cell

any small compartment

"the cells of a honeycomb" (biology) the basic structural and functional unit of all organisms

they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals a device that delivers an electric current as the result of a chemical reaction a small unit serving as part of or as the nucleus of a larger political movement a hand-held mobile radiotelephone for use in an area divided into small sections, each with its own short-range transmitter/receiver small room in which a monk or nun lives a room where a prisoner is kept

ed for keratin (red) and DNA (green)]]The cell is the structural and functional unit of all lifeliving organisms, and are sometimes called the "building blocks of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion or 10¹⁴ cells). The cell theory, first developed in 1839 by Matthias Jakob SchleidenSchleiden and Theodor SchwannSchwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the geneticshereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.The word ''cell'' comes from the Latin ''cella'', a small room. The name was chosen by Robert Hooke when he compared the cork (material)cork cells he saw to small rooms monks lived in.

Overview -

Properties of cells - s across.]]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 share several abilities:

Reproduction by cell division.

cell metabolismMetabolism, including taking in raw materials, building cell components, creating 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 derived from metabolic pathways.

protein biosynthesisSynthesis of proteins, the functional workhorses of cells, such as enzymes. A typical mammalmammalian cell contains up to 10,000 different proteins.

Response to external and internal signal transductionstimuli such as changes in temperature, pH or nutrient levels.

traffic (locational)Traffic of vesicle (biology)vesicles.

Types of cells - One way to classify cells is whether they live alone or in groups. Organisms vary from single cells (called single-celled or unicellular organisms) that function and survive more or less independently, through ''colonial'' forms with cells living together, to multicellular forms in which cells are specialized. 220 types of cells and tissues make up the multicellular human body.Cells can also be classified into two categories based on their internal structure.

''prokaryoteProkaryotic'' cells are structurally simple. They are found only in single-celled and Colony (biology)colonial organisms. In the three-domain system of scientific classification, prokaryotic cells are placed in the domains Archaea and Eubacteria.

''eukaryoteEukaryotic'' cells have organelles with their own membranes. Single-celled eukaryotic organisms such as amoebae and some fungi are very diverse, but many colonial and multicellular forms such as plants, animals, and brown algae also exist.

Subcellular components - s: (1) nucleolus (2) cell nucleusnucleus (3) ribosome (4) vesicle (biology)vesicle,(5) rough endoplasmic reticulum (ER), (6) Golgi apparatus, (7) Cytoskeleton, (8) smooth ER, (9) mitochondrionmitochondria, (10) vacuole, (11) cytoplasm, (12) lysosome, (13) centrioles]] All cells whether prokaryotic or eukaryotic have a cell membranemembrane, which envelopes the cell, separates its interior from its environment, controls what moves in and out, and maintains the cell potentialelectric 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, which contain the information necessary to gene expressionbuild 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 protective coat - ''Main article Cell membraneThe cytoplasm of a eukaryotic cell is surrounded by a ''plasma membrane''. A form of plasma membrane is also found in prokaryotes, but is usually referred to as the ''cell membrane''. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a lipid bilayerdouble layer of lipids (fat-like molecules) and proteins. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell.

Cytoskeleton - a cell's scaffold - ''Main article CytoskeletonThe cytoskeleton is an important, complex, and dynamic cell component. It 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 moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell's structure by directing, bundling, and aligning filaments.

Genetic material - Two different kinds of genetic material exist: DNAdeoxyribonucleic acid (DNA) and RNAribonucleic acid (RNA). Most organisms use DNA for their long term information storage, but some viruses (retroviruses) have RNA as their genetic material. The biological information contained in an organism is Genetic codeencoded in its DNA or RNA sequence. RNA is also used for information transport (e.g. mRNA) and enzymeenzymatic functions (e.g. ribosomeribosomal RNA) in organisms that use RNA for the genetic code itself. 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, e.g. has genetic material in the nucleus (the genomenuclear genome) and in the mitochondria (the mitochondrial genome). The nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins.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.

Organelles - ''Main article OrganelleThe human body contains many different Organ (anatomy)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. Membrane-bound organelles are only found in eukaryotes.

'''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 occur. The nucleus is spheroid 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. In prokaryotes, DNA processing takes place in the cytoplasm.

Ribosomes - the protein production machine: Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation (genetics)translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.

Mitochondria and chloroplasts - the power generators: MitochondrionMitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex metabolic pathways. Chloroplasts are larger than mitochondria, and convert solar energy into a chemical energy ("food") via photosynthesis. Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes like plants and algae. There are a number of plant organelles that are modified chloroplasts; they are broadly called plastids and are often involved in storage.

Endoplasmic reticulum and Golgi apparatus - macromolecule managers:: 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 the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be ''exported'' from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid synthesis, detoxification and as a calcium reservoir. The Golgi apparatus, sometimes called a ''Golgi body'' or ''Golgi complex'' is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily folded membranes.

Lysosomes and peroxisomes - the cellular digestive system: Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.

Centrioles - They help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells.

Vacuoles-They store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane.

Anatomy of cells -

Prokaryotic cells - Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and Piluspili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmcytoplasmic region that contains the genomecell 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'' (a bacterium) and ''Thermoplasma'' (an archaeon)). It consists of ''peptidoglycan'' in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like 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 - There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are usally found in multi-cellular organisms, while prokaryotic cells are usually on their own. EukaryoteEukaryotic 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 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 highly condensed (i.e. folded around histones). All chromosomal DNA is stored in the ''cell nucleus'', separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.

Eukaryotes can move using ''cilia'' or ''flagella''. The flagella are more complex than those of prokaryotes.

Cell functions -

Cell growth and metabolism - ''Main articles Cell growth, Cell metabolismBetween successive cell divisions cells grow through the functioning of cellular metabolism.Cell metabolism is the process by which individual cell (biology)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, where 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 triphosphateATP), 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.

Making new cells - ''Main article Cell division of the cell (''light blue''), genes (DNA, ''dark blue'') are transcription (genetics)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 (genetics)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 transfer RNAtRNA. Newly synthesized proteins (''black'') are often further modified, such as by binding to an effector molecule (''orange''), to become fully active.]]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 tissuetissue) and to procreation (vegetative reproduction) in unicellular organisms.ProkaryoteProkaryotic cells divide by binary fission. EukaryoteEukaryotic 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 - ''Main article Protein biosynthesisProtein synthesis is the process in which the cell builds proteins. DNA transcription (genetics)transcription refers to the synthesis of a messenger RNA (mRNA) molecule from a DNA template. This process is very similar to DNA replication. Once the mRNA has been generated, a new protein molecule is synthesized via the process of translation (genetics)translation.The cellular machinery responsible for synthesizing proteins is the ribosome. The ribosome consists of structural RNA and about 80 different proteins. When the ribosome encounters an mRNA, the process of translation (genetics)translating an mRNA to a protein begins. The ribosome accepts a new transfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an amino acid, the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein.

Origins of cells - ''Main article'': Origin of lifeThe origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.

Origin of first cell - If life is viewed from the point of view of replicatorreplicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymeenzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example, by producing nucleotides) will automatically advantage the neighbouring DNA molecules. This might be viewed as "parasitism by default". Therefore the natural selectionselection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged. If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it.(Note. This is more a metaphor given for simplicity than complete accuracy, since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules, acting both as replicators and enzymes: see RNA world hypothesis . But the core of the reasoning is the same.)Biochemically, cell-like spheroids formed by proteinoidproteinoids are observed by heating amino acidamino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranecell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendancy to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in ones mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a Lipid bilayerbilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble users.rcn.com can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-celluar organisms could be acheived. This theory is expanded upon in the book, ''"The Cell: Evolution of the First Organism"'' by Joseph Panno Ph.D.

Origin of eukaryotic cells - The eukaryotic cell seems to have evolved from a symbiosissymbiotic 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 bacteriumbacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaeaarchaean prokaryote cell – a theory termed the endosymbiotic theory. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.

History -

1632-1723: Antony van Leeuwenhoek teaches himself to grind Lens (optics)lenses, builds a microscope and draws protozoa, such as ''Vorticella'' from rain water, and bacteriumbacteria from his own mouth.

1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.:...I could exceedingly plainly perceive it to be all perforated and porous, much like honeycomb...these pores or cells, were not very deep, but consisted of a great many little boxes...'' – Hooke describing his observations on a thin slice of cork.

1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.

The belief that life forms are able to occur spontaneously (''Abiogenesisgeneratio spontanea'') is contradicted by Louis Pasteur (1822-1895).

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. WatsonWatson and Francis CrickCrick 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.

See also -

Cariology is the study of the cell nucleus.

Cytotoxicity

Plant cell

How to prepare an onion cell slide

Cell types

Syncytium

Cell culture

Stem cell

Plasmolysis

Cytorrhysis

External links - commonsCell (biology) wikibooksparCell Biology

ericdigests.org - Teaching about the Life and Health of Cells.

biopic.co.uk - The cell like a city.

cellsalive.com - Cells Alive!

jcb.org - Journal of Cell Biology

References -

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