A-LevelBiologyNucleic acids and protein synthesis9700/42 · Oct/Nov 2025 · 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.

(a)(i)With reference to Fig. 3.1, suggest and explain how the presence of glucose in the extracellular environment prevents lactose entering the bacterial cell.[3]
(a)(ii)Suggest the advantages for the bacterial cell in preventing the entry of lactose when glucose is present in the extracellular environment.[2]
(a)(iii)The glucose 6-phosphate in the cytoplasm of the bacterial cell is used to produce fructose 1,6-bisphosphate. Fructose 1,6-bisphosphate is used in glycolysis. Outline the events that occur in glycolysis following the production of fructose 1,6-bisphosphate.[3]
(b)(i)With reference to Fig. 3.2, describe the effect of the different diets on the percentage of E. coli that are lac⁺.[2]
(b)(ii)The scientists concluded that lac⁺ E. coli had a selective advantage when colonising the gut, but only in the presence of lactose. Suggest why the presence of lactose caused lac⁺ E. coli to have a selective advantage.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[3] (a)(i) any three from: 1 glucose diffuses into the cell (through, A / transport protein) ; 2 idea that the enzyme / B, loses phosphate group ; 3 (dephosphorylated) B binds to lactose permease ; 4 causes, shape change / allosteric change, to lactose permease ; 5 lactose permease cannot function (so lactose cannot enter) ;
[2] (a)(ii) any two from: 1 no/less, (protein) synthesis of, lactose permease / β-galactosidase / transacetylase or no / less, transcription / expression, of (named structural) genes (of lac operon) ; 2 no / less, ATP / energy, wasted ; 3 no/fewer, amino acids wasted; 4 glucose is respired more easily / AW or fewer steps / less energy, required to break down glucose or lactose has to be broken down (to glucose and galactose) ;
[3] (a)(iii) any three from: 1 splits into / forms, two, triose phosphate / TP (molecules) ; 2 (TP) oxidised / dehydrogenated, to pyruvate ; 3 ATP production; 4 substrate-linked phosphorylation ; 5 production of reduced NAD ;
[2] (b)(i) any two from: lactose diet 1 increases (over six days) ; no lactose diet 2 small increase / increases slightly/ little change / stays around 50% (over six days); 3 comparative data quote – 2 percentages of either diet over time or percentage of each diet on one day ;
[2] (b)(ii) any two from: 1 lactose is the selection pressure ; 2 (lac+), can metabolise / take up / AW, lactose (so survive) or lac, cannot metabolise / take up / AW, lactose (so die); 3 AVP; e.g. ref. glucose has run out

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