Question

when substances cross cell membranes by simple diffusion, they _____.
a move from an area of low concentration to high concentration
b require energy
c can reach a state of equilibrium
d require the assistance of a transport protein

Explanation:

• Option a: Simple diffusion is the movement of substances from an area of high concentration to low concentration (down the concentration gradient), so a is incorrect.
• Option b: Simple diffusion is a passive process and does not require energy (energy - requiring transport is active transport), so b is incorrect.
• Option c: During simple diffusion, substances move until the concentration is equal on both sides of the membrane, which is a state of equilibrium. This is correct.
• Option d: Simple diffusion does not require transport proteins (facilitated diffusion does), so d is incorrect.

Answer:

c. can reach a state of equilibrium