Pack 3 - Transport across cell membranes

Phosphate groups, glycerol and 2 fatty acids make up a phospholipid
1. An ester bond is made and water is removed (condensation)
2. Phospholipids
  1. They are polar molecules
    1. The head (top) is hydrophobic
    2. The tail is hydrophilic
Phospholipid bilayer
1. Functions of the bilayer
  1. Allows lipid-soluble substances to enter and leave the cell
  2. Prevents water-soluble to enter and leave the cell
  3. Makes the membrane flexible and self-sealing
2. Proteins
  1. Proteins can span the membrane or just sit on one of the surfaces of the bilayer
  2. Functions:
    1. Structural
    2. Help cells attach to each other
    3. Channel proteins allow water soluble substances through
    4. They facilitate facilitated diffusion
    5. Carrier proteins help diffusion
      1. Help active transport
    6. Act as cell surface receptors
3. Cholesterol
  1. Functions
    1. Adds strength to the membrane
    2. Reduces movement of other molecules
    3. Makes the membrane less fluid at higher temperatures
    4. Prevents leakage of water and dissolved ions
  2. Found between the phospholipids in the bilayer
4. Glycolipids
  1. Lipid covalently bonded to carbohydrate
    1. Carb extends from bilayer into watery environment
  2. Functions
    1. Act as recognition site
    2. Help maintain stability of membrane
    3. Help cells attach together forming tissues
5. Glycoproteins
  1. Extrinsic protein covalently bonded to carb
    1. Carb extends from the bilayer into water environment around cell
  2. Functions
    1. Recognition sites
    2. Help cells stick together
    3. Allow cells to recognise each other
6. Fluid-mosaic model of the plasma membrane
  1. The molecules act like a fluid
  2. It looks like the tiles on a mosaic
Permeability of plasma membrane
1. Most substances can't pass through the plasma membrane because:
  1. Not lipid soluble
  2. Too large
  3. They're polar molecules
Diffusion
1. The net movement of molecules or ions from a region where they are highly concentrated to a region where their concentration is lower
2. Diffusion is passive as it does not require any ATP
3. When the net flow of molecules from one direction stops it is called dynamic equilibrium
4. Factors that affect the rate:
  1. Temperature
    1. More temp = more KE in particles so they move quicker = faster diffusion
  2. Difference in concentration
    1. Higher difference in concentration = faster diffusion
  3. Distance over which diffusion occurs
    1. Greater distance = slower diffusion
  4. Area over which diffusion occurs
    1. As are increases diffusion rate increases
  5. Nature of any structure across which diffusion occurs
    1. e.g. if a molecule is lipid soluble it will get through plasma membranes fast
  6. Size of diffusing molecule
    1. Smaller means they can pass through pores quickly = faster diffusion
5. Fick's Law
  1. Diffusion rate is proportional to:
    1. (Surface area*conc. difference)/diffusion distance
Facilitated diffusion
1. This is passive, it does not require ATP
  1. Goes with the conc. gradient
2. Proteins
  1. Channel Proteins
    1. These from water filled hydrophilic channels across the membrane
    2. Allows specific water soluble ions to pass through
    3. They are selective
      1. They may only open in the presence of a specific ion
        The binding of the specific ion changes the shape of the protein allowing the ions to pass
  2. Carrier proteins
    1. These span the plasma membrane
      1. A molecule (must be specific and complementary) to bind to the carrier protein
        This causes the protein to change shape
        Then the molecule is released on the opposite side
  3. More carrier proteins and channel proteins = faster diffusion
Osmosis
1. The net movement from an area of less negative water potential to an area of more negative water potential through a semi - permeable membrane
  1. High conc. to low conc.
2. Larger molecules, like glucose, can't fit through the membrane
3. Water potential
  1. Pure water has a WP of 0
    1. Addition of water will lower the WP
      1. Water potential of a solution will always be negative
        More solute = more negative
4. Osmosis in animal cells
  1. If you place a red blood cell in water
    1. The red blood cells have a lower water potential than pure water
      1. So the net movement of water will be into the red blood cells by osmosis
        Red blood cells will swell and lyse
        If you place rbc's in strong solution then rbc's shrink
5. Osmosis in plant cells
  1. In pure water, plant cells have a lower water potential and the net movement will be into the plant
    1. The cell wall becomes rigid and resists the entry of water
      1. It becomes turgid
  2. If plant cells have higher water potential then net movement out of the cell, protoplast comes away from the cell wall
    1. It is plasmolysed
  3. Incipient plasmolysis is when there is no net movement of water, the protoplast is just pulling away from the cell
Active transport
1. The movement of molecules or ions usually from a region of low conc. to a region of high conc.
  1. Against the conc. gradient
2. Requires carrier proteins and ATP
3. e.g. Sodium/potassium pump
  1. Stages are:
    1. 1. Na+ binds to specific receptor sit on the carrier protein
    2. 2. ATP binds to the protein and splits into ADP and a phosphate molecule
    3. 3. Energy is released, the phosphate molecule remains bound to the carrier protein
    4. 4. The carrier protein changes shape, Na+ is released on the opposite side of the membrane
    5. 5. K+ now bind to their receptor sites, causing the protein to change shape again
    6. K+ and phosphate are released on the original side of the membrane leaving the carrier protein to pick up more Na+
Absorption
1. This occurs in the ileum (small intestine)
2. To increase diffusion in the ileum there is microvilli
  1. This increases surface area
3. Starch is broken into maltose then glucose in the small intestine
  1. Enzymes are amylase then maltase
4. After a meal the conc. of glucose etc. will be higher in the ileum than the blood
5. The circulatory system maintains a difference in the conc. gradient by carrying away the products of digestion
6. Co - transport
  1. Glucose or amino acids are drawn into the cell with sodium ions
    1. 1. sodium ions are pumped out of the cell into the blood stream
      1. This requires ATP
        Therefore is active transport
    2. 2. This maintains a conc. gradient of sodium ions from the lumen (high) to the cell (low)
    3. 3. Sodium ions diffuse into the cells down their conc. gradient through the co-transport carrier protein
      1. For this to happen glucose also has to bind to the protein, therefore it is brought through with the sodium ions
      2. Glucose goes against its conc. gradient
    4. 4. Glucose molecules pass into the blood plasma by facilitated diffusion with another carrier protein

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Pack 3 - Transport across cell membranes

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	Creado por Jacob Shepherd hace alrededor de 9 años