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Impermeable

Charged polar molecules

Amino acids, ATP, glucose 6-phosphate, proteins, nucleic acids

Impermeable

centration gradient. As noted in Chapter 2, such transport reactions are spontaneous because they have a positive AS value (increase in entropy) and thus a negative AG (decrease in free energy).

The relative diffusion rate of any substance across a pure phospholipid bilayer is proportional to its concentration gradient across the layer and to its hydrophobicity and size; charged molecules are also affected by any electric potential across the membrane (see below). When a phospholipid bilayer separates two aqueous compartments, membrane permeability can be easily determined by adding a small amount of radioactive material to one compartment and measuring its rate of appearance in the other compartment. The greater the concentration gradient of the substance, the faster its rate of diffusion across a bilayer.

The hydrophobicity of a substance is measured by its partition coefficient K, the equilibrium constant for its partition between oil and water. The higher a substance's partition coefficient, the more lipid-soluble it is. The first and rate-limiting step in transport by passive diffusion is movement of a molecule from the aqueous solution into the hydrophobic interior of the phospholipid bilayer, which resembles oil in its chemical properties. This is the reason that the more hydrophobic a molecule is, the faster it diffuses across a pure phospholipid bilayer. For example, diethylurea, with an ethyl group (CH3CH2—) attached to each nitrogen atom of urea, has a K of 0.01, whereas urea has a K of 0.0002 (see Figure 7-1). Diethy-lurea, which is 50 times (0.01/0.0002) more hydrophobic than urea, will diffuse through phospholipid bilayer membranes about 50 times faster than urea. Diethylurea also enters cells about 50 times faster than urea. Similarly, fatty acids with longer hydrocarbon chains are more hy-drophobic than those with shorter chains and will diffuse more rapidly across a pure phospholipid bilayer at all concentrations.

If a transported substance carries a net charge, its movement is influenced by both its concentration gradient and the membrane potential, the electric potential (voltage) across the membrane. The combination of these two forces, called the electrochemical gradient, determines the energetically favorable direction of transport of a charged molecule across a membrane. The electric potential that exists across most cellular membranes results from a small imbalance in the concentration of positively and negatively charged ions on the two sides of the membrane. We discuss how this ionic imbalance, and resulting potential, arise and are maintained in Sections 7.2 and 7.3.

▲ FIGURE 7-1 Relative permeability of a pure phospholipid bilayer to various molecules. A bilayer is permeable to small hydrophobic molecules and small uncharged polar molecules, slightly permeable to water and urea, and essentially impermeable to ions and to large polar molecules.

Membrane Proteins Mediate Transport of Most Molecules and All Ions Across Biomembranes

As is evident from Figure 7-1, very few molecules and no ions can cross a pure phospholipid bilayer at appreciable rates by passive diffusion. Thus transport of most molecules

ATP-powered pumps

Exterior

Cytosol

Cytosol

Ion channels

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