Diffusion along concentration gradients

Steps:

• Compare concentrations: molecule 1 higher in P than Q, so net diffusion from P to Q.
• For molecule 2, concentrations equal in P and Q, resulting in no net movement.
• For molecule 3, higher concentration in Q than P, so net diffusion from Q to P.
• Initial movement follows these gradients until equilibrium.

Why B is correct:

• Matches diffusion principle: molecules move from high to low concentration, with no net flow at equilibrium (Fick's first law).

Why the others are wrong:

• A: Incorrectly assigns no net movement to molecule 3, which has a gradient.
• C: Swaps movements for molecules 1 and 2, and repeats error on molecule 2.
• D: Misassigns molecule 1's no net movement and swaps directions for 1 and 3.

Final answer: B