[HKALE 85 I] 10. Following the immersion of a moderately turgid plant cell in a sucrose solution, the water potential, osmotic potential and pressure potential of the cell changed as depicted in the three curves given below : (a) Define the water potential, osmotic potential and pressure potential of a plant cell. Describe the relationship between them. (4 marks) (b) Which potential does each of the three curves, X, Y and Z represent ? Give reasons for your answer. (3 marks) (c) From the changes in the three potentials, what can be deduced about (i) the direction of water movement into or out of the cell during this period ?(Explain your answer.) (ii) the water potential of the sucrose solution ? (2 marks) (d) Describe and explain the state of the cell at time B (see graph). (2 marks) (e) If at time C (see graph), the cell is transferred to a solution with an osmotic potential of-5 units, what changes would one expect of X, Y and Z ? Draw three curves to show these changes and explain the results. (5 marks) . (f) Describe a simple method by which the water potential of a named plant tissue can be determined in the laboratory. (4 marks) Ans: Total: 20 marks (a) The water potential of a plant cell is defined as the difference in chemical potential between water inside the cell and pure water at the same temperature and pressure. 1 The osmotic potential of a plant cell is defined as the component of the cell’s water potential that is due to the presence of solutes. 1 The pressure potential of a plant cell is defined as the component of the cell’s water potential that is due to hydrostatic pressure. 1 Water potential = osmotic potential + pressure potential 1 X = pressure potential because it is positive in value 12 12 (4) (b) Y = water potential as it is negative in value and yet always higher than Z 12 12 Z = osmotic potential because it is negative in value and is always lower than Y 12 12 (3) (c) (i) Water should be moving out of the cell as - the pressure potential decreases (i.e. the hydrostatic pressure inside the cell decrease) and - the osmotic potential decrease (i.e. cell contents becoming more concentrated) - the water potential decreases (ii) -2.3 units 12 1 12 (2) (d) The cell is slightly turgid because there is still a slightly positive pressure potential, indicating the presence of some hydrostatic pressure within the cell. 1 The cytoplasm should be swollen and the cell membrane should be in full contact with the cell wall. 1 (2) (e) Correct curve for X Correct curve for Y Correct curve for Z 12 1 1 Water will move out of the cell as the water potential of the cell is higher than that of the surrounding solution. 12 This causes a decrease in the pressure potential, osmotic potential and water potential. 12 When the pressure potential falls to zero, it cannot drop further. 12 Since there is no hydrostatic pressure, water potential always equals to osmotic potential after this point. 12 Osmotic and water potential continue to fall until water potential of the cell drops to -5 units when equilibrium is reached. 12 (5) (f) Marks should be given only to the correct method for the appropriate tissue. 4 (4) [HKALE 89 I-3a] Distinguish between water potential and osmotic potential. (2 marks) Ans: Water potential is a measure of the tendency of water to leave a solution. 1 Osmotic potential is a measure of the tendency of a solution to pull water into it, it always has a 1 negative value.