2017 HCI H2 Biology Prelims Paper 3 Answers

HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 3 / MS 1 HWA CHONG INSTITUTION (COLLEGE SECTION) 2017 JC2 9744 H2 BIOLOGY PRELIMINARY EXAMINATIONS PAPER 3 MARK SCHEME SECTION A Question 1 (a)(i) State why rubisco is said to have quaternary structure. [1] Ref to: more than one polypeptide (a)(ii) Explain what makes a molecule such as rubisco soluble. [1] 1. Ref to: globular protein 2. Ref to: hydrophilic / polar / charged amino acid residues at the surface of the molecule 3. Ref to: hydrophobic / non-polar / non-charged amino acid residues at the inside of the molecule 4. Ref to: hydrogen bonds with water (molecules) (b) Explain why rubisco does not need to be in an active form in the absence of light. [3] 1. Ref to: no light dependent reaction and no light independent reaction 2. Ref to: no ATP / reduced NADP 3. Ref to: no need to fix CO 2 (c) Explain the consequences to the plant of the reaction involving oxygen. [2] 1. Ref to: no CO 2 being fixed 2. Ref to: no (new) PGA / GP being made 3. Ref to: no (new) TP / glucose being made 4. Ref to: no (new) RuBP being (re)generated 5. Ref to: ATP used in making RuBP being wasted 6. Calvin cycle / light independent reaction / Photosynthesis decreased (d) With reference to Fig. 1.2, predict whether C3 or C4 plants are favoured in the context of global warming in the tropics. Explain how these plants are better adapted physiologically. [4] 1. Ref to: C4 plants being favoured over C3 plants at high temperatures above 26 C 2. Ref to: Plants closing their stomata, resulting in reduced CO 2 diffusion into the leaf 3. Ref to: risk of O 2 competing with CO2 for rubisco in C3 plants / AW 4. Ref to: less CO 2 fixed, resulting in lower yield of C3 plants / AW (e)(i) Describe how yeast converts glucose to ethanol. [2] 1. Ref to: anaerobic conditions 2. Ref to: pyruvate forming ethanal 3. Ref to: reduction of ethanal (e)(ii) Explain why it may be better in the long term to use ethanol made from corn starch, rather than petrol, as a fuel for cars. [2] 1. Ref to: corn as renewable resource 2. Ref to: uptake of CO 2 by corn in photosynthesis 3. AVP

HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 3 / MS 2 (f)(i) With reference to Table 1.1, suggest the source of biodiesel chosen by an oil company if reduction of greenhouse gas emissions were the sole criterion. [1] palm oil from Malaysia (f)(ii) Explain your answer to (f)(i). [2] 1. Ref to: lowest emission due to change of land use and from cultivation, processing and distribution 2. Cite appropriate values (g) Identify two criteria, apart from cost, that should also be considered in choosing a source of biodiesel. Explain the significance of each criterion. [4] 1. Ref to: impact on environment / wildlife / biodiversity 2. Ref to: impact on food supply 3. Ref to: loss of carbon sink / carbon dioxide emission 4. Ref to: ownership of land 5. AVP [Total: 25] Question 2 (a) Outline how gel electrophoresis separates DNA fragments. [4] 1. Gel electrophoresis separates a mixture of DNA fragments on the basis of size / molecular weight 2. Negatively-charged DNA fragments are loaded into wells at negatively-charged electrode, migrate towards the positively-charged electrode under application of an direct current 3. Size of the pores in the gel matrix act as a molecular "sieve" to resist / retard the movement of the molecules 4. Smaller / shorter DNA fragments are le ss impeded by the pores than longer ones and migrate faster and further forming a series of discrete size-fractionated bands (b) With reference to Fig. 2.1 and 2.2, explain the DNA profile of Jane. [3] 1. Jane’s is a heterozygous / carrier with 1 normal allele and 1 mutant allele on the homologous chromosomes 2. Restriction digestion of the normal allele generates 1 fragment that has moved least from the cathode and digestion of the mutant allele generates 2 fragments that have moved further from the cathode 3. Due to the presence of restriction site caused by gene mutation of p53 in the mutant allele (c) With reference to development of cancer as a multi-step process, describe how Jane might develop breast cancer. [3] 1. There is an accumulation of mutations in a single cell lineage, ref. to BRCA 1 and BRCA gene mutations 2. Activation of telomerase resulting in lengthening of telomeres, thus evading apoptosis / replicative cell senescence 3. Angiogenesis resulting in formation of new blood vessels, supplying nutrients and oxygen and removing toxic waste products 4. Cancer cells acquired ability to invade to directly migrate and penetrate into neighbouring tissues leading to metastasis [Total: 10]

HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 3 / MS 3 Question 3 (a) With reference to Fig. 3.1, explain how somatic recombination during B cell development results in the formation of millions of different antibody molecules. [4] 1. Ref to process by which different segm ents of V, D, J ligated to form gene encoding different variable regions in heavy chain of the antibody 2. Ref to idea that only 1 segment from each is chosen 3. Ref to idea that remaining gene segments removed 4. Ref to number of gene segments in context of question 5. Ref to different mRNA sequences leading to different amino acid sequences, and subsequently different specific 3D conformation of variable region (b)(i) Compare the increase in mean level of anti-X antibodies after vaccination and after infection with B. pertussis. [2] Any two: 1. Ref to levels of antibody rise earlier after infection 2. Ref to levels of antibody rise faster after infection 3. Ref to levels of antibody rise higher after infection 4. credit comparative manipulation of data (b)(ii) Explain the changes in mean level of anti-X antibodies after infection with B. pertussis. [2] 1. Ref to secondary immune response 2. Ref to memory cells / immunological memory 3. Ref to idea that (on infection / second exposure) memory cells are activated / stimulated (c)(i) Suggest why anti-Y antibodies were not present in the blood of these rats until after infection with B. pertussis. [1] Any one: 1. Ref to idea that antibodies will only be present if antigen present 2. Ref to idea that antigen Y is not present in vaccine 3. Ref to vaccination failed to stimulate immune response (c)(ii) Place a tick in the box next to the term that describes the type of immunity that results in the production of anti-Y antibodies. [1] Natural active (d) Explain how class switching allows for formation of IgG. [3] 1. Ref to one constant region gene segment from IgM is replaced with another of a different segment from IgG 2. Ref to gene segment coding for constant region of IgG is ligated with other exons 3. Ref to somatic recombination in activated B cells (e) Comment on the reliability of the data shown in Fig. 3.2. [2] Any two: 1. Ref to no indication of number of rats used 2. Ref to no data points indicated 3. Ref to no error bars (on graph) / no indication of variability 4. Ref to idea that no indication of experimental details / control group 5. Ref to idea that mean has been used therefore there must have been some repeats carried out [Total: 15]

HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examina