5090_s26_gt — 2026 May-June Biology 5090 Grade Threshold · Cambridge CAIE O Level

Describing antibiotic resistance using Lamarckism instead of natural selection

“Very few candidates could give a coherent description of how antibiotic use has led to the spread of antibiotic-resistant bacteria. Answers often invoked Lamarckism by stating the antibiotics cause a change in the bacteria and that this change is inherited.”— 5090 Paper 2/21, May/June 2023

How to fix: Use the correct natural selection framework: (1) random mutation creates variation in the bacteria population, (2) bacteria with the resistance allele survive antibiotic treatment, (3) surviving bacteria reproduce and pass the beneficial allele to offspring, (4) over generations the resistant strain becomes more common. Never say the antibiotic 'causes' the bacteria to change.

Confusing global warming with ozone depletion — attributing wrong causes and effects

“There is still some confusion between global warming and the destruction of the ozone layer. Very few appreciated that removing trees would remove minerals from the system.”— 5090 Paper 2/21, May/June 2023

How to fix: Global warming: burning fossil fuels releases CO2 and methane which trap infrared radiation (greenhouse effect) causing temperature rise. Ozone depletion: CFCs break down ozone (O3) in the stratosphere allowing more UV radiation to reach Earth. These are two separate environmental problems with different causes, different gases, and different consequences. Never mix them.

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.