RI 2018 H2 Bio Paper 2

2018 H2 A level Paper 2 Answer all questions. 1 Fig. 1.1 represents the molecular structure of a G -protein linked receptor in the cell surface membrane. (a) Discuss how the arrangement of molecules in Fig. 1.1 supports the fluid mosaic model of the cell membrane. [3] 1. It is referred to as ‘fluid” because the cell membrane comprises of phospholipids* and a G- protein linked receptor* which are free to move laterally within a layer; 2. Presence of cholesterol* molecules within each phospholipid layer increases the fluidity of the membrane; 3. It is referred to as ‘mosaic” because the random arrangement of the proteins embedded amongst the phospholipid molecules resemble a mosaic pattern; Teacher’s comment: It is insufficient to state that proteins and lipids are components of the membrane. How they are arranged must also be stated. Using descriptions like the proteins and phospholipids are arranged like ‘mosaic tiles’ is insufficient to gain any marks. (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. G protein linked receptor (GPCR) a seven pass transmembrane protein consisting of 7 α - helices* connected by three intracellular and three extracellular peptide loops. 2. The intracellular domain/cytoplasmic side of GPCR has a G protein binding site* that allows binding of a heterotrimeric G protein complex. 3. The ex tracellular loops has a ligand binding site* at which a specific* signaling molecule can bind to the GPCR. 4. W hen a ligand binds to the ligand binding site at the extracellular side of a GPCR it causes a conformational change* of the intracellular domain /at the cytoplasmic side of the GPCR, 5. The activated GPCR can then activate an associated G protein by exchanging its bound GDP for a GTP. (c) A large range of stimuli can trigger the activation of G-proteins through G-protein linked receptors. These stimuli can 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. The stimuli can trigger a conformational change * in the intracellular domain /at the cytoplasmic side of the GPCR, activating the receptor. (d) Explain how the activation of the 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. When glucagon binds a G-protein linked receptor ( GPCR), the GPCR will undergo a conformation change in intracellular domain allowing G-protein to bind to it; and 2. G- protein is activated when it displaces its attached GDP for GTP; 3. Activated G -protein will translocate along membrane and bind to enzyme adenylyl cyclase and phosphorylate it, thus activating it; 4. Adenylyl cyclase will catalyze conversion of ATP to cAMP, which binds to and activates protein ki