Describe the net movement of water across the plasma membrane, and explain why it occurred.
The net movement of water across plasma membranes is what is refered to as osmosis. It is the movement of water from a region of high water concentration (dilute solution) to a region of lower water concentration (concentrated solution); when the two solutions are separated by the plasma membrane (semi-permeable membrane). A semi-permeable membrane is one that allows certain molecules/ions to pass through it by diffusion and sometimes facilitated diffusion. Water molecules are smaller, and therefore, diffuse down a concentration gradient, across the membrane to the side where there is less water. However, solute particles that are larger and therefore cannot diffuse across the membrane from the opposite side, hence the water continues to diffuse until the concentration gradient becomes zero/ the hydrostatic pressure of water balances the osmotic pressure.