Key Concepts of Section 11.2

• Protein-catalyzed transport of biological solutes across a membrane occurs much faster than simple diffusion, exhibits a V_max when the limited number of transporter molecules are saturated with substrate, and is highly specific for substrate (see Figure 11-4).

• Uniport proteins, such as the glucose transporters (GLUTs), are thought to shuttle between two conformational states, one in which the substrate-binding site faces outward and one in which the binding site faces inward (see Figure 11-5).

• All members of the GLUT protein family transport sugars and have similar structures. Differences in their K_m values, expression in different cell types, and substrate specificities are important for proper sugar metabolism in the body.

• Two common experimental systems for studying the functions of transport proteins are liposomes containing a purified transport protein and cells transfected with the gene encoding a particular transport protein.

• Most cellular membranes are semipermeable, more permeable to water than to ions or most other solutes. Water moves by osmosis across membranes from a solution of lower solute concentration to one of higher solute concentration.

• The rigid cell wall surrounding plant cells prevents their swelling and leads to generation of turgor pressure in response to the osmotic influx of water.

• Aquaporins are water-channel proteins that specifically increase the permeability of cellular membranes to water (see Figure 11-8).

• Aquaporin 2 in the plasma membrane of certain kidney cells is essential for resorption of water from urine being formed; the absence of aquaporin 2 leads to the medical condition diabetes insipidus.