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Cell (biology)

2007 Schools Wikipedia Selection. Related subjects: General Biology

   Drawing of the structure of cork as it appeared under the microscope to
   Robert Hook from Micrographia which is the origin of the word "cell".
   Enlarge
   Drawing of the structure of cork as it appeared under the microscope to
   Robert Hook from Micrographia which is the origin of the word "cell".
   Cells in culture, stained for keratin (red) and DNA (green).
   Enlarge
   Cells in culture, stained for keratin (red) and DNA (green).

   The cell is the structural and functional unit of all living organisms,
   and is sometimes called the "building block 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 trillion or 10^14 cells; a typical cell size is 10 µm, a typical
   cell mass 1 nanogram). The largest known cell is an ostrich egg.

   The cell theory, first developed in 1839 by Schleiden and Schwann,
   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 cells contain the 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, a small room. The name was
   chosen by Robert Hooke when he compared the cork cells he saw to the
   small rooms monks lived in.

Overview

Properties of cells

   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.
   Mouse cells grown in a culture dish. These cells grow in large clumps,
   but each individual cell is about 10 micrometres across.
   Enlarge
   Mouse cells grown in a culture dish. These cells grow in large clumps,
   but each individual cell is about 10 micrometres across.

   All cells share several abilities:
     * 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
       derived from metabolic pathways.
     * Response to external and internal stimuli such as changes in
       temperature, pH or nutrient levels.
     * Cell contents are contained within a cell surface membrane that
       contains proteins and a lipid bilayer.

   Some prokaryotic cells contain important internal membrane-bound
   compartments, but eukaryotic cells have a highly specialized
   endomembrane system characterized by regulated traffic and transport of
   vesicles.

Anatomy of cells

   There are two types of cells, eukaryotic and prokaryotic. Prokaryotic
   cells are usually singletons, while eukaryotic cells are usually found
   in multi-cellular organisms.

Prokaryotic cells

   A typical prokaryotic cell
   Enlarge
   A typical prokaryotic cell

   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 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
       (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"
       ( cytolysis) 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

   A typical eukaryotic cell
   Enlarge
   A typical eukaryotic cell

   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 also contain
       some DNA.
     * Eukaryotes can move using cilia or flagella. The flagella are more
       complex than those of prokaryotes.

   CAPTION: Table 1: Comparison of features of prokaroytic 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
                                   nucleus
                       protein 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 made of
                                   tubulin
   Mitochondria none one to several dozen (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

     CAPTION: 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
     * Vacuoles
     * Lysosomes
     * Centrosome
          + Centrioles

     * Nucleus
          + Nucleolus (within nucleus)
     * Rough ER
     * Smooth ER
     * Ribosomes
     * Cytoskeleton
     * Golgi apparatus ( dictiosomes)
     * Cytoplasm
     * Mitochondria
     * Vesicles
     * Chloroplast and other plastids
     * Central vacuole
          + Tonoplast (central vacuole membrane)
     * Peroxisome
     * Glyoxysome

                            Additional structures
     * Cilium
     * Flagellum
     * Plasma membrane

     * Plasma membrane
     * Cell wall
     * Plasmodesmata
     * Flagellum (only in gametes)

Subcellular components

   The cells of eukaryotes (left) and prokaryotes (right).
   Enlarge
   The cells of eukaryotes (left) and prokaryotes (right).

   All cells, whether prokaryotic or eukaryotic, have a membrane, which
   envelopes 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

          Main article: Cell membrane

   The cytoplasm of a cell is surrounded by a 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 phosphorous
   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

          Main article: Cytoskeleton

   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. 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.

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.

   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 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: Organelle

   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.
   Membrane-bound organelles are found only in eukaryotes.

   Cell nucleus (a cell's information centre)
          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.

   Cell nucleus

   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. As mitochondria contain their own genome that
          is separate and distinct from the nuclear genome of a cell, they
          play a critical role in generating energy in the eukaryotic
          cell, a 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. .

   Endomembrane system

   The ER contains many Ribosomes (the protein production machine)
          The ribosome is a large complex composed of many molecules, only
          exist floating freely in the cytosol, whereas in eukaryotes they
          can be either free or bound to membranes.

   Lysosomes and Peroxisomes (of the eukaryotic cell. The cell could not
          house such destructive enzymes if they were not contained in a
          membrane-bound system.

   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.

   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.

Cell functions

Cell growth and metabolism

          Main articles: Cell growth, Cell 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, wherein
   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.
   An overview of protein synthesis.Within the nucleus of the cell (light
   blue), genes (DNA, dark blue) are 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
   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 tRNA. Newly-synthesized proteins (black) are often further
   modified, such as by binding to an effector molecule (orange), to
   become fully active.
   An overview of protein synthesis.
   Within the nucleus of the cell (light blue), genes (DNA, dark blue) are
   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 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 tRNA. Newly-synthesized proteins (black)
   are often further modified, such as by binding to an effector molecule
   (orange), to become fully active.

Creation of new cells

          Main article: Cell division

   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

          Main article: Protein biosynthesis

   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 complimentary 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 3D protein molecule.

Origins of cells

          Main article: Origin of life

   The 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 the first cell

   If life is viewed from the point of view of replicators, 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 enzymes are not enclosed
   into cells, the enzymes that benefit a given DNA molecule (for example,
   by producing nucleotides) will automatically benefit the neighbouring
   DNA molecules. This might be viewed as " parasitism by default."
   Therefore the selection 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 that acted as both
   replicators and enzymes: see RNA world hypothesis. However, the core of
   the reasoning is the same.)

   Biochemically, cell-like spheroids formed by proteinoids are observed
   by heating amino acids with phosphoric acid as a catalyst. They bear
   much of the basic features provided by cell 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 tendency 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 one's 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 bilayer
   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 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-cellular organisms could be achieved. 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 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 bacteria 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.

History

     * 1632–1723: Antony 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).
     * 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.

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