Membrane Transport Primer

Properties of the cell membrane

Lipid soluble substances (O2, CO2, steroid hormones) dissolve in the lipid bilayer and will cross the cell membrane. Water soluble substances (Na+, Cl-, Glucose, H2O) cannot and require a transporter.

Membrane notes

Membranes contain lipid rafts. These are high in cholesterol (non-fluid) and often have GPI anchored membrane proteins.

Glycosylphosphatidylinositol (GPI) anchors: Allow proteins to attach to the outer leaflet

Peripheral proteins: Can be washed away with salt
Integral proteins: Require a detergent/treatment to be removed. These can be on one side of the bilayer or span both. For integral proteins which span, the portion inside is made of alpha helices/beta barrels which can form pores.

Alpha helices: Hydrophobic amino acids
Beta barrels: Hydrophobic amino acids closer to tails (outside of barrel) while hydrophilic amino acids are in the inside of the barrel

Integral proteins span the entire cell and are embedded into the membrane. These are ion channels or transporters, for example.
Peripheral proteins are located on only one side of the membrane.

Transport types

Simple diffusion

Passive transport. A substance will move down its concentration gradient.
Depends on:

Concentration gradient
Permeability
Surface area

Carrier-mediated transport

All have:

Chemical specificity / stereospecificity for the protein
Competition for the protein
Can saturate of the carrier

Saturability

Carrier proteins have limited binding sites. The rate increases rapidly with increasing concentrations. At high concentrations, binding plateaus. This state is referred to as the Transport Maximum (Tm) or the maximum rate a substance can be reabsorbed.

Types of carrier protein transport

Uniport: Transporting a single substance
Symport (cotransport): Carrying two substances in same direction
Antiport (countertransport or exchange): Transporting substances in opposite directions

Facilitated diffusion

Transport of a substance with concentration gradient, but is facilitated by a carrier protein. Also a type of passive transport. Will be faster than simple diffusion because of the carrier. The rate of diffusion will also reach saturation at higher concentrations.

Examples:

Glucose transport from intestinal epithelium/renal tubules to blood
Transport of glucose to skeletal muscle/adipose cells (via GLUT-4)
Transport of fructose in gut lumen to the blood

Transport rate for simple vs facilitated diffusion.png

Primary active transport

Active transport (requires ATP). Transports solutes against concentration gradient.
Carrier protein has ATPase activity.

Examples:

Sodium-potassium ATPase (Na+/K+ pump)
Calcium ATPase (Ca2+ pump)
Hydrogen-potassium ATPase (H+/K+ pump)

The sodium pump exchanges 3 Na+ (going out) for 2 K+ (going in). This is an example of an antiporter
3 Na-Out / 2 K-In

Secondary active transport

The solute is coupled with sodium, allowing it to be transported against the concentration gradient. Still active transport because the sodium is providing the potential energy as it moves along its own concentration gradient. Inhibiting the Na/K pump will stop this transport.

Example:

Glucose transport in the gut: Sodium-glucose cotransport (SGLT)