Transport across cell membranes

• Name and describe the function of the components of a cell surface membrane

  1. Phospholipids - ensure the membrane is semi-permeable and allows small and non-polar molecules to diffuse through
  2. Intrinsic proteins (carrier/channels) - allow large and polar molecules to diffuse through
  3. Extrinsic proteins - act as antigens, receptors and lipids
  4. Glycoproteins - act as antigens and receptors
  5. Glycolipids - act as receptors
  6. Cholesterol - restrict phospholipid movement. An increased concentration leads to less permeability
• Describe simple diffusion

  Passive movement down a concentration gradient of small and non-polar molecules

  Rate of diffusion is affected by Fick's Law
• Describe facilitated diffusion

  Passive movement down a concentration gradient of large and polar molecules

  Each channel/carrier protein is specific to one molecule

  Rate of diffusion is affected by Fick's Law and the number of channel/carrier proteins
• Describe active transport

  Active movement against a concentration gradient which requires a phosphorylated carrier protein

  Rate is affected by the rate of cellular respiration and the number of carriers
• Describe osmosis

  Passive movement down a water potential gradient through phospholipids and water channels - aquaporins

  Rate of osmosis is affected by Fick's Law and the number of aquaporins
• Describe co-transport

  Movement of two molecules at the same time

  One molecule down a concentration gradient and the other against a concentration gradient

  Requires a special co-transporter protein
• Describe co-transport to allow glucose absorption into the ileum

  -A Na+/K+ pump actively transports Na+ out of the epithelial cell which enters the adjacent capillary

  -This creates a concentration gradient for Na+, lower inside the cell

  -Na+ enters the cell from the lumen of the ileum by facilitated diffusion. Glucose moves through by co-transport

  -This creates a concentration gradient for glucose, higher inside the cell

  -Glucose moves from the cell into the capillary by facilitated diffusion
• Describe exocytosis

  The Golgi vesicle fuses with the cell membrane to release its molecules outside the cell
• Describe endocytosis

  A group of molecules outside the cell bind to receptors on the cell surface membrane which causes the membrane to fold inwards and gather molecules into a vesicle