Which process moves an ion across a cell membrane against the concentration gradient?

• A. Active transport
• B. Osmosis
• C. Facilitated diffusion
• D. Simple diffusion

Answer: (A)

Verified step by step guidance

1. Understand the concept of concentration gradient: A concentration gradient occurs when there is a difference in the concentration of a substance across a space or a membrane.
2. Identify the processes involved: Active transport, osmosis, facilitated diffusion, and simple diffusion are all processes that move substances across cell membranes.
3. Define active transport: Active transport is the movement of ions or molecules across a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient. This process requires energy, usually in the form of ATP.
4. Differentiate between passive and active processes: Osmosis, facilitated diffusion, and simple diffusion are passive processes that do not require energy and move substances down their concentration gradient.
5. Conclude that active transport is the only process that moves ions against the concentration gradient, requiring energy input.