SAJC 2018 H2-Bio-TYS-ANS

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 1 2018 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 Fig. 1.1 represents the molecular structure of a G-protein linked receptor in the cell surface membrane. Fig. 1.1 (a) Discuss how the arrangement of molecules in Fig. 1.1 supports the fluid mosaic model of the cell membrane. ……………………………………………………………………………...……………….. [3] 1 ‘Fluid’ refers to the phospholipids and proteins (such as G-protein linked receptors) being free to move within the membrane laterally 2 and the phospholipids also can move transversely 3 ‘Mosaic’ refers to proteins like G-protein linked receptors being embedded in the phospholipid bilayer in a scattered or random arrangement. Examiner’s comments: This question was well answered by the majority of candidates. Some responses were too vague and lacked specific details. For example, a number of candidates referred to ‘membrane molecules moving’ or ‘being randomly scattered’ without identifying these as proteins or phospholipids. Others provided a general description of the phospholipid bilayer, including details of hydrophobic and hydrophilic interactions, without considering Fig. 1.1.

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 3 (b) Explain how the molecular structure of the protein shown in Fig. 1.1 enables it to function as a G-protein linked receptor. ……………………………………………………………………………...……………….. [3] 1 The protein consists of an extracellular domain that binds to signal molecules/ligand on the extracellular side. [Reject: N and C terminus] 2 It also consists of a cytoplasmic domain that binds to G-proteins (on the cytoplasmic side). [Reject: N and C terminus] 3 It contains seven transmembrane segments which fold such that the hydrophobic amino acid residues are on the outside, to interact with the hydrophobic hydrocarbon chains of the phospholipids of the membrane. 4 Ref. hydrophilic amino acid residues to interact with the polar phosphate head of phospholipids 5 This anchors the G-protein linked receptor in the membrane and allows it to function on both sides of the membrane. [Max 2 for points 3-5] Examiner’s comments: Most candidates provided good responses. Some went into excessive detail about protein structure in general, rather than focusing on how the molecular structure of this protein was related to its function. (c) A large range of stimuli can trigger the activation of G-proteins through G-protein linked receptors. These stimuli include light, calcium ions and hormones such as glucagon. State the common effect that these diverse stimuli have on G-protein linked receptors. ……………………………………………………………………………...……………….. [1] 1 They cause a change in 3D conformation of the (G-protein linked) receptors, exposing its cytoplasmic domain for the binding of a G-protein. Examiner’s comments: The majority of candidates recognised the importance of conformational changes in G-protein linked receptors. (d) Explain how activation of G-protein by the binding of glucagon to the G-protein linked receptor triggers downstream signalling pathways that result in a cascade of enzyme-catalysed reactions. ……………………………………………………………………………...……………….. [3] 1 Binding of glucagon to ligand-binding site of G-protein linked receptor results in conformational changes in the receptor, exposing the cytosolic G- protein binding domain 2 A G-protein binds, exchange of GDP for GTP occurs, GTP-bound G-protein is activated. 3 G-protein dissociates from receptor and moves along the plasma membrane to activate an enzyme (embedded in the plasma membrane). 4 Activated enzyme can trigger the next step in the pathway, leading to a cellular response.

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 4 Examiner’s comments: Most candidates developed detailed responses that showed good understanding of the glucagon signalling pathway. Some referred to the subsequent effects within the cell rather than the effect of the stimulus on the receptor protein itself. [Total: 10]

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 5 QUESTION 2 Fig. 2.1 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. 2.1 (a) Describe, with reference to Fig. 2.1, the effect of temperature on the rate of protein digestion by thermitase. ……………………………………………………………………………...……………….. [3] 1 When temperature increases from 10°C to 76°C, the rate of protein digestion by thermitase increases from 0 to 650 arbitrary units. 2 The optimum temperature for thermitase is at 76°C, where maximum rate of protein digestion is 650 arbitrary units. 3 When temperature increases from 76°C to 90°C, the rate of protein digestion decreases sharply from 650 to 350 arbitrary units. Examiner’s Comments: Most candidates gave sufficiently detailed responses. Some did not adhere to the single command word in the question, ‘describe’, and provided elaborate explanations about why the described changes occurred. Errors included responses that mistakenly referred to all three enzymes and responses that quoted incorrect readings from the graph. (b) Explain the effect on thermitase of increasing the temperature above 80°C.

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 6 ……………………………………………………………………………...……………….. [3] 1 Increasing temperature above 80°C increases the kinetic energy of enzyme molecules, resulting in thermal agitation. 2 Intramolecular bonds such as hydrogen bonds, ionic interactions and hydrophobic interactions between R groups are disrupted. 3 Thermitase is denatured, loses the 3D conformation of its active sites. 4 Substrate molecules can no longer bind effectively to the active sites, and hence decreases rate of protein digestion. Examiner’s Comments: Almost all candidates demonstrated a sound understanding of the relevant principles and many good answers were seen. Some responses commented on conformational changes to the active site without indicating the consequences of this, such as loss of active site or poorer ‘fit’ of substrate. (c) Modified subtilisin is similar to subtilisin, but has had eight of its amino acids replaced with different amino acids. Describe and explain the effect of this modification on the activity of subtilisin. ……………………………………………………………………………...……………….. [4] Describe [max 2 marks] 1 The modified subtilisin has a greater rate of protein digestion from temperatures of 10 - 90°C. 2 Modified subtilisin has a greater optimum temperature of 76°C than that of unmodified subtilisin, which is at 60°C. 3 Modified subtilisin is more thermostable as it is fully denatured at higher temperature of 90°C whereas unmodified subtilisin fully denatures at a lower temperature of 76°C. Explain 4 The eight substituted amino acids are cysteine residues which can form disulfide bonds between their R groups. 5 Disulfide bonds are strong covalent bonds which are not disrupted at high temperatures, they maintain the 3D conformation of the modified subtilisin at temperatures beyond 76°C. Examiner’s Comments: Most candidates were able to describe effects of the described modification on the activity of subtilisin. Fewer were able to explain how changes in the amino acids could result in a more thermostable enzyme. Of those who did, many candidates correctly referred to stronger bonds without providing more specific details. Some candidates did not focus on the modified enzyme shown in the graph but discussed, instead, changes in amino acids that could result in a reduction, or no change, in enzyme activity. [Total: 10]

2018 ‘A’ Levels Examination H2 Biology Mark Scheme 7 QUESTION 3 (a) Define each of the following types