Revision

• Phospholipid 'head'

  Hydrophilic, attracts water
• Glycolipid

  A polysaccharide (carbohydrate) chain is attached to the lipid
• Cholesterol

  Cholesterol molecules fit between phospholipids, binding to the hydrophobic tails and causing them to pack more closely together. This restricts their movement, making the membrane less fluid/more rigid
• Phospholipid bilayer

  • Phospholipids are arranged in a double layer, with their heads facing outwards and their hydrophobic tails facing inwards. This means the centre of the bilayer is hydrophobic, so it acts as a barrier to substances that dissolve in water
• Membrane proteins

  • In a mitochondrial membrane, as in all cell and organelle membranes, proteins are scattered through the phospholipid bilayer like tiles in a mosaic. Some proteins are able to move sideways through the bilayer
• Measuring membrane permeability using a colorimeter

  Pass light through the liquid and measure how much of that light was absorbed
• As the pH increases

  The absorbance decreases, meaning less pigment was released by the beetroot and the beetroot cell membranes were less permeable at a higher pH
• High concentration of cholesterol

  Increases the rigidity of the RBC membranes, affecting the biconcave shape and decreasing the surface area, reducing the rate of diffusion of oxygen across the RBC membrane and oxygen transport
• Increasing the concentration of a solvent (such as alcohol or acetone) increases cell membrane permeability
• Membrane permeability also increases at high temperatures and at very low temperatures (below 0°C)
• Diffusion

  The net movement of particles (molecules or ions) down a concentration gradient
• Channel proteins

  • Form pores in the membrane for charged particles (such as ions) to diffuse through
• Facilitated diffusion

  1. A molecule attaches to a carrier protein that is embedded in the cell membrane

  2. The carrier protein then changes shape and releases the molecule on the opposite side of the membrane
• Diffusion of small and non-polar particles

  • Small particles pass through spaces between the phospholipids
  • Non-polar particles are soluble in lipids, so can dissolve in the phospholipid bilayer
• Smooth endoplasmic reticulum (SER)

  • The folded structure gives the SER a larger surface area
  • This means that more particles can be exchanged in the same amount of time, increasing the rate of diffusion into and out of the SER
• Amino acids are large molecules

  They diffuse directly through membranes very slowly, so they rely on carrier proteins for transport
• Diffusion is a passive process - no metabolic energy is needed
• Water potential

  The potential (likelihood) of water molecules to diffuse out of or into a solution
• Osmosis

  The diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential
• Adding salt to pure water

  Lowers the water potential of the liquid in the beaker
• Experiment to measure water potential

  • Use a cork borer to cut potatoes into identically sized chips
  • Divide the chips into groups of three and measure the mass of each group
  • Place one group into each of the five sucrose solutions
  • Leave the chips in the solutions for at least 20 minutes
  • Remove the chips and pat dry gently with a paper towel
  • Weigh each group again and record the results
  • Calculate the % change in mass for each group
  • Use the results to make a calibration curve, showing % change in mass against sucrose concentration
• The point at which the calibration curve crosses the x-axis, where the % change in mass is 0, is the point at which the water potential of the sucrose solution is the same as the water potential of the potato cells
• The student could repeat the experiment with new sucrose solutions with concentrations between 0.25 and 0.5 mol dm³ to make a calibration curve with a higher resolution and therefore find the water potential of the potato cells to a greater degree of accuracy
• Cell vacuole

  • The net movement of water molecules will be from the vacuole interior to the vacuole exterior because there is a higher water potential inside the vacuole than outside
• As the water potential gradient decreases


  The rate of osmosis decreases
• Making a solution weaker than a given solution

  1. Work out the scale factor

  2. Use 1.25 times less than the desired volume

  3. Top up with distilled water to get the desired volume
• Movement of K ions into the guard cells

  Lowers the water potential of the guard cells
• Active transport

  The use of energy to move molecules and ions across membranes, usually against a concentration gradient
• ATP

  Provides the energy needed for active transport by undergoing a hydrolysis reaction, splitting to form ADP and P
• Increase in the number of carrier proteins

  Increases the rate of active transport
• Active transport vs facilitated diffusion

  • Active transport usually moves solutes from a low to a high concentration, requires metabolic energy, and only uses carrier proteins
  • Facilitated diffusion always moves from high to low concentration, does not require energy, and uses channel proteins too
• Co-transporters

  Work like other carrier proteins but are able to move a molecule against its concentration gradient by simultaneously binding to a second molecule, which is moving down its concentration gradient
• Increase in the extracellular concentration

  May initially increase the rate of active transport, but the rate would then level off as the carrier proteins became saturated
• Sodium ion transport in the ileum

  1. Na-K pumps on the blood side actively transport sodium ions out of the epithelial cells into the blood

  2. This creates a sodium ion concentration gradient between the ileum lumen and the epithelial cell, so sodium diffuses into the cell from the lumen

  3. Sodium-glucose co-transporter proteins on the lumen side carry glucose into the cell with the sodium
• Antigen

  A molecule that can generate an immune response when detected by the body (usually proteins found on the surface of cells)
• Plasma cells

  Clones of B-cells that secrete lots of antibodies
• Cellular immune response

  • T-cells and other immune system cells that they interact with, such as phagocytes, form the cellular response
• Phagocytosis

  1. A phagocyte recognises the foreign antigens on a pathogen

  2. The cytoplasm of the phagocyte moves round the pathogen, engulfing it

  3. The pathogen is contained in a phagocytic vacuole in the cytoplasm of the phagocyte

  4. A lysosome fuses with the phagocytic vacuole and the lysozymes hydrolyse the pathogen
• Helper T-cells

  Release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells, and also activate B-cells
• Exposure to mumps virus antigens

  Produces memory T-cells and B-cells that remain in the body and can respond quickly to a second infection, providing immunity