Diffusion can occur across a cell surface membrane. The image above shows the progression of glucose diffusion; on the left of the diagram there is a steep concentration gradient, and by the end of the process on the right of the diagram the glucose concentration has reached equilibrium.
Factors Affecting Diffusion Table
Channel proteins are pores in the cell membrane; they can open and close.
Carrier proteins change shape to transport molecules from one side of a membrane to the other
Remember that the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein, for example because the molecule is charged and cannot pass directly through the phospholipid bilayer, this is facilitated diffusion.