WATER RELATIONS IN PLANT AND ANIMAL CELLS

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
See also  ACTIVE TRANSPORT

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

  • The rate of formation of contractile vacuoles increases

Also See:

OSMOSIS

COMPARISON BETWEEN ANIMAL CELL AND PLANT CELL

DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE

VARIATION IN POPULATION

COMPARISON BETWEEN ANIMAL CELL AND PLANT CELL

 

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