Membrane Transport Proteins

Lipid Membranes

Transparent to hydrophobic molecules

Small transient holes allow diffusion of small uncharged polar molecules

Other molecules require carriers or channels, which can be regulated

so: Compartmentalization Controls Cellular Concentrations (pH, substrates, etc...)

Ion Flux = Permeability + Driving Force

Driving Force = ion concentration gradient + electric potential

Channels

only small inorganic ions

very fast, but follow concentration gradient

often gated (voltage, ligand, mechanical/stretch)

no interaction with substrate

mostly nerves and epithelial ion absorption/secretion

number of subunits proportional to permeability

Carrier Proteins: Facilitated Diffusion

selective for a particular molecule

follow concentration gradient—"vectorial enzymes"

enzyme kinetics, and can be saturated—Vmax determined by:

Carrier Proteins: Active Transport

move against gradient one of two ways:

many transmembrane regions

conformational change upon binding

complex regulation by glycosylation and phosphorylation of intracellular domain