What is the most important factor that determines the rate of diffusion in biological systems?

The factors that determines the rate of diffusion in the biological system include: temperature, size of the particles, diffusion distance and concentration gradients. Of all these factors, the most important  one is the TEMPERATURE.
The relationship between temperature and the rate of diffusion is a direct one, that is, the higher the temperature the higher the rate of diffusion and vice versa. At high temperature the molecules in the particles move faster because they have higher amount of energy.

Carbon monoxide is the answer: )