A-LevelBiologyCell membranes and transportMay/June 2025Paper 1 Q181 Mark
Samples X, Y and Z are epidermal tissues cut from an onion. Each epidermal tissue was immersed in one of three different concentrations of a salt solution for 30 minutes. A student observed each tissue sample with a light microscope. Then the student estimated the concentration of each salt solution using a scale. The diagram shows the scale. P, Q and R represent the estimated concentration of the three salt solutions. [Figure 18.1 (scale diagram from distilled water to concentrated salt solution with points P, Q, R)] The photomicrographs show the appearance of the tissues in samples X, Y and Z. [Figure 18.2 (photomicrographs of sample X, sample Y, sample Z)] Which row shows the estimated concentrations of the salt solutions in which samples Y and Z were immersed?
Asample Y: P, sample Z: Q
Bsample Y: P, sample Z: R
Csample Y: Q, sample Z: R
Dsample Y: R, sample Z: P
✓ Correct Answer
The correct answer is D: sample Y: R, sample Z: P
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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