Osmosis

Osmosis process

1. OSMOSIS

2. DEFINITION Osmosis is the passive movement of water (solvent molecules) from a region of higher concentration of water molecules across a partially permeable membrane to a region of lower concentration.

3. WATER POTENTIAL Water potential is the tendency of a solution to lose water The symbol used for water potential is Ψ the Greek letter psi, and is usually measured in kilopascal ( kPa ).

4. water moves from a solution with high water potential to one with low water potential. Water potential is decreased by addition of solute. Water potential can be increased by the application of pressure. In pure water or a dilute solution with very few solute molecules, the water molecules have a high free kinetic energy and has the tendency to move very freely. The dilute solution has a high water potential.

5. The water potential of pure water Ψ w at atmospheric pressure is arbitrarily given the value 0 kPa . The addition of solutes lowers the water potential. The water potential of solutions therefore, is lower than pure water and has a negative value. The greater the concentration of solutes, the more negative is the water potential.

6. SOLUTE POTENTIAL Solute potential is the potential or force of attraction towards water molecules caused by dissolved substances (solutes) inside the solution. The attraction between solute molecules and water molecules reduces the random movement of water molecules. The addition of more solute molecules lowers the water potential of a solution.

7. PRESSURE POTENTIAL Pressure potential is the pressure exerted on a fluid by its surrounding. As water diffuses into a plant cell, the cell contents expand causing a pressure ( turgor pressure) on the cellulose cell wall. The cell wall develops an inward pressure (pressure potential) to resist the influx of water.

8. PRESSURE POTENTIAL The pressure potential has a positive value when the plant cell is turgid and 0 kPa when the cell is flaccid The water potential of an animal cell depends on its solute potential because the pressure potential generated by the cell membrane is negligible

9. WATER POTENTIAL IN A PLANT The water potential of a plant cell is the sum of its solute potential and pressure potential.

10. TONICITY OF SOLUTION The term tonicity is a measure of the osmotic pressure gradient of two solutions separated by a semi-permeable membrane. The osmotic pressure gradient is defined by the water potential of the two solutions. Tonicity is influenced only by the solutes that cannot cross the semi-permeable membrane. These solutes can exert an osmotic pressure. Solutes that can move freely across the membrane do not affect tonicity. This is because, these solutes, when reaching equilibrium, will always be in equal concentrations on both sides of the membrane.

11. Hypotonic, hypertonic and isotonic solutions The hypotonic solution is the solution with the lower solute concentration, The hypertonic solution is the solution with the higher solute concentration. An isotonic solution has the same solute concentration as the other solution.

12. OSMOSIS IN PLANT CELLS When a plant cell is placed in a hypotonic solution, water enters the cell by osmosis. The vacuole expands and the cell contents press against the cell wall. As more water enters, the pressure potential produced by the cell wall increases until the pressure potential equals the solute potential.

13. OSMOSIS IN ANIMAL & PLANT CELLS

14. There is no net movement of water in either direction. The plant cell is said to be turgid (Figure 3.12(d)). Turgid cells give support to herbaceous plants, like morning glory and balsam plant. Plant cells do not burst because they are surrounded by a strong, cellulose cell wall.

15. OSMOSIS IN ANIMAL & PLANT CELLS In isotonic solutions, there is no net movement of water molecules and no change in the volume of the cell (Figure 3.12(e)).

16. OSMOSIS IN ANIMAL & PLANT CELLS

17. The protoplast starts to shrink away from the cell wall. Plasmolysis of the cell occurs and the cell becomes flaccid (Figure 3.12(F). When cells become flaccid, they cause the plant to wilt.

18. OSMOSIS IN ANIMALCELLS When an animal cell, for example, a red blood cell, is placed in a hypotonic solution, water enters the cell by osmosis. There is a net movement of water into the cell. The cell expands and the thin plasma membrane bursts, releasing the cell contents. A red blood cell is said to be haemolysed (Figure 3.12(a)). In isotonic solutions, there is no net movement of water molecules and no change in the shape or volume of the cell (Figure 3.12(b)).

19. In hypertonic solutions, there is a net outflow of water by osmosis from the cell (Figure 3.12(c)). The cell shrinks and the plasma membrane has a crinkled appearance. The cell is said to be crenated .