Which example illustrates active transport?

Active transport is characterized by the movement of molecules against their concentration gradient, which requires energy, usually in the form of ATP. The Na+/K+ pump is a prime example of this process. It moves sodium ions out of the cell and potassium ions into the cell against their respective concentration gradients. This mechanism is critical for maintaining cellular function and membrane potential.

In contrast, the movement of glucose into cells may involve facilitated diffusion in some contexts, but it can also utilize active transport in the form of sodium-glucose co-transport, depending on the specific transporter proteins involved. Oxygen diffusion across membranes is a classic example of passive transport, where molecules move from an area of higher concentration to one of lower concentration without energy input. Similarly, the passive movement of water through membranes, such as through aquaporins, also occurs down a concentration gradient without the requirement for energy.

The Na+/K+ pump explicitly highlights the energy-dependent nature of active transport, making it the correct and most illustrative example of this process.