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Transport Through the Membrane - PowerPoint


									The Cell Membrane
     Purpose of the membrane
 1) Transport raw materials into the cell.
 2) Transport manufactured products and
  wastes out of the cell.
 3) Prevent the entry of unwanted matter
  into the cell.
 4) Prevent the escape of matter needed
  for cellular functions.
The cell membrane is composed of
 phospholipid molecules.
    A Phosphate group and two fatty acids
 bonded to a glycerol “backbone”.
    The phosphate group (head) is
 „hydrophillic‟ (likes water) the two fatty acid
 “tails” are „hydrophobic‟ (water hating).
(Pg 51 Fig 2.22)
    Importance of their dual nature
 When phospholipids are spread throughout
water, a curious thing happens. The attractions
  and repulsions of the heads and tails cause
  them to form a cage-like spherical layer. (Pg
  51, fig 2.23).
 -The water attracting heads face in and out of
  the sphere.
 -The non-polar tails face each other.
 -This phospholipid bilayer forms the basis of
  the cell membrane.
          Fluid-Mosaic Model
   There are numerous proteins,
    carbohydrates and other lipids spread and
    moving throughout the membrane
    creating a fluid-like consistency.
    Benefits of the Fluid Mosaic Model
 Cholesterols found in animal cell membranes.
  These keep the membrane fluid at lower
  temperatures. They also prevent some other
  molecules from passing through.
 Protein and carbohydrate arrangements in the
  cell membrane that allow the cell to be
  “recognized” by other cells. (Pg 52, fig 2.24)
              Outside the cell
   In multicellular organisms, the cells are
    bathed in a thin layer of extracellular
    fluid, which is made of water, mineral
    and some cell wastes that have been
Transport Through the
      Passive and Active
     Crossing the Membrane
 The membrane is a cell‟s primary means
  for it to maintain homeostasis (the
  constant steady state of conditions
  achieved to remain healthy).
 The plasma membrane is selectively
  permeable. It allows some materials to
•   Diffusion- a passive way that molecules can
    move across the membrane from an area of
    [high] to [low] ([ ] means concentration).
    - materials that pass easily are:
       gases, small uncharged molecules, and fat
    soluble molecules.
   Any atom above absolute zero (-273 oC) is in
    motion. The random motion within a liquid is
    called Brownian motion. (Think of dye
    spreading through a liquid).
   The difference in concentrations of a substance
    across a membrane is called the concentration
   The diffusion of water is called Osmosis.
    Where water moves from a [high] to [low].
    There are different types of environments that
    will lead to a net direction of movement:
   Isotonic condition – when the concentrations
    of solutes on both sides of the membrane are
    the same causing no net movement of water.
   Hypotonic condition – when the [water] is higher
    ([lower solute]) outside the cell. Water will move
    into the cell to balance out concentrations, causing
    the cell to swell and possibly burst.
   Hypertonic condition – when the [water] is higher
    ([lower solute]) inside the cell. Water will move out
    of the cell, causing the cell to shrivel.
   No energy is required to diffuse water, runs on
    concentration gradient.
           Facilitated Diffusion
   Facilitated Diffusion – molecules that do not
    readily pass, due to size or charge etc.., may
    need assistance to cross the membrane.
    Specialized transport proteins help substances
    move in this case.
   Transport proteins are selective and will only
    move the molecule/ion it was designed to
                Carrier Proteins
   Numerous carrier proteins lie within the membrane to
    facilitate the movement of glucose (sugar). These
    carriers still operate by concentration gradients and
    do not require energy.
               Channel Proteins
   Channel Proteins – are transport proteins that carry
    charged particles. They have a tunnel-like shape and
    have their own charge. They attract the ion like
    attracting a magnet. Size is a limiting factor for using
    these channels.
               Active transport
   Certain cells require materials that may not be
    present in great amounts. In this case, a cell
    may need to pump these materials in or out of
    the cell against the normal concentration
   This moving materials from [low] to [high]
    requires the cell to spend energy, and is
    therefore called Active transport.

   4 Important examples of Active Transport
       - kidney cells pump glucose and Amino
    acids out of the urine and back into the blood.
       - Intestinal cells pump in nutrients from the
       - Root cells pump in nutrients from the soil
       - Gill cells in fish pump out sodium ions.
   Active transport pumps are protein pumps in the
    membrane that grab ions and other materials and
    pump them across the membrane against the
    - one of the best understood is the Na+/K+ pump.
   In some cases the unequal distribution of Na+ (or
    other molecule) can allow another molecule to
    „piggyback‟ into the cell when regular diffusion
 Once you enter this site click on MAIN at the
Bulk Membrane
             Bulk Transport
 Material  too big or material that is
  charged can not freely cross the cell
 The cell must accommodate these
  substances by folding in on itself
  creating a vesicle to take in or expel
  these large molecules.
         2 Types of Bulk transport
   Endocytosis (Endo = into; cyto = cell) – the
    membrane folds inward, trapping matter from the
    extracellular fluid and enters the cell as a vesicle.
            3 Types of Endocytosis
   Phagocytosis (phago = eating) – the membrane folds
    inward, trapping solid matter from the extracellular
   Pinocytosis (pino = drinking) – the membrane folds
    inward trapping fluids from the extracellular fluid.
   Receptor-mediated Endocytosis – the membrane
    takes in particles after they bind to specialized
    receptor proteins embedded in the membrane.
    Cholesterol is brought in this way.
   Exocytosis (exo= exit) – the reverse of endocytosis.
    A vesicle moves out towards the plasma membrane,
    fuses with it and opens up its’ contents into the
    extracellular fluid.
    - This is important in the body secreting manufactured
    products from within the cells.
      Endocytosis and Exocytosis
   http://highered.mcgraw-

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