Cell Biology and Signalling: Membrane Transport

Membrane Transport

• Membranes are selective permeability barriers, blocking the passage of almost all hydrophilic molecules into cells and organelles.
• Small uncharged or hydrophobic molecules can freely cross the membrane by simple diffusion along their concentration gradients.
• Charged polar molecules require specialist proteins (pumps, transporters, pores) to allow them to cross the membrane.

Molecules Crossing Membranes

• Lipid bilayer permeability is higher for molecules that are uncharged, non-polar, and small.
• Examples of molecules that can cross the membrane:
  • Hydrophobic molecules: O2, N2, CO2, benzene, short chain fatty acids.
  • Small uncharged polar molecules: H2O, urea, glycerol.
  • Large uncharged polar molecules: glucose, sucrose.
  • Ions: H+, Na+, Mg2+, HCO3-, K+, Ca2+, Cl-.
  • Charged polar molecules: amino acids, ATP.

Mechanisms of Transport

• Simple passive transport / Diffusion: solutes move down a concentration gradient, crossing the membrane.
• Facilitated diffusion: solutes move down a concentration gradient, crossing the membrane, requiring a membrane protein (ion channel).
• Gated ion channels: allow facilitated diffusion, selective for different ions, and open or close in response to a stimulus.
• Primary active transport: solutes move against a concentration gradient, requiring a membrane protein and energy from ATP hydrolysis.
• Secondary active transport: uses a pre-established gradient to drive transport of solutes across the membrane against a gradient.

Passive Transport

• Solutes move down a concentration gradient, crossing the membrane, at equilibrium [inside cell] = [outside cell].
• Rate of diffusion depends on the Partition Coefficient of the solute.
• Solutes that are more hydrophobic have a higher Partition Coefficient and equilibrate more quickly.

Facilitated Diffusion

• Solutes move down a concentration gradient, crossing the membrane, at equilibrium [inside cell] = [outside cell].
• Requires a membrane protein (ion channel).
• Examples:
  • Cl-/HCO3- channel in erythrocytes.
  • Aquaporin: water channel.
  • GLUT glucose transporters.

Transporter Affinity

• Transporter affinity for solutes is given by the Km (Michaelis constant).
• The lower the Km, the higher the affinity.

GLUT (Glucose Transporter) Family

• Mediates constitutive glucose uptake in many tissues.
• Examples:
  • GLUT1: ubiquitous, low Km (high affinity), abundant in erythrocytes and low in skeletal muscle.
  • GLUT2: liver, pancreatic ß-cells, high Km (low affinity) and large Jmax (high capacity).
  • GLUT3: neurons, low Km (high affinity).
  • GLUT4: muscle, adipocytes, Km similar to fed state blood glucose concentration, regulated by insulin.

Insulin and GLUT4

• Insulin stimulates uptake of glucose in muscle and adipose tissue.
• Insulin increases the amount of GLUT4 in the plasma membrane.
• GLUT4 is present on membrane-bound vesicles in the cytoplasm.
• Insulin triggers the movement of vesicles to the plasma membrane, increasing the level of GLUT4 on the cell surface.
• Increased glucose transporters increase the uptake of glucose into the cell.

Gated Ion Channels

• Ion channels that allow facilitated diffusion, selective for different ions.
• Examples:
  • Ligand-gated: e.g., acetylcholine and acetylcholine-gated Na+/K+ channel.
  • Voltage-gated: e.g., Na+ and K+ channels in axons involved in nerve transduction.

Active Transport

• Solutes move against a concentration gradient, requiring a membrane protein and energy from ATP hydrolysis.
• Examples:
  • Na+/K+ pump (Na+/K+ ATPase) in the plasma membrane.
  • Part of co-transport systems to drive solute movement.

Na+/K+ Pump

• Consists of a tetramer (α2β2).
• Na+ enters the open cytoplasmic access, and phosphorylation from ATP at the cytoplasmic site causes a conformational change.
• Conformational change closes the cytoplasmic access and opens the external access.
• Conformational change means that the pump binds K+ and releases Na+ outside the cell.
• Hydrolysis of the phosphate group closes the external access, opens the cytoplasmic access, and releases K+ into the cell.

Co-Transport Systems

• Pre-established gradient is used to drive transport of solutes across the membrane against a gradient.
• ATP hydrolysis is used to establish the primary gradient.
• Examples:
  • Symport: transport of two solutes in the same direction.
  • Antiport: transport of two solutes in opposite directions.

Na+/Glucose Cotransporter (SGLUT)

• Glucose absorption from the intestine against a gradient.
• SGLUT is a symport.
• Na+ gradient established by the Na+/K+ pump and ATP hydrolysis is used to drive the uptake of glucose into cells.
• Secondary active transport.

Clinical Considerations

• Digitoxin: cardiac glycosides that inhibit the Na+/K+ pump, increasing [Na+] inside heart muscle and contraction.
• Ouabain: cardiac glycosides that inhibit the Na+/K+ pump, increasing [Na+] inside heart muscle and contraction.
• CFTR (Cystic Fibrosis Transmembrane Conductance Regulator): a chloride ion channel in cells responsible for producing mucus, sweat, saliva, and tears.
• Cholera toxin: stimulates an increase in cAMP level, activating CFTR and secretion of chloride ions, followed by Na+ and water into the lumen of the gut.
• Oral rehydration therapy: includes high glucose concentration to drive Na+ uptake into cells via SGLUT, treating cholera.