In passive transport by simple diffusion, how does increasing molecular weight generally affect the rate of diffusion across a membrane (all else being equal)?

• A. It has no effect on diffusion rate as long as the concentration gradient is the same.
• B. It decreases the diffusion rate because larger molecules tend to move more slowly.
• C. It reverses the direction of diffusion so molecules move from low to high concentration.
• D. It increases the diffusion rate because larger molecules have greater momentum.

Answer: (B)