Cell Structure Cell Biology

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    CELL STRUCTURE

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    NUCLEUS

    Nucleus: The nucleus is the most obvious organelle inany eukaryotic cell. It is enclosed in a double membraneand communicates with the surrounding cytosol vianumerous nuclear pores. Within the nucleus is the DNA

    responsible for providing the cell with its uniquecharacteristics. The DNA is similar in every cell of thebody, but depending on the specific cell type, somegenes may be turned on or off - that's why a liver cell isdifferent from a muscle cell, and a muscle cell is different

    from a fat cell. When a cell is dividing, the nuclearchromatin (DNA and surrounding protein) condensesinto chromosomes that are easily seen by microscopy.

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    NUCLEOLUS

    The prominent structure in the nucleus isthe nucleolus. The nucleolus producesribosomes, which move out of the nucleus

    and take positions on the roughendoplasmic reticulum where they arecritical in protein synthesis

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    CYTOSOL

    The cytosol is the "soup" within which allthe other cell organelles reside and wheremost of the cellular metabolism occurs.

    Though mostly water, the cytosol is full ofproteins that control cell metabolismincluding signal transduction pathways,

    glycolysis, intracellular receptors, andtranscription factors

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    CYTOPLASM

    This is a collective term for the cytosolplus the organelles suspended within thecytosol

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    CENTROSOME

    The centrosome, or MICROTUBULEORGANIZING CENTER (MTOC), is anarea in the cell where microtubules are

    produced. Plant and animal cellcentrosomes play similar roles in celldivision, and both include collections of

    microtubules, but the plant cellcentrosome is simpler and does not havecentrioles.

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    During animal cell division, the centriolesreplicate (make new copies) and thecentrosome divides. The result is two

    centrosomes, each with its own pair ofcentrioles. The two centrosomes move toopposite ends of the nucleus, and fromeach centrosome, microtubules grow into

    a "spindle" which is responsible forseparating replicated chromosomes intothe two daughter cells.

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    CENTRIOLE

    Each centriole is a ring of nine groups offused microtubules. There are threemicrotubules in each group. Microtubules

    (and centrioles) are part of thecytoskeleton. In the complete animal cellcentrosome, the two centrioles are

    arranged such that one is perpendicular tothe other.

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    GOLGI APPARATUS

    The Golgi apparatus is a membrane-boundstructure with a single membrane. It is actually astack of membrane-bound vesicles that areimportant in packaging macromolecules for

    transport elsewhere in the cell. The stack oflarger vesicles is surrounded by numeroussmaller vesicles containing those packagedmacromolecules. The enzymatic or hormonalcontents of lysosomes, peroxisomes and

    secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgiapparatus

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    LYSOSOME

    Lysosomes contain hydrolytic enzymesnecessary for intracellular digestion. Theyare common in animal cells, but rare in

    plant cells. Hydrolytic enzymes of plantcells are more often found in the vacuole.

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    PEROXISOMES

    Peroxisomes are membrane-bound packets ofoxidative enzymes. In plant cells, peroxisomesplay a variety of roles including converting fattyacids to sugar and assisting chloroplasts inphotorespiration. In animal cells, peroxisomesprotect the cell from its own production of toxichydrogen peroxide. As an example, white bloodcells produce hydrogen peroxide to kill bacteria.

    The oxidative enzymes in peroxisomes breakdown the hydrogen peroxide into water andoxygen.

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    SECRETORY VESICLES

    Cell secretions - e.g. hormones,neurotransmitters - are packaged insecretory vesicles at the Golgi apparatus.

    The secretory vesicles are thentransported to the cell surface for release

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    CELL MEMBRANE

    Every cell is enclosed in a membrane, a double layer ofphospholipids (lipid bilayer). The exposed heads of thebilayer are "hydrophilic" (water loving), meaning that theyare compatible with water both within the cytosol andoutside of the cell. However, the hidden tails of the

    phosopholipids are "hydrophobic" (water fearing), so thecell membrane acts as a protective barrier to theuncontrolled flow of water. The membrane is made morecomplex by the presence of numerous proteins that arecrucial to cell activity. These proteins include receptors

    for odors, tastes and hormones, as well as poresresponsible for the controlled entry and exit of ions likesodium (Na+) potassium (K+), calcium (Ca++) andchloride (Cl-).

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    MITOCHONDRIA

    Mitochondria provide the energy a cell needs to move,divide, produce secretory products, contract - in short,they are the power centers of the cell. They are aboutthe size of bacteria but may have different shapes

    depending on the cell type. Mitochondria are membrane-bound organelles, and like the nucleus have a doublemembrane. The outer membrane is fairly smooth. Butthe inner membrane is highly convoluted, forming folds(cristae) as seen in the cross-section, above. The cristae

    greatly increase the inner membrane's surface area. It ison these cristae that food (sugar) is combined withoxygen to produce ATP - the primary energy source forthe cell

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    VACUOLE

    A vacuole is a membrane-bound sac that plays roles inintracellular digestion and the release of cellular wasteproducts. In animal cells, vacuoles are generally small.Vacuoles tend to be large in plant cells and play several

    roles: storing nutrients and waste products, helpingincrease cell size during growth, and even acting muchlike lysosomes of animal cells. The plant cell vacuolealso regulates turgor pressure in the cell. Water collectsin cell vacuoles, pressing outward against the cell wall

    and producing rigidity in the plant. Without sufficientwater, turgor pressure drops and the plant wilts.

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    ROUGH ENDOPLASMICRETICULUM

    Rough endoplasmic reticulum appears"pebbled" by electron microscopy due tothe presence of numerous ribosomes on

    its surface. Proteins synthesized on theseribosomes collect in the endoplasmicreticulum for transport throughout the cell

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    RIBOSOMES

    Ribosomes are packets of RNA andprotein that play a crucial role in bothprokaryotic and eukaryotic cells. They are

    the site of protein synthesis. Eachribosome comprises two parts, a largesubunit and a small subunit. MessengerRNA from the cell nucleus is moved

    systematically along the ribosome wheretransfer RNA adds individual amino acidmolecules to the lengthening protein chain

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    CYTOSKELETON

    As its name implies, the cytoskeleton helps tomaintain cell shape. But the primary importanceof the cytoskeleton is in cell motility. The internalmovement of cell organelles, as well as celllocomotion and muscle fiber contraction couldnot take place without the cytoskeleton. Thecytoskeleton is an organized network of threeprimary protein filaments:

    - microtubules- actin filaments (microfilaments)- intermediate fibers

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    CELL CYCLE

    During development from stem to fullydifferentiated, cells in the body alternatelydivide (mitosis) and "appear" to be resting

    (interphase). This sequence of activitiesexhibited by cells is called the cell cycle.

    Interphase, which appears to the eye to be a

    resting stage between cell divisions, is actuallya period of diverse activities. Those interphaseactivities are indispensible in making the next

    mitosis possible.

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    INTERPHASE

    Interphase generally lasts at least 12 to 24hours in mammalian tissue. During thisperiod, the cell is constantly synthesizing

    RNA, producing protein and growing insize. By studying molecular events in cells,scientists have determined that interphase

    can be divided into 4 steps: Gap 0 (G0),Gap 1 (G1), S (synthesis) phase, Gap 2(G2). Gap 0 (G0): There are

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    G0 Phase

    Gap 0 (G0): There are times when a cellwill leave the cycle and quit dividing. Thismay be a temporary resting period or more

    permanent. An example of the latter is acell that has reached an end stage ofdevelopment and will no longer divide (e.g.

    neuron)

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    G1 Phase

    Gap 1 (G1): Cells increase in size in Gap1, produce RNA and synthesize protein.An important cell cycle control mechanism

    activated during this period (G1Checkpoint) ensures that everything isready for DNA synthesis. (C