Osmosis is the diffusion of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane. A dilute solution contains a high concentration of water molecules and is said to have a high water potential, while a concentrated solution contains a low concentration of water molecules and so has a low water potential. Osmosis refers to the movement of water molecules only.
The slideshow shows an example of osmosis showing the direction of movement of water between two different concentrations of sugar solutions:
Osmosis is how plants are able to absorb water from soil. The roots of the plant have a higher solute concentration than the surrounding soil, so water flows into the roots. In plants, guard cells are also affected by osmosis. These are cells on the underside of leaves that open and close to allow gas exchange. When the plant’s cells are full of water, the guard cells swell and open the stomata, small holes that allow the plant to take in carbon dioxide and release oxygen.
Osmosis can have adverse effects on animals such as fish. If freshwater or saltwater fish are put into water that has a different salt concentration than they are used to, they will die from having too much water enter or leave their cells. Osmosis can affect humans as well; in a person infected with cholera, bacteria overpopulate the intestines, leaving the intestines unable to absorb water. The bacteria actually reverse the flow of absorption because osmosis causes water to flow out of the intestinal cells instead of in. This causes severe dehydration and sometimes death.
- Diffusion – a process by which molecules move from areas of high concentration to areas of low concentration. Osmosis is one type of diffusion.
- Solution – a mixture made up of two or more substances where one substance, a solute, is dissolved into another substance, a solvent.
- Semipermeable – also known as selectively permeable, this means that only certain substances can pass through a barrier. Cell membranes are semipermeable.
- Cell – the smallest unit that makes up a living organism. It includes various different parts called organelles that have functions such as storing genetic material and making proteins and energy.
1. When a cell contains a lower concentration of solute than the solvent surrounding it, that cell is said to be in what kind of solution?
A. Hypertonic
B. Hypotonic
C. Isotonic
A is correct. When a solution has a higher concentration of solute than a cell that it surrounds, the solution is hypertonic. When a cell is in a hypertonic solution, osmosis will cause water to flow out of the cell to balance the concentration of solute on either side of the semipermeable membrane. As too much water flows out of the cell, it will shrivel.
2. Isotonic conditions are ideal for which cells?
A. Plant cells
B. Animal cells
C. Both
D. Neither
B is correct. In isotonic solutions, the net movement of water into and out of the cell is the same, which keeps an animal cell balanced and functioning normally. Plant cells fare better in hypotonic solutions where they can be filled to the max with water.
3. What happens to an animal cell in a hypotonic solution?
A. There is no negligible effect, as the concentration of solute on both sides of the membrane is the same.
B. Water will rush out of the cell, making it shrivel.
C. Water will rush into the cell, and it will become turgid.
D. Water will rush into the cell, causing it to lyse (burst).
D is correct. When a cell is in a hypotonic solution, the concentration of solute in the cell is higher than the concentration of solute in the water surrounding it. Water will rush into the cell and can cause it to burst. A describes an isotonic solution, B describes a hypertonic solution, and C describes a plant cell in a hypotonic solution.