Chapter 12 Application

A cell has a high Na+ concentration outside and low Na+ concentration inside. A Na+-glucose symporter imports both substances. Which substance supplies the downhill driving force, and which may move uphill?

Na+ moves downhill into the cell and supplies the driving force; glucose can be transported uphill against its own gradient.

A mutation prevents the Na+/K+ pump from binding ATP. Predict the consequences for ion gradients and neuronal signaling.

The pump cannot maintain normal Na+ and K+ gradients. Over time, membrane potential and action-potential signaling can become abnormal.

A cell's K+ leak channels are closed, and no other ions can cross the membrane. In the simplified slide model, what happens to the membrane potential?

There is no selective K+ movement to establish the modeled charge separation, so the membrane potential remains at zero in that hypothetical setup.

A neuron reaches threshold and opens voltage-gated Na+ channels. What happens next, and why is the response regenerative?

Na+ enters and depolarizes the membrane further, opening more voltage-gated Na+ channels and amplifying the response.

A neuron is stimulated while its voltage-gated Na+ channels are inactivated. Why might it fail to generate another action potential?

Inactivated Na+ channels cannot reopen until they reset after repolarization, creating a refractory period.

A toxin blocks voltage-gated K+ channels. Which phase of the action potential is most directly impaired?

Repolarization may be slowed because K+ efflux through those channels is reduced.

A mutation prevents the acetylcholine receptor from binding acetylcholine. What happens to the normal postsynaptic response at a skeletal muscle?

The transmitter-gated channel does not open normally, reducing the cation current and the usual depolarization.

A cell's Ca2+ pump in the sarcoplasmic reticulum is inhibited. Why might muscle relaxation be impaired?

Ca2+ cannot be returned efficiently to the sarcoplasmic reticulum, so cytosolic Ca2+ remains elevated and contraction-related signaling persists.

A cell's membrane becomes much more permeable to Na+. Why might the resting membrane potential become less negative?

Na+ influx brings positive charge into the cell, tending to depolarize the membrane.

A Na+ channel allows Na+ to pass but cannot distinguish it from K+. Which channel property is defective, and what is the likely consequence?

Selectivity is defective; inappropriate ion movement may disrupt ion gradients and electrical signaling.