The question states that the person is malnourished. This means that he/she is not getting enough nutrients and energy to fuel the body, which directly affects the production of ATP. The correct answer will be a protein that requires energy to transport the molecules (active transport).

Out of the three proteins presented in the question, only one uses ATP to transport molecules: the sodium-potassium pump. It requires ATP because the pump transports sodium (Na) and potassium (K) ions against their respective concentration gradients. The leak channels and the voltage-gated channels use facilitated diffusion and the electrochemical gradients of the ions as the driving force for transport.

Eventually, as ion concentrations fluctuate in the individual, all three types of proteins may be affected, but only as an indirect consequence. Malnourishment will directly affect the available ATP, reducing functionality of the sodium-potassium pump.

The sodium-potassium pump moves sodium to the outside of the cell and potassium to the inside of the cell. Since the pump moves the ions in opposite directions, the pump is classified as an antiporter. If the ions moved in the same direction it would be classified as a symporter.

The activity of the sodium-potassium pump creates an electrochemical gradient. There are more sodium ions outside the cell and more potassium ions inside the cell. Recall from diffusion that molecules will always want to go from a region of high concentration to a region of low concentration; therefore, diffusion will drive sodium ions into the cell and potassium ions out of the cell.

However, these ions can only traverse through the cell membrane through specialized channels, called leak channels. The sodium leak channels will facilitate the movement of sodium ions into the cell and potassium leak channel will facilitate the movement of potassium ions out of the cell. The sodium-potassium pump and the leak channels move ions in opposite directions, which is why the pump requires ATP input and the leak channels are examples of passive diffusion.

The question states that the neuron has a resting membrane potential and threshold stimulus of  and , respectively. The researcher must apply at least  of external stimulus to generate an action potential. Remember that each action potential is an all or nothing phenomenon. The neuron has to experience a single stimulus of  or higher to generate an action potential. Even though the researcher applies a cumulative external stimuli of  (sum of voltages from trials 1 through 5) the neuron will not generate an action potential during any single trial.

Depolarization is the first step in an action potential. In this step the voltage-gated sodium channels open due to an external stimulus. These channels permit rapid flow of sodium ions into the cell. This changes the polarity of the cell and causes the membrane potential to increase. The increase in membrane potential is attributed to the increased amounts of positive ions inside the cell. Recall that the membrane potential is defined as the potential inside the cell minus the potential outside the cell. An increase in positive ions inside the cell will increase the potential inside the cell and, subsequently, increase the membrane potential.

Absolute refractory period occurs because of the inactivation of sodium channels. Since the sodium channels are inactivated, the neuron can’t depolarize and initiate another action potential. Relative refractory period occurs due to the slow inactivation of potassium channels. The voltage-gated potassium channels take a longer time to inactivate, which causes the cell to hyperpolarize. The cell becomes more negative than resting membrane potential as positive potassium ions exit the cell down their gradient. Since the cell has a more negative membrane potential, a larger stimulus is required to reach the threshold.

The absolute refractory period results from the initial gating of voltage-gated sodium channels. This initial mechanism ensures that the channel is incapable of opening, even when stimulated. The channel then shifts to a different gating mechanisms, which can be opened by a large enough electrical stimulus.

Absolute refractory period of a neuron is the period of time during which no amount of external stimulus will generate an action potential. Relative refractory period is the period of time during which only a large stimulus will generate an action potential.

In this question the neuron initially generated an action potential when 20mV was applied; however, five seconds after the initial stimulus the neuron only generated the action potential when a large stimulus (60mV) was applied. We can conclude that the neuron was in its relative refractory period. If the neuron was in its absolute refractory period, then a stimulus of 60mV shouldn’t have generated an action potential.

A neuron never becomes temporarily dead. Also, remember that once the neuron is out of its relative refractory period, the required stimulus for action potential will revert back to normal. At a later time (for example ten seconds after the initial stimulus) the neuron will only require 20mV to generate an action potential. Increasing time doesn’t necessarily increase the magnitude of the external stimuli required to produce an action potential.

All three channels provided in the question are open during depolarization. Open voltage-gated sodium channels characterize depolarization. The flow of sodium ions into the cytosol (facilitated by these channels) causes the cell to depolarize.

The sodium and potassium leak channels are also open during depolarization. Sodium leak channels further enhancing the influx of sodium ions, while potassium leak channels allow potassium ions to diffuse out of the cell. It doesn’t matter if the neuron is at the resting membrane potential, depolarizing, repolarizing, or hyperpolarizing; the leak channels are always open.

Remember that the sodium potassium pump is an integral membrane protein that is essential in maintaining the resting membrane potential in neurons. This protein pumps three sodium ions out of the cell and two potassium ions into the cell, creating an electrochemical gradient. The activity of this pump creates a higher concentration of sodium ions outside the cell and a higher concentration of potassium ions inside the cell. This creates a chemical gradient for sodium to flow into the cell and potassium to flow out of the cell. The net transfer of three ions out of the cell and two ions into the cell also generates an electrical gradient, increasing the potential for positive ions to enter the cell. Together, the chemical gradient and electrical gradient are called an electrochemical gradient, and are responsible for the influx of sodium during the depolarization of an axon.

The axon hillock is the region that is the first to respond to changing sodium levels by triggering an action potential, should threshold potential be reached. This is due to the presence of voltage-gated channels that can detect the voltage difference caused by the influx of sodium making the inside of the cell more positive.

Axons are responsible for the propagation of the action potential down to the axon terminal, but do not initiate the signal. The cell body has no voltage-gated channels, and therefore cannot respond to the influx of sodium ions. Myelin sheaths also do not have voltage-gated channels, and only assist in speeding up the conduction of the signal. Axon terminals form the interface used to innervate a muscle, a gland, or another neuron.