Question

which choice describes diffusion? 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 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 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 across the cell membrane against its concentration with the help of transport proteins and atp.

Explanation:

Diffusion is the passive movement of a substance from an area of high concentration to an area of low concentration, following its concentration gradient. It does not require energy (ATP) or transport proteins as in active - transport cases.

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.