Introduction: Osmosis and diffusion are two processes that occur constantly among cells. When the concentration of a solution differs inside and outside of a cell the solution will diffuse through until it reaches equilibrium. Osmosis occurs in the cell’s permeable membrane. In this lab, dialysis tubing is used to represent the selectively permeable membrane in a cell. Dialysis tubing is made from cellulose or cellophane to allow water or smaller molecules diffuse through it. In selectively permeable membranes, solutions travel across their concentration gradients by osmosis. During osmosis, there are three diverse types of solutions that move across the gradient in different ways. In a hypertonic solution there is a higher concentration of the solute than in the other solution. Therefore, when the cell is put into a hypertonic solution, the water diffuses out of the cell which causes the cell to shrivel. In a hypotonic solution, the solution has a low concentration of the solute, therefore when a cell is added to this solution water diffuses into the cell until it eventually lyses because it is over filled with water. When a solution is said to isotonic it means that there is the same concentration between it and the cell. Here water flows through without changing the mass of the cell. It is vital that osmosis can occur in these situations because without it cells would die because of differing pressure from outside and inside.
The measurement that predicts the movement of the water is ψ, or psi. This is the measure of the water potential inside an individual cell. Ψ can be either positive or negative depending on the solution the cell is in. Ψ is more negative when the solution is hypertonic, so the water is moving out of the cell more quickly than it is coming in. When Ψ is more positive the solution is hypotonic and therefore the water is moving into the cell eventually causing it to explode from expansion.
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