ion gradients, ion transporters, ion channels

ion gradients

Concentration difference in ion concentration across membrane, resulting in membrane potential ranging from -40mV to -80mV

Energy source in transport processes

excitable membranes

essential for neuronal activity (action potential)

ion gradients across membranes

Plasma membrane: low Na+, high K+ inside cell

Intracellular organelles (vacuoles, lysosomes, endosomes, etc): proton gradient

Ca2+ gradients

Generated and maintained by energy dependent ion transport coupled t ATP hydrolysis

Na+, K+ -ATPase (sodium pump)

Maintains sodium/potassium gradient across the plasma membrane

Inside: 10mM Na+, 100mM K+

Outside: 140mM Na+, 10mM K+

Per ATP molecules hydrolysed, three Na+ out and two K+ in

ion channels

Facilitate ion flow across the membrane - form an aqueous pore across the membrane

Ions passing through a channel down the ion gradient - form higher to lower concentration

ion channels are integral membrane proteins

Structural feature: amphipathic helices (hydrophobic side towards lipid environment, hydrophilic side towards pore)

Transmembrane helices tightly packed, form aqueous pore

ion channels are gated

they open only in response to a cellular event