AS Biology 9700: Water Potential, Osmosis and Plasmolysis

A cell taken from the petiole (leaf stalk) of a rhubarb plant, cell Q, has a solute potential (ψs) of −950 kPa. At the moment it is examined, cell Q is not fully turgid, and has a pressure potential (ψp) of +350 kPa.

(a) Using the equation ψ = ψs + ψp, calculate the water potential of cell Q. Show your working. [2]

(b) Cell Q is then placed into an external sucrose solution with a water potential of −1250 kPa. Compare this value with your answer to part (a), predict the direction of net water movement between cell Q and the solution, and explain your reasoning. [3]

(c) Describe how cell Q would appear if examined under a microscope after being left in this solution for some time, and explain, referring to the cell wall and the cell-surface membrane, why it would look this way. [2]

Part (a): Calculating the water potential of cell Q

The water potential of a plant cell is the sum of its solute potential and its pressure potential:

ψ = ψs + ψp

Substituting the given values:

ψ = (−950 kPa) + (350 kPa) = −600 kPa

So cell Q has a water potential of −600 kPa.

Part (b): Predicting the direction of net water movement

The external solution has a water potential of −1250 kPa. Comparing the two values:

Cell Q: ψ = −600 kPa
External solution: ψ = −1250 kPa

−600 kPa is higher (less negative) than −1250 kPa. Water always moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential, moving down the water potential gradient. Since the solution has the more negative water potential, there is a net movement of water out of cell Q, into the external solution.

Part (c): The appearance and cause of plasmolysis

As water continues to leave cell Q by osmosis, the volume of the cytoplasm decreases. The cell-surface membrane is flexible and shrinks with the cytoplasm, but the cell wall is rigid and freely permeable, so it does not shrink to match. Once enough water has left, the cell-surface membrane (with the cytoplasm inside it) pulls away from the cell wall, leaving a visible fluid-filled gap between the wall and the shrunken protoplast. This is described as plasmolysis, and cell Q would appear visibly shrunken away from its cell wall under the microscope. As the protoplast shrinks, its pressure potential falls to zero (it can no longer push outward on the wall), so the cell’s water potential becomes equal to its solute potential alone.

Final answers

(a) ψ = (−950) + (350) = −600 kPa.
(b) Cell Q’s water potential (−600 kPa) is less negative than the solution’s (−1250 kPa), so water moves out of cell Q, down the water potential gradient, into the solution.
(c) Cell Q becomes plasmolysed: the cell-surface membrane and cytoplasm pull away from the rigid cell wall as the protoplast shrinks through water loss.