Question

question 11 of 32
which choice describes diffusion?
the movement of a substance across a cell membrane from an area of low concentration to an area of high concentration, up its concentration gradient.
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 across the cell membrane against its concentration with the help of transport proteins and atp.
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.

Explanation:

Diffusion is the passive movement of particles from an area of higher concentration to lower concentration, following the concentration gradient without energy input. This matches the second option, while others describe active transport (requires ATP/against gradient) or incorrect directionality.

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.