Osmosis is the movement of water across a membrane which is selectively permeable. In osmosis, water moves across a membrane from a region with low solute concentration to a region with high solute concentration. Thus, osmosis tends to equalize the solute concentrations on opposite sides of a membrane.
In living cells, water moves by osmosis across membranes between cells or between membrane-enclosed compartments within an individual cell. All biological membranes are considered selectively permeable since they are highly permeable to water but much less permeable to other substances, such as ions, proteins, and other solutes dissolved in the cell. Osmosis is a passive process, in that it requires no input of energy.
Osmotic pressure is the pressure exerted by dissolved solutes in a solution of water. The stronger the concentration of the dissolved solutes, the greater the movement of water up the concentration gradient, and the greater the osmotic pressure. The significance of osmosis in biology is illustrated by two examples below.
Mammalian red blood cells have a biconcave (doughnut-like) shape. If red blood cells are placed in a 0.3 M NaCl solution, which is the typical concentration of NaCl in a cell, there is little net osmotic movement of water, the size and shape of the cells stay the same. This is because the NaCl concentration on both sides of the membrane is the same; this is described as being isotonic. If red blood cells are placed in a solution with a lower NaCl concentration than is found in the cells (the solution is hypotonic), water moves into the cells by osmosis to try to dilute the NaCl level inside the cell. The cell will swell and can burst. Conversely, if the red blood cells are placed in a solution with a higher solute concentration (a hypertonic solution), water moves out of the cell by osmosis to try to dilute the NaCl outside the cell. The cell becomes smaller and prune-like in shape.
These observations have several important practical implications. First, hospitals must store red blood cells in a plasma solution which has the correct proportions of salts and proteins. The plasma solution is made to be slightly hypertonic to the red cells so that the integrity of the cells is preserved and hemolysis is prevented. Second, when doctors inject a drug intravenously into a patient, the drug is suspended in a saline solution which is slightly hypertonic to red blood cells. Intravenous injection of a drug in pure water will cause some of the patient’s red blood cells to hemolyze because water is hypotonic to the red blood cells.
Plant cells are surrounded by rigid cellulose walls, (unlike animal cells), but plant cells still take in water by osmosis when placed in pure water. However, plant cells do not burst because their cellulose cell walls limit how much water can move in. The cell walls exert pressure, called turgor pressure, as the cells take up water. Turgor pressure is analogous to the air pressure of an inflated tire.
The significance of osmosis to plant function is best appreciated by describing its role in the regulation of guard cells. Guard cells are specialized cells scattered across the surface of plant leaves. Each pair of guard cells surround a special pore, termed a stoma (plural stomata), and control its opening. Guard cells have a special arrangement of microfibrils in their walls, so that when the guard cells swell the stomata open. When the stomata of a plant leaf are open, this increases photosynthetic gas exchange and movement of water out of the plant by transpiration.
In many plants, certain environmental stimuli, such as sunlight, stimulate the guard cells to take up potassium from surrounding cells. This causes their osmotic potential (π) to decrease and water moves in by osmosis. Thus, the guard cells swell, the stomata open, and the rate of gas exchange through the stomata increases. This increases the rate of photosynthesis and plant growth.
Other environmental cues, such as water shortage, cause plants to transmit chemical signals to the guard cells, causing them to release potassium, which increases their osmotic potential, and to lose water by osmosis. This causes the guard cells to shrink, so closing the stomata, and decreasing the rate of water transpiration through the stomata. This reduces water loss and prevents wilting of the plant.
Plants rely upon other environmental signals to regulate the osmotic movement of water into their guard cells and the opening of the stomata, so that the advantage of increased photosynthesis is balanced against the disadvantage of increased water loss.
Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Dennis Bray, Karen Hopkin, Keith Roberts, and Peter Walter. Essential Cell Biology, Second Edition. New York: Garland Science/Taylor & Francis Group, 2003.
Hypertonic —Having a higher osmotic pressure than another fluid.
Hypotonic —A solution with a lower osmotic pressure than another fluid.
Isotonic —Having equal osmotic pressure.
Osmotic potential —Measure of the tendency of water to move into a region due to the presence of solutes.
Pressure potential —Measure of the physical pressure exerted by cell walls upon a cell.
Water potential —Measure of the overall tendency of water to move into a region.
Nelson, David L. and Michael M. Cox. Lehninger Principles of Biochemistry, Fourth Edition. New York: W.H. Freeman, 2004.
Voet, Donald and Judith G. Voet. Biochemistry. New York: John Wiley & Sons, 2006.
Peter A. Ensminger