Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.
Semi-permeable membranes are very thin layers of material that allow some things to pass through them but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide, and oxygen to pass through but do not allow larger molecules like glucose, sucrose, proteins, and starch to enter the cell directly.
The classic example used to demonstrate osmosis and osmotic pressure is to immerse cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution. Figure 126.96.36.199.4 shows what happens in osmosis through the semi-permeable membrane of the cells.
- The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). The net flow or water will be out of the cell.
- The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation, the cell is in an isotonic solution (iso = equal or the same as normal). The amount of water entering the cell is the same as the amount leaving the cell.
- The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). The net flow of water will be into the cell.
Figure 188.8.131.52.5 demonstrates the specific outcomes of osmosis in red blood cells.
- Hypertonic solution. The red blood cell will appear to shrink as the water flows out of the cell and into the surrounding environment.
- Isotonic solution. The red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell.
- Hypotonic solution. The red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell.