1.4 - Membrane Transport

• The phospholipid bilayer is selectively permeable
• Diffusion is the passive net movement of particles from areas of high concentration to areas of low concentration through a partially permeable membrane.
• Factors affecting rate of diffusion:

  • Concentration gradient
  • Surface area
  • Length of diffusion path
• Reducing length of diffusion path:

  • Membranes are incredibly thin
  • Folded membranes increase SA:Vol ratio
  • More membrane in smaller volume means shorter distance molecules must diffuse
• Maximising surface area for absorption:

  • Alveoli in lungs
  • Membrane folds in mitochondria and chloroplasts
  • Root hairs for water and mineral ion uptake
  • Villi for absorption of digested food molecules
• Osmosis

  • May occur when there is a partially permeable membrane, such as a cell membrane.

  When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell)  
• Aquaporin is an integral protein that acts as a pore in the membrane that speeds the movement of water molecules
• Hypertonic - Plasmolysed

  Isotonic - Flaccid

  Hypotonic - Turgid
• Facilitated Diffusion:

  • Large and polar molecules can’t get across the membrane via simple diffusion
  • Transmembrane (polytopic) proteins recognise a particular molecule and help it to move across the membrane. The direction it moves is dependent on the concentration gradient.
• Potassium channels in axons are voltage gated.

  They enable the facilitated diffusion of potassium out of the axon.

  1. At one stage during a nerve impulse there are relatively more positive charges inside.
  2. This voltage change causes potassium channels to open, allowing potassium ions to diffuse out of the axon.
  3. Once the voltage conditions change the channel rapidly closes again.
• Primary active transport requires ATP.

  Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane.

  Molecules are moved against their concentration gradient
• ATP:

  • Hydrolysis of the bond releases one phosphate and a lot of energy
  • Respiration recombines ADP with a phosphate ion to be used for further cellular purposes
• 1. The interior of the sodium potassium pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites.
• 2. ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed.
• 3. The interior of the pump opens to the outside of the axon and the three sodium ions are released.
• 4. Two potassium ions from outside can then enter and attach to their binding sites.
• 5. Binding of potassium causes release of the phosphate group; this causes the pump to change shape again so that it is again only open to the inside of the axon.
• 6. The interior of the pump opens to the inside of the axon and the two potassium ions are released.
• 7. Sodium ions can now enter and bind to the pump again.
•  In secondary active transport, the required energy is derived from energy stored in the form of concentration differences in a second solute.
• Vesicle:

  • They carry proteins produced by ribosomes on the RER to the Golgi apparatus, where they are prepared for export from the cell via another vesicle
• Endocytosis: The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle
• Exocytosis: The release of substances from a cell (secretion) when a vesicle joins with the cell plasma membrane.
• Exocytosis:

  • Constitutive secretion occurs continuously in cells, depending on their function
  • Regulated secretion is in response to a trigger e.g. the release of neurotransmitters
• Phagocytosis - Cell eating

  Pinocytosis - Cell drinking