Question

which choice describes diffusion?
the movement of a substance from a region where it is more concentrated to a region where it is less concentrated, down its concentration gradient.
the movement of a substance against its concentration gradient across a cell membrane from a region where it is less concentrated to a region where it is more concentrated.
the movement of a substance down its concentration gradient from a region where it is less concentrated to an area where it is more concentrated.
the movement of a substance up its concentration gradient using atp and transport proteins across a cell membrane.

Explanation:

To determine the correct description of diffusion, we recall the definition of diffusion: it is the passive movement of a substance from an area of higher concentration to an area of lower concentration, following its concentration gradient (no energy or transport proteins required for simple diffusion).

• The first option states movement from more concentrated to less concentrated down the gradient, which matches diffusion.
• The second option describes movement against the gradient (which is active transport, not diffusion).
• The third option incorrectly says movement from less to more concentrated (opposite of diffusion's direction).
• The fourth option describes active transport (using ATP and transport proteins, moving against the gradient), not diffusion.

Answer:

The movement of a substance from a region where it is more concentrated to a region where it is less concentrated, down its concentration gradient.