A-LevelBiologyNucleic acids and protein synthesisOct/Nov 2025Paper 4 Q312 Marks
The lac operon is present in the genome of the bacterium Escherichia coli. When glucose is not available, the presence of lactose in the extracellular environment leads to the expression of the genes of the lac operon. This leads to an increase in the uptake and metabolism of lactose. When glucose and lactose are available in the extracellular medium, lactose is prevented from entering the bacterial cell. Fig. 3.1 shows what happens when glucose enters a bacterial cell. • Glucose enters the bacterial cell using a transport protein, A. • B is an enzyme found in the cytoplasm of the bacterial cell. • B catalyses the phosphorylation of glucose to produce glucose 6-phosphate.
📋 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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