Osmosis

Water as a solvent in organisms

A solvent is a liquid in which substances dissolve, and water is the important biological solvent for this syllabus because so many of the substances an organism needs or produces can dissolve in it. Cambridge names three specific contexts where this matters.

Digestion

Digestion breaks large food molecules down into smaller ones. Water provides the medium in which many of those smaller molecules can dissolve, which is what allows the soluble products of digestion to be moved and absorbed in the first place.

Transport

Many substances are transported around an organism dissolved in water — carried in body fluids, or as part of the transport fluids of a plant. Once a substance is dissolved, it can travel from one part of the organism to another simply by moving with the water it is dissolved in.

Excretion

Metabolic waste products can also dissolve in water, which is what allows soluble wastes to be carried away from the cells producing them and eventually removed from the organism altogether.

Water’s ability to dissolve substances is therefore not a side detail — it is the thread connecting digestion, transport and excretion across the rest of this syllabus, which is why Cambridge introduces “water as a solvent” here rather than leaving it implicit.

Partially permeable membranes

A partially permeable membrane allows some substances to pass through it more easily than others. For osmosis, the essential fact is narrower than that whole definition: water molecules can pass through it. The cell membrane is the partially permeable membrane through which water moves into and out of a cell, and every Core candidate is expected to know that water diffuses through partially permeable membranes by osmosis on exactly this basis.

Osmosis

At its simplest, the Core-level statement is:

osmosis is the diffusion of water through a partially permeable membrane.

The direction of net water movement depends on the conditions on the two sides of that membrane — which side has more water available to move, relative to the amount of dissolved substance present. For Core answers, that relationship is enough. For Supplement, it has to be expressed more precisely, using water potential.

Water potential — Supplement

The full Supplement definition is:

Osmosis is the net movement of water molecules from a region of higher water potential, which is a dilute solution, to a region of lower water potential, which is a more concentrated solution, through a partially permeable membrane.

For this syllabus, hold onto two fixed facts:

• a dilute solution has a higher water potential;
• a more concentrated solution has a lower water potential.

So the direction of osmosis is always:

higher water potential → lower water potential

as long as the water is moving through a partially permeable membrane.

The concentration being discussed here needs care. Adding more dissolved solute to a solution makes that solution more concentrated in solute, but it lowersits water potential. That is exactly why simply saying osmosis moves “from high concentration to low concentration” is unsafe at Supplement level: it is genuinely ambiguous whether the sentence is talking about the water or about the dissolved substance, and those two concentrations move in opposite directions relative to water potential. For a Supplement answer, always write higher water potential to lower water potential, not a bare concentration statement borrowed from diffusion.

Predicting the direction of osmosis — Supplement

Suppose a cell contains a solution with a lower water potential than the solution surrounding it. Water moves:

surroundings → cell

because the surrounding solution has the higher water potential. If the surroundings instead have the lower water potential, the movement reverses:

cell → surroundings

Notice that the direction never depends on whether the movement happens to be “into” or “out of” the cell as a fixed rule — it depends entirely on which side of the membrane has the higher water potential. If there is no difference in water potential across the membrane, water molecules keep moving in both directions, but there is no net movement of water — the same equilibrium idea that applies to diffusion in general.

You can use exactly that four-step reasoning on any unfamiliar water-potential question: identify the two regions separated by the membrane, decide which has the higher water potential and which has the lower, then move water from higher to lower.

The two required practical investigations built around this relationship — dialysis tubing and plant tissue — are covered on osmosis experiments, and what osmosis actually does to a plant cell’s structure, including turgor, flaccidity and plasmolysis, is covered on osmosis in plant cells.