (chemistry), solutions separated by a Semipermeable membrane, partially-permeable cell Cell membrane, membrane. Tonicity dep ...
environment. Some eukaryotic cells (
plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...
s and
fungal A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
cells) also have a cell wall. * Inside the cell is the cytoplasmic region that contains the
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gene ...
(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 Cell nucleus, nucleus of a Cell (biology), cell. Most DNA in an individual genome is found in chromosomes contained in the nucleus. ...
elements called
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from gDNA, chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; ...
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 ''Borrelia burgdorferi'' is a bacterial species of the Spirochaete, spirochete class in the genus ''Borrelia'', and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme dis ...
'', which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a
nucleoid The nucleoid (meaning ''nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, c ...
. 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. P ...
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#Cellular level, motility. Many protists with flagella are termed as flagellates. A m ...
and pili 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 Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
,
animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...
,
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
,
slime mould Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mul ...
s,
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other microorganisms or o ...
, and
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
are all
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
. 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 (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
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 eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as ma ...
, 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
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s, which are associated with
histone In biology, histones are highly Base (chemistry), basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosom ...
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 eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as ma ...
, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
also contain some DNA. * Many eukaryotic cells are
ciliated The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryote, eukaryotic Cell (biology), cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a ...
with primary cilia. Primary cilia play important roles in chemosensation,
mechanosensation Mechanosensation is the transduction of mechanical stimuli into neural signals. Mechanosensation provides the basis for the senses of light touch, hearing, proprioception, and pain. Mechanoreceptors found in the skin, called cutaneous mechanorecept ...
, 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 The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryote, eukaryotic Cell (biology), cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a ...
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#Cellular level, motility. Many protists with flagella are termed as flagellates. A m ...
. 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 plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s.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 bacteria, Gram-positive coccus, spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the Respiratory tract, upper respir ...
'',  '' E. coli'',  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 A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...
or
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
, have a
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
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 In cell biology, the cytoplasm is all of the material within a eukaryote, eukaryotic Cell (biology), cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear envelope, nuc ...
takes up most of the cell's volume. Except
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...
s, which lack a cell nucleus and most organelles to accommodate maximum space for
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
, all cells possess DNA, the hereditary material of
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanin ...
s, and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
, containing the information necessary to build various
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s such as
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s, 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 developmental biology, development. Biom ...
s in cells. This article lists these primary
cellular component Cellular components are the complex 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, morpho ...
s, then briefly describes their function.


Cell membrane

The
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
, or plasma membrane, is a selectively permeable
biological membrane A biological membrane, biomembrane or cell membrane is a Semipermeable membrane, selectively permeable membrane that separates the interior of a Cell (biology), cell from the extracellular, external environment or creates intracellular compartmen ...
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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
. 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 An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
(partly
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
and partly
hydrophilic A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
). Hence, the layer is called a
phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organis ...
, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
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 A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
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
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...
s.


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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, an ...
, the uptake of external materials by a cell, and
cytokinesis Cytokinesis () is the part of the cell division biological process, process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of Mitosis, nuclear d ...
, the separation of daughter cells after
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
; 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 nanometer, nm and have an inner diameter bet ...
s,
intermediate filament Intermediate filaments (IFs) are cytoskeleton, 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' ...
s and
microfilament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cell (biology), cells that form part of the cytoskeleton. They are primarily composed of biopolymer, polymers of actin, but are modified by and inte ...
s. In the cytoskeleton of a
neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
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 and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
. The subunit of microtubules is a dimeric molecule called
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments include
vimentin Vimentin is a structural protein that in humans is encoded by the ''VIM'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." ...
, desmin,
lamin Lamins, also known as nuclear lamins are fibrous proteins in Intermediate filament#Type V – nuclear lamins, type V intermediate filaments, providing structural function and Transcription (biology), transcriptional regulation in the cell nucleus. ...
(lamins A, B and C),
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers ...
(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 Genetic code, coding, Translation (biology), decoding, Regulatory RNA, regulation and Gene expression, expression of genes. RNA and deoxyribonucleic acid ( ...
(RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is
encoded In communications and information processing, code is a system of rules to convert information—such as a letter (alphabet), letter, word, sound, image, or gesture—into another form, sometimes data compression, shortened or secrecy, secret ...
in its DNA sequence. RNA is used for information transport (e.g.,
mRNA 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 Protein biosynthesis, synthesizing a protein. mRNA is ...
) and
enzymatic Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
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 Messenger RNA, mRNA a ...
(tRNA) molecules are used to add amino acids during protein
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
. Prokaryotic genetic material is organized in a simple
circular bacterial chromosome A circular chromosome is a chromosome in bacteria, archaea, Mitochondrial DNA#Genome structure and diversity, mitochondria, and Chloroplast DNA#Molecular structure, chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromo ...
in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s inside a discrete nucleus, usually with additional genetic material in some organelles like
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
and
chloroplasts 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, converts it, ...
(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 possibly ...
). 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 eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as ma ...
(the
nuclear genome Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryote, eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and Plastid#Structure, plastid ...
) and in the mitochondria (the
mitochondrial genome Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondrion, mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mit ...
). In humans, the nuclear genome is divided into 46 linear DNA molecules called
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s, including 22
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same locus (genetics), loci where they pr ...
pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA 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 In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gene ...
, 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 Organ (biology), 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 Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
and Golgi apparatus) are typically solitary, while others (such as
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
,
chloroplasts 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, converts it, ...
, 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 eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as ma ...
is the most conspicuous organelle found in a
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
synthesis (Transcription (genetics), transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope, 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 Transcription (genetics), transcribed, or copied into a special
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
, called messenger RNA (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 In cell biology, the cytoplasm is all of the material within a eukaryote, eukaryotic Cell (biology), cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear envelope, nuc ...
. * Mitochondria and chloroplasts: generate energy for the cell. Mitochondrion, Mitochondria are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Cellular respiration, Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate adenosine triphosphate, ATP(aerobic respiration). 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 (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 macromolecules such as
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s 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 (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
s, food particles, and engulfed viruses or
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
. Peroxisomes 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 endoplasmic reticulum, 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 Cart wheel, cartwheel, which separate during
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
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 (genus), 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 is a large complex of
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
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 are membrane-bound organelle generally found in plant cells and Euglenid, euglenoids 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. Chloroplast, Chloroplasts(contains chlorophyll and some carotenoid pigments which helps in the tapping of light energy during photosynthesis), Chromoplast, Chromoplasts(contains fat-soluble carotenoid pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage), Leucoplast, Leucoplasts(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 membrane, 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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
. 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 and bacteria cell walls are made up of
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units ...
.


Prokaryotic


Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as ''Bacillus anthracis'' or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink#Uses other than writing, India ink or methyl blue; which allows for higher contrast between the cells for observation.


Flagella

Flagella 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 (bacteriology), 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) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation.


Cellular processes


Replication

Cell division involves a single cell (called a ''mother cell'') dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of biological tissue, tissue) and to procreation (vegetative reproduction) in
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell (biology), cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryoti ...
s. Prokaryote, Prokaryotic cells divide by binary fission, while Eukaryote, eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called
cytokinesis Cytokinesis () is the part of the cell division biological process, process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of Mitosis, nuclear d ...
. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid 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 (naturally occurring), DNA damage and carry out DNA repair, 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. Escherichia coli, ''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, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) homologous recombination, recombinational repair and (5) light-dependent repair (photolyase, photoreactivation).


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, in which the cell breaks down complex molecules to produce energy and Reducing agent, reducing power, and anabolism, 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 (adenosine triphosphate, ATP), 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 Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription (genetics), transcription and translation (genetics), 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 (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes 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 (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#Cellular level, motility. Many protists with flagella are termed as flagellates. A m ...
and cilia. In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. 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 Chemotaxis, navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants 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,
neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
s, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are Genetics, genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential Regulation of gene expression, expression of the
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanin ...
s they contain. Most distinct cell types arise from a single totipotent cell, called a zygote, that Cellular differentiation, differentiates into hundreds of different cell types during the course of Development (biology), 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
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s during cell division, division).


Origin of multicellularity

Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like cyanobacteria, 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 animals, once for brown algae, and perhaps several times for
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
, Mycetozoa, slime molds, and red algae. Multicellularity may have evolved from Colony (biology), colonies of interdependent organisms, from cellularization, or from organisms in Symbiosis, symbiotic relationships. The first evidence of multicellularity is from cyanobacteria-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 experimental evolution, 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 polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
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 cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
and the
chloroplasts 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, converts it, ...
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 (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
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 cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
, 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 Fellow of the Royal Society, 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 ...
discovered cells in Cork (material), cork, then in living plant tissue using an early compound microscope. He coined the term ''cell'' (from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''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 Theodor Schwann (; 7 December 181011 January 1882) was a German physician and physiology, physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the d ...
and
Matthias Jakob Schleiden Matthias Jakob Schleiden (; 5 April 1804 – 23 June 1881) was a German Botany, botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow. Career Matthias Jakob Schleiden was born in Hamburg. on 5 April 1804. His fat ...
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 Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
(''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 possibly ...
.


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 Processes

MBInfo – Cellular Organization

Inside 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