cell membrane and transport

plasma membrane
1. The plasma membrane is the barrier through which all material entering the cell must pass
  1. gives cell structure
  2. to allow substances to exit and enter the cell
  3. cell-cell recognition and cell-cell signalling
  4. it is selectively permeable
2. Structure
  1. mainly consists of phospholipid molecules arranged into a bilayer
    1. 7-8nm
  2. the hydrophillic heads face outwardsand the hydophoic tails face inward.
    1. this forms a hydrophobic, non-polar region in the middle of the bilayer. this prevents polar molecules such as glucose from passing through.
  3. intrinsic proteins lie across both the layers of the membrane (carrier proteins etc.)
  4. extrinsic proteins are either in one layer of the membrane or on the surface of the membrane glycoprotein etc.)
  5. glycoproteins are protiens with a carbohydrate chain attached. Glycolipids are lipids with a carbohydrate chain attached. These are involved in cell-cell recognition.
  6. The membrane also contains cholesterol.Cholesterol also helps to increse the ridgidity of the membrane.
3. factors affecting permeability
  1. when the temperature rises,the molecules in the membrane gain kinetic energy and move at a faster rate. this cause the membrane to become more fluid and therefore more permeable
  2. organic solvents can dissolve the membrane making it permeable
4. fluid mosaic model
  1. fluid - all parts of the membrane can move relative to each other
  2. mosaic - proteins are dotted throughout the membrane like mosaic tiles
membrane transport
1. lipid soluble substances can pass through the phospholipid bilayer
  1. water soluble substances are prevented from passing through
  2. water soluble molecules have to pass through carrier proteins or protein channles
  3. lipid soluble molecules (glycerol) or small, uncharged molecules (O2 and CO2) can pass through more easily
  4. the hydrophobic core prevents polar molecules or ions passing through
2. Diffusion
  1. diffusion is the movement of molecules or ions from an area of higher concentration to an area of a lower concentration (down the concentration gradient)
  2. diffusion occurs through the phospholipid bilayer.
  3. due to the hydrophobic tails only lipid soluble molecules, which are uncharged and non-polar, can diffuse
  4. ATP is not needed
  5. does not involve proteins
3. Faciliated diffusion
  1. large, water soluble and charged molecules have to pass through via faciliated diffusion
  2. it involves molecules passing through a channel protein or carrier protein.
    1. the proteins have a hydrophilic pore down the centre, to allow molecules through, whilst the outside is hydrophobic, so it fits in membrane
  3. the molecules travel from an area of high concentration th an area of low concentration and no ATP is used
  4. as the concentration gradient increases the rate of diffusion increases
    1. but when the rate gets to a point the rate will get no faster because all the proteins are in use (saturated)
  5. temperature and number of carrier/channel proteins affects the rate
4. Active transport
  1. active transport is the movement of molecules from an area of low concentration to an area of high concentration (against the concentration gradient, using chemical energy in the form of ATP
  2. a carrier protein in the membrane is used to pump
  3. e.g. root hair cell in the roots
  4. active transport can be stopped by adding cyanide. It stops ATP being produced.
5. Osmosis
  1. osmosis is the movement of water molecules from a high water potential to a lower water potential across a partialy permiable membrane
    1. water potential is the potential energy of a solution relative to pure water
    2. the partial permiable membrane is important because it ensures the solute does not diffuse to counteract the effect of osmosis
    3. water potential of a cell = solute potential + pressure potential
      1. the solute potential is generated by the solutes dissolved in the water
      2. the pressure potential is generated by the cytoplasm pushing on the cell wall of a plant cell
  2. pure water has a water potential of 0KPa. This is the highest possible water potential, so all solutions have a negative water potential
    1. osmosis can be described as the movement from a region of more negative to a region of less negative
    2. a solution with higher water potential is called hypotonic.
    3. a solution with a lower water potential is hpertonic
    4. solution with the same water potential is isotonic
  3. hypotonic solution
    1. plant cells will gain water, swell and become turdgid. This is good because it ensures the plant remains upright.
    2. animal cells will swell and bust (lysed)
  4. iotonic solution
    1. plant and animal cells will be normal
  5. hypertonic solution
    1. plant cells will lose water and will become flascid and plasmolysed.
      1. incipient plasmolysis is the point at which the cytoplasm begins to come away from the cell wall
    2. animal cells will lose water and become shrivelled
6. Bulk transport
  1. exocytosis
    1. large molecules can be released from cells by exocytosis
    2. a vesicle fuses with the plasma membrane and the molecule is then released to the outside
    3. the molecule could be a modified protein from the golgi body (E.g. hormone)
    4. the surface area of the plasma membrane increases after the vesicle fuses
  2. endocytosis
    1. large substances can be taken into the cell by endocytosis
    2. the plasma membrane folds around the molecule and engulfs it. this means that the substance is in the cell in a vesicle
    3. phagocytosis - large solid substances e.g. white blood cell ingesting bacteria
    4. pinocytosis - fluids
    5. the cell membrane decreases in size

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	Created by greencat over 10 years ago