3.2.3 - Transport across cell membranes


Cell Membrane Structure

  • Fluid-mosaic:
    • Phospholipid bilayer (about 7nm thick) - fluid because the phospholipid are constantly moving and can move to fit perfectly in place
    • Cholesterol molecules
      • A type of lipid - present in all cell membranes (except bacterial cells)
      • Fits between the phospholipids, and bind to the hydrophobic tails. This makes them pack closer together, making the cell more rigid
      • Helps maintain the shape of animal cells
    • Proteins scattered throughout the bilayer, like in a mosaic
      • channel proteins, carrier proteins allow large molecules and ions into the cell.
      • Receptor proteins on the surface allow the cell to detect chemicals released from other cells
      • Some proteins are able to move sidways through the bilayer, while others are fixed in position
      • Some have a polysaccharide attached to them, and these are called glycoproteins
      • Some lipids have a polysaccharide attached to them, and these are called glycolipids
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Factors which affect Cell Membrane Permeability

  • Experiment:
    • Use a scalpel to carefully cut 5 equal sized pieces of beetroot. Rince the pieces to remove any pigment released during cutting
    • Add the 5 pieces to 5 different test tubes, each containing 5cm^3 of water. Use a measuring cylinder or pipette to measure the water
    • Place each test tube in a water bath at a different temperature (10*C, 20*C, 30*C, 40*C, 50*C) for the same length of time (using a stopwatch)
    • Remove the pieces of beetroot from the tubes, leaving just the coloured fluid.
    • Use a colorimeter to show how much light is absorbed - the higher the absorbance, the more pigment released, so the higher the permeability of the membrane
  • Increasing the temperature increases membrane permeability
    • Temperatures below 0*C - don't have much energy, can't move much. Channel proteins and carrier proteins deform (more permeable). Ice crystals may pierce the membrane, making it highly permeable when it thaws
    • Temperatures between 0*C and 45*C - membrane is partially permeable, phospholipids can move around, not as tight together. As temp. increases, phopholipids move more and so increase the permeability
    • Temperatures above 45*C - the bilayer starts to melt and the membrane becomes more permeable. Water in the cell expands, applying pressure. Channel and carrier proteins deform, can't control what enters. Increases permeability
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Simple Diffusion

  • Passive - requires no energy
  • The net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
  • Molecules will diffuse both ways, but the net movement will be to the area of lower concentration.
  • Oxygen and carbon dioxide can diffuse easily through cell membranes because they're small and lipid-soluble (they're non-polar)
  • Known as simple diffusion when a molecule directly diffuses through a cell membrane
  • Depends on:
    • The concentration gradient -  the higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium
    • The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of diffusion
    • The surface area - the larger the surface area, the faster the rate of diffusion
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Facilitated Diffusion

  • Uses carrier proteins and protein channels
  • Required to let larger molecules into the cells (or polar molecules or ions)
  • Particles still move down a concentration gradient, from a higher to lower concentration
  • Doesn't use energy (passive)
  • Carrier Proteins:
    • Move large molecules across membranes. Different carrier proteins facilitate the diffusion of different molecules
    • First, a large molecule attaches to a carrier protein in the membrane. Then, the protein changes shape. This releases the molecule of the opposite side of the membrane
  • Channel Proteins:
    • These form pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles
  • Depends on:
    • The concentration gradient
    • The number of channel or carrier proteins (greater number of cc proteins, faster the rate of facilitated diffusion
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  • The diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential
  • Water potential is the potential of water molecules to diffuse out of or into a solution
  • Pure water has the highest water potential (0 or between 0 and -10), all other solutions have a lower water potential than pure water
  • If two solutions have the same water potential, they're isotonic
  • If the solution has a higher water potential than in a cell, the solution is hypotonic (lysed)
  • If the solution has a lower water potential than in a cell, the solution is hypertonic (shrivelled)
  • Depends on:
    • The water potential gradient, which levels off over time as the difference in water potential on either side of the membrane decreases
    • The thickness of the exchange surface
    • The surface area of the exchange surface
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Investigations into Water Potential

  • Serial Dilutions:
    • Line up 5 test tubes in a rack
    • Add 10cm^3 of the initial 2M sucrose solution to the first test tube and 5cm^3 of distilled water to the other 4 test tubes
    • Then, using a pipette, draw 5cm^3 of the solution from the first test tube and muix thoroughly. You now have 10cm^3 of solution that's now half as concentrated as the solution in the first test tube
    • Repeat 3 more times)
  • Scale Factor:
    • Start with a solution of a known concentration, e.g. 1M
    • Find the scale factor by dividing the conc. of this solution by the conc. you want to make. Place these in individual test tubes
    • Top up the rest of the tube with ditilled water
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Active Transport - Carrier Proteins

  • Fairly similar to facilitated diffusion, however:
    • Active transport moves solutes from a low to high concentration
    • Active transport requires energy
      • ATP is released during respiration
      • ATP undergoes a hydrolysis reaction, splitting into ADP and Pi. This releases energy so that the solutes can be transported
  • Co-transporters:
    • They bind two molecules at a time
    • The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient (e.g. Na+ with amino acids or glucose)
  • Affected by:
    • The speed of individual carrier proteins
    • The number of carrier proteins present
    • The rate of respiration in the cell and the availability of ATP
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Active Transport - Co-transport

  • Glucose enters the ileum epithelium with sodium ions
    • Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates a concentration gradient - there's now a higher concentration of sodium in the lumen of the ileum than the cell
    • This causes sodium ions to diffuse from the ileum into the epithelial cell, down their concentration gradient
    • The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.
    • Glucose diffuses out of the cell, into the blood, down its concentration gradient through a channel protein, by facilitated diffusion
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