SUMMARY

• 12.1 Phospholipids and Glycolipids Form Bimolecular Sheets

  Membrane lipids spontaneously form extensive bimolecular sheets in aqueous solutions. The driving force for membrane formation is the hydrophobic effect. The hydrophobic tails then interact with one another by van der Waals interactions, while the hydrophilic head groups interact with the aqueous medium. Lipid bilayers are cooperative structures, held together by many weak bonds. These lipid bilayers are highly impermeable to ions and most polar molecules, yet they are quite fluid, which enables them to act as a solvent for membrane proteins.

• 12.2 Membrane Fluidity Is Controlled by Fatty Acid Composition and Cholesterol Content

  The degree of fluidity of a membrane partly depends on the chain length of its lipids and the extent to which their constituent fatty acids are unsaturated. In animals, cholesterol content also regulates membrane fluidity.

• 12.3 Proteins Carry Out Most Membrane Processes

  Specific proteins mediate distinctive membrane functions such as transport, communication, and energy transduction. Many integral membrane proteins span the lipid bilayer, whereas others are only partly embedded in the membrane. Peripheral membrane proteins are bound to membrane surfaces or integral membrane proteins by electrostatic and hydrogen-bond interactions. Membrane-spanning proteins have regular structures, including β strands, although the a helix is the most common membrane-spanning structure.

• 12.4 Lipids and Many Membrane Proteins Diffuse Laterally in the Membrane

  Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flopping) is usually very slow. Proteins do not rotate across bilayers; hence, membrane asymmetry can be preserved.

  220

• 12.5 A Major Role of Membrane Proteins Is to Function as Transporters

  Polar or charged molecules require proteins to form passages through the hydrophobic barrier. Passive transport or facilitated diffusion takes place when an ion or polar molecule moves down its concentration gradient. If a molecule moves against a concentration gradient, an external energy source is required; this movement is referred to as active transport and results in the generation of concentration gradients.

  Active transport is often carried out at the expense of ATP hydrolysis. P-type ATPases pump ions against a concentration gradient and become transiently phosphorylated in the process of transport. Membrane proteins containing ATP-binding-cassette domains are another family of ATP-dependent pumps.

  Many active-transport systems couple the uphill flow of one ion or molecule to the downhill flow of another. These membrane proteins, called secondary transporters or cotransporters, can be classified as antiporters or symporters. Antiporters couple the downhill flow of one type of ion in one direction to the uphill flow of another in the opposite direction. Symporters move both ions in the same direction.

  Ion channels allow the rapid movement of ions across the hydrophobic barrier of the membrane. In regard to K⁺ channels, hydrated potassium ions must transiently lose their coordinated water molecules as they move to the narrowest part of the channel, termed the selectivity filter.