Transport across membranes

• The phospholipid bilayer- this comprises of a glycerol molecule with two fatty acids attached to it and a phosphate group. Water is found on the inside and outside of cells. Dissolved in the water are ions and large polar molecules.

  anything that is lipid soluble or non polar is able to pass directly through the phospholipid bilayer to enter the cell.
• Intrinsic proteins- permanently embedded in the membrane. Most are transmembrane proteins, which penetrate the entire phospholipid bilayer and are usually involved in transport.
• Extrinsic proteins- these sit on one side of the bilayer only, usually bound to the polar heads. These may provide mechanical support or act as receptors when attached to glycolipids.
• Cholesterol- found in the cell membrane of animal cells. Provides extra turgidity- it reduces the movement of the phospholipid and prevents the membrane becoming too fluid at high temperatures. Does not fix the phospholipids in a rigid position so cell shape can change. It reduces the loss of water and ions.
• Diffusion- the net movement of particles from an area of high concentration to an area of low concentration. It is a passive process that doesn't require energy.
• Factors affecting rate of diffusion


  • Concentration gradient
  • Temperature
  • Surface area of the membrane
  • Diffusion distance
  • Size of molecules diffusing
• Concentration gradient


  A large difference in concentration will increase the rate in order to balance out the concentration of particles on either side. Higher chance of net movement.
• Temperature

  An increase in temperature increases the particles' kinetic energy and so increases the rate. Particles are more likely to collide with the membrane and spread out.
• Surface area of the membrane

  Large surface area so there will be more space for particles to diffuse so more can diffuse at once.
• Diffusion distance

  Shorter pathway, so particles don't go far.
• Size of molecules diffusing

  Smaller ones means faster rate as more particles can move through the membrane.
• Carrier proteins- formed from intrinsic protein molecules that allow polar molecules to pass through. Molecules bind to the carrier proteins as they gave a complementary shape. The protein undergoes a shape change and this transports the molecule to the other side. No input metabolic energy required.
• Ion channels- these are proteins which are lined with polar groups that allow charged ions to pass through them. Most are gated so that they can open or close to control the movement of substances.
• Osmosis- the net movement of water particles from a high water potential to a low water potential through a semi-permeable membrane.
• Cytosol- the liquid part of the cytoplasm.
• Isotonic- the same water potential inside and outside of the cell.
• Hypotonic- lower water potential inside the cell.
• Hypertonic- higher water potential inside the cell.
• Water potential- the ability of water molecules to move. The collisions against the membrane exerts a pressure on the membrane.
• Active transport- movement of molecules from a low concentration to a high concentration against a concentration gradient. This requires energy from respiration. It needs a carrier protein.
• Co-transport- describes the movement of two or more substances using the same protein carrier.
• Absorption of glucose


  1. Sodium ions actively transported out of epithelial cells into blood by sodium-potassium pump using ATP

  2. Concentration of sodium ions in lumen of small intestine much higher than inside epithelial cells

  3. Sodium ions diffuse into epithelial cells through co-transport protein

  4. Glucose molecule moves into cell at same time using same protein carrier, against concentration gradient

  5. Glucose moves into blood plasma by facilitated diffusion using another carrier

  6. Sodium ion concentration gradient powers movement of glucose, not ATP