cell membranes let stuff diffuse in and out of the cell. this is because they are partially permable.
1) big molecules like starch and proteins cant fit through the holes in cell membranes, but, very small molecules can diffuse through - glucose, amino acids, water and oxygen.
2) molecules flow from where theres a high concentration to where theres a lower concentration.
3)they actually move both ways but if theres alot more particles on one side of the membrane, theres a net movement from that side.
the larger the surface area of the membrane, the faster the diffusion rate. this is because more particles can pass through at the same time.