Pack 3 - Transport across cell membranes

Pack 3 - Transport across cell membranes

1. Phosphate groups, glycerol and 2 fatty acids make up a phospholipid

   • An ester bond is made and water is removed (condensation)
   • Phospholipids
     • They are polar molecules
       • The head (top) is hydrophobic
       • The tail is hydrophilic

2. Phospholipid bilayer

   • Functions of the bilayer
     • Allows lipid-soluble substances to enter and leave the cell
     • Prevents water-soluble to enter and leave the cell
     • Makes the membrane flexible and self-sealing
   • Proteins
     • Proteins can span the membrane or just sit on one of the surfaces of the bilayer
     • Functions:
       • Structural
       • Help cells attach to each other
       • Channel proteins allow water soluble substances through
       • They facilitate facilitated diffusion
       • Carrier proteins help diffusion
         • Help active transport
       • Act as cell surface receptors
   • Cholesterol
     • Functions
       • Adds strength to the membrane
       • Reduces movement of other molecules
       • Makes the membrane less fluid at higher temperatures
       • Prevents leakage of water and dissolved ions
     • Found between the phospholipids in the bilayer
   • Glycolipids
     • Lipid covalently bonded to carbohydrate
       • Carb extends from bilayer into watery environment
     • Functions
       • Act as recognition site
       • Help maintain stability of membrane
       • Help cells attach together forming tissues
   • Glycoproteins
     • Extrinsic protein covalently bonded to carb
       • Carb extends from the bilayer into water environment around cell
     • Functions
       • Recognition sites
       • Help cells stick together
       • Allow cells to recognise each other
   • Fluid-mosaic model of the plasma membrane
     • The molecules act like a fluid
     • It looks like the tiles on a mosaic

3. Permeability of plasma membrane

   • Most substances can't pass through the plasma membrane because:
     • Not lipid soluble
     • Too large
     • They're polar molecules

4. Diffusion

   • The net movement of molecules or ions from a region where they are highly concentrated to a region where their concentration is lower
   • Diffusion is passive as it does not require any ATP
   • When the net flow of molecules from one direction stops it is called dynamic equilibrium
   • Factors that affect the rate:
     • Temperature
       • More temp = more KE in particles so they move quicker = faster diffusion
     • Difference in concentration
       • Higher difference in concentration = faster diffusion
     • Distance over which diffusion occurs
       • Greater distance = slower diffusion
     • Area over which diffusion occurs
       • As are increases diffusion rate increases
     • Nature of any structure across which diffusion occurs
       • e.g. if a molecule is lipid soluble it will get through plasma membranes fast
     • Size of diffusing molecule
       • Smaller means they can pass through pores quickly = faster diffusion
   • Fick's Law
     • Diffusion rate is proportional to:
       • (Surface area*conc. difference)/diffusion distance

5. Facilitated diffusion

   • This is passive, it does not require ATP
     • Goes with the conc. gradient
   • Proteins
     • Channel Proteins
       • These from water filled hydrophilic channels across the membrane
       • Allows specific water soluble ions to pass through
       • They are selective
         • 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
     • Carrier proteins
       • These span the plasma membrane
         • 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
     • More carrier proteins and channel proteins = faster diffusion

6. Osmosis

   • The net movement from an area of less negative water potential to an area of more negative water potential through a semi - permeable membrane
     • High conc. to low conc.
   • Larger molecules, like glucose, can't fit through the membrane
   • Water potential
     • Pure water has a WP of 0
       • Addition of water will lower the WP
         • Water potential of a solution will always be negative
           • More solute = more negative
   • Osmosis in animal cells
     • If you place a red blood cell in water
       • The red blood cells have a lower water potential than pure water
         • 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
   • Osmosis in plant cells
     • In pure water, plant cells have a lower water potential and the net movement will be into the plant
       • The cell wall becomes rigid and resists the entry of water
         • It becomes turgid
     • If plant cells have higher water potential then net movement out of the cell, protoplast comes away from the cell wall
       • It is plasmolysed
     • Incipient plasmolysis is when there is no net movement of water, the protoplast is just pulling away from the cell

7. Active transport

   • The movement of molecules or ions usually from a region of low conc. to a region of high conc.
     • Against the conc. gradient
   • Requires carrier proteins and ATP
   • e.g. Sodium/potassium pump
     • Stages are:
       • 1. Na+ binds to specific receptor sit on the carrier protein
       • 2. ATP binds to the protein and splits into ADP and a phosphate molecule
       • 3. Energy is released, the phosphate molecule remains bound to the carrier protein
       • 4. The carrier protein changes shape, Na+ is released on the opposite side of the membrane
       • 5. K+ now bind to their receptor sites, causing the protein to change shape again
       • K+ and phosphate are released on the original side of the membrane leaving the carrier protein to pick up more Na+

8. Absorption

   • This occurs in the ileum (small intestine)
   • To increase diffusion in the ileum there is microvilli
     • This increases surface area
   • Starch is broken into maltose then glucose in the small intestine
     • Enzymes are amylase then maltase
   • After a meal the conc. of glucose etc. will be higher in the ileum than the blood
   • The circulatory system maintains a difference in the conc. gradient by carrying away the products of digestion
   • Co - transport
     • Glucose or amino acids are drawn into the cell with sodium ions
       • 1. sodium ions are pumped out of the cell into the blood stream
         • This requires ATP
           • Therefore is active transport
       • 2. This maintains a conc. gradient of sodium ions from the lumen (high) to the cell (low)
       • 3. Sodium ions diffuse into the cells down their conc. gradient through the co-transport carrier protein
         • For this to happen glucose also has to bind to the protein, therefore it is brought through with the sodium ions
         • Glucose goes against its conc. gradient
       • 4. Glucose molecules pass into the blood plasma by facilitated diffusion with another carrier protein

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