Barriers, Pores, Pumps and Gates

3.10.05

Cell membranes
Channels and pores
• Membrane-spanning proteins.
• Hydrophilic domain allows water molecules and ions to cross membranes.
• Selective.
• Gated-channels.

Selectivity
• Main cations: Na+, K+, H+, Ca2+ + non-selective cation channels.
• Main anion channels: Cl-, HCO3-.

Ion movement through a channel or pore is always DOWN the electrochemical potential gradient of the ion.

Nernst equation: membrane potential.

Movement of an ion or any other substance against its EPG requires active transport.

Active transport
Involves an ENERGY INPUT into the transport process.

Primary active transport makes direct use of metabolic energy in the form of ATP.
Sodium pump = sodium/potassium ATPase. 3 Na+s out, 2 K+s in.

• Na+ pump.
• Ca2+ pump.
• H+ pump.
• No ANION pumps.
• No sugar or amino acid pumps.
• No water molecule pumps.

Secondary active transport: Utilises energy stored in EPG of one substance to drive the movement of another.

Cotransport: Na+ +
• Glucose (epithelial cells) }
• Amino acids } symports
• Also antiports - Na+ in, something else (eg. H+) out.

• Glucose, amino acids, bicarbonate, ions etc.
• Most commonly utilised driving force - inwardly directed Na+ gradient.
• Na+ gradient created by primary active transport (Na+ pump).

Transepithelial water movement
Na+ outside much much greater than Na+ inside.
K+ inside much much greater than K+ outside