TMJC H2 Bio data response questions (Ans)

Tampines Meridian Junior College JC2 H2/9744 Biology Data Response Practice (Ans) PRACTICE 1 Pyruvate dehydrogenase is a tetrameric enzyme found in the mitochondria that catalyses the conversion of pyruvate to acetyl CoA during cellular respiration. The activity of pyruvate dehydrogenase is inhibited by acetyl Co-A. Fig. 1 shows the relationship between the initial rate of reaction of pyruvate dehydrogenase activity and the concentration of pyruvate at the optimal temperature and pH of the enzyme. All other variables are kept constant. Fig 1 OR

(a) Comment on the trend shown on the graph. [HI2] [6] • [Describe] As the concentration of pyruvate increases from 0 to 0.02 moldm-3, the rate of reaction increases gradually from 0 molmin-1 to 0.01 molmin-1. • [Explain] At low pyruvate concentrations, the inactive form of the pyruvate dehydrogenase is favoured. / Only a small number of enzymes are able to form enzyme-substrate complexes per unit time. • [Describe] As the concentration of pyruvate increases from 0.02 moldm-3 to 0.04 moldm- 3, the rate of reaction increases steeply from 0.01 molmin-1 to (0.044 - 0.045) molmin-1 (sigmoidal curve) • [Explain] The substrate binds to one active site in the enzyme and causes a conformational change in the enzyme, hence, stabilising the other subunits in the active form and increases the affinity for binding to its substrate molecules. • [Describe] As the concentration of pyruvate increases from (0.057-0.059) moldm-3 to 0.10 moldm-3, the rate of reaction plateaus/remains constant at 0.05 molmin-1. • [Explain] All active sites of all pyruvate dehydrogenase molecules are saturated with pyruvate molecules. (b) On Fig 1, sketch and label a curve to show the effect of adding acetyl Co-A on the initial rate of reaction of pyruvate dehydrogenase activity. [HI 1][1] • Graph should show a lower initial rate of reaction over the entire range of pyruvate concentrations. Graph may reach the Vmax of the normal graph or not reach it. Describe + Explain

PRACTICE 2 Fig. 2 shows the effect of increasing temperature on the activity of three protein -digesting enzymes: • thermitase from thermophilic Thermoactinomyces vulgaris • subtilisin from Bacillus subtilis • modified subtilisin Fig. 2 (a) Describe, with reference to Fig. 2, the effect of temperature on the rate of protein digestion by thermitase. [HI-1] [3] • As temperature increases from 10C to 40 C, rate of protein digestion increases gradually from 0 a.u. to 120 a.u. • As temperature increases from 40C to 76C, the rate increases steeply from 120 a.u. to 650 a.u. • At optimum temperature of 76C, rate of protein digestion is at its maximum. • As temperature increases from 76C to 90C, the rate decreases steeply from 650 a.u. to 350 a.u. Describe means “describe trend of graph and cite data only! No need to explain why!!!

(b) Modified subtilisin is similar to subtilisin but had eight of its amino acids replaced with different amino acids. Describe and explain the effect of this modification on the activity of subtilisin. [HI-2] [4] • [Describe] Rate of protein digestion is higher with modified subtilisin at every temperature investigated as compared to subtilisin, [Cite data – at any temperature] At 30C, the rate of reaction of subtilisin is 10 a.u. whereas rate of modified substilisin is 40 a.u. • [Explain] The replaced amino acids might be the catalytic amino acids. The new amino acids may have higher catalytic ability • [Describe] Modified subtilisin has higher optimal temperature of 76C as compared to subtilisin with optimal temperature of 58C • [Explain] The new amino acids might be cysteine that forms strong disulfide (covalent) bonds that hold the 3D structure of the enzyme intact at higher temperature / thermostable enzyme. PRACTICE 3 In 1954, an article was published in the British Medical Journal entitled, The mortality of doctors in relation to their smoking habits. One aspect of the investigation studied a very large number of doctors in the UK aged 35 years and older. A survey established the quantity of tobacco smoked per day. Twenty-nine months later, the case of any deaths in the study group was recorded. Table 3 summarises the results obtained. Table 3 Cause of death Number of deaths Death rate per year per 1000 men in the study Non- smokers Smokers, tobacco smoked / g day-1 1-14 15-24 25 and above Coronary thrombosis (heart attack) 235 3.89 3.91 4.71 5.15 Lung cancer 36 0.00 0.48 0.67 1.14 Important: CONTEXTUALISE your answers. DO NOT give vague answers by mere description of how primary structure is changed and how it subsequently changes the secondary and tertiary structure of protein.

(a) State which group in the study is most at risk from dying of lung cancer. [HI-1] [1] • Smokers who smoked 25g day-1 and more tobacco. (idea of tobacco smoked with units must be clear) Examiner’s comments • A handful of candidates were not specific and identified ‘smokers’ without specifying the different groups. • Some candidates lost marks because units were either not given or wrongly identified. (b) Using information from Table 3 to support your answer, discuss the evidence linking tobacco smoking to coronary thrombosis and early death. [HI-3] [4] • Death rate caused by coronary thrombosis is higher in smokers than non-smokers. • [cite relevant data] Smokers who smoked 1-14 g day-1 has a death rate of 3.91 per year per 1000 men which is higher than non-smokers which have a death rate of 3.89. • As the amo unt of tobacco smoked increased, the death rate per year per 1000 men caused by coronary thrombosis increases. • [cite data] As the amount of tobacco smoked increased from 1-14 g day-1 to 25 g day-1 and above, the death rate per year per 1000 men increased from 3.91 to 5.15. Examiner’s comments • Candidates did not read the question carefully and end up discuss about lung cancer when it is not needed. • Weaker students failed to cite data despite the phrase ‘Using information from Table3.1’. • Most only gave 1 conclusion of trend seen. (c) Suggest a significant limitation of this study. [KU-3] [1] • No females are included in the study. • Deaths as a proportion of the sample is lacking. • Information (e.g. age group, family history) are not considered. • Other diseases and deaths caused by smoking tobacco are not included. • AVP Examiner’s comments • Sample size of 1000is a substantial number. • Period of investigation too short was not accepted.

PRACTICE 4 p53 is a tumor suppressor protein which plays an important role at cell cycle checkpoints. An experiment was carried out to investigate the effect of p53 on the cell cycle in human liver cells. Two types of cells were used in the investigation, one with normal p53 gene and the other with mutated p53 gene, resulting in p53-deficient cells. Both cell types were subjected to γ-radiation, which is a DNA damaging agent. Mitotic index of the cells were then measured and the results are shown in Fig. 4. Mitotic index reflects the percentage of cells in a population that are dividing. It is calculated by counting the number of cells with condensed chromosomes and dividing it by the total number of cells observed. Fig. 4 a) [HI-2] With reference to Fig. 4 , account for the difference in results obtained after 12 hours. [3] [Describe] • [trend] Cells with mutated p53 gene has a higher mitotic index at all times/from 12h-60h compared to cells with normal p53 gene. • [cite data] the mitotic index for cells with normal p53 gene is always below 5% whereas the mitotic index for cells with mutated p53 gene increased steeply