2024 TJC H2 Bio P2 MS

1 JC2 PRELIMS 2024 H2 PAPER 2 1 (a) Fig. 1.1 is a diagram of a section through a mitochondrion. Fig. 1.1 (i) On Fig 1.1 , use label lines and letters to label the positions where the following reactions take place: [3] X - Link reaction Y - Krebs cycle Z - Oxidative phosphorylation [occurs in inner mitochondrial membrane] 1 mark each (ii) The four arrows, A, B, C and D, show the movement of molecules and ions. Use the letters to identify all the arrows (one or more) that show: [2] Active transport of protons D Diffusion of carbon dioxide C and D (b) Compare the process of oxidative phosphorylation with photophosphorylation. [2] SIMILARITIES (Any 1): S1. Electrons passed down electron carriers of decreasing energ y level in electron transport chain in both processes. S2. Pumping of H+ across membrane to create steep proton gradient in both processes. S3. D i f f u s i o n of H+ via hydrophilic channel of ATP synthase (stalked particle) to synthesize ATP in both processes. S4. Use of energy released from electrons transported down energy level i n electron transport chain to pump H+ to create proton gradient in both processes. DIFFERENCES (Any 1): X and Y Z [occurs in matrix of mitochondrion] O2 and pyruvate (from glycolysis) enter the mitochondrion CO2 leaves the mitochondrion CO2 leaves the mitochondrion & H+ pumped from matrix to IMS H+ diffuses from IMS into matrix

2 Feature of comparison Photophosphorylation Oxidative phosphorylation D1. Location  Thylakoid of chloroplasts  Inner mitochondrial membrane D2. Source of electrons  [Non-cyclic] Water  [ C y c l i c ] PS I  NADH  FADH2 D3. Final electron acceptor  [Non-cyclic] NADP+  [ C y c l i c ] PS I  O2 D4. Products formed  NADPH  H2O D5. Requirement of light energy  Yes for photolysis of water  No D6. Source of energy  Light  Oxidation of glucose D7. Direction of H+ pumped to generate steep proton gradient  Pumped from stroma to thylakoid space  Pumped from matrix to intermembrane space D8. Direction of H+ diffusion to synthesize ATP  Diffusion from thylakoid space to stroma  Diffusion from intermembrane space to matrix (c) Apart from channel proteins that allow transport of ions , plant and animals cells also have channel proteins such as aquaporins which permits the movement of water across membranes. Explain why aquaporins are necessary. [3] 1. Cell surface membrane is made up of phospholipid bilayer 2. Has a hydrophobic boundary/core due to presence of non-polar fatty acid tails 3. Water molecules are small and polar 4. Only small number of water molecules can move directly across the cell surface membrane (i.e. rate of movement of water molecules is slow) 5. Aquaporins provide hydrophilic channel (due to polar amino acids line the interior part of aquaporins to interact with the water molecules) 6. Allowing large number of water molecules (i.e. rate of movement of water molecules is faster) to move across membrane via osmosis. [Total: 10] reasons

3 2 Fig. 2.1 shows the primary structure of a section of a polypeptide chain of collagen Fig. 2.1 (a) Explain how the primary structure shown in Fig. 2.1 indicates that the structure of the polypeptide is suited to be a component of a collagen molecule. [3] Max marking (1/2m each point) 1. Every third amino acid in the polypeptide is glycine. 2. The R-group of glycine is a H atom and is the only R-group t hat is small enough to fit into the centre of the triple helix (Note: not collagen). 3. This allows close association of the three polypeptide chains (note: not collagen). 4. Glycine (-NH) can form hydrogen bonds with C=O group in prol ine other polypeptides of triple helix. (Accept: idea that H bonds can form with other polypeptides in the triple helix) 5. resulting in a stable helical structure 6. Hydrophobic R-groups of pro line residues will project on the exterior of the triple helix. 7. insoluble molecule. 8. consists mainly of repeated glycine X Y sequences. 9. repeating organisation, 10. contributes to a stable helical structure. Fibroblasts are cells that synthesize and secrete collagen, which forms the extracellular matrix. Hydrolytic enzymes, known as collagenases, are secreted by some cells during wound healing. These cells also secrete inhibitors of collagenases. The activity of the enzymes and inhibitors is regulated so that the development and maintenance of the extracellular matrix is controlled. (b) State and explain what the outcome will be for the composition of the extracellular matrix if collagenase inhibitor activity is high. [2] 1. Higher collagen concentrati on / more collagen present. [must have, 1/2m] 2. Collagen not hydrolysed. Accept: less hydrolysis, 3. If competitive inhibitor, o compete with collagen for the active site, block collagen from binding to active site OR 3. If non-competitive inhibitor, o bind to a site other than active site and change the shape of the (active site) enzyme, collagen cannot bind to active site. 4. no / few, ESC / enzyme substrate complexes form. Thus collagen not hydrolyze. reasons

4 Collagenase has several important medical uses, such as in the treatment of burnt skin. Scientists investigated the effect of pH on the activity of collagenase at 37 °C. The results of their investigation are shown in Fig. 2.2. Fig. 2.2 (c) Explain why the activity of collagenase is lower at pH 8.0 than at the optimum pH. [3] ½ mark each 1. At pH lower than optimum pH, H + concentration is changed / decreased. 2. This alters ionic charges on the basic and acidic R-groups o f amino acid residues on enzyme molecule. 3. Ionic bonds are disrupted, a nd substrate binding is affected. 4. Shape of active site is changed and is less complementary to shape of substrate. 5. Rate of effective collision decreases and less enzyme-substr ate complex formed per unit time. 6. Less products formed. Synthetic inhibitors have been trialed as potential treatment for diseases which are caused by a lack of regulation of collagenase activity. Fig. 2.3 shows the rate of reaction of collagenase in the absence of the synthetic inhibitor. Fig. 2.3 Collagen

5 (d) Sketch on Fig. 2.3 the curve that is expected if the synthetic inhibitor used in the trial is a non-competitive inhibitor. [1] Note: The 2 graphs cannot overlap at the initial rate of rxn. Separate right for the start. [Total: 9] Collagen

6 3 Adult stem cells in a tissue are often at different stages of the cell cycle. (a) Fig. 3.1 shows cells at different stages of the cell cycle. Fig. 3.1 (i) Identify the stages of mitosis occurring in the cells labelled B and C in Fig. 3.1. [2] B prophase C metaphase (ii) Describe the behaviour of the chromosomes in the stage of mitosis shown in cell A.[2] 1. The centromere of each chromosome divides (reject split), 2. causing the sister chromatids of each chromosome to separate. 3. The sister chromatids move to opposite poles (reject ends/ respective ends) of the cell, centromeres first/ led by centromeres first. 4. This is due to the shortening of the spindle fibres. (b) Distinguish between adult stem cells and zygotic