ii. Lipid solubility of diffusing molecule
iii. Composition of lipid bilayer
1. Cholesterol slows
2. Protein channels accelerate
2. Flux: diffusion rate per unit surface area of membrane
a. Flux = conc. gradient x membrane permeability
III. Protein-Mediated Transport: carried out by transporter proteins
A. Facilitated diffusion (passive transport): protein-mediated transport; moves polar molecules down
their conc. gradient; no energy req; net movement stops when equilibrium is reached.
i. Cells can avoid reaching equilibrium by keeping the concentration of the substrate in the
1. Ex, glucose is phosphorylated to G6P after it enters the cell => prevents build-up
of glucose in cell and prevents glucose from leaving cell.
ii. Channel proteins: create a water-filled passageway that directly links the ICM to the
1. Rapid transport
2. Limited to small ions and water
3. Ion channels may be specific for one ion or may allow ions of similar size and
charge to pass.
a. #Example: K leak channels
i. Allow facilitated diffusion of K down its concentration gradient.
ii. Found on basolateral membrane of epithelial cells.
4. Selectivity of channel determined by diameter of its central pore and by the
electrical charge of the amino acids that line the channel.
a. (If channel amino acids are positive, then negative ions can pass thru
5. Open channels allow ions to cross membrane with no regulation
6. Gated channels regulate movement of ions