Osmosis in Living Systems

Osmosis is physical process during which the solvent moves from a region of higher solvent concentration to a region of lower solvent concentration across a semi-permeable or a selectively permeable membrane. This process does not require the input of energy.

When two solutions having a difference in their concentrations are separated by a semi-permeable membrane, the net movement of the solvent takes place from a hypotonic solution (less-concentrated due to lesser amount of salt) to a hypertonic solution (more concentrated due to higher amount of salt). This process of osmosis tries to reduce the concentration gradient between the two solutions.

The pressure that is required to maintain a state of dynamic equilibrium between the two solutions is known as osmotic pressure . At this stage of dynamic equilibrium there is no net movement of solvent across the membrane.

Effect of Osmosis in Plant Cell

•    Hypotonic condition – When a plant cell is placed in a solution containing lower concentration of solute and a higher concentration of water, the cell swells up due to the movement of water into the cell. The cell becomes turgid and the rigid cell wall tries to hold this excess amount of water.
•    Isotonic condition – When the plant cell is placed in a solution of similar concentration, water moves across the cell membrane in both the directions. The net movement of water is zero with no change in the cell size. The cell at this stage is said to be in flaccid condition.
•    Hypertonic solution – When the plant cell is placed in a solution containing higher concentration of solute and a lower concentration of water, the cell shrinks as greater amount of water leaves the cell. The vacuole shrinks and the cytoplasm gets peeled off from the cell wall. The cell is said to be plasmolysed.

Effect of Osmosis in an Animal Cell

Animal cells do not have a rigid cell wall to support the cell membrane. When these cells are placed in a hypotonic solution, the cells absorb water, get swollen and burst. They do not have a capacity to hold water. On the other hand when they are placed in a hypertonic solution the animal cells loose water and shrink. It is very important in animals that the extracellular fluid is maintained at the same concentration as the cell cytoplasm. This balance in concentration is maintained by kidneys which regulate water and salt concentration of the body.

Applications of Osmosis in a Living System

•    Many biological membranes are selectively permeable in nature, that is, they allow only water and small uncharged solute molecules to pass through, whereas they inhibit larger molecules like polysaccharides, proteins etc., from passing across the membrane.
•    A cell adjusts in various salt concentrations due to the semi-permeability of the cell membrane. Osmosis is a most important mechanism by which water gets transported from external environment to internal environment of the cell and vice-versa.
•    In animals, the distribution of nutrients within the body and release of metabolic waste from the body takes place by the process of osmosis. The kidneys are the vital organs of the body which maintain the fluid balance within the organism. Kidneys help in the maintenance of the correct concentration of the plasma within the body.
•    Many water animals adjust to the changes in the water concentrations by the process of osmosis.
•    Our entire ecosystem depends on plants and they absorb water from soil by the process of osmosis. Cells of the root hairs have a concentrated cell sap where as water in soil is a weaker solution. This promotes the absorption of water by the cells of the root hairs.