2022 J2 ASRJC H2 Biology Prelims Paper 3 (Ans)

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P3 [Turn over H2 ANDERSON SERANGOON JUNIOR COLLEGE HIGHER 2 ANSWERS 2022 JC2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS INDEX NUMBER BIOLOGY 9744/03 PAPER 3 LONG STRUCTURED AND FREE RESPONSE QUESTIONS Candidates answer on the Question Paper. No Additional Materials are required. 15 SEPTEMBER 2022 THURSDAY 2 HOURS READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graph Do not use paper clips, highlighters, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 13 printed pages and 1 blank page For Examiner’s Use 1 / 30 2 / 10 3 / 10 4 /5 / 25 Total / 75

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P3 Section A Answer all the questions in this section. 1 Lactose is a disaccharide found in milk. Lactase , an enzyme found in mammals and some fungi, catalyse the breakdown of lactose. The lactase enzyme is made up of 4 identical polypeptide chains. In humans, molecules of lactase are embedded in the cell surface membrane of epithelial cells lining the small intestine. As the lactose molecules float by in the lumen, they are broken down. (a) Explain how the polypeptide chains in lactase are held together and how they interact with the cell surface membrane. How polypeptide chains are held together 1. by R group interactions such as 2 named bonds: ionic bonds, hydrogen bonds, hydrophobic interactions ( accept disulfide bonds) how they interact with the cell surface membrane 2. Part of polypeptide chain with polar/charged (acidic and basic) / hydrophilic amino acids are able to associate with polar/ charged/ hydrophilic phosphate head of phospholipids via hydrogen/ ionic/ bonds. Reject polar/ charged protein 3. Parts of polypeptide chain with non-polar/ hydrophobic amino acids associate with non-polar/ hydrophobic fatty acid tails of phospholipids via hydrophobic interactions [3]

3 ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P3 [Turn over (b) The products of lactose digestion, glucose and galactose are actively absorbed by intestinal epithelial cells. This absorption is carried out by the sodium-glucose linked transporter (SGLT). SGLT is a secondary active transporter that works together with sodium-potassium (Na+-K+) pump. SGLT transports glucose and galactose concurrently with sodium ions (Na+) into the intestinal epithelial cells. This transport of glucose and galactose uses the driving force generated by the sodium ion gradient created by the Na+-K+ pump. Fig. 1.1 illustrates the transport process. Fig. 1.1 Cyanide is a poison that binds with cytochrome oxidase, one of the electron carriers in the mitochondrial membrane. It has been observed that the absorption of glucose and galactose from the lumen of the small intestine is reduced if the intestinal epithelial cells are treated with cyanide. Using Fig. 1.1 and all the information provided, explain why absorption of glucose and galactose from the lumen of the small intestine is reduced when the intestinal epithelial cells are treated with cyanide. 1. Less ATP produced via oxidative phosphorylation 2. less sodium ions actively transported out of intestinal cells via sodium potassium pump 3. high sodium concentration inside intestinal cells means less steep sodium concentration gradient across the intestinal membrane 4. less diffusion of sodium into cell via SGLT means less driving force generated for the 5. less active transport of products of lact ose digestion into cells, against their concentration gradients [4]

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P3 (c) Many human adults do not produce lactase and are lactose intolerant. This means they cannot digest lactose. Lactose intolerance leads to side -effects such as abdominal pain after eating food containing lactose Scientists have investigated ways to produce low -lactose cow’s milk from normal cow’s milk for people who are lactose intolerant. One method involved extracting lactase from fungi and mixing the extracte d lactase with normal cow’s milk. This method is, however, ineffective because one of the product s of lactose digestion, galactose, is an inhibitor of lactase. (i) Explain the effect of galactose on lactase activity. 1. Galactose has a shape complementary to allosteric site/ site away from active site on lactase 2. Binds (to allosteric site)and change the shape of the active site → no longer complementary to substrate lactose 3. Less successful / effective collisions between lactase and lactose → less ES complexes formed per unit time ( reduces lactase activity) Reject competitive inhibition because questions hinted “production inhibition” Accept allosteric inhibition [2] (ii) Explain why product inhibition is useful when lactase is acting as an intracellular enzyme in fungi cells but can be a disadvantage when extracted lactase is used free in solution for the production of low-lactose cow’s milk. (any 2) 1. idea of control / maintaining balance / efficient metabolism: e.g. if, (enough) glucose / galactose / monosaccharides, present then no need for, uptake / breakdown, of lactose 2. avoids osmotic problems as there is no build-up of monosaccharides accept converse for extracted lactase [2] Another method of producing low -lactose cow’s milk involved immobilising extracted lactase within alginate beads and putting in high-lactose cow’s milk. As the high-lactose cow’s milk comes into contact with the alginate beads, the immobilised lactase hydrolyses the lactose. Fig. 1.2 shows the set-up. Fig. 1.

5 ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P3 [Turn over (iii) Suggest how using immobilised lactase for the production of low -lactose cow’s milk helps to reduce the problem of product inhibition. 1. Galactose / end-product inhibitor and lactase kept separated 2. Galactose/ end-product inhibitor removed immediately Accept idea of enzyme are kept inside the beads because opening of the tape ensures that only product will leave / Keeping the enzymes on the beads ensure it will not flow out together with the product when the tap is opened But reject if answer talks about objective is to ensure enzyme can be re- used. [1] (d) A company producing low-lactose cow’s milk carried out an investigation to study the effect of drinking normal high-lactose cow’s milk and the com