What is Osmosis?
Here’s the definition of osmosis that you will see in most textbooks:
In biology, osmosis is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell’s partially permeable membrane.
A partially permeable membrane (sometimes called a selectively permeable membrane) only allows certain molecules or ions to cross it
In the diagram above, the higher concentration of water molecules to the left of the partially permeable membrane makes it likely that a large number of water molecules will collide with the membrane and pass through it.
The lower concentration of water molecules on the right-hand side of the partially permeable membrane in the diagram makes it likely that fewer water molecules will collide with the membrane and pass through it.
This means that more water molecules move from left to right on this diagram than move from right to left, and so the overall movement (net movement) is to the right. It is important, though, to stress to students that water molecules are moving in both directions.
You will often see this described as movement ‘down the concentration gradient’, meaning the water is moving from a higher concentration of water (in this case, the dilute sucrose solution), to a lower concentration of water (the concentrated sucrose solution).
If a plant cell is surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cell, water will enter the cell by osmosis and the plant cell will become turgid (firm).
The pressure that develops inside a plant cell when it becomes turgid is called turgor pressure. Turgid plant cells help a stem to stay upright.
If a plant cell is surrounded by a solution that contains a lower concentration of water molecules than the solution inside the plant cell, water will leave the cell by osmosis and the plant cell will become flaccid (soft).
If the cells in a plant stem become flaccid the turgor pressure inside them will decrease and the stem will wilt.
If a plant cell is surrounded by a solution that contains the same concentration of water molecules as the solution inside the plant cell, there is no overall net flow of water. The movement of water molecules into and out of the cell, through the partially permeable membrane, balances out.
Description of Osmosis Process
Osmosis is the movement of a solvent across a semipermeable membrane toward a higher concentration of solute. In biological systems, the solvent is typically water, but osmosis can occur in other liquids, supercritical liquids, and even gases.
When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to high solute concentration.
For example, if the cell is submerged in saltwater, water molecules move out of the cell. If a cell is submerged in freshwater, water molecules move into the cell.
When the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at exactly the same rate. There is no net flow of water through the membrane.
Osmosis can be demonstrated when potato slices are added to a high salt solution. The water from inside the potato moves out to the solution, causing the potato to shrink and to lose its ‘turgor pressure’. The more concentrated the salt solution, the bigger the loss in size and weight of the potato slice.
Chemical gardens demonstrate the effect of osmosis in inorganic chemistry.
Mechanism of Osmosis in Biology
The mechanism responsible for driving osmosis has commonly been represented in biology and chemistry texts as either the dilution of water by solute or by a solute’s attraction to water. Both of these notions have been conclusively refuted.
The diffusion model of osmosis is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water.
The “bound water” model is refuted by the fact that osmosis is independent of the size of the solute molecules a colligative property or how hydrophilic they are.
It is difficult to describe osmosis without a mechanical or thermodynamic explanation, but essentially there is an interaction between the solute and water that counteracts the pressure that otherwise free solute molecules would exert.
One fact to take note of is that heat from the surroundings is able to be converted into mechanical energy (water rising).
Many thermodynamic explanations go into the concept of chemical potential and how the function of the water on the solution side differs from that of pure water due to the higher pressure and the presence of the solute counteracting such that the chemical potential remains unchanged.
The virial theorem demonstrates that attraction between the molecules (water and solute) reduces the pressure, and thus the pressure exerted by water molecules on each other in solution is less than in pure water, allowing pure water to “force” the solution until the pressure reaches equilibrium.
Role of Osmosis in living things
Osmotic pressure is the main agent of support in many plants. The osmotic entry of water raises the turgor pressure exerted against the cell wall, until it equals the osmotic pressure, creating a steady state.
When a plant cell is placed in a solution that is hypertonic relative to the cytoplasm, water moves out of the cell and the cell shrinks. In doing so, the cell becomes flaccid. In extreme cases, the cell becomes plasmolyzed – the cell membrane disengages with the cell wall due to lack of water pressure on it.
When a plant cell is placed in a solution that is hypotonic relative to the cytoplasm, water moves into the cell and the cell swells to become turgid.
Osmosis is responsible for the ability of plant roots to draw water from the soil. Plants concentrate solutes in their root cells by active transport, and water enters the roots by osmosis. Osmosis is also responsible for controlling the movement of guard cells.
Osmosis also plays a vital role in human cells by facilitating the movement of water across cell membranes. This process is crucial for maintaining proper cell hydration, as cells can be sensitive to dehydration or overhydration.
In human cells, osmosis is essential for maintaining the balance of water and solutes, ensuring optimal cellular function. Imbalances in osmotic pressure can lead to cellular dysfunction, highlighting the importance of osmosis in sustaining the health and integrity of human cells.
In unusual environments, osmosis can be very harmful to organisms. For example, freshwater and saltwater aquarium fish placed in water of a different salinity than that to which they are adapted to will die quickly, and in the case of saltwater fish, dramatically. Another example of a harmful osmotic effect is the use of table salt to kill leeches and slugs.
Suppose an animal or a plant cell is placed in a solution of sugar or salt in water.
- If the medium is hypotonic relative to the cell cytoplasm, the cell will gain water through osmosis.
- If the medium is isotonic, there will be no net movement of water across the cell membrane.
- If the medium is hypertonic relative to the cell cytoplasm, the cell will lose water by osmosis.
This means that if a cell is put in a solution which has a solute concentration higher than its own, it will shrivel, and if it is put in a solution with a lower solute concentration than its own, the cell will swell and may even burst.
Examples of Osmosis
Here are examples of osmosis in animal and plant cells.
Osmosis in animal cells
In biological systems, osmosis is essential since many biological membranes are semipermeable, and it leads to different physiological effects.
For example, when animal cells are exposed to a hypertonic surrounding (or lower water concentration) the water will leave the cells causing the cells to shrink. This condition is referred to as crenation.
Conversely, when the animal cells are placed in a hypotonic surrounding (or higher water concentration), the water molecules will move into the cells causing them to swell. If osmosis continues and becomes excessive the cells will eventually burst.
Osmosis in plant cells
The cell bursting due to too much water influx does not happen in plant cells. Plants can counter excessive osmosis through their cell walls and vacuoles. The cell wall exerts osmotic pressure that stabilizes the plant cell.
Osmotic pressure is what makes plants stay upright. The large vacuole inside the plant cell also helps through osmoregulation, a regulatory process where water potential is regulated so that the osmotic pressure inside the cell is kept within the optimal range.
The plant cells, though, are not protected by water efflux. When a plant cell is placed in a hypertonic surrounding, the cell wall cannot prevent the cell from losing water. This leads to cell shrinking or the cell becoming flaccid.