Transport by transporters can be either active or passive, whereas transport by channels

Transport by transporters can be either active or passive, whereas transport by channels is always passive.

Transporters saturate at high concentrations of the transported molecule when all their binding sites are occupied; channels, on the other hand, do not bind the ions they transport and thus the flux of ions through a channel does not saturate.

The membrane potential arises from movements of charge that leave ion concentrations practically unaffected, causing only a very slight discrepancy in the number of positive and negative ions on the two sides of the membrane.

Order Ca2+, CO2, ethanol, glucose, RNA, and H2O according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order.

How is it possible for some molecules to be at equilibrium across a biological membrane and yet not be at the same concentration on both sides?

Ion transporters are linked togethernot physically, but as a consequence of their actions. For example, cells can raise their intracellular pH, when it becomes too acidic, by exchanging external Na+ for internal H+, using a Na+H+ antiporter. The change in internal Na+ is then redressed using the Na+-K+ pump. A. Can these two transporters, operating together, normalize both the H+ and the Na+ concentrations inside the cell? B. Does the linked action of these two pumps cause imbalances in either the K+ concentration or the membrane potential? Why or why not?

Microvilli increase the surface area of intestinal cells, providing more efficient absorption of nutrients. Microvilli are shown in profile and cross section in Figure Q111. From the dimensions given in the figure, estimate the increase in surface area that microvilli provide (for the portion of the plasma membrane in contact with the lumen of the gut) relative to the corresponding surface of a cell with a flat plasma membrane.

According to Newtons laws of motion, an ion exposed to an electric field in a vacuum would experience a constant acceleration from the electric driving force, just as a falling body in a vacuum constantly accelerates due to gravity. In water, however, an ion moves at constant velocity in an electric field. Why do you suppose that is?

In a subset of voltage-gated K+ channels, the N-terminus of each subunit acts like a tethered ball that occludes the cytoplasmic end of the pore soon after it opens, thereby inactivating the channel. This ball-andchain model for the rapid inactivation of voltage-gated K+ channels has been elegantly supported for the shaker K+ channel from Drosophila melanogaster. (The shaker K+ channel in Drosophila is named after a mutant form that causes excitable behavioreven anesthetized flies keep twitching.) Deletion of the N-terminal amino acids from the normal shaker channel gives rise to a channel that opens in response to membrane depolarization, but stays open instead of rapidly closing as the normal channel does. A peptide (MAAVAGLYGLGEDRQHRKKQ) that corresponds to the deleted N-terminus can inactivate the open channel at 100 M. Is the concentration of free peptide (100 M) that is required to inactivate the defective K+ channel anywhere near the local concentration of the tethered ball on a normal channel? Assume that the tethered ball can explore a hemisphere [volume = (2/3)r3] with a radius of 21.4 nm, which is the length of the polypeptide chain (Figure Q112). Calculate the concentration for one ball in this hemisphere. How does that value compare with the concentration of free peptide needed to inactivate the channel?

The giant axon of the squid (Figure Q113) occupies a unique position in the history of our understanding of cell membrane potentials and nerve action. When an electrode is stuck into an intact giant axon, the membrane potential registers 70 mV. When the axon, suspended in a bath of seawater, is stimulated to conduct a nerve impulse, the membrane potential changes transiently from 70 mV to +40 mV