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Cell (biology)

The '''cell''' is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.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". Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 10¹⁴ cells; a typical cell size is 10 micrometre|µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilised ostrich Ovum|egg cell. In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The 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 all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the genetics|hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word ''cell'' comes from the Latin ''cellula'', meaning, a small room. The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the Cork (material)|cork cells he saw through his microscope to the small rooms monks lived in."... 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. Robert Hooke

Anatomy of cells

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Prokaryotic cells

The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a cell nucleus|nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar overall structure. A prokaryotic cell has three architectural regions:
- On the outside, flagella and Pilus|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;
- Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a bacterial capsule|capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as ''Mycoplasma'' (bacteria) and ''Thermoplasma'' (archaea). 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 finally bursting (cytolysis) from osmotic pressure against a Tonicity#Hypotonicity|hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall;
- Inside the cell is the cytoplasm|cytoplasmic region that contains the genome|cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium ''Borrelia burgdorferi'', which causes Lyme disease). Though not forming a ''nucleus'', the DNA is condensed in a ''nucleoid''. Prokaryotes can carry extrachromosomal DNA elements called ''plasmids'', which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.

Eukaryotic cells

Eukaryote|Eukaryotic cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true 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. Some eukaryotic organelles such as mitochondria also contain some DNA.
- Many eukaryotic cells are cilium|ciliated with ''primary cilia''. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular Antenna (biology)|antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
- Eukaryotes can move using ''motile cilia'' or ''flagella''. The flagella are more complex than those of prokaryotes.

Structures outside the cell wall

Capsule

A gelatinous capsule is present in some bacteria outside the 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 ordinary stain and can be detected by India ink#Uses other than writing|special stain. The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes.

Flagella

Flagella are the organelles of cellular mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread-like appendages, protein in nature. Are most commonly found in bacteria cells but are found in animal cells as well.

Fimbriae (pili)

They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in the process of conjunction.

Cell functions

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 a less chemically-complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (adenosine triphosphate|ATP), a form of energy, via two different pathways. The first pathway, glycolysis, requires no oxygen and is referred to as Fermentation (biochemistry)|anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

Creation of new cells

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 biological tissue|tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryote|Prokaryotic cells divide by binary fission. Eukaryote|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, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

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 (genetics)|transcription and translation (genetics)|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.

Cell movement or motility

Cells can move during many processes: such as wound healing, the immune response and cancer metastasis. For wound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill the microorganisms that cause infection.
At the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumor development, cells from a primary tumor move away and spread to other parts of the body. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. 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 of these steps is driven by physical forces generated by unique segments of the cytoskeleton.Alberts B, Johnson A, Lewis J. et al. Molecular Biology of the Cell, 4e. Garland Science. 2002Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell Movement. Int J Biol Sci 2007; 3:303–317. http://www.biolsci.org/v03p0303.htm

Evolution

The origin of cells has to do with the origin of life, which began the timeline of evolution|history of life on Earth.

Origin of the first cell

There are a number of theories about the origin of small molecules that could lead to life in an early Earth. One is that they came from meteorites (''see Murchison meteorite''). Another is that they were created at Hydrothermal vent|deep-sea vents. A third is that they were synthesized by lightning in a reducing atmosphere (''see Miller–Urey experiment''); althougyze chemical reactions (''see RNA world hypothesis''). But some other entity with the potential to self-replicate could have preceded RNA, like clay or peptide nucleic acid. Cells emerged at least 4.0–4.3 billion years ago. The current belief is that these cells were heterotrophs. An important characteristic of cells is the cell membrane, composed of a bilayer of lipids. 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 Vesicle (biology)|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.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiosis|symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory. There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells. Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests that the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing co-evolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.

History

- 1632 – 1723: Antonie van Leeuwenhoek teaches himself to grind Lens (optics)|lenses, builds a microscope and draws protozoa, such as ''Vorticella'' from rain water, and Bacterium|bacteria from his own mouth.
- 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.
- 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate 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.
- The belief that life forms are able to occur spontaneously (''Abiogenesis|generatio spontanea'') is contradicted by Louis Pasteur (1822 – 1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
- 1855: Rudolph Virchow states that cells always emerge from cell divisions (''omnis cellula ex cellula'').
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously-unresolvable organelles.
- 1953: James D. Watson|Watson and Francis Crick|Crick made their first announcement on the double-helix structure for DNA on February 28.
- 1981: Lynn Margulis published ''Symbiosis in Cell Evolution'' detailing the endosymbiotic theory.

References

External links

- Inside the Cell
- Virtual Cell's Educational Animations
- The Inner Life of A Cell, a flash video showing what happens inside of a cell
- The Virtual Cell
- Cells Alive!
- Journal of Cell Biology
- The Biology Project > Cell Biology
- Centre of the Cell online
- The Image & Video Library of The American Society for Cell Biology, a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
- Gall JG, McIntosh JR, eds (2001). ''Landmark Papers in Cell Biology''. Bethesda, MD and Cold Spring Harbor, NY: The American Society for Cell Biology and Cold Spring Harbor Laboratory Press; 2001. Commentaries and links to original research papers published in the ASCB Image & Video Library

Textbooks

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- Category:Cell biology zh-min-nan:Sè-pau be-x-old:Вуза simple:Cell zh-yue:細胞 bat-smg:Lāstelė

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