Exchange across cell membrane

• Diffusion

  • the net movement of molecules/ions from an area of higher concentration to an area of lower concentration, moving down a concentration gradient
  • a passive process: does not require metabolic energy
  • small molecules: can pass through spaces between phospholipids
  • non-polar molecules: can dissolve in hydrophobic centre of the bilayer
• Facilitated diffusion

  • a passive process: does not require metabolic energy
  • large molecules (eg. glucose, amino acids) do not as they are too big
  • polar molecules do not as they are water soluble and the centre of the bilayer is hydrophobic
  • these substances diffuse through carrier proteins or channel proteins instead
• Carrier proteins

  • moves large molecules across membranes
  • when a molecule specific to the protein is present, it binds with the protein, causing it to change shape so that the molecule is released to the opposite side of the membrane
• Channel proteins

  • form water filled channels which allow water-soluble ions to pass through
  • selective-opens in the presence of a specific ion - allows control over the entry and exit of ions
• Factors affecting rate of diffusion

  • concentration gradient
  • temperature
  • surface area
  • thickness of membrane
  • number of carrier/channel proteins
• Aquaporins: channel proteins that allow the facilitated diffusion of water through cell membranes
• Osmosis

  • the diffusion of water molecules from an area of higher water potential to an area of lower water potential, across a partially permeable membrane
  • a passive process: does not require metabolic energy
• Water potential:

  • the potential of water molecules to diffuse into/out of a solution
  • pure water has a water potential of 0kPa. adding solutes to water reduces the water potential ( it becomes negative)
• Isotonic solution: same water potential inside and outside the cell, movement of water molecule occurs at the same rate (no net movement)
• Hypertonic solution:

  • the solution outside of the cell has a higher solute concentration than the inside of the cell: water moves out of the cell
  • plant cell shrinks and detaches from the cell wall (phasmocytosis)
  • animal cell shrinks
• Hypotonic solution:

  • the solution outside of the cell has a lower solute concentration than the inside of the cell: water moves into the cell
  • plant cell becomes turgid
  • animal cell bursts (cytolysis) as it has no cell wall to withstand the increased pressure
• Active transport

  • movement of molecules from an area of lower concentration to an area of higher concentration, against a concentration gradient
  • requires carrier proteins: changes shape to transport substances across the membrane, releasing it on the other side
  • requires energy provided by ATP produced during respiration
• Co-transport

  • carrier proteins bind to 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
  • involves both facilitated diffusion and active transport
• Co-transport in the Mammalian Ileum:

  1. Sodium ions are actively transported out of the ileum epithelial cells into the blood, by the sodium-potassium pump.
  2. Concentration gradient created - Na ions diffuse from the lumen of the ileum into the epithelial cell, via the sodium-glucose co-transporter proteins.
  3. The co-transporter carries glucose into the cell with the sodium.
  4. Glucose concentration inside cell increases, so diffuses out of the cell into the blood through a protein channel, by facilitated diffusion
• Increasing temperature increases permeability of membrane

  • phospholipid molecules have more kinetic energy and move further apart from each other, which destroys the cell memrabne structure
  • membrane proteins denature, so they lose control of what enters and exits the membrane
• Increasing alcohol concentration increases membrane permeability

  • lipids dissolve in alcohol, so cell membrane breaks down