Question

how does a substance cross the cell membrane in diffusion?
a flowing down the concentration gradient
b binding to a carrier protein
c going through a pump
d going through a channel protein

Explanation:

Diffusion is the passive movement of substances from an area of high - concentration to an area of low - concentration (down the concentration gradient). Binding to a carrier protein is a feature of facilitated diffusion or active transport. Going through a pump is active transport. Going through a channel protein is also a form of facilitated diffusion. The basic principle of simple diffusion is flowing down the concentration gradient.

Answer:

A. flowing down the concentration gradient