Chapter 5 Summary

Summary

Passive Solute Transport by Simple Diffusion

A mechanism of solute transport is passive if it transports only toward electrochemical equilibrium. For an uncharged solute, the electrochemical equilibrium is the same as concentration equilibrium. For a charged solute, the electrochemical equilibrium is achieved when the voltage difference exactly counterbalances the solute’s concentration gradient across a cell membrane or epithelium.
Simple diffusion is the most straightforward type of passive solute transport. The fundamental mechanism of simple diffusion is exemplified most clearly by the diffusion of an uncharged solute in an aqueous solution. Molecular agitation tends by simple statistics to carry more molecules of such a solute out of regions of high concentration than into such regions, thereby tending to produce equality of concentration everywhere. If a solute is charged and diffusing where a voltage difference exists, forces of electrical attraction and repulsion affect the paths followed by molecules or ions during molecular agitation and thus contribute to diffusion.
Voltage differences can occur across cell membranes or epithelia because the lipid layers in cell membranes act as capacitors, permitting positive and negative charges to be unequally distributed on the two sides. Bulk solutions are electrically neutral, however, and voltage differences are therefore not a factor in simple diffusion within bulk solutions.
When lipid solutes such as steroid hormones and fatty acids cross cell membranes by simple diffusion, they typically dissolve—because of their hydrophobic nature—in the lipid layers of the membranes. Ions, however, cannot cross membranes by this mechanism because of their poor solubility in lipids; thus their diffusion requires mediation by membrane proteins. Inorganic ions typically cross membranes by simple diffusion through ion channels. Channels are often gated—that is, opened and closed by factors such as ligand binding.
The permeability of a membrane to a lipid solute depends on how readily the solute dissolves in and moves through the membrane lipid layer. The permeability of a membrane to an inorganic ion depends on the number of channels for the ion per unit of membrane area, and can be changed by the opening and closing of the channels.

Active Transport

Active-transport mechanisms—also called uphill-transport mechanisms or pumps—are able to convert energy obtained from the catabolism of foodstuffs into solute motive energy and thus can transport solutes away from electrochemical equilibrium. Such mechanisms are known for many solutes but not for O₂ or H₂O.
Solutes must bind noncovalently to a transporter protein for active transport to occur. Thus active transport is a type of carrier-mediated transport. A second type is facilitated diffusion, which differs in that it cannot tap metabolic energy and is therefore strictly toward equilibrium.
Active transport is primary if the transporter protein is an ATPase and thus draws energy directly from ATP bonds. One of the best-known transporters of this sort is Na⁺–K⁺-ATPase (a P-type ATPase), which is found in all animal cells, including the basolateral membranes of all epithelial cells. Primary-active-transport mechanisms pump ions. Active transport is secondary if the immediate source of energy for transport is a solute electrochemical gradient, rather than ATP. Secondary-active-transport mechanisms depend on transporter proteins—cotransporters or countertransporters—that obligatorily transport two solutes simultaneously. Secondary-active-transport mechanisms pump organic solutes and ions.
Active transport of ions is electrogenic if it produces a voltage difference but electroneutral if it does not.

Osmotic Pressure and Other Colligative Properties of Aqueous Solutions

Colligative properties depend on the number of dissolved entities per unit of volume of solution rather than the chemical nature of the dissolved entities. The three colligative properties of greatest importance in biology are the osmotic pressure, freezing point, and water vapor pressure. Because the three are quantitatively related, any one can be calculated from the others.
Osmotic pressures are generally expressed in osmolar units by biologists today. A 1-osmolar (Osm) solution behaves osmotically as if it has 1 Avogadro’s number (6 • 10²³) of independent dissolved entities per liter.
Physical chemists measure osmotic pressures by using hydrostatic pressure, explaining why osmotic pressures are called pressures.

Osmosis

Osmosis is the passive transport of water across a membrane, such as a cell membrane or an epithelium. Osmosis always occurs from the solution of lower osmotic pressure (more abundant water) to the solution of higher osmotic pressure (less abundant water). Osmosis is always toward equilibrium, tending to bring the solutions on the two sides of a membrane to equal osmotic pressure.
Two solutions are isosmotic if they have the same osmotic pressure. If two solutions have different osmotic pressures, they are described as being hyposmotic and hyperosmotic to each other, the one with the lower osmotic pressure being the hyposmotic one.
A single solution exerts no increase in hydrostatic pressure because of its osmotic pressure. However, hydrostatic pressures can be generated by osmosis between two solutions interacting across a membrane.
In some cell membranes, the only mechanism by which osmosis occurs is that H₂O molecules dissolve in the membrane and move through by molecular agitation (simple diffusion). Other cell membranes, such as the membranes of red blood cells and certain kidney tubules, however, have aquaporins, providing a second path for osmosis to occur and increasing the rate of osmosis 5- to 50-fold in comparison with membranes without aquaporins. Aquaporins permit highly specific, channel-mediated H₂O transport.