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)

Verified step by step guidance

1. Understand that passive transport by simple diffusion depends on the movement of molecules from an area of higher concentration to an area of lower concentration across a membrane without energy input.
2. Recall that the rate of diffusion is influenced by factors such as the concentration gradient, membrane permeability, temperature, and the size (molecular weight) of the diffusing molecules.
3. Recognize that larger molecules (with higher molecular weight) generally diffuse more slowly because their size hinders their ability to pass through the membrane easily.
4. Note that the concentration gradient remains the driving force for diffusion direction, so increasing molecular weight does not reverse diffusion direction.
5. Conclude that increasing molecular weight decreases the diffusion rate because larger molecules have reduced mobility, leading to slower diffusion across the membrane.