MODES OF MEMBRANE TRANSPORT

A. Cell Membrane Structure

1. Phospholipid bilayer

Note: Lipid core is impermeable to charged particles (ions) and polar substances

2. Proteins, including membrane-spanning proteins (integral proteins) which can aid in membrane transport

3. Carbohydrates, associated with membrane proteins (glycoprotein) or membrane lipids (glycolipid)

4. Cholesterol (membrane flexibility)

B. Dissolve in Membrane Lipid

1. Only applicable to lipid-soluble substances

2. Permeability depends on degree of lipid solubility (sometimes measured by oil-water partition coefficient)

3. Substances utilizing this mechanism include

a. small, lipid-soluble organic molecules
b. dissolved gases (O2, CO2, etc.)
c. water (!)

C. Simple, Passive Movement Through Channels

1. Channel: membrane-spanning macromolecule with an aqueous pore in its center

Note the above channel on the right is "gated"; that is, the permeability can change depending of the degree to which the gate is open or closed

2. Aqueous-soluble particles can move through pores (charged particles and polar particles) (Note: "Polar" means that the particle is neutral but that its charge is not uniformly distributed)

3. Some channels are selective for one or a small group of substances (e.g. Na+-channel, K+-channel); some channels are non-selective (e.g. “leak” channel)

4. Permeability depends upon

a. particle size (generally only small particles can pass through pores)
b. channel density (number of channels/area)
c. channel gates (element of a channel that can open or close)
d. gated channels are classified by their control

1) voltage-gated or electrically-gated: open or close depends upon membrane potential
2) ligand-gated: open or close depends upon a chemical substance binding to a membrane receptor
3) physically-gated: open or close depends upon a physical influence -- mechanical, thermal, etc.

Note: Ligand: chemical that can bind to a receptor

D. Facilitated Diffusion (increases rate of membrane movement)

1. Define: passive movement by binding to specific sites on membrane-spanning or membrane-mobile proteins

2. Mechanism

a. particle on one side of the membrane binds to receptor site
b. channel reconfigures, exposing the receptor site to the solution on the other side of the membrane
c. particle dissociates from the receptor and diffuses into the solution on the other side
d. may move several particles simultaneously in the same direction (symport) or in opposite directions (antiport)

3. Selectivity: facilitated diffusion channels are generally selective because of specificity of receptor binding sites

4. Permeability depends upon

a. same factors as simple, passive movement through pores
b. affinity of receptor for transported particle
c. number of receptors; note saturation (maximum transport rate) as concentration of transported particles increases and all receptors are occupied
d. possible presence of other particles that can bind to the same receptors (agonist, antagonist)

Note:
Agonist: substance that binds to a receptor and has the same effect as the substance that normally binds to the receptor
Antagonist (or Blocking Agent): substance that binds to the receptor preventing the agonist from binding and/or preventing the normal response

Note: Facilitated Diffusion still requires a passive driving force -- concentration gradient and/or and electrical potential difference -- for particles to move, and the direction of movement is determined by the concentration/electrical gradient.