Which process requires energy to move substances across the cell membrane?

Active transport is the process that requires energy to move substances across the cell membrane. This is because active transport is against the concentration gradient, meaning that it moves substances from an area of lower concentration to an area of higher concentration. The energy used in active transport typically comes from ATP, which is the energy currency of the cell.

This process is essential for maintaining cellular homeostasis, allowing cells to accumulate necessary ions and molecules that are present at lower concentrations outside the cell. For instance, the sodium-potassium pump, a well-known example of active transport, pumps sodium ions out of the cell while bringing potassium ions in, which is vital for numerous cellular functions including nerve impulse transmission and muscle contraction.

In contrast, facilitated diffusion, passive transport, and simple diffusion do not require energy. Facilitated diffusion and simple diffusion occur down the concentration gradient, where substances move from areas of higher concentration to areas of lower concentration, highlighting the energy-free nature of these processes. Passive transport is a broader term that encompasses both facilitated diffusion and simple diffusion, reinforcing that none of these processes require cellular energy.