Cell (biology)
Cell | |
---|---|
prokaryotic cell (right) | |
Identifiers | |
MeSH | D002477 |
TH | H1.00.01.0.00001 |
FMA | 686465 |
Anatomical terminology] |
The cell is the basic structural and functional unit of all
Cells are broadly categorized into two types:
Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, 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.
Number of cells
The number of cells in plants and animals varies from species to species; it has been estimated that the human body contains around 37 trillion (3.72×1013) cells,[1] and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female).[2] The human brain accounts for around 80 billion of these cells.[3]
Cell types
Cells are broadly categorized into two types:
Prokaryotic cells
A prokaryotic cell has three regions:
- Enclosing the cell is the fungalcells) also have a cell wall.
- Inside the cell is the antibiotic resistancegenes.
- On the outside, some prokaryotes have flagella and pilithat project from the cell's surface. These are structures made of proteins that facilitate movement and communication between cells.
Eukaryotic cells
- 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 mitochondriaalso contain some DNA.
- Many eukaryotic cells are
- Motile eukaryotes can move using flagella. Motile cells are absent in conifers and flowering plants.[citation needed] Eukaryotic flagella are more complex than those of prokaryotes.[9]
Prokaryotes | Eukaryotes | |
---|---|---|
Typical organisms | bacteria, archaea
|
protists, algae, fungi, plants, animals |
Typical size | ~ 1–5 μm[10] | ~ 10–100 μm[10] |
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 in the cytoplasm
protein synthesis |
Ribosomes | 30S
|
40S
|
Cytoplasmic structure | very few structures | highly structured by endomembranes and a cytoskeleton |
Cell movement | flagella made of flagellin | flagella and |
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
Cell membrane
The
Cytoskeleton
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during
Genetic material
Two different kinds of genetic material exist:
Prokaryotic genetic material is organized in a simple
A
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
Organelles are parts of the cell that 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).[6] 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
Eukaryotic
- Cell nucleus: A cell's information center, the 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.[6]
- Mitochondria and chloroplasts: generate energy for the cell. binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.
- Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes.[6] The smooth ER plays a role in calcium sequestration and release and also helps in synthesis of lipid.
- Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as 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, Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[6]
- Centrosome: the cytoskeleton organizer: The mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
- Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled spaces and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.
Eukaryotic and prokaryotic
- Ribosomes: The ribosome is a large complex of RNA and protein molecules.[6] 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).[14]
- Plastids: Plastid are membrane-bound organelle generally found in plant cells and euglenoids and contain specific pigments, thus affecting the colour of the plant and organism. And these pigments also helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments. Chloroplasts contain chlorophyll and some carotenoid pigments which helps in the tapping of light energy during photosynthesis. Chromoplasts contain fat-soluble carotenoid pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage. Leucoplasts are non-pigmented plastids and helps in storage of nutrients.[15]
Structures outside the cell membrane
Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.
Cell wall
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.
Prokaryotic
Capsule
A gelatinous
Flagella
Fimbriae
A
Cellular processes
Replication
Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in
DNA replication, or the process of duplicating a cell's genome,[6] 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
DNA repair
Cells of all organisms contain enzyme systems that scan their DNA for DNA damage and carry out repair processes when damage is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: nucleotide excision repair, DNA mismatch repair, non-homologous end joining of double-strand breaks, recombinational repair and light-dependent repair (photoreactivation).[18]
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 can be broken down into simpler sugar molecules called
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
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.
Motility
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include
In multicellular organisms, cells can move during processes such as wound healing, the immune response and
In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to
Multicellularity
Cell specialization/differentiation
Multicellular organisms are
In complex multicellular organisms, cells specialize into different
Most distinct cell types arise from a single
Origin of multicellularity
Multicellularity has evolved independently at least 25 times,
The first evidence of multicellularity is from
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in
Origins
The origin of cells has to do with the
Origin of the first cell
Small molecules needed for life may have been carried to Earth on meteorites, created at
Cells emerged around 4 billion years ago.[30][31] The first cells were most likely heterotrophs. The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA.[32][33]
Origin of eukaryotic cells
History of research
In 1665,
- 1632–1723: bacteria from his own mouth.[43]
- 1665: Robert Hooke discovered cells in cork, then in living plant tissue using an early compound microscope. He coined the term cell (from Latin cellula, meaning "small room"[44]) in his book Micrographia (1665).[45][43]
- 1839: Theodor Schwann[46] and Matthias Jakob Schleiden elucidated the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
- 1855: Rudolf Virchow stated that new cells come from pre-existing cells by cell division (omnis cellula ex cellula).
- 1931: University of Berlin.[47]By 1935, he had built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing how eukaryotic cells were created by symbiogenesis.[48]
See also
References
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- ^ NCBI. 30 March 2004. Archived from the originalon 2009-12-08. Retrieved 3 May 2013.
- ^ European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi Archived 2013-05-06 at the Wayback Machine, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012
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- ^ Campbell Biology – Concepts and Connections. Pearson Education. 2009. p. 138.
- ^ Snustad, D. Peter; Simmons, Michael J. Principles of Genetics (5th ed.). DNA repair mechanisms, pp. 364–368.
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... 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 Archived 1997-06-06 at the Wayback Machine
Further reading
- Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Morgan, David; Raff, Martin; Roberts, Keith; Walter, Peter (2015). Molecular Biology of the Cell (6th ed.). Garland Science. p. 2. ISBN 978-0815344322.
- Alberts, B.; et al. (2014). Molecular Biology of the Cell (6th ed.). Garland. ISBN 978-0815344322. Archived from the original on 2014-07-14. Retrieved 2016-07-06.; The fourth edition is freely available Archived 2009-10-11 at the Wayback Machine from National Center for Biotechnology InformationBookshelf.
- Lodish, Harvey; et al. (2004). Molecular Cell Biology (5th ed.). New York: WH Freeman. ISBN 978-0716743668.
- Cooper, G. M. (2000). The cell: a molecular approach (2nd ed.). Washington, D.C: ASM Press. ISBN 978-0878931026. Archivedfrom the original on 2009-06-30. Retrieved 2017-08-30.
External links
- MBInfo – Descriptions on Cellular Functions and Processes
- Inside the Cell Archived 2017-07-20 at the ePub.
- 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 Archived 2011-06-10 at the Wayback Machine, a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
- WormWeb.org: Interactive Visualization of the C. elegans Cell lineage – Visualize the entire cell lineage tree of the nematode C. elegans