VJC 2021 H2 Bio 9744 P2 Answers

1 Victoria Junior College Biology Department 2021 Prelims H2 Paper 2 – Proposed Answers 1 (a) Outline how the enzymes are packaged into vesicles and release to the outside of the cell. [4] 1. Enzymes synthesised in rough endoplasmic reticulum (rER) are packaged into transport vesicles pinched off from rER and fuse with cis/forming face of Golgi apparatus; 2. At Golgi apparatus, modification, sorting and packaging of enzymes occur; 3. Secretory vesicles (A: Golgi vesicles) containing enzymes buds off trans/maturing face of Golgi apparatus; 4. Secretory vesicles move to and fuse with cell surface membrane, releasing the enzymes via exocytosis; [Points 5 and 6 – 1m max] 5. Vesicles move towards cell surface membrane with the help of microtubules; 6. ATP is required for exocytosis; (b) Explain how the structure of the vesicle allows it to serve its function shown in Fig. 1. 1. [3] 1. Structure of vesicle: made up of a phospholipid bilayer; 2. Phospholipids (PL) provides fluidity, which allows for fusion of vesicle membrane with cell surface membrane for release of enzymes to the outside of the cell; 3. The hydrophilic phosphate heads of the PL interact with the aqueous environment/cytoplasm and within the vesicle for stability; Or hydrophilic phosphate heads of the PL interact with the aqueous environment of the cytoplasm to allow for transport of vesicles within the cell; 4. Hydrocarbon tails of the phospholipids forms the hydrophobic core of the membrane prevents the enzymes from moving out of the vesicles while they are being transported; 5. AVP; (c) Suggest and explain one modification to the vesicles to allow them to deliver the drugs to specific cells. [2] • Modification to vesicle [Any one below] ➢ Add glycoproteins/ glycolipids add to the phospholipids on the outer surface of the vesicle; ➢ Attach carbohydrate specific epitopes/ antigen to the phospholipids on the outer surface of the vesicle; which will recognise and bind to specific receptors found only on the specific cells by complementary shape; • How uptake can take place: (binding of glycoproteins on vesicle to specific receptors on specific cell results in ) receptor-mediated endocytosis/ endocytosis/ fusion, allowing uptake by cells;

2 (d) Describe two differences between the release of enzymes shown in Fig 1.1 and the process in which flu virus leave the host cells. [2] Release of enzymes Process which flu virus leave the host cells [Any 1 of the following] • Exocytosis • involves fusion of vesicle membrane with cell surface membrane • Addition of cell membrane • Budding • Involves evagination of host cell membrane; • Involves removal/ loss of cell membrane; R: Budding vs no budding, fusion vs no fusion, loss vs no loss of membrane etc – when there is an equivalent process that occurs, students should describe the process. [Any 1 of the following] • Vesicle can exit / leave / fuse at any part of the cell membrane Any idea of non-specific sites vs specific sites/ do not require specific glycoproteins for exit vs require specific glycoproteins for exit. • Viruses leave the host cells a specific exit points that have haemagglutinin and neuraminidase / glycoproteins embedded; R: release of viruses results in lysis of host cells whereas release of enzyme does not as there is no lysis. Host cells die when substantial amount of cell membrane is lost via budding. R: release of viruses kills the host cell as it depends on the extent of budding. Not all host cells will die. 2 Fig 2.1 shows the structure of a natural triglyceride. Fig. 2.1 (a) (i) On Fig. 2.1, circle and label all the saturated and unsaturated fatty acid chains. [2] 1. Correct circle and label of saturated fatty acid chain; 2. Correct circle and label of unsaturated fatty acid chains; Or

3 3. Correct circle for saturated and unsaturated fatty acid chains 4. Correct label of saturated and unsaturated fatty acid chains (ii) Explain how the structure and properties of triglyceride is related to its role in living organisms. [3] 1. 3 long hydrocarbon chains/ large number of carbon-hydrogen bonds for energy store/ which can be oxidised to provide energy/ form ATP; 2. The hydrocarbon chains are also hydrophobic / non-polar which enables the molecule to be insoluble in water such that it will not affect the water potential of the cells; 3. Being non-polar / dehydrated means triglyceride can be packed compactly within cells as energy store; 4. AVP – maximum two Large number of hydrogen atoms which yields metabolic water upon oxidation; Less dense than water which provides buoyancy to aquatic mammals; Good heat/ thermal insulator which prevents excessive heat loss in animals/ keep aquatic mammals warm; (b) Triglycerides, such as the one shown in Fig. 2.1, can be catalysed by lipase. Fig. 2.2 shows the reaction. With reference to Fig. 2.2, explain the mode of action of lipase. [4] 1. Lipase catalyses the hydrolysis of triglyceride to form fatty acids and glycerol; 2. Binding of triglyceride to the active site of lipase causes lipase to have a slight conformation change, resulting in the triglyceride fitting more snugly in the active site; Ref. to the induced-fit hypothesis; 3. Formation of the enzyme-substrate c omplex lowers the activation energy of triglyceride hydrolysis; 4. By placing the ester bond between glycerol and a fatty acid chain is placed under physical stress; 5. And catalytic residues in the active site alter the distribution of electrons within the ester bond; 3(a) (i) With reference to Fig. 3.1, describe one difference and two similarities between the organization of polypeptide chains in keratin and collagen. [3] • difference – 3 polypeptide chains wound around each other to form a tropocollagen vs 2 polypeptide chains assemble to form each dimer; • similarities – aggregration of fibrils to form fibres in both keratin and collagen; • staggered arrangement of tropocollagen (collagen) and the keratin dimer (keratin) to form fibril; (ii) Suggest how the structural organisation of keratin enable it to fulfil its function. [2] • linear structure increase surface area for intra-chain bonds to form between chains / aggregration of fibrils into fibres → confer high tensile strength and ability to withstand stress; • staggered arrangement – remove presence of weak areas in the fibril; • no available OH-groups for interaction with H2O → insoluble in water; (b)(i) Explain the significance of the amino acid sequence to the function of collagen. [4] • rich in amino acids glycine and proline / Every 3rd amino acid in the chain is a glycine (smallest amino acid) / repeating sequence of gly-x-y → forming a kinked helix;

4 • idea of each polypeptide chain able to wind more tightly around each other to form tropocollagen; • contains modified amino acids hydroxyproline and/or hydroxylysine; • idea that the OH groups (of modified amino acids) allow for formation of H b onds between polypeptide chains → high tensile strength; • presence of lysine residues allowing for covalent cross -links to be forme d between tropocollagen; Max 4 (ii) Suggest why assembly of collagen cannot take place in the cytoplasm. [1] • after assembly, it will be too large to pass through cell membrane (phospholipid bilayer) directly; 4(a) With reference to Fig 4.1 and your knowledge, suggest how phosphorylation of condensins and lamins prepares the cell for mitosis. [2] • Phosphorylation changes conformation of condensins/ activates condensins, allowing for chromatin to pack more tightly/ condense into distinct chromosomes; • Phosphorylation changes conformation/stability of lamins / deactivates/inactivates lamins, resulting in the breaks down / disintegration of the nuclear membrane / envelope into smal