What form of transport requires energy input?

• A. Passive transport
• B. Diffusion
• C. Facilitated diffusion
• D. Active transport

Answer: (D)

Active transport requires an energy input because it involves the movement of molecules across a cell membrane against their concentration gradient. This process is essential for maintaining the proper concentrations of ions and nutrients within the cell, which is critical for various cellular functions. Active transport relies on energy, typically in the form of ATP, to function.

In contrast, passive transport, diffusion, and facilitated diffusion do not require energy input as they rely on the natural movement of particles from areas of higher concentration to areas of lower concentration. Passive transport includes simple diffusion, while facilitated diffusion involves specific transport proteins that help move substances across the membrane without expending energy. Active transport is unique in that it actively manipulates the movement of substances to achieve desired concentrations, making energy utilization essential for its function.