**Question:** Which of the following does not affect rate of diffusion?

A. Temperature (C)
B. Pressure (mmHg)
C. Molecular weight
D. Surface area to volume ratio

**Core Concept:** Diffusion is the passive movement of molecules across a selectively permeable membrane from an area of higher concentration to an area of lower concentration. Rate of diffusion is influenced by factors like temperature, pressure, and surface area to volume ratio. Molecular weight is directly related to the size of molecules, but it does not directly affect diffusion rate.

**Why the Correct Answer is Right:** Diffusion is a passive transport process, which means its rate is not directly influenced by molecular weight. The rate of diffusion depends on factors like surface area to volume ratio, pressure, and temperature. Higher surface area to volume ratio allows for more frequent collisions between molecules and membrane, thus increasing diffusion rate. Increasing pressure and temperature also increase the kinetic energy of molecules, causing them to collide more frequently and diffuse faster. Molecular weight is related to the size of molecules, but it does not directly affect the rate of diffusion because diffusion is a passive process.

**Why Each Wrong Option is Incorrect:**

A. **Temperature (C):** Increasing temperature increases the kinetic energy of molecules, causing more frequent collisions between molecules and the membrane. This results in an increased rate of diffusion, so option C (temperature) is incorrect.

B. **Pressure (mmHg):** Higher pressure increases the concentration gradient, causing more molecules to move across the membrane to balance the concentration gradient. This results in an increased rate of diffusion, so option B (pressure) is incorrect.

C. **Molecular weight:** As mentioned earlier, molecular weight is related to the size of molecules, but it does not directly affect the rate of diffusion because diffusion is a passive transport process. Therefore, option C (molecular weight) is incorrect.

**Core Concept:** The rate of diffusion is influenced by factors like surface area to volume ratio, pressure, and temperature. These options increase the rate of diffusion by affecting the frequency and energy of collisions between molecules and the membrane. Molecular weight, being related to molecule size, is not directly involved in controlling the rate of diffusion.