Water potential — CCEA AS Biology Practice Questions

Question 1

Plant cells were placed in a concentrated sucrose solution. Explain, in terms of water potential, what happens to the cells.

Accepted Answer

The concentrated sucrose solution has a lower, more negative water potential than the contents of the cell. Water therefore moves out of the cell by osmosis, down a water potential gradient, through the partially permeable cell surface membrane. As water leaves, the cell loses turgor pressure and becomes flaccid; if enough water is lost the protoplast shrinks and pulls away from the cell wall, and the cell is plasmolysed.

Question 2

Explain the difference between water potential, solute potential and pressure potential in a plant cell, and state the water potential of pure water.

Accepted Answer

Water potential (Ψ) is the tendency of water molecules to move from one place to another. Solute potential (Ψs) is the reduction in water potential caused by dissolved solutes, so it is always negative; the more concentrated the solution, the more negative it is. Pressure potential (Ψp) is the pressure exerted by the cell wall on the cell contents as the cell becomes turgid, and it is usually positive. The two combine as Ψ = Ψs + Ψp. Pure water at standard temperature and pressure has a water potential of 0 kPa, which is the maximum possible value.

Water potential: CCEA Biology practice questions

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Plant cells were placed in a concentrated sucrose solution. Explain, in terms of water potential, what happens to the cells.

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