SAJC_2014_H2-Bio-TYS-ANS

2014 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2014 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2014 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 (a) With reference to Fig. 1.1, describe the curve between points A and B. [2] 1 Initially, rate of reaction increases gradually e.g. from 0 to 8 arbitrary unit (a.u) as temperature increases from 35 oC (point A) to 50oC. 2 Subsequently, the rate of reaction increases rapidly e.g. from 18 to 65 a.u as temperature increases from 55 oC to 65oC (point B). Examiner’s comment: The majority of candidates correctly described the change in the rate of the reaction and supported their answers by quoting figures from the graph from both axes. Candidates who considered explanation of the shape of the graph had no t recognised the significance of the instruction to ‘describe’ the curve. (b) Explain why the reaction rate changes from: (i) A to B, [3] 1 (As temperature increases,) heat increases and increases the kinetic energy of enzymes and substrates. 2 Increasing frequency of effective collision between enzyme and substrates. 3 More enzyme-substrate complexes formed per unit time, hence the rate of reaction increases. (ii) C to D. [3] 1 At the optimum temperature at point C, the rate of reaction is at its highest. As temperature increases beyond the optimum temperature, the rate of reaction decreases drastically. 2 Increased heat increases the thermal agitation of the enzyme molecule and put strain on the bonds holding the tertiary structure of the enzyme. Eventually high enough temperature leads to the disruption of weak bonds e.g hydrogen bonds, ionic bonds and hydrophobic interaction that stabilised the conformation of the enzyme and its active site. 3 The three dimensional (3D) conformation of enzyme is eventually broken permanently. Enzyme is denatured and binding of the substrate is affected. (c) Suggest how the structural features of Taq polymerase make it thermostable. [2] 1 (Taq polymerase is a protein whose structural features are influenced by) the number, types and sequence of the different amino acids in the primary structure which determines how the polypeptide chain coils and folds into tertiary structure. Tertiary structure is maintained by bonds formed by chemical interactions between the different R groups of the amino acids. 2 (To be thermostable, Taq polymerase) would have more cysteine amino acids (to form disulfide bonds) and polar or acidic / basic amino acids with polar or charged R groups, respectively (to form ionic bonds). More disulfide bonds and ionic bonds would require higher temperature to break these bonds to denature the protein. Examiner’s comment: Candidates needed to consider features of proteins that are likely to make them more thermostable. This proved challenging for many.

2014 UCLES ‘A’ Level H2 Biology Mark Scheme 3 QUESTION 2 Fig.2.1 shows the molecular structure of malt