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The cell Part II

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					      Endomembrane System
• Includes all membrane-enclosed organelles, and
  is derived from the endoplasmic reticulum
• Endoplasmic reticulum, Golgi apparatus,
  vacuoles, vesicles, lysosomes, microbodies,
  peroxisomes, glyoxysomes
          Endoplasmic Reticulum
• Structure: Flattened, fluid-filled,
  membrane enclosed sacs that
  compartmentalize the cell with a
  series of channels formed between
  overlapping membranes.

• Two types of Endoplasmic
  Reticulum:
   – A. Rough ER
   – B. Smooth ER

   – The Nuclear envelope connects to
     the Rough ER which then
     connects to the Smooth ER.
   Rough Endoplasmic Reticulum
• Structure: (continuous with nuclear
  envelope) tubular canals with attached
  ribosomes
• Function: produces proteins for export
  via smooth ER and Golgi apparatus
• Ex. hormones, proteins for the cell
  membrane, digestive enzymes, mucus
• Proteins made in the rough ER are
  further modified with a “mailing label”
  (oligosaccharide) which states where
  the protein should go and helps to sort
  proteins.
• Then proteins move to the smooth ER
   Smooth Endoplasmic Reticulum
• Structure: membrane enclosed sacs that lack
  ribosomes
• Function: membranes are embedded with
  enzymes that catalyze many reactions and
  then send products to the Golgi apparatus:
   – packages proteins into vesicles for transport
     within the cell, abundant in secretory cells
     (hormones), muscles and liver cells
   – produces lipids like phospholipids and
     produces vesicles to replace the cell
     membrane
   – Produces steroid hormones in testes and
     adrenal cortex
   – Stores enzymes to break/detoxify drugs and
     poisons in liver cells, some drugs after being
     broken down are packaged into vesicles and
     transported to another organelle for further
     breaking down (Peroxisomes, Glyoxysomes)
   – Stores calcium ions in muscle cells, Ca2+ also
     used in controlling, building, or disassembling
     cytoskeleton elements.
                      Golgi Apparatus
• Discovered in 1898 by Camillo Golgi;
  Animal cells 10-20, Plant cells 100’s

• Structure: stacks of flattened saccules
  similar to hollow pancakes; inner face
  directed toward nucleus and ER, outer face
  directed toward cell membrane

• Function: (Post Office) vesicles from the
  ER move to the inner face of the Golgi body
  where they are then collected, stored,
  modified, concentrated as they move from
  layer to layer, and then repackaged into
  vesicles and pinched off the outer face of
  the Golgi body.

• Ex. Proteins that leave the Golgi apparatus:
  hormones, digestive enzymes,
  glycoproteins, glycolipids, protein fibers that
  strengthen organs like the skin
        Golgi Apparatus Functions
– Modifies “mailing labels” with
  polysaccharides, glycoproteins and
  glycolipids, so that the proteins can be
  sorted and packaged. If there are
  errors with mailing labels, then the
  Golgi body sends the protein back to
  the ER.

– Regulates the surface area of the cell
  membrane by producing vesicles that
  can rupture and attach to the cell
  membrane or vesicles move from the
  cell membrane back to the Golgi body.

– Free ribosomes send their products
  here for modification and packaging
                                 Vacuoles
• Structure: large, long-lived membrane enclosed
  sacs that are relatively static; both plants and
  animals but greatest development in plants
• Function: stores water, sugars, amino acids,
  salts, pigments (beets), organic acids, wastes,
  and toxic substances (toxins or tannins that
  deter herbivores); called cell sap
    – Vacuoles may start as many small vacuoles that
      as they fill and mature, combine to form one
      large Central vacuole that can occupy 90% of
      the plant cell volume
    – Pushes cytoplasm to the periphery against the
      cell wall (turgor pressure); provides structure
      and stiffness

    – Contractile vacuoles in protists are used to expel
      water and waste, propeling the protist (Ex.
      amoeba)
    – Food vacuoles in animals and protists store and
      digest food (Ex. paramecium food vacuoles
      stained red)
                   Vesicles
• Structure: small, short-lived membrane enclosed
  sacs, which move around cell
• Function: vehicles to move substances from one
  place to another in the cell and to the cell
  membrane for export. May also bud off of
  vacuoles.
               Lysosomes
• Structure: Membrane-bounded vesicles formed
  by the Golgi apparatus
• Function: Contains very acidic hydrolytic
  enzymes (pH 5 vs cytosol pH 7.2) used in the
  break down of macromolecules
                 Lysosomes: Functions
•   Contains very acidic hydrolytic enzymes (pH 5
    vs cytosol pH 7.2) used in the break down of
    macromolecules
     – White blood cells digest bacteria and dead/worn
       out cells (neutrophils), take in antigens by
       professional antigen-presenting cells (B-cells)
     – Digest worn out cell organelles (Ex. mitochondria
       every 10 days)
     – Digest food molecules taken in by endocytosis
     – Digests and releases products obtained from other
       vesicles
     – Used to believe involved in autodigestion when
       lysosome breaks down and enzymes pour into the
       cell digesting the whole cell. (Ex. tadpoles tail,
       human embryo’s webbed fingers) Today know that
       Apoptosis or Programmed cell death involves cells
       shrinking, mitochondria breaking down, chromatin
       degrading/breaking into small fragments, and then
       these engulfed by macrophages
     – Tay-sachs disease – lysosome is missing a
       digestive enzyme so nerve cells can not transmit
       nerve impulses
                              Microbodies
•   Structure: single membrane-bound sacs
    containing specific enzymes in a dense
    crystalline core. Enzymes and proteins used
    here are made by free ribosomes.
     – Two types:
          • Animal/Plant cells called Peroxisomes
          • Plant cells called Glyoxysomes

•   Function: Conducts enzymatic reactions that
    build up and break down specific molecules
    often used for respiration, therefore they often
    lie next to mitochondria or chloroplasts.

•   Some sources do not include microbodies in
    the endomembrane system because they do
    not bud from the ER, instead they increase in
    number by splitting in two and grow in size by
    incorporating proteins from free ribosomes,
    lipids from the ER, and lipids made within
    themselves.
     Peroxisomes                              (animal cells and plant cells)
•   Contain many enzymes used to rid the cell of toxic
    substances by transferring hydrogen from the poisons
    to molecules of oxygen (oxidation) to form H2O2.
    Hydrogen peroxide is highly reactive so catalase is
    used to break down hydrogen peroxide into water and
    oxygen.

•   Liver peroxisomes – detoxify alcohol and other harmful
    compounds, remove amino group from amino acids,
    break down fatty acids. These reactions make
    molecules used in respiration and other biochemical
    pathways so they are transferred to a mitochondrion.

•   Some peroxisomes produce phospholipids,
    cholesterol, bile acids, and lipids that make myelin
    around neurons.

•   Kidney peroxisomes - breaks down excess purines to
    yield uric acid

•   Converts fat to carbohydrates.

•   Able to self-replicate but has no internal DNA and must
    import proteins it needs to make copies of itself
               Glyoxysomes                           (plant cells)
•   Contain specific enzymes that make molecules for photosynthesis, respiration,
    and other biochemical pathways.

•   Breaks down fats and converts fatty acids to carbohydrates, so often found in
    the fat-storing tissues of plant seeds.

•   Plays key role during germination and growth because a lot of carbohydrates are
    needed to produce cell walls (cellulose) and the emerging seedling can use the
    sugars produced as a source of energy and carbon until it can produce its own
    sugar by photosynthesis.
   Energy Related Organelles:
• 1. Mitochondrion
• 2. Chloroplast

• Endosymbiotic hypothesis – how these
  organelles were derived

• Not part of the endomembrane system because they are
  composed of a double membrane, their proteins are
  made by free ribosomes or themselves and not by the
  ER, and they can grow and self-replicate.
                             Mitochondria
•   Found in both plant and animal cells, can number into
    the hundreds or thousands per cell.

•   Structure: double membrane-bound organelle with
    inner membrane forming shelves / infoldings called
    “cristae” that project into a matrix (inner space) and
    contain embedded enzymes in cristae, enzymes in
    the matrix, energy-rich reactants, DNA, and
    ribosomes. Cristae provide large surface area to
    enhance productivity. (1-10 µm long, move around,
    change shapes, and divide in two)

•   Function: “Power house” of the cell, produces ATP
    for cellular energy by aerobic respiration =
     – Glucose + Oxygen → Carbon dioxide + Water + ATP
     – C6H12O6 + 6O2 →        6CO2      + 6H20 + 36ATP

•   Extract energy from food to make little packets of
    energy that is just right for cellular activities.
                    Chloroplasts
• Found only in plant cells and photosynthetic protists (algae)

• Structure: double membrane-bound organelle that contains
  thylakoids (flattened sacs) piled up into stacks called grana
  and the fluid-filled space around the grana is called stroma,
  containing DNA, ribosomes, and enzymes. Grana are joined
  together by lamella. (2-5 µm long, move around, change
  shapes, and divide in two)
        Chloroplasts: Function
• Function: Carries out photosynthesis which
  captures sunlight energy and transforms it into
  food energy (glucose).
• Sunlight + CO2 + H20 → C6H12O6 + O2

• Chlorophyll is a pigment on the surfaces of
  thylakoid membranes that absorbs solar energy
  and makes leaves appear green.
        Endosymbiotic Hypothesis
•   Contends that some early prokaryotes were engulfed by larger
    bacterial cells and became mitochondria and chloroplasts found in
    modern eukaryotic cells. (Proposed by Lynn Margulis)

•   On prehistoric earth, cyanobacteria began to produce oxygen after 3.5 bya
    which made it hard for anaerobic bacteria to survive. Perhaps around 1.5
    bya, anaerobic bacteria engulfed bacteria that could use and survive in
    oxygen. The new engulfed bacteria resisted digestion, and began to live
    symbiotically with the new host and helped the host to carry out metabolic
    processes to make ATP or synthesize food. (mitochondria were aerobic
    heterotrophic bacteria and chloroplasts were cyanobacteria)

•   There are many symbiotic associations between Prokaryotes and
    Eukaryotes like Coral which is cyanobacterium living inside an animal cell.

•   Some believe that peroxisomes, basal bodies, centrioles, and nematocyts,
    flagella (spirochete prokaryote) also have endosymbiotic origins.




•   Ch 25 pg 412 Mader
     Evidence for Endosymbiosis
– Size similar to bacterium (1-3 µm)

– Double membrane came from engulfing a bacterium
  so that the inner membrane = bacteria’s membrane
  and outer membrane = plasma membrane of host

– Own DNA in a circular loop like a bacterium’s DNA;
  can be copied but once incorporated most bacterial
  genes must have been transferred into the host
  cell’s DNA (for metabolism).

– Own Ribosomes similar in size and RNA base
  sequence to bacterial ribosomes; can copy its DNA
  and use ribosomes to make proteins for use within
  the mitochondrion or chloroplast.

– Able to Reproduce / Divide by fission similar to the
  way in which bacteria divide into two. This is the
  only way to transfer mitochondria/chloroplasts into
  new cells.

– Able to build their own membranes
                       Cytoskeleton
• Network of protein fibers crisscrossing the cell and functions to help
  maintain the cell’s shape, provide mechanical strength (gelatin-like
  consistency of cytosol), anchor organelles and enzymes, allows
  cells and its organelles to move, and helps to regulate the activities
  of the cell by transmitting mechanical forces through the
  cytoskeleton to influence cell activity. Can be broken down and
  reassembled in a new location to change the shape of a cell. (The
  muscles and skeleton of the cell) (Discovered in mid 1970’s)

• Three types:
    – Actin filaments / microfilaments
    – Intermediate filaments
    – Microtubules
Actin Filaments / Microfilaments
• Structure: are long, thin fibers composed of two chains of the
  subunit protein actin twisted together (7 nm diameter).

• Functions: Attached to cell membrane to provide mechanical
  strength and maintain shape (mostly concentrated near the plasma
  membrane), links plasma membrane proteins to cytoplasmic
  proteins, anchors centrosomes to opposite poles during mitosis (cell
  division), allow microvili to shorten and extend into the intestine, aid
  in muscle contraction with myosin motor protein walking along the
  actin, help pseudopods to change shape for cell motility, involved in
  cytoplasmic streaming, forms cleavage furrow / pinches cell inward
  in a dividing animal cell by forming a constriction ring with myosin.
            Intermediate Filaments
•   Structure: Composed of 5 types of protein subunits (such as vimentin and
    keratin). Two Fibrous polypeptides / chains wrap themselves together in a
    rope-like fashion to form dimer. Then 2 dimers join with other dimers head
    to tail forming strands. Eight strands bind in a hollow tube, or coil together
    to make thicker cables (8-12 nm diameter, different depending on the
    protein). More permanent structures, unlike actin and microtubules which
    assemble and disassemble.

•   Functions: Provides structure and tensile strength for the cell to prevent
    excessive stretching, forms nuclear lamina providing structure to nucleus as
    well as holding the nucleus in place in a basket-like network of filaments,
    attaches ribosomes and other organelles within the cytoplasm, aids in cell-
    to-cell junctions by joining plasma membranes, makes up keratin in
    epithelial cells even after cell death (hair and nails), anchors the actin and
    myosin filaments together in muscle cells, and provides mechanical strength
    to long axons in neuron cells.
                 Microtubules
• Structure: Hollow cylinders composed of 13 helical rows
  of tubulin dimers forming a circle (2 tubulin proteins,
  alpha and beta, make a dimer). (25 nm diameter)
  Assembly of microtubules is under the control of the
  Centrosome, area located just outside the nucleus, and
  in animal cells there is a pair of centrioles within the
  centrosome which microtubules grow out of. Grows by
  adding tubulin and shrinks by taking away tubulin.
        Microtubule: Functions
• Functions: Plays a role in cell structure, helps stretch
  out ER, but mainly involved in movement of cell parts;
  moves vesicles and other organelles on “tracks” or
  “conveyor belts”, moves chromosomes apart (spindle)
  during cell division, and involved in cell motility by
  moving cilia and flagella
• Associated with two types of motor molecules:
   – Kinesin – moves vesicles and organelles (vesicles out toward
     plasma membrane) by walking along microtubules.
   – Dynein – moves cilia and flagella (vesicles in toward the
     nucleus)
  Centrioles, Cilia, and Flagella
• All made from microtubules but differ in
  their arrangement of microtubules.

• All involved in movement
                            Centrioles
• Structure: Ring of nine sets of three microtubules and no
  microtubules in the center = 9 + 0 pattern. Only found in animal
  cells. Two centrioles lie at right angles to one another near the
  nucleus and replicate before cell division with one pair lying on each
  side of the nucleus.

• Functions: During cell division, centrioles (within the centrosome)
  sends out microtubule fibers to attach to chromosomes (spindle).
  Chromosomes move along the microtubules to divide genetic
  material while at the ends the microtubules disassemble. (Not
  necessary for cell division because plants lack centrioles.) Also
  produce basal bodies which anchor cilia and flagella.
                       Cilia and Flagella
•   Structure: Hair-like extensions of the cell made of nine microtubule doublets arranged
    in a circle around two central microtubules = 9 + 2 pattern. Plasma membrane
    extends to cover the cilia and flagella.
•   Cilia are 2-20 µm in length and often found in large numbers on the cell surface.
•   Flagella are 10-200 µm in length and there is usually one to a few per cell.
•   Anchored by basal bodies with a 9 + 0 pattern.

•   Function: Provide movement across the cell or for the entire cell.
•   Cilia beat in unison in oar-like fashion to move some protists (paramecia), sweep
    debris trapped in mucus out of the lungs back up into the throat in the respiratory
    tract, in a woman’s fallopian tubes moves the egg to the uterus, and are the sensory
    hairs in cochlea of ear.
•   Flagella move in an undulating motion to propel sperm. (Sperm’s basal bodies of the
    flagella become the centrioles in the new fertilized egg)
•   Movement caused when microtubule doublets slide past one another with the aid of
    bending dynein molecules attached as side-arms to the microtubule doublets and
    ATP.

				
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