Which statement best defines active transport across a biological membrane?

• A. Movement of a solute down its concentration gradient through a membrane channel without direct energy input.
• B. Nonselective movement of small molecules directly through the lipid bilayer at a rate proportional to membrane surface area.
• C. Movement of a solute across a membrane against its electrochemical gradient, requiring an energy source (e.g., ATP hydrolysis or coupling to another gradient).
• D. Net movement of water across a semipermeable membrane driven solely by differences in solute concentration.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of active transport in cell biology. Active transport refers to the movement of molecules across a biological membrane against their concentration or electrochemical gradient, which requires an input of energy.
2. Step 2: Recognize that moving a solute down its concentration gradient without energy input describes passive transport, not active transport.
3. Step 3: Identify that nonselective movement of small molecules directly through the lipid bilayer is characteristic of simple diffusion, which is also passive and does not require energy.
4. Step 4: Note that net movement of water across a semipermeable membrane due to solute concentration differences is osmosis, a passive process driven by water potential, not active transport.
5. Step 5: Conclude that the correct definition of active transport is the movement of a solute across a membrane against its electrochemical gradient, requiring an energy source such as ATP hydrolysis or coupling to another gradient.