Water Relation in Plants and Animals
Water Relation in Animals
As discussed earlier, the cell membrane is semi permeable. Let us discuss what would happen if an animal cell say red blood cell is placed in solutions of varying concentrations
a) Red blood cell in hypotonic solution e.g. distilled water
Distilled water has a higher concentration of water molecules compared to the red blood cell cytoplasm. When a red blood cell is placed in a hypotonic solution, water will move into the cell through osmosis. The cell will swell and burst. Swelling of red blood cell when placed in a hypotonic solution is referred to as haemolysis. The cell is said to be haemolysed.
b) Red blood cell in hypertonic solution
A hypertonic solution has a low concentration of water molecules compared to the red blood cell cytoplasm. Water will, therefore, be drawn out of the cell into the hypertonic solution. The cell will shrink and become small. The cell is said to be crenated. The process by which animal cells shrink and become smaller when placed in hypertonic solutions is referred to as crenation.
c) Red blood cell in isotonic solution
When placed in an isotonic solution, the cell remains unchanged. This is because there will be no net inflow or outflow of water between the cell and the solution.Note:
When the cell becomes haemolysed or crenated, its functioning is impaired. This implies that the body fluids and blood plasma surrounding the cells must be kept at the same concentration as the animal cells. This will prevent bursting or shrinking of the cells that would otherwise impair their physiology.
The body has a mechanism through which these concentrations are maintained at a nearly same concentration.
Water Relations in Plants
Water relations in plant cells differ with that in animal cells.
A plant cell has both a cellulose cell wall and cell membrane. The centre of the cell contains vacuole with sap. The sap is a solution of salts and sugars and is bound by a membrane, the tonoplast.
The cell membrane and tonoplast are semi permeable while the cellulose cell wall is fully permeable.
a) Plant cell in hypotonic solution e.g. distilled water
If a plant cell is placed in water or hypotonic solution, the cell will draw water from the hypotonic solution through osmosis causing the cell to distend.
The cellulose cell wall is rigid and does not allow plant cells to burst as in the case of animal cells.
As the cell gains more water, the vacuole enlarges and exerts an outward an outward pressure on the cell wall called turgor pressure.
The turgor pressure increases as more water is taken into the vacuole causing the cell tostretch until the cell cannot stretch any more. The cell becomes firm and is said to be turgid.
Turgor pressure is the outward pressure that the cell cytoplasm exerts on the cell wall as it gains more water through osmosis.
When the cell wall is being stretched towards the outside, it will develop a resistant pressure to stretching that is equal and opposite to turgor pressure called wall pressure.
b) A plant cell in a hypertonic solution
When placed in a hypertonic solution, the plant cell will lose water to the solution through osmosis. As the water moves out of the cell, the cell starts to shrink, becomes less rigid or flabby and is said to be flaccid.
If the cell loses more water, its contents reduce in size and the plasma membrane pulls away from the cell wall towards the centre. The process through which plant cells lose water, shrink and become flaccid is called plasmolysis.
Plasmolysis can be reversed when a flaccid cell is placed in distilled water in a process called deplasmolysis.
Plants always lose water to the atmosphere through transpiration and evaporation. Simultaneously, the plant cells lose water and draw more from the soil.
Wilting is a phenomenon that occurs when plant cells lose more water than they draw from the soil making the plant cells to lose their turgor pressure and droop.
At night, plants always recover from wilting since stomata are closed and water loss through evapotranspiration is significantly reduced.
Where water supply from the soil is inadequate, the plants may fail to recover from wilting and instead undergo permanent wilting.