Have you ever forgotten to water a plant for a few days, then come back to find your once-perky arugula a wilted mess? If so, you already know that water balance is very important for plants. When a plant wilts, it does so because water moves out of its cells, causing them to lose the internal pressure—called turgor pressure—that normally supports the plant.
How it works
Why does water move from areas where solutes are less concentrated to areas where they are more concentrated?
In the case of osmosis, you can once again think of molecules—this time, water molecules—in two compartments separated by a membrane. If neither compartment contains any solute, the water molecules will be equally likely to move in either direction between the compartments. But if we add solute to one compartment, it will affect the likelihood of water molecules moving out of that compartment and into the other—specifically, it will reduce this likelihood.
Why should that be? There are some different explanations out there. The one that seems to have the best scientific support involves the solute molecules actually bouncing off the membrane and physically knocking the water molecules backwards and away from it, making them less likely to cross
Regardless of the exact mechanisms involved, the key point is that the more solute water contains, the less apt it will be to move across a membrane into an adjacent compartment. This results in the net flow of water from regions of lower solute concentration to regions of higher solute concentration.
Osmolarity
Osmolarity describes the total concentration of solutes in a solution. A solution with a low osmolarity has fewer solute particles per liter of solution, while a solution with a high osmolarity has more solute particles per liter of solution. When solutions of different osmolarities are separated by a membrane permeable to water, but not to solute, water will move from the side with lower osmolarity to the side with higher osmolarity.
Three terms—hyperosmotic, hypoosmotic, and isoosmotic—are used to describe relative osmolarities between solutions. For example, when comparing two solution that have different osmolarities, the solution with the higher osmolarity is said to be hyperosmotic to the other, and the solution with lower osmolarity is said to be hypoosmotic. If two solutions have the same osmolarity, they are said to be isoosmotic.
Tonicity
In healthcare settings and biology labs, it’s often helpful to think about how solutions will affect water movement into and out of cells. The ability of an extracellular solution to make water move into or out of a cell by osmosis is known as its tonicity. Tonicity is a bit different from osmolarity because it takes into account both relative solute concentrations and the cell membrane’s permeability to those solutes.
Three terms—hyerptonic, hypotonic, and isotonic—are used to describe whether a solution will cause water to move into or out of a cell:
If a cell is placed in a hypertonic solution, there will be a net flow of water out of the cell, and the cell will lose volume. A solution will be hypertonic to a cell if its solute concentration is higher than that inside the cell, and the solutes cannot cross the membrane.
If a cell is placed in a hypotonic solution, there will be a net flow of water into the cell, and the cell will gain volume. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.
If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell, and the cell’s volume will remain stable. If the solute concentration outside the cell is the same as inside the cell, and the solutes cannot cross the membrane, then that solution is isotonic to the cell.
Tonicity in living systems
If a cell is placed in a hypertonic solution, water will leave the cell, and the cell will shrink. In an isotonic environment, there is no net water movement, so there is no change in the size of the cell. When a cell is placed in a hypotonic environment, water will enter the cell, and the cell will swell.
In the case of a red blood cell, isotonic conditions are ideal, and your body has homeostatic (stability-maintaining) systems to ensure these conditions stay constant. If placed in a hypotonic solution, a red blood cell will bloat up and may explode, while in a hypertonic solution, it will shrivel—making the cytoplasm dense and its contents concentrated—and may die.
Maintaining this balance of water and solutes is very important to the health of the plant. If a plant is not watered, the extracellular fluid will become isotonic or hypertonic, causing water to leave the plant's cells. This results in a loss of turgor pressure, which you have likely seen as wilting. Under hypertonic conditions, the cell membrane may actually detach from the wall and constrict the cytoplasm, a state called plasmolysis (left panel below).
