WATER RELATIONS IN PLANT AND ANIMAL CELLS

The medium (solution) surrounding cells or organisms is described by the terms hypotonic, hypertonic and isotonic. A solution whose solute concentration is more than that of the cell sap is said to be hypertonic

A cell placed in such a solution loses water to the surroundings by osmosis

A solution whose solute concentration is less than that of the cell sap is said to be hypotonic

A cell placed in such solution gains water from the surroundings by osmosis

A solution which has the same solute concentration as the cell sap is said to be isotonic

When a cell is placed in such a solution there will be no net movement of water either into or out of the cell

Osmotic Pressure

The term osmotic pressure describes the tendency of the solution with a high solute concentration to draw water into itself when it is separated from distilled water or dilute solution by a semi-permeable membrane
Osmotic pressure is measured by an osmometer
When plant cells are placed in distilled water or in a hypotonic solution, the osmotic pressure in the cells is higher than the osmotic pressure of the medium
This causes the water to enter the cells by osmosis
The water collects in the vacuole which increases in size
As a result the cytoplasm is pushed outwards and it in turn presses the cell membrane next to the cell wall
This builds up water pressure (hydrostatic pressure) inside the cell
When the cell is stretched to the maximum, the cell wall prevents further entry of water into the cell
Then the cell is said to be fully turgid
The hydrostatic pressure developed is known as turgor pressure

Plasmolysis

When a plant cell is placed in a hypertonic medium, it loses water by osmosis
The osmotic pressure of the cell is lower than that of the medium
The vacuole decreases in size and the cytoplasm shrinks as a result of which the cell membrane loses contact with the cell wall
The cell becomes flaccid

The whole process is described as plasmolysis

Incipient plasmolysis is when a cell membrane just begins to lose contact with the cell wall
Plasmolysis can be reversed by placing the cell in distilled water or hypotonic solution
However, full plasmolysis may not be reversed if cell stays in that state for long

Wilting

The term wilting describes the drooping of leaves and stems of herbaceous plants after considerable amounts of water have been lost through transpiration
It is observed in hot dry afternoons or in dry weather
This is when the amount of water lost through transpiration exceeds the amount absorbed through the roots
Individual cells lose turgor and become plasmolysed and the leaves and stems droop
The condition is corrected at night when absorption of water by the roots continue while transpiration is absent
Eventually, wilting plants may die if the soil water is not increased through rainfall or watering

Haemolysis

Haemolysis is the bursting of cell membrane of red blood cells releasing their haemoglobin
It occurs when red blood cells are placed in distilled water or hypotonic solution
This is because the cell membrane does not resist further entry of water by osmosis after maximum water intake

Crenation

Takes place when red blood cells are placed in hypertonic solution
They lose water by osmosis, shrink and their shape gets distorted
Animal cells have mechanisms that regulate their salt water balance (osmoregulation) to prevent above processes that lead to death of cells
An Amoeba placed in distilled water, i.e

hypotonic solution, removes excess water using a contractile vacuole