Opening and closing ion channels alters the membrane potential

Opening and closing ion channels alters the membrane potential

In a neuron, the resting membrane potential is closer to the potassium equilibrium potential than it is to the sodium equilibrium potential. That's because the resting membrane is much more permeable to $\text K^+$start text, K, end text, start superscript, plus, end superscript than to $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript.

• If more potassium channels were to open up—making it even easier for $\text{K}^+$start text, K, end text, start superscript, plus, end superscript to cross the cell membrane—the membrane would hyperpolarize, getting even closer to the potassium equilibrium potential.
• If, on the other hand, additional sodium channels were to open up—making it easier for $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript to cross the membrane—the cell membrane would depolarize toward the sodium equilibrium potential.

Changing the number of open ion channels provides a way to control the cell’s membrane potential and a great way to produce electrical signals. (We will see the opening and closing of channels again when we discuss action potentials.)

The $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript-$\text K^+$start text, K, end text, start superscript, plus, end superscriptpump maintains 

$\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text{K}^+$start text, K, end text, start superscript, plus, end superscript gradients

The $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text{K}^+$start text, K, end text, start superscript, plus, end superscript concentration gradients across the membrane of the cell (and thus, the resting membrane potential) are maintained by the activity of a protein called the $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript-$\text K^+$start text, K, end text, start superscript, plus, end superscript ATPase, often referred to as the sodium-potassium pump. If the $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript-$\text K^+$start text, K, end text, start superscript, plus, end superscriptpump is shut down, the $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript concentration gradients will dissipate, and so will the membrane potential.

[Why is a pump needed to maintain the concentration gradients?]

$\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript$\text K^+$start text, K, end text, start superscript, plus, end superscript$\text K^+$start text, K, end text, start superscript, plus, end superscript$\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript$\text K^+$start text, K, end text, start superscript, plus, end superscript$\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript

$\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript$\text K^+$start text, K, end text, start superscript, plus, end superscript

Like the ion channels that allow $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript to cross the cell membrane, the $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript-$\text K^+$start text, K, end text, start superscript, plus, end superscript pump is a membrane-spanning protein. Unlike potassium channels and sodium channels, however, the $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript-$\text K^+$start text, K, end text, start superscript, plus, end superscript pump doesn’t just give $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript a way to move down their electrochemical gradients. Instead, it actively transports $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text{K}^+$start text, K, end text, start superscript, plus, end superscript against their electrochemical gradients.

The energy for this "uphill" movement comes from ATP hydrolysis (the splitting of ATP into ADP and inorganic phosphate). For every molecule of ATP that's broke down, $3$3 $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript ions are moved from the inside to the outside of the cell, and $2$2 $\text K^+$start text, K, end text, start superscript, plus, end superscript ions are moved from the outside to the inside.

1. Three sodium ions bind to the sodium-potassium pump, which is open to the interior of the cell.
2. The pump hydrolyzes ATP, phosphorylating itself (attaching a phosphate group to itself) and releasing ATP. This phosphorylation event causes a shape change in the pump, in which it closes off on the inside of the cell and opens up to the exterior of the cell. The three sodium ions are released, and two potassium ions bind to the interior of the pump.
3. The binding of the potassium ions triggers another shape change in the pump, which loses its phosphate group and returns to its inward-facing shape. The potassium ions are released into the interior of the cell, and the pump cycle can begin again.

_Image modified from "The sodium-potassium exchange pump," by Blausen staff

[

1. To begin, the pump is open to the inside of the cell. In this form, the pump binds readily to $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript ions (has a high affinity for them) and will take up three of them.
2. When the $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
3. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer binds readily to $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript ions (has a low affinity for them), so the three $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript ions are released outside the cell.
4. In its outward-facing form, the pump switches allegiances and now readily binds to (has a high affinity for) $\text K^+$start text, K, end text, start superscript, plus, end superscript ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
5. With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
6. In its inward-facing shape, the pump no longer readily binds to $\text {K}^+$start text, K, end text, start superscript, plus, end superscript ions, so the two $\text {K}^+$start text, K, end text, start superscript, plus, end superscript ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.

1. $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript
2. $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript
3. $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript$\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript
4. $\text K^+$start text, K, end text, start superscript, plus, end superscript
6. $\text {K}^+$start text, K, end text, start superscript, plus, end superscript$\text {K}^+$start text, K, end text, start superscript, plus, end superscript

Because $3$3 $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript are exported for every $2$2 $\text K^+$start text, K, end text, start superscript, plus, end superscript brought into the cell, the pump makes a small direct contribution to the resting membrane potential (making it slightly more negative than it would otherwise be). The pump's big contribution to the membrane potential, however, is indirect: It maintains steady $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript gradients, which give rise to the membrane potential as $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript move down their respective concentration gradients through leak channels.