Wikipedia

Cell (biology)

This article is about the basic unit of lifeforms. For the branch of biology that studies them, seeCell biology.

Thecellis the basic structural and functional unit of allforms of life.Every cell consists ofcytoplasmenclosed within amembrane;many cells containorganelles,each with a specific function. The term comes from theLatinwordcellulameaning 'small room'. Most cells are only visible under amicroscope.Cellsemerged on Earthabout 4 billion years ago. All cells are capable ofreplication,protein synthesis,andmotility.

Cell
Onion(Allium cepa) root cells in different phases of thecell cycle(drawn byE. B. Wilson,1900)
Aeukaryoticcell (left) andprokaryoticcell (right)
Identifiers
MeSHD002477
THH1.00.01.0.00001
FMA686465
Anatomical terminology

Cells are broadly categorized into two types:eukaryotic cells,which possess anucleus,andprokaryotic cells,which lack a nucleus but have a nucleoid region. Prokaryotes aresingle-celled organismssuch asbacteria,whereas eukaryotes can be either single-celled, such asamoebae,ormulticellular,such as somealgae,plants,animals,andfungi.Eukaryotic cells contain organelles includingmitochondria,which provide energy for cell functions;chloroplasts,which create sugars byphotosynthesis,in plants; andribosomes,which synthesise proteins.

Cells were discovered byRobert Hookein 1665, who named them after their resemblance tocellsinhabited byChristian monksin a monastery.Cell theory,developed in 1839 byMatthias Jakob SchleidenandTheodor 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.

Cell types

Main article:Cell type

Cells are broadly categorized into two types:eukaryotic cells,which possess anucleus,andprokaryotic cells,which lack a nucleus but have a nucleoid region. Prokaryotes aresingle-celled organisms,whereas eukaryotes can be either single-celled ormulticellular.

Prokaryotic cells

Main article:Prokaryote
Structure of a typicalprokaryoticcell

Prokaryotesincludebacteriaandarchaea,two of thethreedomains of life.Prokaryotic cells were the first form oflifeon Earth, characterized by having vitalbiological processesincludingcell signaling.They are simpler and smaller than eukaryotic cells, and lack anucleus,and other membrane-boundorganelles.TheDNAof a prokaryotic cell consists of a singlecircular chromosomethat is in direct contact with thecytoplasm.The nuclear region in the cytoplasm is called thenucleoid.Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter.[1][page needed]

A prokaryotic cell has three regions:

  • Enclosing the cell is thecell envelope,generally consisting of aplasma membranecovered by acell wallwhich, for some bacteria, may be further covered by a third layer called acapsule.Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such asMycoplasma(bacteria) andThermoplasma(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 ofpeptidoglycanin bacteria and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) fromosmotic pressuredue to ahypotonicenvironment. Some eukaryotic cells (plant cellsandfungalcells) also have a cell wall.
  • Inside the cell is thecytoplasmic regionthat contains thegenome(DNA), ribosomes and various sorts of inclusions.[2]The genetic material is freely found in the cytoplasm. Prokaryotes can carryextrachromosomal DNAelements calledplasmids,which are usually circular. Linear bacterial plasmids have been identified in several species ofspirochetebacteria, including members of the genusBorrelianotablyBorrelia burgdorferi,which causes Lyme disease.[3]Though not forming a nucleus, theDNAis condensed in anucleoid.Plasmids encode additional genes, such asantibiotic resistancegenes.
  • On the outside, some prokaryotes haveflagellaandpilithat project from the cell's surface. These are structures made of proteins that facilitate movement and communication between cells.

Eukaryotic cells

Main article:Eukaryote
Structure of a typical animal cell
Structure of a typicalplant cell

Plants,animals,fungi,slime moulds,protozoa,andalgaeare alleukaryotic.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 iscompartmentalization:the presence of membrane-boundorganelles(compartments) in which specific activities take place. Most important among these is acell nucleus,[2]an organelle that houses the cell'sDNA.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, calledchromosomes,which are associated withhistoneproteins. All chromosomal DNA is stored in thecell nucleus,separated from the cytoplasm by a membrane.[2]Some eukaryotic organelles such asmitochondriaalso contain some DNA.
  • Many eukaryotic cells areciliatedwithprimary cilia.Primary cilia play important roles in chemosensation,mechanosensation,andthermosensation.Each cilium may thus be "viewed as a sensory cellularantennaethat coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation. "[4]
  • Motile eukaryotes can move usingmotile ciliaorflagella.Motile cells are absent inconifersandflowering plants.[citation needed]Eukaryotic flagella are more complex than those of prokaryotes.[5]
Comparison of features of prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Typical organisms bacteria,archaea protists,algae,fungi,plants,animals
Typical size ~ 1–5μm[6] ~ 10–100 μm[6]
DNA Innucleoid region Innucleuswith double membrane
Chromosomes Single, usuallycircular Multiple paired linear chromosomes withhistoneproteins
RNA/proteinsynthesis coupled in thecytoplasm RNA synthesisin the nucleus
protein synthesisin the cytoplasm
Ribosomes 50Sand30S 60Sand40S
Cytoplasmic structure Microcompartments with proteins,cytoskeleton Endomembrane system,cytoskeleton
Cell movement flagella flagella andcilia;lamellipodiaandfilopodia
Mitochondria none one to several thousand
Chloroplasts none inalgaeandplants
Organization single cells, colonies,biofilms single cells, colonies,multicellular organismswith specialized cells
Cell division binary fission(simple division) mitosis(fission or budding)
meiosis
Membranes cell membrane Cell membrane and membrane-bound organelles

Many groups of eukaryotes are single-celled. Among the many-celled groups are animals and plants. The number of cells in these groups vary with species; it has been estimated that thehuman bodycontains around 37 trillion (3.72×1013) cells,[7]and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female).[8]

Subcellular components

All cells, whetherprokaryoticoreukaryotic,have amembranethat envelops the cell, regulates what moves in and out (selectively permeable), and maintains theelectric potential of the cell.Inside the membrane, thecytoplasmtakes up most of the cell's volume. Exceptred blood cells,which lack a cell nucleus and most organelles to accommodate maximum space forhemoglobin,all cells possessDNA,the hereditary material ofgenes,andRNA,containing the information necessary tobuildvariousproteinssuch asenzymes,the cell's primary machinery. There are also other kinds ofbiomoleculesin cells. This article lists these primarycellular components,then briefly describes their function.

Cell membrane

Main article:Cell membrane
Detailed diagram of lipid bilayer of cell membrane

Thecell membrane,or plasma membrane, is a selectively permeable[citation needed]biological membranethat 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 acell wall.This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from adouble layer of phospholipids,which areamphiphilic(partlyhydrophobicand partlyhydrophilic). Hence, the layer is called aphospholipid bilayer,or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called theporosomethe universal secretory portal in cells and a variety ofproteinmolecules that act as channels and pumps that move different molecules into and out of the cell.[2]The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (moleculeorion) pass through freely, to a limited extent or not at all.[citation needed]Cell surface membranes also containreceptorproteins that allow cells to detect external signaling molecules such ashormones.[9]

Cytoskeleton

Main article:Cytoskeleton
Further information:Morphogenesis
A fluorescent image of an endothelial cell. Nuclei are stained blue,mitochondriaare stained red, and microfilaments are stained green.

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps duringendocytosis,the uptake of external materials by a cell, andcytokinesis,the separation of daughter cells aftercell division;and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed ofmicrotubules,intermediate filamentsandmicrofilaments.In the cytoskeleton of aneuronthe intermediate filaments are known asneurofilaments.There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.[2]The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape,polarityand cytokinesis.[10]The subunit protein of microfilaments is a small, monomeric protein calledactin.The subunit of microtubules is a dimeric molecule calledtubulin.Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments includevimentin,desmin,lamin(lamins A, B and C),keratin(multiple acidic and basic keratins), andneurofilament proteins(NF–L,NF–M).

Genetic material

Main articles:DNAandRNA
Deoxyribonucleic acid(DNA)

Two different kinds of genetic material exist:deoxyribonucleic acid(DNA) andribonucleic acid(RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism isencodedin its DNA sequence.[2]RNA is used for information transport (e.g.,mRNA) andenzymaticfunctions (e.g.,ribosomalRNA).Transfer RNA(tRNA) molecules are used to add amino acids during proteintranslation.

Prokaryotic genetic material is organized in a simplecircular bacterial chromosomein thenucleoid regionof the cytoplasm. Eukaryotic genetic material is divided into different,[2]linear molecules calledchromosomesinside a discrete nucleus, usually with additional genetic material in some organelles likemitochondriaandchloroplasts(seeendosymbiotic theory).

Ahuman cellhas genetic material contained in thecell nucleus(thenuclear genome) and in the mitochondria (themitochondrial genome). In humans, the nuclear genome is divided into 46 linear DNA molecules calledchromosomes,including 22homologous chromosomepairs and a pair ofsex chromosomes.The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although themitochondrial DNAis very small compared to nuclear chromosomes,[2]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 calledtransfection.This can be transient, if the DNA is not inserted into the cell'sgenome,or stable, if it is. Certainvirusesalso insert their genetic material into the genome.

Organelles

Main article:Organelle

Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to theorgansof the human body (such as the heart, lung, and kidney, with each organ performing a different function).[2]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 thenucleusandGolgi apparatus) are typically solitary, while others (such asmitochondria,chloroplasts,peroxisomesandlysosomes) can be numerous (hundreds to thousands). Thecytosolis the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

Human cancer cells, specificallyHeLa cells,with DNA stained blue. The central and rightmost cell are ininterphase,so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going throughmitosisand its chromosomes have condensed.
  • Cell nucleus:A cell's information center, thecell nucleusis the most conspicuous organelle found in aeukaryoticcell. It houses the cell'schromosomes,and is the place where almost allDNAreplication andRNAsynthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called thenuclear 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,DNAistranscribed,or copied into a specialRNA,calledmessenger RNA(mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. Thenucleolusis a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in thecytoplasm.[2]
  • Mitochondria and chloroplasts:generate energy for the cell.Mitochondriaare self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.[2]Respirationoccurs in the cell mitochondria, which generate the cell's energy byoxidative phosphorylation,usingoxygento release energy stored in cellular nutrients (typically pertaining toglucose) to generateATP(aerobic respiration). Mitochondria multiply bybinary fission,like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates throughphotosynthesis.
Diagram of theendomembrane system
  • Endoplasmic reticulum:Theendoplasmic 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.[2]The smooth ER plays a role in calcium sequestration and release and also helps in synthesis oflipid.
  • Golgi apparatus:The primary function of the Golgi apparatus is to process and package themacromoleculessuch asproteinsandlipidsthat are synthesized by the cell.
  • Lysosomes and peroxisomes:Lysosomescontaindigestive enzymes(acidhydrolases). They digest excess or worn-outorganelles,food particles, and engulfedvirusesorbacteria.Peroxisomeshave enzymes that rid the cell of toxicperoxides,Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[2]
  • Centrosome:the cytoskeleton organizer: Thecentrosomeproduces themicrotubulesof a cell—a key component of thecytoskeleton.It directs the transport through theERand theGolgi apparatus.Centrosomes are composed of twocentrioleswhich lie perpendicular to each other in which each has an organization like acartwheel,which separate duringcell divisionand help in the formation of themitotic spindle.A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles:Vacuolessequester 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 notablyAmoeba,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 a membrane which transports ions against concentration gradients.

Eukaryotic and prokaryotic

  • Ribosomes:Theribosomeis a large complex ofRNAandproteinmolecules.[2]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).[11]
  • Plastids:Plastidare membrane-bound organelle generally found in plant cells andeuglenoidsand contain specificpigments,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.Chloroplastscontainchlorophylland some carotenoid pigments which helps in the tapping of light energy during photosynthesis.Chromoplastscontain fat-solublecarotenoidpigments like orange carotene and yellow xanthophylls which helps in synthesis and storage.Leucoplastsare non-pigmented plastids and helps in storage of nutrients.[12]

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 cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Further information:Cell wall

Many types of prokaryotic and eukaryotic cells have acell wall.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 ofcellulose,fungi cell walls are made up ofchitinand bacteria cell walls are made up ofpeptidoglycan.

Prokaryotic

Capsule

A gelatinouscapsuleis present in some bacteria outside the cell membrane and cell wall. The capsule may bepolysaccharideas inpneumococci,meningococciorpolypeptideasBacillus anthracisorhyaluronic acidas instreptococci. Capsules are not marked by normal staining protocols and can be detected byIndia inkormethyl blue,which allows for higher contrast between the cells for observation.[13]: 87 

Flagella

Flagellaare 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

Afimbria(plural fimbriae also known as apilus,plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein calledpilin(antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved inbacterial conjugation.

Cellular processes

Prokaryotesdivide bybinary fission,whileeukaryotesdivide bymitosisormeiosis.

Replication

Main article:Cell division

Cell division involves a single cell (called amother cell) dividing into two daughter cells. This leads to growth inmulticellular organisms(the growth oftissue) and to procreation (vegetative reproduction) inunicellular organisms.Prokaryoticcells divide bybinary fission,whileeukaryoticcells usually undergo a process of nuclear division, calledmitosis,followed by division of the cell, calledcytokinesis.Adiploidcell may also undergomeiosisto produce haploid cells, usually four.Haploidcells serve asgametesin multicellular organisms, fusing to form new diploid cells.

DNA replication,or the process of duplicating a cell's genome,[2]always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of thecell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs beforemeiosis I.DNA replication does not occur when the cells divide the second time, inmeiosis II.[14]Replication, like all cellular activities, requires specialized proteins for carrying out the job.[2]

DNA repair

Main article:DNA repair

Cells of all organisms contain enzyme systems that scan their DNA fordamageand carry outrepair processeswhen it 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 tomutation.E. colibacteria are a well-studied example of a cellular organism with diverse well-definedDNA repairprocesses. These include:nucleotide excision repair,DNA mismatch repair,non-homologous end joiningof double-strand breaks,recombinational repairand light-dependent repair (photoreactivation).[15]

Growth and metabolism

Main articles:Cell growth,Metabolism,andPhotosynthesis

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 andreducing power,andanabolism,in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.

Complex sugars can be broken down into simpler sugar molecules calledmonosaccharidessuch asglucose.Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP),[2]a molecule that possesses readily available energy, through two different pathways. In plant cells,chloroplastscreate sugars byphotosynthesis,using the energy of light to join molecules of water andcarbon dioxide.

Protein synthesis

Main article:Protein biosynthesis

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 fromamino acidbuilding blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps:transcriptionandtranslation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to givemessenger RNA(mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes calledribosomeslocated in thecytosol,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 totransfer RNA(tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

Main article:Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion includeflagellaandcilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response andcancer 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.[16]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.[17][16]

See also:Cybernetics § In biology

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able tonavigateefficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffusedchemoattractantswhich enable them to sense upcoming maze junctions before reaching them, including around corners.[18][19][20]

Multicellularity

Main article:Multicellular organism

Cell specialization/differentiation

Main article:Cellular differentiation
Staining of aCaenorhabditis eleganshighlights the nuclei of its cells.

Multicellular organisms areorganismsthat consist of more than one cell, in contrast tosingle-celled organisms.[21]

In complex multicellular organisms, cells specialize into differentcell typesthat are adapted to particular functions. In mammals, major cell types includeskin cells,muscle cells,neurons,blood cells,fibroblasts,stem cells,and others. Cell types differ both in appearance and function, yet aregeneticallyidentical. Cells are able to be of the samegenotypebut of different cell type due to the differentialexpressionof thegenesthey contain.

Most distinct cell types arise from a singletotipotentcell, called azygote,thatdifferentiatesinto hundreds of different cell types during the course ofdevelopment.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 ofmoleculesduringdivision).

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,[22]including in some prokaryotes, likecyanobacteria,myxobacteria,actinomycetes,orMethanosarcina.However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.[23]It evolved repeatedly for plants (Chloroplastida), once or twice foranimals,once forbrown algae,and perhaps several times forfungi,slime molds,andred algae.[24]Multicellularity may have evolved fromcoloniesof interdependent organisms, fromcellularization,or from organisms insymbiotic relationships.

The first evidence of multicellularity is fromcyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.[22]Other early fossils of multicellular organisms include the contestedGrypaniaspiralisand the fossils of the black shales of thePalaeoproterozoicFrancevillian Group FossilB Formation inGabon.[25]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, inevolution experimentsusing predation as theselective pressure.[22]

Origins

Main article:Evolutionary history of life

The origin of cells has to do with theorigin of life,which began thehistory of lifeon Earth.

Origin of life

Further information:AbiogenesisandEvolution of cells
Stromatolitesare left behind bycyanobacteria,also called blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is fromGlacier National Parkin the United States.

Small molecules needed for life may have been carried to Earth on meteorites, created atdeep-sea vents,orsynthesized by lightning in a reducing atmosphere.There is little experimental data defining what the first self-replicating forms were.RNAmay have beenthe earliest self-replicating molecule,as it can both store genetic information and catalyze chemical reactions.[26]

Cells emerged around 4 billion years ago.[27][28]The first cells were most likelyheterotrophs.The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayeredvesiclesin water, and could have preceded RNA.[29][30]

First eukaryotic cells

Main article:Eukaryogenesis
In the theory ofsymbiogenesis,a merger of anarchaeanand an aerobic bacterium created the eukaryotes, with aerobicmitochondria,some 2.2 billion years ago. A second merger, 1.6 billion years ago, addedchloroplasts,creating the green plants.[31]

Eukaryoticcells were created some 2.2 billion years ago in a process calledeukaryogenesis.This is widely agreed to have involvedsymbiogenesis,in whicharchaeaandbacteriacame together to create the first eukaryotic common ancestor. This cell had a new level of complexity and capability, with a nucleus[32][33]and facultatively aerobicmitochondria.[31]It evolved some 2 billion years ago into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. It featured at least onecentrioleandcilium,sex (meiosisandsyngamy),peroxisomes,and a dormantcystwith a cell wall ofchitinand/orcellulose.[34][35]In turn, the last eukaryotic common ancestor gave rise to the eukaryotes'crown group,containing the ancestors ofanimals,fungi,plants,and a diverse range of single-celled organisms.[36][37]The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that addedchloroplasts,derived fromcyanobacteria.[31]

History of research

Main article:Cell theory
Robert Hooke's drawing of cells incork,1665

In 1665,Robert Hookeexamined a thin slice of cork under hismicroscope,and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like aHoney-comb,but that the pores of it were not regular ".[38]To further support his theory,Matthias SchleidenandTheodor Schwannboth also studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were not only fundamental to plants, but animals as well.[39]

See also

References

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