Write A Short Note on Osmosis in Plants

Osmosis is a type of diffusion of water through a semipermeable membrane. In osmosis, the water molecules move from a higher concentration to a lower concentration across a semipermeable membrane.

The area of higher concentration might have a lower solute concentration, and the area of lower concentration will have a higher solute concentration.

Having osmotically active substances across the membrane can increase the rate of osmosis. Osmosis in plants helps maintain cell turgor and also aids in cell growth and development.

Phenomenon of Osmosis in Plants

Pure water has high water potential. Water will move into a hypertonic solution that has a higher solute concentration. Thus, pure water could be considered a hypotonic solution. The movement of water to the hypertonic solution continues until the concentration of water molecules becomes equal on both sides of the membrane.

When the concentration of the solution on both sides of the membrane is the same, they are called isotonic solutions. Thus, there is no flow between isotonic solutions.

If the solution on the outside has a higher concentration than the other one, it is called a hypertonic solution. The solution that has a lower concentration is called a hypotonic solution.

Let’s see the three scenarios when a hypertonic, isotonic, and a hypotonic cell is placed in solution.

A hypertonic cell or a cell in a hypotonic solution absorbs more water and swells. The absorbed water passes through the plasma membrane and enters the tonoplast or the vacuole. This causes an increase in osmotic pressure (water pressure) inside the tonoplasts. This pressure creates turgor pressure, resulting in the compression of the cytosol against the cell wall. This works as a mechanism to prevent bursting. It causes the plant tissues to stiffen and become turgid. When the osmotic pressure outside and the turgor pressure inside the cells become equal, water stops entering the cell.
A hypotonic cell or a cell in a hypertonic cell has less solute concentration and therefore loses water, causing it to shrink. The shrinkage pulls the plasma membrane away from the cell wall in a phenomenon called plasmolysis. During plasmolysis, some bits of the plasma membrane remain attached to the cell wall at various points.
In an isotonic solution, the solute concentration inside and outside the cell is the same; therefore, there is no movement of water in or out of the cell.

Role of Osmotic Pressure

Osmotic pressure is the pressure required to prevent the net movement of solvent molecules across the semipermeable membrane. It is directly proportional to the solute concentration, which increases and decreases with an increase or decrease in solute concentration. It is this osmotic pressure that helps plants pull water from the soil.

During transpiration, when plants lose water, it creates a higher solute concentration and a high osmotic pressure. This causes the cell to absorb from the nearby cell. This continues as a series until it reaches the root tip. The root tips absorb water via osmosis as the osmotic pressure in the soil is higher than in the water. Thus, transpiration creates an osmotic pressure that enables the root tips to absorb water from the soil.

Reverse Osmosis

By applying pressure, the process of osmosis may be stopped or sometimes reversed. Such reversal is called reverse osmosis. In reverse osmosis, the solvent molecules move from an area of lower concentration to a higher concentration. This process is commercially used to purify water.

In RO systems, there will be membrane filters. Water flows from the lower concentration to a higher concentration. Pure water is collected, and the impure or deionised water comes out of another outlet.

Significance of Osmosis

All living cells are solutions having varying solute concentrations. Their survival and maintenance are through the maintenance of their solute concentration by osmoregulation. Each living organism will have a specific mechanism for this purpose or to conduct osmosis.
Plant cells are mainly hypertonic due to higher solute concentration in their vacuoles. This helps maintain high hydrostatic pressure or turgor pressure. This turgor pressure maintains the shape of the cell and keeps it turgid.
During osmosis, the cell swells up. To maintain its turgidity, it loses solutes in the vacuole. Osmosis in plants is essential for the transport of water and nutrients from the roots to the leaves. It also transports carbohydrates from the leaves to the roots and other parts.
The opening and closing of stomata is also an osmosis-dependent process. Touch-response in plants is another process regulated by osmosis.

References

S. C Bhatla, M. A. Lal, Plant Physiology, Development and Metabolism, https://doi.org/10.1007/978-981-13-2023-1_1
Zheng, S., Li, Y., Shao, Y., Li, L., & Song, F. (2023). Osmotic Pressure and Its Biological Implications. International Journal of Molecular Sciences, 25(6), 3310. https://doi.org/10.3390/ijms25063310