Which type of transport requires energy to move molecules across a membrane?

Active transport is the process that requires energy to move molecules across a membrane against their concentration gradient. This means that molecules are transported from an area of lower concentration to an area of higher concentration, which is not a spontaneous process and cannot occur without the input of energy. The energy for this transport often comes from ATP.

In contrast, facilitated diffusion, passive diffusion, and osmosis are all forms of passive transport. These processes do not require additional energy, as they rely on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration until equilibrium is reached. Facilitated diffusion involves specific transport proteins, but still occurs along a concentration gradient. Passive diffusion and osmosis describe the movement of small or uncharged molecules and water, respectively, without energy input.

Understanding the differences in these transport mechanisms is crucial, as it illustrates how cells maintain homeostasis and regulate internal environments through energy expenditure in active transport.