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Cell (biology)

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Not to be confused with Cell biology

Basic structural and functional unit of organisms

Cell
Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E. B. Wilson, 1900)
A eukaryotic cell (left) and prokaryotic cell (right)
Identifiers
MeSH	D002477
TH	H1.00.01.0.00001
FMA	68646
Anatomical terminology [edit on Wikidata]

The cell (from Latin cella, meaning "small room"^[1]) is the basic structural, functional, and biological unit of all known organismslifecell biology

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids^[2] Most plant and animal cells are only visible under a microscope, with dimensions between 1 and 100 micrometres^[3] Organisms can be classified as unicellular (consisting of a single cell such as bacteria) or multicellular (including plants and animals^[4] Most unicellular organisms are classed as microorganisms

The number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion (4×10¹³) cells.^[a][5] The human brain^[6]

Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery^[7][8] Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann^[9] Cells emerged on Earth at least 3.5 billion years ago.^[10][11][12]

Contents

• 1 Cell types
  • 1.1 Prokaryotic cells
  • 1.2 Eukaryotic cells
• 2 Subcellular components
  • 2.1 Membrane
  • 2.2 Cytoskeleton
  • 2.3 Genetic material
  • 2.4 Organelles
    • 2.4.1 Eukaryotic
    • 2.4.2 Eukaryotic and prokaryotic
• 3 Structures outside the cell membrane
  • 3.1 Cell wall
  • 3.2 Prokaryotic
    • 3.2.1 Capsule
    • 3.2.2 Flagella
    • 3.2.3 Fimbriae
• 4 Cellular processes
  • 4.1 Replication
  • 4.2 DNA repair
  • 4.3 Growth and metabolism
  • 4.4 Protein synthesis
  • 4.5 Motility
• 5 Multicellularity
  • 5.1 Cell specialization
  • 5.2 Origin of multicellularity
• 6 Origins
  • 6.1 Origin of the first cell
  • 6.2 Origin of eukaryotic cells
• 7 History of research
• 8 See also
• 9 References
• 10 Notes
• 11 Further reading
• 12 External links

Cell types

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryoticsingle-celled organismsmulticellular

Prokaryotic cells

Main article: Prokaryote

Structure of a typical prokaryotic cell

Prokaryotes include bacteria and archaea, two of the three domains of lifelife on Earth, characterized by having vital biological processes including cell signalingnucleusorganellesDNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasmnucleoid^[13]

A prokaryotic cell has three regions:

• Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsuleMycoplasma (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 peptidoglycancytolysis) from osmotic pressure due to a hypotonicplant cells and fungal
• Inside the cell is the cytoplasmic region that contains the genome^[4] The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmidsspirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease.^[14] Though not forming a nucleus, the DNA is condensed in a nucleoidantibiotic resistance
• On the outside, flagella and pili

Structure of a typical animal cell

Structure of a typical plant cell

Eukaryotic cells

Main article: Eukaryote

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryoticcompartmentalizationorganellescell nucleus,^[4] an organelle that houses the cell's DNA

• The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
• The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histonecell nucleus^[4] Some eukaryotic organelles such as mitochondria
• Many eukaryotic cells are ciliated with primary ciliamechanosensationantennae^[15]
• Motile eukaryotes can move using motile cilia or flagellaconifers and flowering plants^[16] Eukaryotic flagella are more complex than those of prokaryotes.^[17]

Comparison of features of prokaryotic and eukaryotic cells

	Prokaryotes	Eukaryotes
Typical organisms	bacteria, archaea	protists, fungi, plants, animals
Typical size	~ 1–5 µm[18]	~ 10–100 µm[18]
Type of nucleus	nucleoid region; no true nucleus	true nucleus with double membrane
DNA	circular (usually)	linear molecules (chromosomes) with histone proteins
RNA/protein synthesis	coupled in the cytoplasm	RNA synthesis in the nucleus protein synthesis in the cytoplasm
Ribosomes	50S and 30S	60S and 40S
Cytoplasmic structure	very few structures	highly structured by endomembranes and a cytoskeleton
Cell movement	flagella made of flagellin	flagella and cilia containing microtubules; lamellipodia and filopodia containing actin
Mitochondria	none	one to several thousand
Chloroplasts	none	in algae and plants
Organization	usually single cells	single cells, colonies, higher multicellular organisms with specialized cells
Cell division	binary fission (simple division)	mitosis (fission or budding) meiosis
Chromosomes	single chromosome	more than one chromosome
Membranes	cell membrane	Cell membrane and membrane-bound organelles

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cellcytoplasmred blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymesbiomoleculescellular components

Membrane

Main article: Cell membrane

Detailed diagram of lipid bilayer cell membrane

The cell membrane, or plasma membrane, is a biological membranecell walldouble layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilicphospholipid bilayerporosome the universal secretory portal in cells and a variety of protein^[4] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ionreceptor proteins that allow cells to detect external signaling molecules such as hormones

Cytoskeleton

Main article: Cytoskeleton

A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell divisionmicrofilaments, intermediate filaments and microtubules^[4] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity^[19] The subunit protein of microfilaments is a small, monomeric protein called actintubulinvimentin, desmin, lamin (lamins A, B and C), keratin

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acidencoded^[4] RNA is used for information transport (e.g., mRNA) and enzymaticribosomalTransfer RNA (tRNA) molecules are used to add amino acids during protein translation

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region^[4] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genomechromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomesmitochondrial DNA is very small compared to nuclear chromosomes,^[4] 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 transfectiongenomeviruses

Organelles

Main article: Organelle

Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs^[4] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomescytosol

Eukaryotic

Human cancer cells, specifically HeLa cellsinterphasemitosis

• Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryoticchromosomes, and is the place where almost all DNA replication and RNA synthesis (transcriptionnuclear envelopeDNA is transcribed, or copied into a special RNA, called messenger RNAnucleoluscytoplasm^[4]
• Mitochondria and Chloroplasts: generate energy for the cell. Mitochondria^[4] 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 ATPbinary fissionphotosynthesis

Diagram of the endomembrane system

• Endoplasmic reticulum: The endoplasmic reticulum^[4] The smooth ER plays a role in calcium sequestration and release.
• Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids
• Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolasesorganelles, food particles, and engulfed viruses or bacteriaPeroxisomes have enzymes that rid the cell of toxic peroxides^[4]
• Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeletonER and the Golgi apparatuscentrioles, which separate during cell division and help in the formation of the mitotic spindleanimal cells
• Vacuoles: VacuolesAmoeba, 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.

Eukaryotic and prokaryotic

• Ribosomes: The ribosome is a large complex of RNA and protein^[4] 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).^[20]

Structures outside the cell membrane

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane

Cell wall

Further information: Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wallcellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan

Prokaryotic

Capsule

A gelatinous capsulepolysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococciIndia ink or methyl blue^[21]:87

Flagella

Flagella

Fimbriae

A fimbria (plural fimbriae also known as a piluspilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesionbacterial conjugation

Cellular processes

Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis

Replication

Main article: Cell division

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 tissue) and to procreation (vegetative reproduction) in unicellular organismsProkaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesisdiploid cell may also undergo meiosisHaploid cells serve as gametes

DNA replication, or the process of duplicating a cell's genome,^[4] 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 Imeiosis II^[22] Replication, like all cellular activities, requires specialized proteins for carrying out the job.^[4]

An outline of the catabolism of proteins, carbohydrates and fats

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes^[23] 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 damages that could lead to mutationE. coliDNA repairnucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joiningrecombinational repairphotoreactivation

Growth and metabolism

An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNAmRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translationribosomes (purple) that match the three-base codonstRNAblack) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Main articles: Cell 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 power, and anabolismmonosaccharides such as glucoseATP),^[4] a molecule that possesses readily available energy, through two different pathways.

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 from amino acidtranscription and translation

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNAribosomes located in the cytosoltransfer RNA

Motility

Main article: Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis^[24] 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.^[25][26]

Multicellularity

Main article: Multicellular organism

Cell specialization

Staining of a Caenorhabditis elegans which highlights the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms^[27]

In complex multicellular organisms, cells specialize into different cell typesskin cells, muscle cells, neurons, blood cells, fibroblasts, stem cellsgeneticallygenotype but of different cell type due to the differential expression‎ of the genes

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of developmentmolecules during division

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,^[28] 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.^[29] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae^[30] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships

The first evidence of multicellularity is from cyanobacteria^[28] 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^[31]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure^[28]

Origins

Main article: Evolutionary history of life

The origin of cells has to do with the origin of life, which began the history of life

Origin of the first cell

Stromatolites are left behind by cyanobacteriaGlacier National Park

Further information: Abiogenesis and Evolution of cells

There are several theories about the origin of small molecules that led to life on the early EarthMurchison meteorite), created at deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller–Urey experimentRNARNA world hypothesisclay or peptide nucleic acid^[32]

Cells emerged at least 3.5 billion years ago.^[10][11][12] The current belief is that these cells were heterotrophsvesicles^[33]

Origin of eukaryotic cells

Further information: Evolution of sexual reproduction

The eukaryotic cell seems to have evolved from a symbiotic communitymitochondria and the chloroplastsproteobacteria and cyanobacteria, respectively, which were endosymbiosed by an ancestral archaean

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis

History of research

Main article: Cell theory

Hooke's drawing of cells in cork, 1665

• 1632–1723: Antonie van Leeuwenhoek taught himself to make lenses, constructed basic optical microscopes and drew protozoa, such as Vorticella from rain water, and bacteria
• 1665: Robert Hooke discovered cells in corkcell (from Latin cella, meaning "small room"^[1]) in his book Micrographia (1665).^[34]
• 1839: Theodor Schwann and Matthias Jakob Schleiden
• 1855: Rudolf Virchowcell division (omnis cellula ex cellula).
• 1859: The belief that life forms can occur spontaneously (generatio spontanea) was contradicted by Louis Pasteur (1822–1895) (although Francesco Redi
• 1931: Ernst Ruska built the first transmission electron microscope (TEM) at the University of Berlin
• 1953: Based on Rosalind Franklin's work, Watson and Crick made their first announcement on the double helix
• 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory

See also

• Biology portal

• Cell cortex
• Cell culture
• Cellular model
• Cytorrhysis
• Cytoneme
• Cytotoxicity
• Human cell
• Lipid raft
• Outline of cell biology
• Plasmolysis
• Syncytium
• Tunneling nanotube
• Vault (organelle)

References

1. ^ Jump up to: ^{a b} "Cell". Online Etymology Dictionary. Retrieved 31 December 2012.
2. ^ 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 embryosamino acids as "molecular building blocks
3. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring LifeISBN 9780132508827
4. ^ Jump up to: ^{a b c d e f g h i j k l m n o p q r}  This article incorporates public domain material from the NCBI document: "What Is a Cell?". 30 March 2004.
5. ^ Jump up to: ^{a b c} Bianconi, Eva; Piovesan, Allison; Facchin, Federica; Beraudi, Alina; Casadei, Raffaella; Frabetti, Flavia; Vitale, Lorenza; Pelleri, Maria Chiara; Tassani, Simone (November 2013). "An estimation of the number of cells in the human body". Annals of Human Biology. 40 (6): 463–471. doi:10.3109/03014460.2013.807878. ISSN 0301-4460. PMID 23829164. These partial data correspond to a total number of 3.72±0.81×10¹³ [cells].
6. ^ Azevedo, FA; Carvalho, LR; Grinberg, LT; Farfel, JM; Ferretti, RE; Leite, RE; Jacob Filho, W; Lent, R; Herculano-Houzel, S (10 April 2009). "Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain". The Journal of Comparative Neurology. 513 (5): 532–41. doi:10.1002/cne.21974PMID 19226510
7. ^ Karp, Gerald (19 October 2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley & Sons. p. 2. ISBN 9780470483374Hooke called the pores cells because they reminded him of the cells inhabited by monks living in a monastery.
8. ^ Tero AC (1990). Achiever's Biology. Allied Publishers. p. 36. ISBN 9788184243697In 1665, an Englishman, Robert Hooke observed a thin slice of" cork under a simple microscope. (A simple microscope is a microscope with only one biconvex lens, rather like a magnifying glass). He saw many small box like structures. These reminded him of small rooms called "cells" in which Christian monks lived and meditated.
9. ^ Maton A (1997). Cells Building Blocks of LifeISBN 9780134234762
10. ^ Jump up to: ^{a b} Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB (2007). "Evidence of Archean life: Stromatolites and microfossils". Precambrian Research. 158 (3–4): 141–55. Bibcode:2007PreR..158..141S. doi:10.1016/j.precamres.2007.04.009.
11. ^ Jump up to: ^{a b} Schopf JW (2006). "Fossil evidence of Archaean life". Philos Trans R Soc Lond B Biol Sci. 29 (361(1470)): 869–885. doi:10.1098/rstb.2006.1834. PMC 1578735. PMID 16754604.
12. ^ Jump up to: ^{a b} Raven PH, Johnson GB (2002). Biology. McGraw-Hill Education. p. 68. ISBN 9780071122610. Retrieved 7 July 2013.
13. ^ Microbiology : Principles and Explorations By Jacquelyn G. Black
14. ^ European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi
15. ^ Satir P, Christensen ST (June 2008). "Structure and function of mammalian cilia"Histochemistry and Cell Biology. 129 (6): 687–93. doi:10.1007/s00418-008-0416-9PMC 2386530. PMID 18365235
16. ^ PH Raven, Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York
17. ^ Blair, David F.; Dutcher, Susan K. (1992-01-01). "Flagella in prokaryotes and lower eukaryotes". Current Opinion in Genetics & Development. 2 (5): 756–767. doi:10.1016/S0959-437X(05)80136-4ISSN 0959-437XPMID 1458024
18. ^ Jump up to: ^{a b} Campbell Biology—Concepts and Connections. Pearson Education. 2009. p. 320.
19. ^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton"Annual Review of Biochemistry. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452PMID 16756499
20. ^ Ménétret JF, Schaletzky J, Clemons WM, Osborne AR, Skånland SS, Denison C, Gygi SP, Kirkpatrick DS, Park E, Ludtke SJ, Rapoport TA, Akey CW (December 2007). "Ribosome binding of a single copy of the SecY complex: implications for protein translocation". Molecular Cell. 28 (6): 1083–92. doi:10.1016/j.molcel.2007.10.034PMID 18158904
21. ^ ProkaryotesISBN 9780080984735
22. ^ Campbell Biology—Concepts and Connections. Pearson Education. 2009. p. 138.
23. ^ D. Peter Snustad, Michael J. Simmons, Principles of Genetics – 5th Ed. (DNA repair mechanisms) pp. 364-368
24. ^ Ananthakrishnan R, Ehrlicher A. "The Forces Behind Cell Movement". Retrieved 2009-04-17.
25. ^ Alberts, Bruce (2002). Molecular biology of the cell (4th ed.). Garland Science. pp. 973–975. ISBN 0815340729
26. ^ Ananthakrishnan R, Ehrlicher A (June 2007). "The forces behind cell movement"International Journal of Biological Sciences. 3 (5): 303–17. doi:10.7150/ijbs.3.303PMC 1893118. PMID 17589565
27. ^ Becker WM, et al. (2009). The world of the cell. Pearson Benjamin CummingsISBN 9780321554185
28. ^ Jump up to: ^{a b c} Grosberg RK, Strathmann RR (2007). "The evolution of multicellularity: A minor major transition?" (PDF). Annu Rev Ecol Evol Syst. 38: 621–54. doi:10.1146/annurev.ecolsys.36.102403.114735.
29. ^ Popper ZA, Michel G, Hervé C, Domozych DS, Willats WG, Tuohy MG, Kloareg B, Stengel DB (2011). "Evolution and diversity of plant cell walls: from algae to flowering plants" (PDF). Annual Review of Plant Biology. 62: 567–90. doi:10.1146/annurev-arplant-042110-103809hdl:10379/6762PMID 21351878
30. ^ Bonner JT"The Origins of Multicellularity" (PDF). Integrative Biology: Issues, News, and Reviews. 1 (1): 27–36. doi:10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6ISSN 1093-4391the original (PDF, 0.2 MB) on March 8, 2012.
31. ^ El Albani ANature. 466 (7302): 100–04. Bibcode:2010Natur.466..100Adoi:10.1038/nature09166PMID 20596019
32. ^ Orgel LE (December 1998). "The origin of life – a review of facts and speculations". Trends in Biochemical Sciences. 23 (12): 491–95. doi:10.1016/S0968-0004(98)01300-0PMID 9868373
33. ^ Griffiths G (December 2007). "Cell evolution and the problem of membrane topology". Nature Reviews. Molecular Cell Biology. 8 (12): 1018–24. doi:10.1038/nrm2287PMID 17971839
34. ^ Hooke R (1665). Micrographia: …" … I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular […] these pores, or cells, […] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this … " – Hooke describing his observations on a thin slice of cork. See also: Robert Hooke

Notes

1. ^ An approximation made for someone who is 30 years old, weighs 70 kilograms (150 lb), and is 172 centimetres (5.64 ft) tall.^[5] The approximation is not exact, this study estimated that the number of cells was 3.72±0.81×10¹³.^[5]

Further reading

• Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P (2015). Molecular Biology of the Cell (6th ed.). Garland Science. p. 2. ISBN 9780815344322
• Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2014). Molecular Biology of the CellISBN 9780815344322; The fourth edition is freely available from National Center for Biotechnology Information
• Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipurksy SL, Darnell J (2004). Molecular Cell BiologyISBN 9780716743668
• Cooper GM (2000). The cell: a molecular approachISBN 9780878931026

External links

Wikimedia Commons has media related to Cell biology.

Wikiquote has quotations related to: Cell (biology)

• 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!
• Cell Biology in "The Biology Project" of University of Arizona
• Centre of the Cell online
• The Image & Video Library of The American Society for Cell Biology
• HighMag Blog
• New Microscope Produces Dazzling 3D Movies of Live Cells, 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 C. elegans
• Cell Photomicrographs

show v t e Structures of the cell / organelles
Endomembrane system	Cell membrane Nucleus Endoplasmic reticulum Golgi apparatus Parenthesome Autophagosome Vesicle Exosome Lysosome Endosome Phagosome Vacuole Acrosome Cytoplasmic granule Melanosome Microbody Glyoxysome Peroxisome Weibel–Palade body
Cytoskeleton	Microfilament Intermediate filament Microtubule Prokaryotic cytoskeleton Microtubule organizing center Centrosome Centriole Basal body Spindle pole body Myofibril
Endosymbionts	Mitochondrion Plastid Chloroplast Chromoplast Gerontoplast Leucoplast Amyloplast Elaioplast Proteinoplast Tannosome
Other internal	Nucleolus RNA Ribosome Spliceosome Vault Cytoplasm Cytosol Inclusions Proteasome
External	Undulipodium Cilium Flagellum Axoneme Radial spoke Archaellum Cell wall Extracellular matrix

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Hierarchy of life

• Biosphere > Biome > Ecosystem > Biocenosis > Population > Organism > Organ system > Organ > Tissue > Cell > Organelle > Biomolecular complex > Macromolecule > Biomolecule

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