2023 TMJC A Level Paper 2 (A)

1 2023 ‘A’ Level Examinations H2/9744 Biology Paper 2 General comments by Cambridge examiners The candidates’ responses were generally of a high standard and displayed very good subject knowledge. Most candidates were able to accurately interpret graphical information and apply their knowledge to novel situations. Candidates did not always use correct scientific terminology and there was a tendency for responses to include irrelevant details that did not address the requirements of the question. Explanations were sometimes not fully developed.

2 QUESTION 1 Fig. 1.1 shows the effect of increasing the external concentration of glucose or fatty acids on the rate of uptake of these molecules into a cell. Fig. 1.1 (a) With reference to Fig. 1.1, explain the effect of increasing the external concentration of glucose on the rate of uptake of glucose into a cell. [3] 1. [describe] Rate of uptake into the cell via facilitated diffusi on increases as external concentration increases. 2. [explain] Due to increase in glucose concentration gradient. 3. [describe] Rate no longer increases beyond a certain external concentration. 4. [explain] The glucose transporters are all saturated with glucose. Examiners’ Comments Many responses clearly explained the relationship between glucose concentration and glucose uptake. Weak responses often did not consider the specific mechanism by which glucose molecules cross cell surface membranes or recognise the role of specific membrane components in this process. Some described the relationship shown in the graph rather than trying to explain it. (b) Explain why there is no uptake of glucose into the cell at X even though glucose is present outside the cell. [1] 1. The concentration gradient is not steep enough for glucose to diffuse into the cell / the concentration of glucose outside and inside the cell are at equilibrium. 2. The affinity of glucose transporter is not high enough to bind to glucose at such low concentrations. Examiners’ Comments Most candidates were able to give a correct explanation. Some responses incorrectly referred to a lag time or a lack of ATP.

3 (c) One suggested mechanism for the transport of fatty acids into cells is shown in Fig. 1.2. This mechanism is referred to as a flip-flop mechanism. Fig. 1.2

4 With reference to Fig. 1.2, explain the flip-flop mechanism for the transport of fatty acid molecules across the cell surface membrane, including the role of hydrogen ions. [5] 1. Fatty acid contains a long hydrophobic hydrocarbon and a carboxylate group / COO− 2. Due to the charged COO− group, fatty acid cannot interact with the hydrophobic fatty acid tail of the membrane to be directly transported. 3. The hydrocarbon of the fatty acid first inserts into the outer phospholipid layer of the membrane, with the COO− interacting with the phosphate grou