Cell (biology)

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

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.[2] Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals).[3] While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (1013) cells.[4] Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.[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.[9][10][11]

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)
Anatomical terminology
Structure of animal cell
Structure of an animal cell


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

Average prokaryote cell- en
Structure of a typical prokaryotic cell

Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single 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.[12]

A prokaryotic cell has three architectural 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 capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions.[3] The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease.[13] Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, flagella 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.
Animal cell structure en
Structure of a typical animal cell
Plant cell structure-en
Structure of a typical plant cell

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,[3] an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane.[3] Some eukaryotic organelles such as mitochondria also contain some DNA.
  • Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia 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."[14]
  • Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants.[15] Eukaryotic flagella are more complex than those of prokaryotes.[16]
Comparison of features of prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, fungi, plants, animals
Typical size ~ 1–5 µm[17] ~ 10–100 µm[17]
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)
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 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.


Cell membrane detailed diagram en
Detailed diagram of lipid bilayer 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 variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.[3] 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.


A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

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.[3] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.[18] 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.[3] 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,[3] 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,[3] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

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


Organelles 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).[3] 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.


HeLa cells stained with Hoechst 33258
Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed.
  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, 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.[3]
  • Mitochondria and Chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.[3] 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 ATP. 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.
Endomembrane system diagram en
Diagram of the endomembrane system
  • 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.[3] 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 that are synthesized by the cell.
  • Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[3]
  • Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules.[3] 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).[19]

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.[20]:87


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 also known as a pilus is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae, or pili are formed of a protein called pilin (antigenic) and are responsible for attachment of bacteria to specific receptors of human cell (cell adhesion). There are special types of specific pili involved in bacterial conjugation.

Cellular processes

Three cell growth types
Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.


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,[3] 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.[21] Replication, like all cellular activities, requires specialized proteins for carrying out the job.[3]

Catabolism schematic
An outline of the catabolism of proteins, carbohydrates and fats

Growth and metabolism

An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

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),[3] a molecule that possesses readily available energy, through two different pathways.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

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


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.[22] 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.[23][24]


Cell specialization

C elegans stained
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.[25]

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,[26] 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.[27] 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.[28] 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.[26] 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.[29]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[26]


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

Stromatolites are left behind by cyanobacteria, also called blue-green algae. They are the oldest known fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

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.[30]

Cells emerged at least 3.5 billion years ago.[9][10][11] 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.[31]

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

Hooke's drawing of cells in cork, 1665

See also


  1. ^ 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 embryos. It is also common to describe small molecules such as amino acids as "molecular building blocks".
  3. ^ 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.
  4. ^ Alberts, p. 2.
  5. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 978-0-13-250882-7.
  6. ^ Karp, Gerald (19 October 2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley & Sons. p. 2. ISBN 978-0-470-48337-4. Hooke called the pores cells because they reminded him of the cells inhabited by monks living in a monastery.
  7. ^ Tero AC (1990). Achiever's Biology. Allied Publishers. p. 36. ISBN 978-81-8424-369-7. 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.
  8. ^ Maton A (1997). Cells Building Blocks of Life. New Jersey: Prentice Hall. ISBN 978-0-13-423476-2.
  9. ^ 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.
  10. ^ a b Schopf JW (2006). "Fossil evidence of Archaean life". Philos Trans R Soc Lond B Biol Sci. 29 (361(1470)): 869–885.
  11. ^ a b Raven PH, Johnson GB (2002). Biology. McGraw-Hill Education. p. 68. ISBN 978-0-07-112261-0. Retrieved 7 July 2013.
  12. ^ Microbiology : Principles and Explorations By Jacquelyn G. Black
  13. ^ European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012
  14. ^ Satir P, Christensen ST (June 2008). "Structure and function of mammalian cilia". Histochemistry and Cell Biology. 129 (6): 687–93. doi:10.1007/s00418-008-0416-9. PMC 2386530. PMID 18365235. 1432-119X.
  15. ^ PH Raven, Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York
  16. ^ 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)80136-4. ISSN 0959-437X.
  17. ^ a b Campbell Biology—Concepts and Connections. Pearson Education. 2009. p. 320.
  18. ^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton". Annual Review of Biochemistry. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499.
  19. ^ 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.2007.10.034. PMID 18158904.
  20. ^ Prokaryotes. Newnes. Apr 11, 1996. ISBN 978-0-08-098473-5.
  21. ^ Campbell Biology—Concepts and Connections. Pearson Education. 2009. p. 138.
  22. ^ Ananthakrishnan R, Ehrlicher A. "The Forces Behind Cell Movement". Biolsci.org. Retrieved 2009-04-17.
  23. ^ Alberts B, Johnson A, Lewis J. et al. Molecular Biology of the Cell, 4e. Garland Science. 2002
  24. ^ Ananthakrishnan R, Ehrlicher A (June 2007). "The forces behind cell movement". International Journal of Biological Sciences. 3 (5): 303–17. doi:10.7150/ijbs.3.303. PMC 1893118. PMID 17589565.
  25. ^ Becker WM, et al. (2009). The world of the cell. Pearson Benjamin Cummings. p. 480. ISBN 978-0-321-55418-5.
  26. ^ 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.
  27. ^ 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-042110-103809. hdl:10379/6762. PMID 21351878.
  28. ^ Bonner JT (1998). "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;2-6. ISSN 1093-4391. Archived from the original (PDF, 0.2 MB) on March 8, 2012.
  29. ^ 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 (July 2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago". Nature. 466 (7302): 100–04. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019.
  30. ^ 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)01300-0. PMID 9868373.
  31. ^ Griffiths G (December 2007). "Cell evolution and the problem of membrane topology". Nature Reviews. Molecular Cell Biology. 8 (12): 1018–24. doi:10.1038/nrm2287. PMID 17971839.
  32. ^ Hooke R (1665). Micrographia: …. London, England: Royal Society of London. p. 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

Further reading

External links

American Society for Cell Biology

The American Society for Cell Biology (ASCB) is a professional society that was founded in 1960.Its mission statement says:

ASCB is an inclusive community of biologists studying the cell, the fundamental unit of life. We are dedicated to advancing scientific discovery, advocating sound research policies, improving education, promoting professional development, and increasing diversity in the workforce.


CFU-GM (or "GMP", for "granulocyte-macrophage progenitor") is a colony forming unit. It is derived from CFU-GEMM.

The "GM" stands for "granulocyte, monocyte".It is the precursor for monoblasts and myeloblasts.

Production is stimulated by granulocyte macrophage colony-stimulating factor (GM-CSF).

Cell biology

Cell biology (also called cytology, from the Greek κύτος, kytos, "vessel") is a branch of biology that studies the structure and function of the cell, which is the basic unit of life. Cell biology is concerned with the physiological properties, metabolic processes, signaling pathways, life cycle, chemical composition and interactions of the cell with their environment. This is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells. Knowing the components of cells and how cells work is fundamental to all biological sciences; it is also essential for research in bio-medical fields such as cancer, and other diseases. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology and cytochemistry .

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 the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment consisting of a lipid bilayer with embedded proteins. The cell membrane controls the movement of substances in and out of cells and organelles. In this way, it is selectively permeable to ions and organic molecules. In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall, the carbohydrate layer called the glycocalyx, and the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.

Class II PI 3-kinases

Class II PI 3-kinases are a subgroup of the enzyme family, phosphoinositide 3-kinase that share a common protein domain structure, substrate specificity and method of activation.

Class II PI 3-kinases were the most recently identified class of PI 3-kinases and little is currently known about these enzymes.

There are three class II PI 3-kinase isoforms expressed in mammalian cells;

PI3K-C2α encoded by the PIK3C2A gene

PI3K-C2β encoded by the PIK3C2B gene

PI3K-C2γ encoded by the PIK3C2G gene

Class I PI 3-kinases

Class I PI 3-kinases are a subgroup of the enzyme family, phosphoinositide 3-kinase that possess a common protein domain structure, substrate specificity, and method of activation. Class I PI 3-kinases are further divided into two subclasses, class IA PI 3-kinases and class IB PI 3-kinases.

Delta cell

Delta cells (δ-cells or D cells) are somatostatin-producing cells. They can be found in the stomach, intestine and the pancreatic islets. In rodents, delta-cells are located in the periphery of the islets; in humans the islet architecture is generally less organized and delta-cells are frequently observed inside the islets as well. In both species, the peptide hormone Urocortin3 (Ucn3) is a major local signal that is released from beta cells (and alpha cells in primates) to induce the local secretion of somatostatin. Ghrelin can also strongly stimulate somatostatin secretion, thus indirectly inhibiting insulin release. Viewed under an electron microscope, delta-cells can be identified as cells with smaller and slightly more compact granules than beta cells.

D cells in the stomach contain CCKBR (which respond to gastrin) and M3 receptors (which respond to Ach). Respectively, these receptors will increase somatostatin output and decrease somatostatin output from the D cells. VIP, vasoactive intestinal peptide, acts positively on D cells resulting in more somatostatin being released.

In the stomach, somatostatin acts directly on the acid-producing parietal cells via a G-protein coupled receptor (which inhibits adenylate cyclase, thus effectively antagonising the stimulatory effect of histamine) to reduce acid secretion. Somatostatin can also indirectly decrease stomach acid production by preventing the release of other hormones, including gastrin, secretin and histamine which effectively slows down the digestive process.


In cell biology, molecular biology and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid (see extracellular matrix). The term is used in contrast to intracellular (inside the cell).

According to the Gene Ontology, the extracellular space is a cellular component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. For multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism that are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".The composition of the extracellular space includes metabolites, ions, various proteins and non-protein substances (e.g. DNA, RNA, lipids, microbial products etc.) that might affect cellular function. For example, hormones, growth factors, cytokines and chemokines act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes (Trypsin, Pepsin), extracellular proteinases (Matrix metalloproteinases, ADAMTSs, Cathepsins) and antioxidant enzymes (extracellular superoxide dismutase). Often, proteins present in the extracellular space are stored outside the cells by attaching to various extracellular matrix components (Collagens, Proteoglycans, etc.). In addition, extracellular matrix proteolytic products are also present in the extracellular space, especially in tissues undergoing remodelling [2].

Granule (cell biology)

In cell biology, a granule is a small particle. It can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.

In utero

In utero is a Latin term literally meaning "in the womb" or "in the uterus". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to referred to an unborn child, i.e., gestational age until birth. Under common law, unborn children are still considered to exist for property transfer purposes.


Interphase is the phase of the cell cycle in which a typical cell spends most of its life.

During this phase, the cell copies its DNA in preparation for mitosis. Interphase is the 'daily living' or metabolic phase of the cell, in which the cell obtains nutrients and metabolizes them, grows, reads its DNA, and conducts other "normal" cell functions. The majority of eukaryotic cells spend most of their time in interphase. This phase was formerly called the resting phase. However, interphase does not describe a cell that is merely resting; rather, the cell is living, and preparing for later cell division, so the name was changed. A common misconception is that interphase is the first stage of mitosis. However, since mitosis is the division of the nucleus, prophase is actually the first stage.In interphase, the cell gets itself ready for mitosis or meiosis. Somatic cells, or normal diploid cells of the body, go through mitosis in order to reproduce themselves through cell division, whereas diploid germ cells (i.e., primary spermatocytes and primary oocytes) go through meiosis in order to create haploid gametes (i.e., sperm and ova) for the purpose of sexual reproduction. Chromosomes are copied.


In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular (outside the cell). The cell membrane (and, in many organisms, the cell wall) is the barrier between the two, and chemical composition of intra- and extracellular milieu (Milieu intérieur) can be radically different. In most organisms, for example, a Na+/K+ ATPase maintains a high potassium level inside cells while keeping sodium low, leading to chemical excitability.


Karyorrhexis (from Greek κάρυον karyon, "kernel, seed or nucleus", and ῥῆξις rhexis, "bursting") is the destructive fragmentation of the nucleus of a dying cell whereby its chromatin is distributed irregularly throughout the cytoplasm. It is usually preceded by pyknosis and can occur as a result of either programmed cell death (apoptosis), senescence, or necrosis.

In apoptosis, the cleavage of DNA is done by Ca2+ and Mg2+ -dependent endonucleases.


Lysis ( LY-sis; Greek λύσις lýsis, "a loosing" from λύειν lýein, "to unbind") refers to the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" LIT-ək) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a lysate. In molecular biology, biochemistry, and cell biology laboratories, cell cultures may be subjected to lysis in the process of purifying their components, as in protein purification, DNA extraction, RNA extraction, or in purifying organelles.

Many species of bacteria are subject to lysis by the enzyme lysozyme, found in animal saliva, egg white, and other secretions. Phage lytic enzymes (lysins) produced during bacteriophage infection are responsible for the ability of these viruses to lyse bacterial cells. Penicillin and related β-lactam antibiotics cause the death of bacteria through enzyme-mediated lysis that occurs after the drug causes the bacterium to form a defective cell wall. If the cell wall is completely lost, the bacterium is referred as a protoplast if penicillin was used on gram-positive bacteria, and spheroplast when used on gram-negative bacteria.

Max Planck Institute of Molecular Cell Biology and Genetics

The Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG) is a biology research institute located in Dresden, Germany. It was founded in 1998 and was fully operational in 2001. Twenty-four research groups work in molecular biology, cell biology, developmental biology,and biophysics supported by various facilities.

Megakaryocyte–erythroid progenitor cell

The megakaryocyte–erythroid progenitor cell (or MEP, or hMEP to specify human) is a cell that gives rise to megakaryocytes and erythrocytes.It is derived from the common myeloid progenitor.

Nature Reviews Molecular Cell Biology

Nature Reviews Molecular Cell Biology is a peer-reviewed monthly review journal that was established in October 2000 and is published by Nature Publishing Group. It covers all aspects of molecular and cell biology.

According to the Journal Citation Reports, the journal had a 2016 impact factor of 46.602, ranking it first in the category "Cell Biology". In 2016, it has an h-index of 324.

Outline of cell biology

The following outline is provided as an overview of and topical guide to cell biology:

Cell biology – A branch of biology that includes study of cells regarding their physiological properties, structure, and function; the organelles they contain; interactions with their environment; and their life cycle, division, and death. This is done both on a microscopic and molecular level. Cell biology research extends to both the great diversities of single-celled organisms like bacteria and the complex specialized cells in multicellular organisms like humans. Formerly, the field was called cytology (from Greek κύτος, kytos, "a hollow;" and -λογία, -logia).

Stem cell

Stem cells are cells that can differentiate into other types of cells, and can also divide in self-renewal to produce more of the same type of stem cells.

In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm (see induced pluripotent stem cells)—but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.

There are three known accessible sources of autologous adult stem cells in humans:

bone marrow, adipose tissue, and blood. Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell types, autologous harvesting involves the least risk. By definition, autologous cells are obtained from one's own body, just as one may bank his or her own blood for elective surgical procedures.

Adult stem cells are frequently used in various medical therapies (e.g., bone marrow transplantation). Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves. Embryonic cell lines and autologous embryonic stem cells generated through somatic cell nuclear transfer or dedifferentiation have also been proposed as promising candidates for future therapies. Research into stem cells grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s.

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