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 kinase A (PKA);

5. Activation of PKA will initiate a sequential activation of kinases resulting in a (phosphorylation cascade) to eventually activate glycogen phosphorylase for breakdown of glycogen; [Total: 10] 2 Fig. 2.1 shows the effect of increasing temperature on the activity of three protein -digesting enzymes: • thermitase from thermophilic Termoactinomyces vulgaris • subtilisin from Bacillus subtilis • modified subtilisin (a) Describe, with reference to Fig. 2.1, the effect of temperature on the rate of protein digestion by thermitase. [3] 1. As temperature increases from 10°C to 76°C , the rate of protein digestion by thermitase increases from 0 arbitrary units(a.u) to 650 arbitrary units(a.u); 2. The optimum temperature of the enzyme thermitase is 76°C where there is maximum rate of protein digestion; 3. Further increase in temperature from 76°C to 90°C will cause the rate of protein digestion decreases sharply from 650 a.u to 355 a.u; (b) Explain the effect on thermitase of increasing the temperature above 80°C. [3] 1. As the temperature increases above 80°C, there will be greater (intramolecular) vibrations/ thermal agitation of the enzyme thermitase; 2. which breaks hydrogen, ionic bonds and other weak interactions that stabilizes the 3D conformation resulting in denaturation*; 3. The enzyme active site* no longer complementary in shape* and charge to the substrate; and 4. the rate of reaction decreases steeply from a rate of 570 a.u. to 340 a.u.; (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 1. Modified subtilisin has a higher rate of protein digestion across all temperatures compared to subtilisin; or 2. Modified subtilisin has a wider range of temperatures in can work in, from 10 °C to 90°C compared to subtilisin which works from 10°C to 74°C; 3. Modified subtilisin has an optimum temperature of 76°C compared to subtilisin which has an optimum temperature of 59°C; 4. The maximum rate of protein digestion in the modified subtilisin is 4 times higher than in subtilisin (i.e. 319 a.u for modified subtilisin and 78 a.u for subtilisin); Explain 5. The replacement of eight amino acids in the modified subtilisin has resulted in a more thermostable enzyme;

6. The R groups* of these amino acids are able to form stronger bonds, such as strong covalent bonds like disulfide linkages between cysteine residues, which can only be broken at higher temperatures; 7. H ence, the conformation of the active site remains stable and is able to catalyse the reaction longer and at higher temperatures; [Total: 10] 3(a) Define each of the following types of stem cell and name a naturally occurring example of each. (i) totipotent [2] 1. They have the ability to differentiate* into all cell types that make up an organism including extra-embryonic tissue* such as placenta*, and hence are able to form entire organism; 2. e.g. zygotic stem cells/ fertilised egg up to 8 cell stage/first three divisions; (ii) pluripotent [2] 1. They have the ability to differentiate* into almost all of the cell types that make up an organism except extraembryonic tissue* such as placenta* but cannot form the entire organism alone; 2. e.g. inner cell mass of blastocyst*; (iii) multipotent [2] 1. They have the ability to differentiate* into only a limited and related range of cell types and tissues in an organism to replace dead cells that died; 2. e.g blood/haematopoietic stem cells in bone marrow; (b) Induced pluripotent stem cells (iPSCs) are a type of pluripotent stem cell that are produced directly from adult cells. To produce iPSCs, four genes need to be introduced into adult cells. These genes code for transcription factors. Explain why these transcription factors are necessary for the production of iPSCs from adult cells. [2] 1. These transcription factors such as repressors/activators* bind to the silencers/enhancers*; 2. initiate the transcription of inactive genes in the differentiated somatic adult cells; 3. and altered the gene expression patterns of the cells such that it was similar to that of embryonic stem cells; resulting in the production of iPSCs A: upregulate proto oncogene (see below for example)/inhibit tumour suppressor gene. 4. One of these transcriptions factors is an activator that binds to the enhancer of a proto oncogene; 5. That upregulates transcription leading to increased cell proliferation; 6. Example is a protein that promotes cell division e.g. cyclin, growth factors; (c) Explain how the use of iPSCs may overcome some of the ethical concerns of using other types of stem cells in medical research and treatment. [2] 1. As iPS cells are not from an embryo, A. there will be no dest