explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells (knowledge of solute potential and pressure potential is not expected)

Published by Patrick Mutisya · 14 days ago

Movement into and out of Cells – A‑Level Biology 9700

Movement of Water Between Cells and Solutions

Water moves across cell membranes by diffusion. The direction of movement is determined by the

water potential (Ψw) of the two compartments. Water always moves from a region of higher water

potential to a region of lower water potential until equilibrium is reached.

Water potential is a single value that reflects the combined effect of solutes and pressure on the

tendency of water to move. In the context of this topic we treat it qualitatively:

  • Pure water has the highest water potential (set as 0).

  • Adding solute lowers the water potential.

  • Increasing pressure raises the water potential.

Thus, water will flow from a solution with a higher (less negative) water potential to one with a lower

(more negative) water potential.

Key Points

  1. Water moves by osmosis – diffusion of water through a semi‑permeable membrane.

  2. The net movement continues until the water potentials on both sides are equal.

  3. Cell membranes are selectively permeable: they allow water to pass but restrict most solutes.

Effects of Water Movement on Plant Cells

Plant cells have a rigid cell wall that influences how they respond to changes in water potential.

  • Turgid (full) cells: When water enters a plant cell, the vacuole expands and the

    cell becomes turgid. The cell wall resists further expansion, generating turgor pressure that

    supports the plant.

  • Plasmolysed cells: If water leaves the cell (e.g., placed in a hypertonic solution),

    the plasma membrane pulls away from the cell wall, reducing turgor and causing wilting.

Suggested diagram: A plant cell showing turgid and plasmolysed states.

Effects of Water Movement on Animal Cells

Animal cells lack a cell wall, so they are more vulnerable to changes in volume caused by water movement.

  • Swelling (lysis): In a hypotonic environment, water enters the cell, the cell swells,

    and may burst (lysis) because there is no rigid wall to contain the pressure.

  • Shrinking (crenation): In a hypertonic environment, water leaves the cell,

    causing it to shrink and become crenated.

Suggested diagram: An animal cell undergoing lysis and crenation.

Comparison of Plant and Animal Cell Responses

ConditionPlant Cell ResponseAnimal Cell Response
Hypotonic solution (water potential outside > inside)Turgid – cell swells until the cell wall exerts pressure; no bursting.Swelling – may lead to lysis because there is no cell wall.
Isotonic solution (water potentials equal)Cell remains turgid; normal metabolic function.Cell retains its normal shape; no net water movement.
Hypertonic solution (water potential outside < inside)Plasmolysis – plasma membrane pulls away from cell wall; loss of turgor.Crenation – cell shrinks and becomes irregularly shaped.

Summary

Understanding water potential allows us to predict the direction of water movement between cells

and their environment. The presence of a rigid cell wall in plants provides protection against

excessive swelling, whereas animal cells rely on maintaining an isotonic environment to avoid

lysis or crenation.