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

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

Answer: (C)

Active transport is the process that requires energy to move substances across a cell membrane. This mechanism is essential for transporting molecules against their concentration gradient, which means moving them from an area of lower concentration to an area of higher concentration. This is contrary to passive processes, such as diffusion and facilitated diffusion, which rely on the natural movement of particles from an area of higher concentration to a lower concentration without the use of cellular energy.

In active transport, energy is typically derived from ATP (adenosine triphosphate), which provides the necessary power for transport proteins, such as pumps, to carry out their functions. This allows cells to maintain concentration gradients of certain ions or molecules that are crucial for cellular processes, including nerve signal transmission and muscle contraction.

Understanding the distinction between active transport and passive transport processes helps clarify why active transport is indispensable for cellular function. For instance, while diffusion, passive transport, and facilitated diffusion operate without energy expenditure, they cannot effectively concentrate substances within a cell when it is necessary for the cell's functionality.