What type of transport requires energy to move substances across a cell membrane?

Active transport is the process that requires energy to move substances across a cell membrane against their concentration gradient. This means that substances are transported from areas of lower concentration to areas of higher concentration, which is contrary to the natural flow that would occur through diffusion. In active transport, cells utilize adenosine triphosphate (ATP) as a source of energy to facilitate this movement.

The necessity of energy in active transport is significant because it allows cells to maintain essential concentrations of ions and molecules that are critical for various cellular functions. For example, maintaining high levels of potassium ions inside the cell and low levels of sodium ions is crucial for the generation of electrical signals in nerve and muscle cells.

In contrast, diffusion and facilitated diffusion are types of passive transport that do not require energy. In diffusion, substances move from an area of high concentration to an area of low concentration until equilibrium is reached. Facilitated diffusion involves the use of protein channels but still occurs along the concentration gradient, therefore not requiring any energy input from the cell. Passive transport processes rely on the natural tendency of substances to move towards lower concentrations.