Cell (biology)

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=Cell+Movements+and+the+Shaping+of+the+Vertebrate+Body+AND+mboc4%5Bbook%5D+AND+374635%5Buid%5D&rid=mboc4.section.3919 Cell Movements and the Shaping of the Vertebrate Body] in Chapter 21 of " [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=cell+biology+AND+mboc4%5Bbook%5D+AND+373693%5Buid%5D&rid=mboc4 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 " [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 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 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.Fact|date=September 2008

In 1837 before the final cell theory was developed, a Czech 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. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. [cite book
last = Maton
first = Anthea
authorlink =
coauthors = Hopkins, Jean Johnson, Susan LaHart, David Quon Warner, Maryanna Wright, Jill D
title = Cells Building Blocks of Life
publisher = Prentice Hall
date = 1997
location = New Jersey
pages =
url =
doi =
id =
isbn = 0-13-423476-6]

The word "cell" comes from the Latin "cellula", meaning, a small room. The descriptive name for the smallest living biological structure was chosen by Robert Hooke in a book he published in 1665 when he compared the 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. [http://www.ucmp.berkeley.edu/history/hooke.html Robert Hooke] ]

General principles

Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.

All cells have several different abilities: [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 The Universal Features of Cells on Earth] in Chapter 1 of the Alberts textbook (reference #1, above).]
* Reproduction by cell division: (binary fission/mitosis or meiosis).
* Use of enzymes and other proteins coded for by DNA genes and made via messenger RNA intermediates and ribosomes.
* Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is released and then used in metabolic pathways.
* Response to external and internal stimuli such as changes in temperature, pH or levels of nutrients.
* Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in it.

Some prokaryotic cells contain important internal membrane-bound compartments, [cite journal | last = L.M. | first = Mashburn-Warren | coauthors =Whiteley, M. | title =Special delivery: vesicle trafficking in prokaryotes. | journal =Mol Microbiol | volume =61 | issue =4| pages =839-46| date =2006 | doi =10.1111/j.1365-2958.2006.05272.x | doi = 10.1111/j.1365-2958.2006.05272.x | pmid = 16879642 ] but eukaryotic cells have a specialized set of internal membrane compartments. Material is moved between these compartments by regulated traffic and transport of small spheres of membrane-bound material called "vesicles".A. Rose, S. J. Schraegle, E. A. Stahlberg and I. Meier (2005) "Coiled-coil protein composition of 22 proteomes--differences and common themes in subcellular infrastructure and traffic control" in "BMC evolutionary biology" Vulume 5 article 66. Entrez Pubmed|16288662
Rose "et al." suggest that coiled-coil alpha helical vesicle transport proteins are only found in eukaryotic organisms.]

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

Prokaryotes differ from eukaryotes since they lack a nuclear envelope and a cell nucleus. Prokaryotes also lack most of the intracellular organelles and structures that are seen in eukaryotic cells. There are two kinds of prokaryotes, bacteria and archaea, but these are similar in the overall structures of their cells. Most functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic cell's plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili — proteins attached to the cell surface; a cell envelope - consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
* The "plasma membrane" (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
* Most prokaryotes have a "cell wall" (some exceptions are "Mycoplasma" (bacteria) and "Thermoplasma" (archaea)). This wall consists of "peptidoglycan" in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" (cytolysis) from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like plants (cellulose) and fungi, but has a different chemical composition.
* A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium "Borrelia burgdorferi", which causes Lyme disease). Even without a real "nucleus", the DNA is condensed in a "nucleoid". Prokaryotes can carry extrachromosomal DNA elements called "plasmids", which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.

Eukaryotic cells

) cell, showing subcellular components.

Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles within centrosome]

Eukaryotic cells are about 10 times the size of 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.
* Eukaryotes can move using "cilia" or "flagella". The flagella are more complex than those of prokaryotes.

ubcellular components

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells 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 will list these primary components of the cell, then briefly describe their function.

Cell membrane: A cell's defining boundary

The cytoplasm of a cell is surrounded by a cell membrane or "plasma membrane". The plasma membrane in plants and prokaryotes 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 lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called 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. The membrane is said to be 'semi-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 signalling molecules such as hormones.

Cytoskeleton: A cell's scaffold

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 is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. [cite journal |author=Michie K, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu Rev Biochem |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 | doi = 10.1146/annurev.biochem.75.103004.142452 ]

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses (e.g., retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the genetic code itself. Transfer RNA (tRNA) molecules are used to add specific amino acids during the process of protein translation.

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, 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 in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called 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, it codes for 13 proteins involved in mitochondrial energy production as well as 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

The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions.

There are several types of organelles within an animal cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

{|
-|; 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 in shape 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 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. |
-

; Mitochondria and Chloroplasts (the power generators) : Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by the process of oxidative phosphorylation, utilizing oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by splitting in two.

:Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through the process of photosynthesis, which utilizes solar energy to generate carbohydrates and oxygen from carbon dioxide and water.

:Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, strongly supporting the evolutionary theory of endosymbiosis; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.
-|; Endoplasmic reticulum (eukaryotes only) : The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release.
-|; Golgi apparatus (eukaryotes only) : 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. It is particularly important in the processing of proteins for secretion. The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells. Vesicles that enter the Golgi apparatus are processed in a cis to trans direction, meaning they coalesce on the cis side of the apparatus and after processing pinch off on the opposite (trans) side to form a new vesicle in the animal cell.|
-

; Ribosomes : The ribosome is a large complex of RNA and protein molecules. This is where proteins are produced. Ribosomes can be found either foating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). [cite journal |author=Ménétret JF, Schaletzky J, Clemons WM, "et al" |title=Ribosome binding of a single copy of the SecY complex: implications for protein translocation |journal=Mol. Cell |volume=28 |issue=6 |pages=1083–92 |year=2007 |month=December |pmid=18158904 |doi=10.1016/j.molcel.2007.10.034]
-
colspan="2"
; Lysosomes and Peroxisomes (eukaryotes only) : 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. These organelles are often called a "suicide bag" because of their ability to detonate and destroy the cell.
-
colspan="2"
; 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.
-
colspan="2"
; Vacuoles : Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably "Amoeba", have contractile vacuoles, which are able to pump water out of the cell if there is too much water.

tructures outside the cell wall

Capsule

It is present only in some bacteria outside the cell wall. It is gelatinous in nature. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as bacillus anthracis or hyaluronic acid as in streptococci.Capsules not stained by ordinary stain and can detected by 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 mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread like appendages, protein in nature, formed of flagellin protein (antigenic). They can not be stained by gram stain. They have a special stain. According to their arrangement they may be monotrichate, amphitrichate, lophotrichate, peritrichate.

Fimbriae (pili)

They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachement 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 (ATP), a form of energy, via two different pathways.

The first pathway, glycolysis, requires no oxygen and is referred to as 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 tissue) and to procreation (vegetative reproduction) in unicellular organisms.

Prokaryotic cells divide by binary fission. 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 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.

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. [ [http://www.biolsci.org/v03p0303.htm The Forces Behind Cell Movement ] ] The process is divided into three steps - protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion 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. 2002] Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell Movement. Int J Biol Sci 2007; 3:303-317. http://www.biolsci.org/v03p0303.htm]

Origins of cells

The origin of cells has to do with the origin of life, which began the history of life on Earth. The birth of the cell marked the passage from prebiotic chemistry to life.

Origin of the first cell

:"For more information, RNA world hypothesis":"For more information, Last universal ancestor"

The unit of selection in modern organisms and populations of organisms is not clear, with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species.cite journal |author=Gould SJ |title=Gulliver's further travels: the necessity and difficulty of a hierarchical theory of selection |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9533127 |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=353 |issue=1366 |pages=307–14 |year=1998 |pmid=9533127 |doi=10.1098/rstb.1998.0211] [cite journal |author=Mayr E |title=The objects of selection |url=http://www.pnas.org/cgi/content/full/94/6/2091 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=94 |issue=6 |pages=2091–94 |year=1997 |pmid=9122151 | doi = 10.1073/pnas.94.6.2091 ] None of these models are mutually-exclusive and selection may act on multiple levels simultaneously. [cite journal |author=Maynard Smith J |title=The units of selection |journal=Novartis Found. Symp. |volume=213 |pages=203–11; discussion 211–17 |year=1998 |pmid=9653725] However, in a gene-centered view of evolution, life is regarded in terms of replicators—that is the DNA molecules in the organism. If freely-floating DNA molecules that code for enzymes are not enclosed in cells, the enzymes that benefit a given replicator (for example, by producing nucleotides) may do so less efficiently, and may in fact benefit competing replicators. If the entire DNA molecule of a replicator is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The replicator will directly benefit from its encoded enzymes.

Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear many of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules may have been the first cellular life forms on Earth. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane composed of proteins and phospholipid bilayer membranes.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a 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 coevolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.cite journal | author = Sterrer W | title = On the origin of sex as vaccination | journal = Journal of Theoretical Biology | volume = 216 | pages = 387-396 | year = 2002 | pmid = 12151256 | doi = 10.1006/jtbi.2002.3008 ]

History

* 1632 – 1723: Antonie van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as "Vorticella" from rain water, and 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 ("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: Watson and 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.

ee also

References

*

External links

* [http://vcell.ndsu.nodak.edu/animations/ Virtual Cell's Educational Animations] .
* [http://www.ixedu.com/es/celula/Introduccion.html Ixedu.com] 3D Animations, Virtual Microscope, Activities, a Game and more! All about the cells.
* [http://www.studiodaily.com/main/searchlist/6850.html The Inner Life of A Cell] , a flash video showing what happens inside of a cell
* [http://www.ibiblio.org/virtualcell/tour/cell/cell.htm The Virtual Cell]
* [http://www.cellsalive.com/ Cells Alive!]
* [http://www.jcb.org/ Journal of Cell Biology]
* [http://members.optusnet.com.au/exponentialist/Cells.htm A comparison of the generational and exponential growth of cell populations]
* [http://brainmaps.org/index.php?q=cell High-resolution images of brain cells]
* [http://www.biology.arizona.edu/cell_bio/cell_bio.html The Biology Project > Cell Biology]
* [http://cellimages.ascb.org/ The Image & Video Library] of [http://www.ascb.org/ 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.
* [http://www.centreofthecell.org/ Centre of the Cell online]
* [http://publications.nigms.nih.gov/insidethecell/ Inside the Cell]

Online textbooks

*

*

*

* Gall JG, McIntosh JR, eds (2001)." [http://cellimages.ascb.org/cdm4/browse.php?CISOROOT=/p4041coll11 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