Lecture 2 ion channels, ionic equilibria, and resting membrane potential. Learning objectives: explain how biological membranes act as a barrier between the ecf and icf and how they act as capacitors. Biological membranes are permeable only to molecules with certain properties. At rest the icf (intracellular fluid) is ~-70 mv relative to the ecf (extracellular fluid: due to higher concentration of na+ outside the cell and higher k+ inside. Bilipid membrane separates the icf and ecf spaces: hydrophobic inside and hydrophilic outside, lipid bilayer creates a physical barrier/capacitor, able to carry a charge, explain how ion channels permit the movement of ions. Ion channels are selectively permeable and only allow certain ions to pass: can be selective or non-selective, usually gated. Ions will diffuse down their gradients (electrical or chemical) Equilibrium -> equal opposing electrical and concentration gradients: gap-junction channel allows flow of ions directly from intracellular space of one cell to the intracellular space of another cell.