In passive transport by simple diffusion, how does the magnitude of the concentration gradient across a membrane affect the net rate of diffusion (all else equal)?

• A. The concentration gradient has no effect on net diffusion rate; only temperature matters.
• B. A steeper concentration gradient reverses diffusion so solutes move from low to high concentration.
• C. A steeper concentration gradient decreases the net rate of diffusion.
• D. A steeper concentration gradient increases the net rate of diffusion.

Answer: (D)

Verified step by step guidance

1. Understand that passive transport by simple diffusion involves the movement of solute molecules from an area of higher concentration to an area of lower concentration across a membrane, driven by the concentration gradient.
2. Recall Fick's first law of diffusion, which states that the net rate of diffusion (flux) is proportional to the concentration gradient. The law can be expressed as: \[ J = -D \times \frac{\Delta C}{\Delta x} \] where \(J\) is the diffusion flux, \(D\) is the diffusion coefficient, \(\Delta C\) is the concentration difference across the membrane, and \(\Delta x\) is the thickness of the membrane.
3. Recognize that the negative sign indicates diffusion occurs down the concentration gradient (from high to low concentration), so a steeper concentration gradient (larger \(\Delta C\)) results in a larger magnitude of \(J\), meaning a higher net rate of diffusion.
4. Note that temperature and membrane properties also affect diffusion rate, but if all other factors are held constant, the concentration gradient is the key driving force determining the net diffusion rate.
5. Conclude that increasing the concentration gradient increases the net rate of diffusion, consistent with the correct answer: 'A steeper concentration gradient increases the net rate of diffusion.'