Diffusion and Osmosis
Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration.
Cell membranes hold the cell together, but still let substances get in and out of them. Things like amino acids, glucose, water and oxygen. Big Molecules like starch and proteins cannot fit through the membrane.
Osmosis is the movement of water from an area of high water potential to an area of lower water potential down the water potential gradient through a semi permeable membrane.