Rate of diffusion across the cell membrane depends on which of the following factors?
Size of the substances
Solubility in lipids
The diffusion rate depends on the size of the substances and smaller substances diffuse faster. Diffusion of any substance across the cell membrane also depends upon it's solubility in lipids. This is because the cell membrane is made of lipids. The rate of diffusion of a particular substance also depends on it's concentration gradient. More the difference between the concentration, higher is the rate of diffusion. It is not dependent on the living system itself.