Which example illustrates active transport across a cell membrane?

Active transport across a cell membrane is the process by which cells move substances against their concentration gradient, from an area of low concentration to an area of high concentration, using energy in the form of ATP. A classic example of active transport is the sodium-potassium pump.

Here's how the sodium-potassium pump operates:

1. The pump binds three sodium ions (Na+) from inside the cell.

2. ATP (adenosine triphosphate) provides energy by transferring a phosphate to the pump, a process called phosphorylation.

3. Phosphorylation changes the shape of the pump, releasing the sodium ions outside the cell.

4. The shape change also allows the pump to bind two potassium ions (K+) from outside the cell.

5. The phosphate group is then released, which causes the pump to revert to its original shape.

6. This shape change releases the potassium ions inside the cell while preparing the pump for another round of sodium ion transport.

This is a clear example of active transport because it requires energy to move ions against their concentration gradient, meaning in a direction that would not naturally occur due to diffusion.