A-LevelBiologyEnzymes9700/22 · Feb/Mar 2017 · Q311 Marks

Catalase is an enzyme that catalyses the breakdown of hydrogen peroxide, a toxic waste product of metabolism. 2H2O2 catalase 2H2O + O2 Fig. 3.1 shows the results of an investigation into the effect of hydrogen peroxide concentration on the rate of the catalase-controlled reaction, with and without the presence of two different inhibitors. [Figure 3.1]

(a)The inhibitors used in the investigation have different modes of action. Identify which of curves X, Y and Z are the results for: • the reaction with the non-competitive inhibitor • the reaction with the competitive inhibitor • the reaction without any inhibitor. non-competitive inhibitor competitive inhibitor without any inhibitor[1]
(b)With reference to Fig. 3.1, compare the maximum rate of reaction, Vmax, and the Michaelis-Menten constant, Km, for curves X, Y and Z.[4]
(c)Hydrogen peroxide has a harmful effect on cells. One effect is to damage DNA. Describe the structure of DNA.[4]
(d)The cell has mechanisms to repair the damage to DNA caused by hydrogen peroxide. Errors in repair may cause a change to the structure of DNA. Studies have investigated possible risks associated with foods and drinks that contain hydrogen peroxide. This is because the compound can be considered a mutagen. Mutagens cause mutations. Explain why hydrogen peroxide can be considered a mutagen.[2]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2017). The mark or mark type is shown in brackets.

[1] 3(a) all three correct; with the non-competitive inhibitor Z with the competitive inhibitor Y without any inhibitor X
[4] 3(b) four from: Vmax 1 X and Y same Vmax of 10 au; 2 Vmax of, X/Y, higher than Z/ORA; A (Vmax Of), X/Y, 10 au v Z 5 au A (Vmax of), X/Y, double the Vmax of Z Km 3 X and Z same Km ; A Km of both is 4 mmol dm³ 4 X/Z, lower Km than Y/ORA ; A K㎞ of, X/Z, 4 mmol dm¯³ v Y 6.5 mmol dm³ 5 reference to affinity for substrate ;
[4] 3(c) four from: 1 double helix; 2 strands are held together by hydrogen bonds (between bases) ; 3 complementary base pairing/described as A-T and C-G; A purine pairs with pyrimidine R thiamine 4 antiparallel stands/strands are 3' to 5' and 5' to 3'; A strands run in opposite directions 5 (each strand has a sugar phosphate backbone with) phosphodiester bonds; 6 (monomers/units/DNA) are (DNA) nucleotides/polynucleotide strands ; 7 (nucleotide = ) deoxyribose sugar, phosphate, nitrogenous (organic) base; A points from a diagram
[2] 3(d) two from: 1 idea that, hydrogen peroxide, damage/breaks, DNA and repair errors (may) occur ; 2 (so leads to) incorrect, nucleotide/base, inserted (during replication)/ change in, nucleotide/base, sequence (of DNA/RNA); 3 new allele (may be) formed; 4 may result in an altered polypeptide/AW ;

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