Chapter 12

Name some of the features common to all membranes.

What conditions are required for a small molecule to spontaneously pass through a membrane?

Match each term with its description. (a) Integral membrane protalin _____________ (b) Peripheral membrane protein_________________ (c) Channel__________________ (d) Passive transport___________________ (e) Active transport_______________________ (f) \(\mathrm{Na}^{+}-\mathrm{K}^{+}\) ATPase_______________ (g) Secondary transporter_________________ (h) Antiporter___________________ (i) Symporter______________________ (j) Ion channel______________________ 1\. Facilitated diffusion 2\. Uses the energy of one gradient to create another 3\. Interacts tightly with the membrane interior 4\. Molecules moving in opposite directions 5\. Interacts with the border of a membrane 6\. Allows rapid movement of molecules down a gradient across a membrane 7\. Movement against a concentration gradient 8\. Molecules moving in the same direction 9\. Can be voltage-gated or ligand-gated 10\. Inhibited by digitalis

Arrange the following substances in order of increasing permeability through a lipid bilayer: (a) glucose; (b) glycerol; (c) \(\mathrm{Cl}^{-} ;\) (d) indole; (e) tryptophan.

Differentiate between simple diffusion and facilitated diffusion.

Differentiate between passive transport and active transport.

Proper membrane fluidity is vital to membrane-protein function. Suggest how a loss of fluidity and how too much fluidity might affect membrane-protein function.

Outline the relation between the \(\mathrm{Na}^{+}-\mathrm{K}^{+}\) ATPase and the strength of a heart contraction. Identify the relevant primary and secondary active-transport components. How do cardiotonic steroids affect the strength of a heartbeat?

Somali hunters use arrows that have been dipped in high concentrations of the cardiac glycoside ouabain to kill game. Indeed, there are reports that animals the size of a hippopotamus can be killed by ouabain-treated arrows. Suggest a biochemical basis for the lethal action of ouabain.

What are two fundamental properties of all ion channels?

Differentiate between ligand-gated and voltage-gated channels.

Is the following statement true or false? Explain. The sodium-glucose linked transporter does not depend on the hydrolysis of ATP.

List two forms of energy that can power active transport.

A stretch of 20 amino acids is sufficient to form an \(\alpha\) helix long enough to span the lipid bilayer of a membrane. How could this piece of information be used to search for membrane proteins in a data bank of primary sequences of proteins?

Lipid bilayers are self-sealing. If a hole is introduced, the hole is filled in immediately. What is the energetic basis of this self-sealing?

Differentiate between peripheral proteins and integral proteins.

The \(\mathrm{K}^{+}\) channel and the \(\mathrm{Na}^{+}\) channel have similar structures and are arranged in the same orientation in the cell membrane. Yet the Na^{+ chan- } nel allows sodium ions to flow into the cell and the \(\mathrm{K}^{+}\) channel allows potassium ions to flow out of the cell. Explain.

Explain why an \(\alpha\) helix is especially suitable for a transmembrane- protein segment.

The neurotransmitter acetylcholine, which activates a ligand-gated ion channel, is rapidly destroyed by the enzyme acetylcholinesterase. This enzyme, which has a turnover number of 25,000 per second,has attained catalytic perfection with a \(k_{\text {cat }} / K_{\mathrm{M}}\) of \(2 \times 10^{8}\) \(\mathrm{M}^{-1} \mathrm{s}^{-1} .\) Why is the efficiency of this enzyme physiologically crucial?

Both aspirin and ibuprofen inhibit prostaglandin \(\mathrm{H}_{2}\) synthase-1 and relieve inflammation. Aspirin functions by blocking a channel in the enzyme, thereby preventing access to the substrate. Ibuprofen does not block this channel but still inhibits the synthase. How might ibuprofen function?

Certain fish living in desert streams alter their membrane-lipid composition in the transition from the heat of the day to the cool of the night. Predict the nature of the changes.

Phospholipids form lipid bilayers in water. What structure might form if phospholipids were placed in an organic solvent?

Cone snails are carnivores that inject a powerful set of toxins into their prey, leading to rapid paralysis. Many of these toxins are found to bind to specific ion-channel proteins. Why are such molecules so toxic? How might such toxins be useful for biochemical studies?

Provide an energetic explanation for how the potassium channel allows passage of the potassium ion but not the smaller sodium ion.