Which of the following is not listed as a factor affecting diffusion rate?

• A. Gravity
• B. Temperature
• C. Surface area
• D. Membrane permeability

Answer: (A)

Diffusion rate is driven by the random motion of particles and the driving force of a concentration gradient, with several key factors shaping how fast it happens. Temperature boosts molecular kinetic energy, so higher temperature makes molecules move faster and diffusion speeds up. Surface area matters because more area gives more space for molecules to cross at once, increasing the overall rate. Membrane permeability determines how easily particles cross the barrier; a more permeable membrane allows quicker diffusion.

Gravity doesn’t drive diffusion at the scale of cells. The random motion of molecules and the concentration difference between sides of a membrane are the main forces, not gravitational effects. So gravity isn’t listed as a factor affecting diffusion rate.