Membrane Transport

Diffusion

  • If a substance can permeate a membrane, the substance will move from higher concentration to lower concentration through the membrane - diffusion
  • If the membrane is impermeable to the substance, no diffusion will occur

Fick’s Law of Diffusion

  • Factors which influence the rate of net diffusion
    • ↑ concentration gradient (𝝙C) = ↑ rate of diffusion (Q)
    • ↑ surface area of membrane (A) = ↑ rate of diffusion
    • ↑ lipid solubility = ↑ rate of diffusion
    • ↑ molecular weight of substance = ↓ rate of diffusion
    • ↑ distance (thickness) = ↓ rate of diffusion
  • Q ∝ 𝝙C · A · P
    • P = permeability

Electrochemical gradient

  • Electrical gradient: difference in charge between two adjacent areas, promotes the movement of ions to the area of opposite charge
  • Electrochemical gradient: electrical and concentration (chemical) gradient acting on an ion simultaneously

Osmosis

  • The net diffusion of water down its own concentration gradient through a selectively permeable membrane
  • Osmolarity: the concentration of osmotically active particles in a solution (Osm/l)
    • Body fluids ~300 mOsm/l
  • Tonicity: the effect a solution has on cell volume
    • Isotonic: no net movement of water, no change in cell volum
    • Hypotonic: water diffuses into cells, cells swell
    • Hypertonic: water diffuses out of cells, cells shrink

Carrier mediated transport

  • Substance binds onto a carrier which undergoes a conformational change, which transports the substance
  • Factors that determine kind/amount of material transported:
    • Specificity
    • Saturation - transport maximum, Tm
    • Competition - if a carrier can transport two substances, the presence of both diminishes the rate of transfer for either

Facilitated diffusion

  • Facilitated diffusion: uses a carrier to facilitate the transfer of a substance across the membrane from high to low concentration

Active transport

  • Active transport: requires the carrier to expend energy to transfer a substance against a concentration gradient
  • Primary active transport: energy (ATP) is directly required to move a substance against its concentration gradient
    • e.g. Na+-K+ ATPase (Na+/K+ pump) - helps establish concentration gradient across the plasma membrane, helps regulate cell volume and provides energy for secondary active transport (creates the ion concentration gradient - 3 Na+ out for every 2 K+ in)
  • Secondary active transport: carrier moves molecule against concentration gradient by using secondhand energy stored as an ion concentration gradient (usually Na+
    • Symport: solute and Na+ move in the same directio
    • Antiport: solute and Na+ move in opposite directions (Na+ in, solute out)

Vesicular transport

  • Requires energy
  • Exocytosis: vesicle fuses with plasma membrane, releasing its contents to the ECF
  • Endocytosis: 'pinching off’ of membrane to engulf substance