Cell (biology)The cell is the basic structural and functional unit of all known living It is the smallest unit of an organism that is classified as a living thing， and is often called the building block of [1] Some organisms， such as most bacteria， are unicellular (consist of a single cell) Other organisms， such as humans， are (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 ) The largest known cell is an unfertilized ostrich egg [2]In 1835 before the final cell theory was developed， Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a 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 hereditary information necessary for regulating cell functions and for transmitting information to the next generation of [3]The word cell comes from the Latin cellula， meaning， a small 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 cells he saw through his microscope to the small rooms monks lived [4][edit] General principles Mouse cells grown in a culture These cells grow in large clumps， but each individual cell is about 10 micrometres acrossEach 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 Each cell stores its own set of instructions for carrying out each of these All cells have several different abilities:[5]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 Metabolism， including taking in raw materials， building cell components， converting energy， molecules and releasing by- The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic This energy is released and then used in metabolic Response to external and internal stimuli such as changes in temperature， pH or levels of Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in Some prokaryotic cells contain important internal membrane-bound compartments，[6] but eukaryotic cells have a specialized set of internal membrane [edit] Anatomy of cellsThere are two types of cells: eukaryotic and Prokaryotic cells are usually independent， while eukaryotic cells are often found in multicellular [edit] Prokaryotic cellsMain article: Prokaryote Diagram of a typical prokaryotic cellThe prokaryote cell is simpler than a eukaryote cell， lacking a nucleus and most of the other organelles of There are two kinds of prokaryotes: bacteria and archaea; these share a similar overall A prokaryotic cell has three architectural regions:on the outside， flagella and pili project from the cell's 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 The envelope gives rigidity to the cell and separates the interior of the cell from its environment， serving as a protective 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 It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic Some eukaryote cells (plant cells and fungi cells) also have a cell wall; inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of 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 Prokaryotes can carry extrachromosomal DNA elements called plasmids， which are usually Plasmids enable additional functions， such as antibiotic [edit] Eukaryotic cellsMain article: Eukaryote Diagram of a typical animal (eukaryotic) cell， showing subcellular 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 centrosomeEukaryotic cells are about 15 times the size of a typical prokaryote and can be as much as 1000 times greater in The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take 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 " Other differences include:The plasma membrane resembles that of prokaryotes in function， with minor differences in the Cell walls may or may not be The eukaryotic DNA is organized in one or more linear molecules， called chromosomes， which are associated with histone All chromosomal DNA is stored in the cell nucleus， separated from the cytoplasm by a Some eukaryotic organelles such as mitochondria also contain some DNA Many eukaryotic cells are ciliated with primary Primary cilia play important roles in chemosensation， mechanosensation， and Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways， sometimes coupling the signaling to ciliary motility or alternatively to cell division and "[7] Eukaryotes can move using motile cilia or The flagella are more complex than those of Table 1: Comparison of features of prokaryotic and eukaryotic cells Prokaryotes Eukaryotes Typical organisms bacteria， archaea protists， fungi， plants， animals Typical size ~ 1–10 µm ~ 10–100 µm (sperm cells， apart from the tail， are smaller) Type of nucleus nucleoid region; no real nucleus real nucleus with double membrane DNA circular (usually) linear molecules (chromosomes) with histone proteins RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the nucleusprotein synthesis in cytoplasm Ribosomes 50S+30S 60S+40S Cytoplasmatic 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 (though some lack mitochondria) 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 Table 2: Comparison of structures between animal and plant cells Typical animal cell Typical plant cell Organelles Nucleus Nucleolus (within nucleus) Rough endoplasmic reticulum (ER) Smooth ER Ribosomes Cytoskeleton Golgi apparatus Cytoplasm Mitochondria Vesicles Lysosomes Centrosome Centrioles Vacuoles Nucleus Nucleolus (within nucleus) Rough ER Smooth ER Ribosomes Cytoskeleton Golgi apparatus (dictiosomes) Cytoplasm Mitochondria [edit] Subcellular components The cells of eukaryotes (left) and prokaryotes (right)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 Inside the membrane， a salty cytoplasm takes up most of the cell 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 There are also other kinds of biomolecules in This article will list these primary components of the cell， then briefly describe their [edit] Cell membrane: A cell's defining boundaryMain article: Cell membraneThe cytoplasm of a cell is surrounded by a cell membrane or plasma The plasma membrane in plants and prokaryotes is usually covered by a cell 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 Hence， the layer is called a phospholipid It may also be called a fluid mosaic 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 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 Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as [edit] Cytoskeleton: A cell's scaffoldMain article: Cytoskeleton Bovine Pulmonary Artery Endothelial cell: nuclei stained blue， mitochondria stained red， and F-actin， an important component in microfilaments， stained Cell imaged on a fluorescent 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 The eukaryotic cytoskeleton is composed of microfilaments， intermediate filaments and There is a great number of proteins associated with them， each controlling a cell's structure by directing， bundling， and aligning The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape， polarity and [8][edit] Genetic materialTwo 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 (， retroviruses) have RNA as their genetic The biological information contained in an organism is encoded in its DNA or RNA RNA is also used for information transport (， mRNA) and enzymatic functions (， ribosomal RNA) in organisms that use DNA for the genetic code Transfer RNA (tRNA) molecules are used to add specific amino acids during the process of protein Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the 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 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 tRNAForeign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called This can be transient， if the DNA is not inserted into the cell's genome， or stable， if it Certain viruses also insert their genetic material into the [edit] OrganellesMain article: OrganelleThe human body contains many different organs， such as the heart， lung， and kidney， with each organ performing a different Cells also have a set of "little organs，" called organelles， that are adapted and/or specialized for carrying out one or more vital There are several types of organelles within an animal 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 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 Mitochondria play a critical role in generating energy in the eukaryotic 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 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 [citation needed] Mitochondria and chloroplasts each contain their own genome， which is separate and distinct from the nuclear genome of a 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 [citation needed] Ribosomes The ribosome is a large complex of RNA and protein They each consist of two subunits， and act as an assembly line where mRNA from the nucleus is used to synthesise proteins from amino Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes， or the cell membrane in prokaryotes)[9] Cell nucleus – a cell's information center The cell nucleus is the most conspicuous organelle found in a eukaryotic It houses the cell's chromosomes， and is the place where almost all DNA replication and RNA synthesis (transcription) The nucleus is spherical in shape and separated from the cytoplasm by a double membrane called the nuclear The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its 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 The nucleolus is a specialized region within the nucleus where ribosome subunits are In prokaryotes， DNA processing takes place in the Diagram of a cell nucleus 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 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 Smooth ER plays a role in calcium sequestration and 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 It is particularly important in the processing of proteins for The Golgi apparatus forms a part of the endomembrane system of eukaryotic 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 [citation needed] Diagram of an endomembrane system Lysosomes and Peroxisomes – eukaryotes only Lysosomes contain digestive enzymes (acid hydrolases) They digest excess or worn-out organelles， food particles， and engulfed viruses or Peroxisomes have enzymes that rid the cell of toxic The cell could not house these destructive enzymes if they were not contained in a membrane-bound These organelles are often called a "suicide bag" because of their ability to detonate and destroy the [citation needed] Centrosome – the cytoskeleton organiser The centrosome produces the microtubules of a cell – a key component of the It directs the transport through the ER and the Golgi Centrosomes are composed of two centrioles， which separate during cell division and help in the formation of the mitotic A single centrosome is present in the animal They are also found in some fungi and algae [citation needed] Vacuoles Vacuoles store food and Some vacuoles store extra They are often described as liquid filled space and are surrounded by a Some cells， most notably Amoeba， have contractile vacuoles， which are able to pump water out of the cell if there is too much [edit] Structures outside the cell wall[edit] CapsuleA gelatinous capsule is present in some bacteria outside the cell The capsule may be polysaccharide as in pneumococci， meningococci or polypeptide as bacillus anthracis or hyaluronic acid as in [citation needed] Capsules not marked by ordinary stain and can detected by special The capsule is The capsule has antiphagocytic function so it determines the virulence of many It also plays a role in attachment of the organism to mucous [citation needed]
