5090_s26_ms_32 — 2026 May-June Biology 5090 Mark Scheme 3 Variant 2 · Cambridge CAIE O Level

Describing osmosis using vague concentration language instead of water potential

“When explaining osmosis, the best answers referred to the differences in water potential inside and outside the cell. Candidates should be encouraged to use this terminology, as stated in the syllabus. If they describe concentration differences, it is often unclear whether they are referring to water or solute concentration.”— 5090 Paper 2/21, May/June 2023

How to fix: Always use the term 'water potential' when explaining osmosis. The correct definition: osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Saying 'high concentration to low concentration' is ambiguous and loses marks.

Graph drawing errors — non-linear scales, missing units on axes, joining points incorrectly

“Full marks could not be awarded where the graphs were too small or where the scales were not linear. A common error on this graph was to have a scale on the y-axis from -0.1(g) to +0.1(g), instead of -1.0(g) to +1.0(g).”— 5090 Paper 3/32, May/June 2023

How to fix: Graph checklist: (1) Use a linear scale — equal spacing for equal intervals, (2) Label both axes with the variable name AND units, (3) Use more than half the grid area, (4) Read the question — if it says 'ruled lines' join points with a ruler, do NOT draw a curve or line of best fit, (5) Plot all data points including zero values, (6) Show interpolation working as drawn lines from axis to graph line.

Confusing reliability with accuracy when discussing repeated experiments — claiming repeats improve accuracy

“Some candidates discussed reducing the effect or removal of the anomalous result rather than its identification and a large number referred to increased accuracy rather than just increased reliability; neither of these responses could be credited. It should also be noted that repetition in itself does not reduce errors, so answers to this effect did not gain credit.”— 5090 Paper 3/32, May/June 2025

How to fix: Use the three terms precisely: ACCURACY = how close a measurement is to the true value (improved by better equipment / calibration, NOT by repetition). RELIABILITY = consistency across repeats (improved by repeating the experiment and taking a mean). VALIDITY = whether the experiment actually tests what it claims to (improved by controlling all variables). Repeats improve reliability and let you identify anomalies — they do NOT improve accuracy and do NOT remove errors by themselves.