Cell membranes

• the cell membrane controls which substances can enter and leave the cell.

  • They are partially permeable so can let some molecules through.
  • Substances can move across by diffusion, osmosis & active transport.
• the cell membrane is made up of phospholipids.
• Cholesterol is present in the bilayer which helps maintain the shape of the cell.

  • it does this by binding to the hydrophobic tails causing them ot pack closer together, restricting movement & making the membrane more rigid.
• proteins are scattered throughout the bilayer, these include channel proteins and carrier proteins.

  • these allow large molecules & ions to pass through the membrane.
• phospholipids have;

  • the head is hydrophilic
  • the tail is hydrophobic
  • the molecules arrange themselves so the heads face outwards & tails inwards.
  • this means the centre of the bilayer is hydrophobic so wont allow water soluble substances through.
• investigating permeability of the cell membrane:

  1. cut 5 equally sized pieces of beetroot & rinse to remove any pigment released in cutting.
  2. add the 5 pieces to different test tubes, each containing an equal volume of water.
  3. Place each test tube in a water bath at different temperatures (10, 20, 30, 40, 50) for the same amount of time.
  4. remove the beetroot pieces from the now coloured liquid.
  5. Use a colorimeter to see how much pigment is released & compare.
• why is it good to use a colorimeter ?

  it gives a quantitive result which is easily comparable
• as temperature increases, the permeability of the membrane increases
• at temperatures between 0 - 45 the phospholipids can move & arent as tightly packed - the membrane is partially permeable.
• at temperatures above 45 the phospholipid bilayer begins to melt and the membrane becomes more permeable.

  • channel and carrier proteins denature so change shape and can no longer control what enters or leaves the cell.
• diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
• diffusion is a passive process so doesnt require any energy
• facilitated diffusion uses channel and carrier proteins to transport larger molecules down a concentration gradient. e.g glucose & amino acids.

  • water soluble molecules, ions & polar molecules are transported this way.
  • this doesnt require energy
• channel proteins form pores in the membrane for charged particles to diffuse through down a concentration gradient.

  • different channel proteins facilitate diffusion for different charged particles.
• carrier proteins move large molecules across membranes, down a concentration gradient.

  1. first the large molecule attaches to a carrier protein
  2. then the carrier protein changes shape.
  3. This releases the molecule on the other side of the membrane.
• the rate of diffusion depends on:

  • the steepness of the concentration gradient.
  • the thickness of the exchange surface
  • the surface area of the exchange surface
• the rate of facilitated diffusion depends on:

  • the steepness of the concentration gradient
  • the number of channel and carrier proteins present in the membrane.
• osmosis is the diffusion of water molecules across a partially permeable membrane from an area of high water potential to an area of low water potential.
• water potential is the potential of water to diffuse out of a solution
• if two solutions have the same water potential then it is isotonic
• the rate of osmosis depends on:

  • the steepness of the water potential gradient
  • the thickness of the exchange surface
  • the surface area of the exchange surface
• investigating water potential:

  • make up several solutions of different known concentrations of sucrose.
  • use a cork borer to cut potatoes into identical sized cylinders.
  • divide the chips into groups of 3 and measure the mass of each group.
  • place on group in each of the sucrose solutions & leave then for 20 minutes.
  • remove the chips and dry them.
  • weigh each group again and calculate the change in mass.
  • the chips will gains water in high water potential solutions and will loose water in low water potential solutions, altering their weight.
• active transport uses energy (ATP) to move molecules & ions across membranes against a concentration gradient.
• active transport involves use of carrier proteins and ATP
• co-transport proteins are a type of carrier protein that bind two molecules at a time.
• rate of active transport depends on:

  • the speed of individual carrier proteins
  • the number of carrier proteins present
  • the rate of respiration in the cell providing the ATP for active trasnport
• glucose is absorbed into the blood in the small intestine / the ileum
• active transport of glucose:

  1. sodium ions are actively transported out of the ileum epithelial cells into the blood.
  2. this creates a concentration gradient causing sodium to diffuse from the ileum lumen into the epithelial cell.
  3. It moves via a sodium-glucose co-transport protein, carrying glucose with it down a concentration gradient.
  4. This creates a high concentration of glucose in the epithelial cell so glucose moves by facilitated diffusion into the blood.