Question

which of the following options describes active transport in a cell?
a protein pump that exports protons and requires an energy input
water moving across a membrane from a high concentration of water to a low concentration of water
a protein channel allowing ions into the cell that does not require energy

Explanation:

1. Recall the definition of active transport: movement of molecules across a cell membrane against their concentration gradient, requiring energy (usually ATP) and often a protein pump.
2. Analyze Option 1: A protein pump exporting protons (against gradient) and requiring energy input matches active transport.
3. Analyze Option 2: Water moving from high to low water concentration is osmosis (passive, no energy), so incorrect.
4. Analyze Option 3: A protein channel (like ion channel) with no energy requirement is facilitated diffusion (passive), so incorrect.

Answer:

a. a protein pump that exports protons and requires an energy input