Osmosis

In the syllabus and the exam

Osmosis anchors an entire block of Paper 2 and Paper 3 questions on plant and animal cells placed in solutions of different concentrations, and candidates are expected to predict and explain what happens to a cell (swelling, becoming turgid, becoming flaccid, plasmolysing or, in animal cells, lysing) based on whether the surrounding solution is hypotonic, hypertonic or isotonic relative to the cell. A well-graded answer names the direction of water movement (into or out of the cell), links it to the difference in water potential across the partially permeable membrane, and states the visible result for the specific cell type in the question, since plant and animal cells respond differently due to the presence or absence of a cell wall. A very common error is defining osmosis using the older 'concentration' wording only (movement of water from a dilute to a concentrated solution) without connecting this to water potential, which the current syllabus expects candidates to use as the more precise explanation. Another frequent mistake is applying osmosis to solute particles, such as writing that 'salt moves out of the cell by osmosis'; osmosis strictly describes water movement only, so any answer describing the movement of dissolved substances such as ions or sugars should refer to diffusion or active transport instead. Potato-strip experiments, where equal-sized strips are soaked in solutions of different sucrose concentrations and their change in mass or length is measured, are a standard Paper 3 practical used to test whether candidates can plan, tabulate and correctly interpret an osmosis investigation from start to finish.

Example

When a raisin is placed in distilled water, it swells because water moves into the raisin's cells by osmosis, from the higher water potential outside to the lower water potential inside.

Related terms

DiffusionHypotonic solutionHypertonic solution

Source:SRC-DSKP-EN