Diffusion: • Passive, non-mediated, no channels. • Exergonic process (ΔG < 0). • Spontaneous movement of material from a region of high concentration to a region of low concentration. • Change in Gibbs free energy (ΔG) during diffusion of nonelectrolytes depends on the concentration gradient. • Change in ΔG during diffusion of electrolytes dependent on electrochemical gradient. • Water moves from low [solute] to high [solute] by osmosis. • Many cells contain aquaporins (integral membrane proteins) that act as water channels for when osmosis is not rapid enough
Ion Channels: • • • •
Diffusion of ions, passive, non-mediated. Selective and bidirectional (dependent on direction of electrochemical gradient). Many under regulatory control – gated channels. 3 categories of gated channels: Voltage-gated channels (dependent on electrochemical gradient). Ligand-gated channels (dependent on binding of ligand e.g. cAMP). Mechano-gated channels (dependent on mechanical forces e.g. stretch).