Under which conditions will a substance be likely to enter a cell through diffusion ?

A substance will enter into a cell through diffusion if a concentration gradient is present. The process of diffusion will only occur if the concentration of a substance is higher outside the cell as compared to the inside of the cell. Diffusion is the movement of particles along the concentration gradient i.e from an area of higher concentration to an area of lower concentration.

The movement of molecules against a concentration gradient is termed as active transport.

"a.) well–developed vascular system."