VS 2018 O Level Pure Biology P2 MS w Feedback

SUGGESTED MARK SCHEME – BIOLOGY 2018 Answer Key – Paper 1 1 2 3 4 5 6 7 8 9 10 A A B B C D C A B A 11 12 13 14 15 16 17 18 19 20 C B C D A B C C B D 21 22 23 24 25 26 27 28 29 30 B D A D C C D B B A 31 32 33 34 35 36 37 38 39 40 C D A C D A C B D D Paper 2

Section A Qn No Answers Mark Feedback 1(a)(i) chromatid 1 1(a)(ii) stage 3: metaphase stage 4: anaphase 1 1 1(b)(i) 2 1 1(b)(ii) plant 1: AA plant 3: aa 1 1 2(a)(i) any three from 1. produces bile for emulsification 2. deamination of excess amino acids 3. detoxification of alcohol 4. onverts excess glucose to glycogen 1 1 1 Most knew the 2 correct functions. Some weaker answers lacked precision e.g. ref to maintaining blood glucose levels. 2(a)(ii) C 1 C = pancreas exocrine: secretes enzymes in pancreatic juice) endocrine: secretes insulin & glucagon 2(a)(iii) bile emulsifies large fat molecules to smaller fat globules to increase SA:V (for efficient digestion by lipase) 1 1 2(b)(i) 40 °C 1 2(b)(ii) rennin has denatured at 60 °C and is no longer able to catalyse the clotting of milk. 1 Denaturation was invariably correctly stated. Some candidates included a detailed description of denaturation which was not required. 2(c) clotting of the milk proteins allows the proteins to retain longer in the stomach; provides more time for proteins to be digested to polypeptides. 1 1 Many candidates correctly referred to rennin causing milk to clot or become solid 3(a)(i) 3 minutes 1 units required

3(a)(ii) increase of temperature from 15°C to 25°C results in a decrease of time from 24 to 6 minutes (manipulated data ); further increase of temperature from 25°C to 35°C reduces time from 6 to 2.5 minutes. 1 1 Correctly quoted data in support of their statement 3(b) clotting of blood stops the loss of blood when wounds occur; prevents bacteria and other pathogens from entering the body; 1 1 Weaker students ref to prevention of unqualified ‘foreign n harmful particles’ 4(a) 1. each alveolus has a one-cell thick wall and is very thin → short distance for higher rate of diffusion of oxygen and carbon dioxide between alveoli and capillaries 2. inner wall of each alveolus has a thin film of moisture on it → allows oxygen to dissolve before diffusing 3. each alveolus is surrounded by a network of capillarie s → to maintain a steep concentration gradient for higher rate of diffusion of gases 1 1 1 Most candidates knew the adaptations of alveoli for the exchange of gases. Imprecise answers included stating that the alveolus itself was one cell thick (rather than referring to its wall) or not emphasising that the alveolus was well supplied with blood. 4(b) carbonic anhydrase, found in RBC, catalyses the reaction of carbon dioxide with water to form carbonic acid, which allows it to be transported in blood with reduced carbonic anhydrase, less carbon dioxide can be excreted at the lungs. high concentration of carbon dioxide in the blood stimulates breathing; leads to hyperventilation/ breathing excessively 1 1 1 4(c)(i) during breathing in the 1st sec, pressure i n alveoli decreases from 0 to -1.0 arbitrary units; in the 2nd sec, pressure increases by 1 unit gradually back to 0 1 1 Almost all candidates chose the correct part of the graph to describe and identified a pressure decrease followed by an increase. Data quotes were usually correct; the most common error was to omit the time from the quote.

4(c)(ii) when breathing in, diaphragm muscle contracts and flattens; external intercostal muscles contract (internal intercostal muscles relax); ribcage swings upwards and outwards; lungs volume increases which leads to drop in pressure in the thoracic cavity; atmospheric pressure > pressure in the thoracic cavity; air rushes in 1 1 1 There were some good responses to this question, with candidates showing a sound understanding of the roles of the external intercostal muscles and the diaphragm. Weaker responses omitted the role of the ribcage or contained contradictions, e.g. the diaphragm contracts and rises. 5 (a)(i) 173.5 (glucose/ g) 1 5 (a)(ii) percentage of water that was excreted = 180.0 - 175.5 / 180.0 X 100% = 2.5 % 1 5 (a)(iii) urea 1 5(b) Ultrafiltration: A has minute pore s allowing only small substances to pass through Ultrafiltration occurs at A to force small substances like glucose, urea and water out of the glomerulus into the Bowman’s capsule, due to the high hydrostatic pressure formed. Large substances like proteins are unable to pass through the basement membrane. Selective reabsorption: blood circulating constantly through B creates a steep concentration gradient Useful substances such as water and glucose are then selectively reabsorbed back into the capillarie s at the proximal convoluted tubule by diffusion and active transport . Blood passes through B and returns to the main circulation via renal vein. Waste materials including excess water, salts and nitrogenous waste that are not reaborbed form urine. 1 1 1 Candidates showed a good knowledge of the role of structure A; imprecise answers were often seen. For example, the description of the blood in the glomerulus as having ‘hydrostatic pressure’, rather than having ‘high hydrostatic pressure’; or small molecules being forced out of the glomerulus, without identifying where they were going. The term ultrafiltration was well known. The role of structure B was less well known with many describing it as simply ‘returning blood to the body or the heart.’ 6(a) Contains xylem vessel and phloem vessel; Almost all candidates knew that A transported water and food, which was

Both of them are parts of the plant transport system; xylem vessel transports water and dissolved mineral salt s from the leaf stalk to the different parts of the leaf for photosynthesis; while phloem transports excess manufactured food like sucrose and amino acids away from the leaf to other parts of the plant . 1 1 sometimes clarified as sucrose. A common misconception was that glucose, rather than sucrose, was transported by the phloem. 6(b)(i) sucrose 1 6(b)(ii) phloem (tissue) 1 6(b)(iii) rate of movement = 65 cm / 2.5 h ; = 26 cm per hour ; 1 1 7(a) During the process P, living organisms like animals and green plants oxidise glucose to release energy for their life processes; respiration, they will also give out carbon dioxide into the atmosphere. During process Q, which is photosynthesis , plants take in CO2 to manufacture glucose, which is essential as a source of food for all organisms and the locked C serves as a carbon sink. Process S, combustion, where energy is released from fossil fuel and CO2 is released to t