1Why energy, and why a protein

Ions cannot cross the hydrophobic core (2.4), so any ion transport needs a protein. In facilitated diffusion the protein is only a route, and the gradient drives the flow. In active transport the protein must also supply the push, because the flow runs uphill.

The energy comes from ATP. A pump binds ATP and splits off one phosphate group; the phosphate attaches to the pump, and this phosphorylation changes the pump's shape. The change in shape is what carries the ion across and releases it on the other side. When the phosphate later comes off, the pump returns to its first shape, ready to go again.

So active transport has two requirements: a membrane protein, and metabolic energy. Remove either and it stops. Pumps are specific, like channels: each carries particular ions or molecules.

Active transport needs two things: a specific membrane protein, and energy, usually from ATP.

2Electrochemical gradients

An ion feels two pushes across a membrane.

• Its concentration gradient: it tends to move from where it is more concentrated to where it is less.
• The electrical gradient: a positive ion is attracted toward the negative side, and a negative ion toward the positive side.

The two together make the electrochemical gradient. Cells spend metabolic energy to establish these gradients and, because ions keep leaking back through channels, to maintain them. A large part of a resting animal cell's ATP goes on this work.

A typical animal cell, at rest:

The inside is also negative compared with the outside. So Na⁺ is pulled inward by both its concentration gradient and the charge; K⁺ is pushed outward by its concentration gradient.

3The Na⁺/K⁺ ATPase

The Na⁺/K⁺ ATPase is a pump in the plasma membrane of animal cells. Its name says what it does: it moves Na⁺ and K⁺, and it is an enzyme that breaks down ATP (an ATP-ase). Each cycle, shown as four steps in Figure 1:

1. The pump is open to the cytoplasm. Three Na⁺ bind inside it.
2. ATP transfers a phosphate to the pump. The pump changes shape and opens to the outside.
3. The three Na⁺ are released outside. Two K⁺ from outside bind.
4. The phosphate leaves. The pump returns to its first shape, opens to the inside, and releases the two K⁺ into the cytoplasm.

Each step in Figure 1 moves an ion against its gradient: Na⁺ out to where Na⁺ is already concentrated, K⁺ in to where K⁺ is already concentrated. That is why ATP is spent.

4How the pump keeps the membrane potential

The membrane potential is the difference in electrical charge across the membrane. In a resting animal cell the inside is negative relative to the outside.

The pump contributes in two ways.

• It builds the gradients. By keeping K⁺ high inside, it lets K⁺ leak out through K⁺ channels (2.6). Each K⁺ that leaves takes a positive charge out, making the inside more negative. This leak is the larger part of the resting potential, and it depends on the gradient the pump maintains.
• It moves charge directly. Each cycle sends out three positive charges and brings in two, a net loss of one positive charge from inside, which adds a little more negativity.

If the pump stops, nothing happens at once, because the gradients are still there. But Na⁺ keeps leaking in and K⁺ keeps leaking out, the gradients run down, and over minutes the membrane potential gets smaller and smaller. Nerve and muscle cells, whose signals depend on these gradients, stop working.

5Gradients as stored energy

Once a gradient exists, letting ions flow back down it releases energy, much as water flowing downhill can turn a wheel. Cells use this.

• Nerve cells open Na⁺ channels; Na⁺ rushes in down its electrochemical gradient, which is the start of a nerve impulse (Unit 4).
• Some transport proteins let an ion flow down its gradient and use that flow to carry a second substance up its own gradient. The energy still comes from ATP, but indirectly, through the gradient the pump built.

6All the mechanisms side by side