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Chapter 5 Cell Membrane

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					Chapter 5 Cell Membrane
           Plasma Membrane
• --the fluid mosaic model (S.J Singer)
• -- semi-permeable
• --fluid portion is a double layer of
  phospholipids, called the phospholipid bilayer
      Jobs of the cell membrane
• Isolate the cytoplasm from the external
  environment
• Regulate the exchange of substances
• Communicate with other cells
• Identification
          Phospholipid Bilayer
• Phospholipids contain a hydrophilic head and a
  nonpolar hydrophobic tail
• Hydrogen bonds form between the phospholipid
  "heads" and the watery environment inside and
  outside of the cell
  Hydrophobic interactions force the "tails" to face
  inward
  Phospholipids are not bonded to each other,
  which makes the double layer fluid
• Cholesterol embedded in the membrane makes it
  stronger and less fluid
 Proteins embedded in membrane
• 1. Channel Proteins - form small openings for
  molecules to difuse through
  2. Carrier Proteins- binding site on protein
  surface "grabs" certain molecules and pulls them
  into the cell
  3. Receptor Proteins - molecular triggers that set
  off cell responses (such as release of hormones or
  opening of channel proteins)
  4. Cell Recognition Proteins - ID tags, to idenitfy
  cells to the body's immune system
  5. Enzymatic Proteins - carry out metabolic
  reactions
    Transport Across Membrane
• Passive Transport
• Simple Diffusion - water, oxygen and other
  molecules move from areas of high
  concentration to areas of low concentration,
  down a concentration gradient
• Facilitation Diffusion - diffusion that is
  assisted by proteins (channel or carrier
  proteins)
                   Osmosis
• diffusion of water.
  Osmosis affects the turgidity of cells, different
  solution can affect the cells internal water
  amounts
• Contractiles Vacuoles are found in freshwater
  microorganisms - they pump out excess water

• Turgor pressure occurs in plants cells as their
  central vacuoles fill with water.
        Types of Osmosis Solution
• Isotonic: equal amount solute inside and outside
  of the cell.
  – Cell will stay normal
• Hypertonic: More solute on the outside of the cell
  – Water will move out of the cell
  – Cell will shrivel up (animal)
  – Plant cell plasmolyzed (wilt and death)
• Hypotonic: More solute inside the cell
  – Water will move in the cell
  – Cell will burst
  – Plants will have turgid (exerts pressure) (Normal for
    plant cells)
Osmosis without walls
Osmosis with walls
              Active Transport
• involves moving molecules "uphill" against the
  concentration gradient, which requires energy
• Endocytosis - taking substances into the cell
  (pinocytosis for water, phagocytosis for solids)
  Exocytosis - pushing substances out of the cell,
  such as the removal of waste
  Sodium-Potassium Pump - pumps out 3 sodiums
  for every 2 potassium's taken in against gradient
•
Sodium Potassium Pump
Observation of elodea cells in salt
 water. What happens and why?
     Modifications of Cell Surfaces
• Tight junctions are composed of protein fibers
  that seal adjacent cells to prevent leakage,
  something which can be useful in organs such as
  the bladder and the lining of the digestive tract.
  Tight junctions literally fuse the cells together
  forming a sheet of cells restricting molecules to
  one side of the sheet or the other.
• Tight junctions can also partition the cells in which
  they are found. Certain membrane proteins can be
  restricted to one side of the junction, as well, since
  the tight junction prevents protein migration
  within the membrane.
                    Continue…
• Desmosomes anchor adjacent cells together by making
  connections that work like staples or rivets that attach
  to components of the cytoskeleton. Many epithelial
  cells must adhere to adjacent membranes to prevent
  free passage or free movement, and to not break apart
  under stress.
• Desmosome filaments are composed of specialized
  glycoproteins proteins. Intermediate filaments of
  keratin in the desmosomes help strengthen the
  junction.
• Actin microfilaments can also attach to desmosomes,
  but have less strength
              Continue…
Plants have plasmodesmata - channels
between the cell wall that cytosol can
pass through
                 Continue…
• Gap junctions are protein channels, called
  connexons, between adjacent cells that
  permit the transfer of small molecules, such as
  nutrient monomers, between the cells. They
  are common in brain cells, forming the
  synapse, in many glands, and in cells in the
  heart muscle that coordinate contraction for
  heartbeat. Gap junctions can be gated.

				
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