When an ion crosses the membrane and binds to a protein, which transport method is used?

The correct answer encompasses both facilitated diffusion and active transport because both methods involve proteins aiding in the transport of ions across the membrane.

When an ion crosses the membrane, it cannot typically pass directly through the lipid bilayer due to its charge and polarity. Instead, it requires the aid of specific membrane proteins. Facilitated diffusion involves the use of channel proteins or carrier proteins that allow ions to move down their concentration gradient without the expenditure of energy. This is a vital process for ions such as sodium or potassium, which need to balance concentrations across membranes for cellular function.

On the other hand, active transport involves proteins that move ions against their concentration gradient, which requires energy, usually derived from ATP. This mechanism is crucial for maintaining specific ion concentrations within cells, such as the sodium-potassium pump, which actively transports sodium out of the cell and potassium into the cell.

Thus, since both facilitated diffusion (which does not require energy) and active transport (which requires energy) involve the binding of ions to transport proteins, the most inclusive and comprehensive answer is that either of these transport methods could be utilized when an ion binds to a protein during membrane crossing.