Key Concepts of Section 11.2

Key Concepts of Section 11.2

Facilitated Transport of Glucose and Water

  • Protein-catalyzed transport of biological solutes across a membrane occurs much faster than simple diffusion, exhibits a Vmax 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 Km 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.