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 candidat