SAJC_2016_H2-Bio-TYS-ANS

2016 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2016 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2016 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 Fig.1.1 shows the effect of increasing substrate co ncentration on the rate of an enzyme-catalysed reaction in the presence and absence of a non-competitive inhibitor. Fig. 1.1 (a) Explain why, in the reaction with the enzyme only, as substrate concentration increases : (i) the rate of reaction increases at first [2] 1 Active sites of available enzyme molecules are not fully occupied by substrate, increase in frequency of effective collisions between enzyme and substrate molecules (as substrate concentration increases) ; 2 Increase in concentration of enzyme-substrate complexes formed per unit time, increase in concentration of products formed per unit time ; 3 Substrate concentration is limiting factor at low substrate concentration, (ii) the rate of reaction becomes constant [ 2 ] 1 Active sites of available enzyme molecules are saturated with substrate molecules ; 2 Any extra substrate molecule has to wait until the E-S complex in the active site of the enzyme is released as products before the substrate can enter the active site ; 3 Enzyme concentration is limiting factor at high substrate concentration ;

2016 UCLES ‘A’ Level H2 Biology Mark Scheme 3 (b) Explain why, in Fig. 1.1, the addition of a non-competitive inhibitor causes the reaction to become constant at a lower rate. [2] 1 Non-competitive inhibitor binds to the enzyme at its allosteric site, results in change in 3D conformation of the enzyme, 3D conformation at active site altered ; 2 Substrate cannot bind to the active site, decrease in frequency of effective collisions between enzymes and substrate molecules / decrease in concentration of enzyme-substrate complexes formed per unit time, decrease in concentration of products formed per unit time ; (c) Draw, on Fig. 1.1, the approximate shape of the curve if a competitive inhibitor were added to the enzyme instead of a non-competitive inhibitor. [2] 1 (Labelled) Curve beginning at 0 ; 2 Vmax reached at higher substrate concentration ; enzyme + competitive inhibitor

2016 UCLES ‘A’ Level H2 Biology Mark Scheme 4 (d) The antibiotic penicillin irreversibly inhibits the activity of transpeptidase. Transpeptidase is a bacterial enzyme that cross-links cell wall peptides during the formation of bacterial cell walls. Fig.1.2 shows part of each of the molecular structure of a cell wall peptide and penicillin. Fig. 1.2 Suggest why the penicillin molecule is an effective inhibitor of transpeptidase.[2] 1 Penicillin has similar 3D conformation to the cell wall peptide (substrate for transpeptidase) and competes for binding to the sam e active site on transpeptidase as the cell wall peptide ; 2 Prevents c