Cell (biology) explained

Cell (biology) should not be confused with Cell biology.


The cell (from Latin cella, meaning "small room"[1]) is the basic structural, functional, and biological unit of all known organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology, cellular biology, or cytology.

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). 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.[4] The human brain accounts for around 80 billion of these cells.[5]

Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery.[6] [7] Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells.[8] Cells emerged on Earth at least 3.5 billion years ago.

Cell types

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

See main article: Prokaryote.

Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.[9]

A prokaryotic cell has three regions:

Eukaryotic cells

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus,[10] an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

Comparison of features of prokaryotic and eukaryotic cells
Typical organismsbacteria, archaeaprotists, fungi, plants, animals
Typical size~ 1–5 µm[15] ~ 10–100 µm
Type of nucleusnucleoid region
no true nucleus
true nucleus with double membrane
DNAcircular (usually)linear molecules (chromosomes) with histone proteins
RNA/protein synthesiscoupled in the cytoplasmRNA synthesis in the nucleus
protein synthesis in the cytoplasm
Ribosomes50S and 30S60S and 40S
Cytoplasmic structurevery few structureshighly structured by endomembranes and a cytoskeleton
Cell movementflagella made of flagellinflagella and cilia containing microtubules; lamellipodia and filopodia containing actin
Mitochondrianoneone to several thousand
Chloroplastsnonein algae and plants
Organizationusually single cellssingle cells, colonies, higher multicellular organisms with specialized cells
Cell divisionbinary fission (simple division)mitosis (fission or budding)
Chromosomessingle chromosomemore than one chromosome
Membranescell membraneCell 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 cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells (except red 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 enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components, then briefly describes their function.


See main article: Cell membrane. The cell membrane, or plasma membrane, is a biological membrane 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. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.[10] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.


See main article: Cytoskeleton. 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 division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.[10] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.[16] The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence.[10] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer 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 of the cytoplasm. Eukaryotic genetic material is divided into different,[10] 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 genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes,[10] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.


See 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 of the human body (such as the heart, lung, and kidney, with each organ performing a different function).[10] 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 lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.


Eukaryotic and prokaryotic

Structures outside the cell membrane

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wall. 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.



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 or methyl blue; which allows for higher contrast between the cells for observation.[18]


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.


A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) 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


See 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 organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. 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,[10] 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.[19] Replication, like all cellular activities, requires specialized proteins for carrying out the job.[10]

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes when damages are detected.[20] 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 mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair (photoreactivation).

Growth and metabolism

See main article: 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 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 (ATP),[10] a molecule that possesses readily available energy, through two different pathways.

Protein synthesis




See 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. 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.[21] 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.[22] [23]


Cell specialization

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms.[24]

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,[25] 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.[26] 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.[27] 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-like organisms that lived between 3 and 3.5 billion years ago.[25] 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.[28]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[25]


The origin of cells has to do with the origin of life, which began the 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 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 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 clay or peptide nucleic acid.[29]

Cells emerged at least 3.5 billion years ago.[30] [31] [32] 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 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.[33]

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are descended from ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, which were endosymbiosed by an ancestral archaean prokaryote.

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

History of research

See main article: Cell theory.

See also

Further reading

External links

Notes and References

  1. Cell. Online Etymology Dictionary. 31 December 2012.
  2. https://www.ncbi.nlm.nih.gov/books/NBK26863/ Cell Movements and the Shaping of the Vertebrate Body
  3. Book: Campbell, Neil A.. Biology: Exploring Life. Brad Williamson. Robin J. Heyden. Pearson Prentice Hall. 2006. 9780132508827. Boston, Massachusetts.
  4. 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. en. 40. 6. 463–471. 10.3109/03014460.2013.807878. 23829164. 0301-4460. These partial data correspond to a total number of 3.72&plusmn;0.81×1013 [cells]..
  5. Azevedo . FA . Carvalho . LR . Grinberg . LT . Farfel . JM . Ferretti . RE . Leite . RE . Jacob Filho . W . Lent . R . Herculano-Houzel . S . Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. . The Journal of Comparative Neurology . 10 April 2009 . 513 . 5 . 532–41 . 10.1002/cne.21974 . 19226510.
  6. Book: Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments. 19 October 2009. John Wiley & Sons. 9780470483374. 2. Hooke called the pores cells because they reminded him of the cells inhabited by monks living in a monastery..
  7. Book: Tero, Alan Chong . Achiever's Biology. 1990 . Allied Publishers . 9788184243697. 36. In 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.. vanc .
  8. Book: Maton, Anthea . vanc . Cells Building Blocks of Life . Prentice Hall . 1997 . New Jersey . 9780134234762 .
  9. Microbiology : Principles and Explorations By Jacquelyn G. Black
  10. 30 March 2004.
  11. European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012
  12. Satir P, Christensen ST . Structure and function of mammalian cilia . Histochemistry and Cell Biology . 129 . 6 . 687–93 . June 2008 . 18365235 . 2386530 . 10.1007/s00418-008-0416-9 . Søren T. Christensen . 1432-119X .
  13. PH Raven, Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York
  14. Blair. David F.. Dutcher. Susan K.. 1992-01-01. Flagella in prokaryotes and lower eukaryotes. Current Opinion in Genetics & Development. 2. 5. 756–767. 10.1016/S0959-437X(05)80136-4. 1458024. 0959-437X.
  15. Book: Campbell Biology—Concepts and Connections . Pearson Education . 2009 . 320.
  16. Michie KA, Löwe J . Dynamic filaments of the bacterial cytoskeleton . Annual Review of Biochemistry . 75 . 467–92 . 2006 . 16756499 . 10.1146/annurev.biochem.75.103004.142452 .
  17. Ménétret JF, Schaletzky J, Clemons WM, Osborne AR, Skånland SS, Denison C, Gygi SP, Kirkpatrick DS, Park E, Ludtke SJ, Rapoport TA, Akey CW . Ribosome binding of a single copy of the SecY complex: implications for protein translocation . Molecular Cell . 28 . 6 . 1083–92 . December 2007 . 18158904 . 10.1016/j.molcel.2007.10.034 .
  18. Book: Prokaryotes . Newnes . Apr 11, 1996 . 9780080984735.
  19. Book: Campbell Biology—Concepts and Connections. 2009. Pearson Education. 138.
  20. D. Peter Snustad, Michael J. Simmons, Principles of Genetics – 5th Ed. (DNA repair mechanisms) pp. 364-368
  21. Web site: Revathi . Ananthakrishnan . Allen . Ehrlicher . vanc . The Forces Behind Cell Movement . Biolsci.org . 2009-04-17.
  22. Book: Alberts . Bruce . Molecular biology of the cell . 2002 . Garland Science . 0815340729 . 973–975 . 4th.
  23. Ananthakrishnan R, Ehrlicher A . The forces behind cell movement . International Journal of Biological Sciences . 3 . 5 . 303–17 . June 2007 . 17589565 . 1893118 . 10.7150/ijbs.3.303 .
  24. Book: Becker, Wayne M. . vanc . The world of the cell . . 2009 . 9780321554185 . 480. etal.
  25. Grosberg RK, Strathmann RR . The evolution of multicellularity: A minor major transition? . Annu Rev Ecol Evol Syst . 2007 . 38 . 621–54 . 10.1146/annurev.ecolsys.36.102403.114735 .
  26. Popper ZA, Michel G, Hervé C, Domozych DS, Willats WG, Tuohy MG, Kloareg B, Stengel DB . Evolution and diversity of plant cell walls: from algae to flowering plants . Annual Review of Plant Biology . 62 . 567–90 . 2011 . 21351878 . 10.1146/annurev-arplant-042110-103809 . 10379/6762 .
  27. Bonner . John Tyler . vanc . John Tyler Bonner . 1998 . The Origins of Multicellularity . Integrative Biology: Issues, News, and Reviews . 1 . 1 . 27–36 . PDF, 0.2 MB . 1093-4391 . 10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 . dead . https://web.archive.org/web/20120308175112/http://courses.cit.cornell.edu/biog1101/outlines/Bonner%20-Origin%20of%20Multicellularity.pdf . March 8, 2012 .
  28. El Albani A, Bengtson S, Canfield DE, Bekker A, Macchiarelli R, Mazurier A, Hammarlund EU, Boulvais P, Dupuy JJ, Fontaine C, Fürsich FT, Gauthier-Lafaye F, Janvier P, Javaux E, Ossa FO, Pierson-Wickmann AC, Riboulleau A, Sardini P, Vachard D, Whitehouse M, Meunier A . Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago . Nature . 466 . 7302 . 100–04 . July 2010 . 20596019 . 10.1038/nature09166 . Abderrazak El Albani . 2010Natur.466..100A .
  29. Orgel LE . The origin of life – a review of facts and speculations . Trends in Biochemical Sciences . 23 . 12 . 491–95 . December 1998 . 9868373 . 10.1016/S0968-0004(98)01300-0 .
  30. Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB . 2007 . Evidence of Archean life: Stromatolites and microfossils . Precambrian Research . 158 . 3–4 . 141–55 . 2007PreR..158..141S . 10.1016/j.precamres.2007.04.009 .
  31. Schopf JW . 2006 . Fossil evidence of Archaean life . Philos Trans R Soc Lond B Biol Sci . 29 . 361(1470) . 869–885 . 10.1098/rstb.2006.1834 . 16754604 . 1578735 .
  32. Book: Peter Hamilton . Raven . George Brooks . Johnson . vanc . Biology. registration . 7 July 2013. 2002. McGraw-Hill Education. 9780071122610. 68.
  33. Griffiths G . Cell evolution and the problem of membrane topology . Nature Reviews. Molecular Cell Biology . 8 . 12 . 1018–24 . December 2007 . 17971839 . 10.1038/nrm2287 .
  34. Book: Hooke. Robert . vanc . Micrographia: …. 1665. Royal Society of London. London, England. 113. " … 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