Tonicity & Cells

Predict asks you to state a likely outcome — but a bare outcome is worth little. The marks come from predicting and giving the osmotic reasoning that makes the prediction follow, for all six combinations.

6 marks, six cases: three solutions against two cell types. Set up the rule once, then run it through each case. A table in your plan, prose in your answer.

Water moves by osmosis across a partially permeable membrane from a region of higher water concentration to one of lower water concentration, which is to say towards the higher solute concentration. A hypertonic solution has more solute than the cell, so water leaves; a hypotonic solution has less, so water enters; an isotonic solution has the same, so there is no net movement.

The two cell types respond differently because of one structural difference: a plant cell has a rigid cellulose cell wall outside its membrane, whereas an animal cell has only the membrane. The wall is fully permeable, so it does not affect the direction of water movement, but it is strong enough to resist the pressure that builds as water enters.

In a hypotonic solution water enters an animal cell continuously, and with no wall to resist the increasing pressure the membrane stretches until the cell bursts, a process called lysis, or haemolysis in a red blood cell. In a hypertonic solution water leaves, the cell shrinks and its membrane becomes wrinkled, known as crenation. In an isotonic solution there is no net water movement and the cell keeps its normal shape and volume, which is why isotonic surroundings are the only safe ones for animal cells and why blood plasma is maintained isotonic with the cells it carries.

In a hypotonic solution water enters a plant cell and the vacuole expands, pushing the membrane against the cell wall; the wall resists, turgor pressure builds, and the cell becomes turgid but does not burst, which is the healthy state that holds a non-woody plant upright. In a hypertonic solution water leaves the vacuole, the cell becomes flaccid, and if enough water is lost the membrane pulls away from the wall, a condition called plasmolysis, seen in a wilting plant. In an isotonic solution there is no net movement, the cell is flaccid and no turgor pressure supports the tissue, so the plant begins to droop even though no cell has been damaged.

The direction of water movement is the same for both cell types in every case, because osmosis depends only on the solute gradient; what differs is the consequence, because the cell wall converts the water gain that destroys an animal cell into the turgor pressure that supports a plant one.

An explain question set in a scenario is marked on whether the biology accounts for this observation. Answer in the gardener’s terms as well as in osmotic ones, or half the explanation is left implied.

5 marks. Identify the tonicity of the soil solution, give the direction of water movement and why, name what happens at cell level, then connect that to the visible wilting.

A concentrated fertiliser solution has a high concentration of dissolved mineral salts, so the soil water surrounding the roots now has a higher solute concentration, and therefore a lower water concentration, than the cytoplasm and vacuolar sap of the root cells. The external solution has become hypertonic to the root cells.

Water therefore moves by osmosis out of the root cells and into the soil solution, down the water concentration gradient and across the partially permeable cell membranes. The direction is the reverse of normal uptake, so the plant is not merely failing to absorb water — it is actively losing it to the soil, while transpiration continues to draw water out of the leaves.

As water leaves, the vacuole of each cell shrinks and the turgor pressure that pushed the membrane against the cell wall falls, so the cells become flaccid. If enough water is lost the membrane pulls away from the cell wall altogether, a condition called plasmolysis, and the cell can no longer support itself.

A non-woody plant is held upright by the combined turgor pressure of millions of cells pressing against their walls, so when that pressure is lost the stems and leaves can no longer be supported and the plant droops, which is the wilting the gardener observed. Watering thoroughly with plain water would dilute the soil solution, restoring a hypotonic external environment so water re-enters the cells and turgor is regained, provided plasmolysis has not progressed far enough to kill the cells.