What is likely to occur when a plant cell is placed into salty water?

Question

BlackTom AI · Accepted Answer

Water potential determines the direction of water movement: water moves from regions of higher water potential (less negative) to regions of lower water potential (more negative), so placing a plant cell in salty (hypertonic) water causes net water loss from the cell.

1) Relevant knowledge points and concepts
- Water potential (Ψ) = solute potential (Ψs) + pressure potential (Ψp). Solutes lower Ψs (make it more negative); positive turgor pressure raises Ψp.
- Osmosis is water movement across a semipermeable membrane from higher Ψ to lower Ψ.
- In a hypertonic (salty) external solution the external Ψ is lower (more negative) than the cell’s internal Ψ.
- Plant cells have rigid cell walls; walls prevent bursting in hypotonic media but do not stop water from leaving in hypertonic conditions. Loss of water causes the protoplast to shrink (plasmolysis) and turgor pressure to drop.

2) Step-by-step reasoning
- External salty water increases external solute concentration → external Ψs becomes more negative → external overall Ψ decreases.
- The cell’s internal Ψ becomes relatively higher than outside.
- Water moves by osmosis from the inside (higher Ψ) to the outside (lower Ψ) until equilibrium or until the protoplast pulls away from the cell wall (plasmolysis).
- Therefore net movement is out of the cell.

3) Example calculation (intermediate steps)
- Suppose internal Ψs = −0.5 MPa and internal Ψp = +0.3 MPa, so internal Ψ = −0.2 MPa.
- External salty solution has Ψs = −1.0 MPa and Ψp ≈ 0, so external Ψ = −1.0 MPa.
- Internal Ψ (−0.2 MPa) > external Ψ (−1.0 MPa) → water flows out of the cell.

4) Why the correct answer is correct
Because the salt lowers external water potential, water moves down its water potential gradient out of the cell, reducing turgor and potentially causing plasmolysis. The cell wall does not prevent this outward water movement.

5) Why other options are incorrect
- "Nothing. The plant cell wall will prevent water movement." Incorrect: the cell wall does not stop osmosis; it only resists expansion.
- "Salts will enter the cell down its concentration gradient." Incorrect: if external salt concentration is higher, passive entry would move ions into the cell only if channels/transports permit; but the dominant immediate effect is water loss by osmosis, not net salt influx.
- "Overall, water will enter the cell down its water potential gradient." Incorrect: water would enter only if external Ψ were higher (less negative), i.e., in hypotonic conditions. Here external Ψ is lower.

6) Conclusion
Overall, water will leave the cell down its water potential gradient. This matches the provided correct answer.

What is likely to occur when a plant cell is placed into salty water?Single choice

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