Key Concepts of Section 11.1

• Cellular membranes regulate the traffic of molecules and ions into and out of cells and their organelles. The rate of simple diffusion of a substance across a membrane is proportional to its concentration gradient and hydrophobicity.

• With the exception of gases (e.g., O₂ and CO₂) and small, uncharged, water-soluble molecules, most molecules cannot diffuse across a pure phospholipid bilayer at rates sufficient to meet cellular needs.

• Membrane transport proteins provide a hydrophilic passageway for molecules and ions to travel through the hydrophobic interior of a membrane.

• Three classes of transmembrane proteins mediate transport of ions, sugars, amino acids, and other metabolites across cellular membranes: channels, transporters, and ATP-powered pumps (see Figure 11-2).

• Channels form a hydrophilic “tube” through which water or ions move down a concentration gradient, a process known as facilitated transport.

• Transporters fall into three groups. Uniporters transport a molecule down its concentration gradient (facilitated transport); symporters and antiporters couple movement of a substrate against its concentration gradient to the movement of a second substrate down its concentration gradient, a process known as secondary active transport or cotransport (see Table 11-1).

• ATP-powered pumps couple the movement of a substrate against its concentration gradient to ATP hydrolysis, a process known as active transport.

• Conformational changes are essential to the function of all membrane transport proteins; speed of transport depends on the number of substrate molecules or ions that can pass through a protein at once.