RI Nov 17 H2 P2 ans

Raffles Institution Nov 2017 (H2 Biology) Paper 2 2023 Nov 2017 H2 Bio Paper 2 N17P2Q1 (a) (i) State the number of glycogen molecules shown in Fig. 1.1. [1] 2 (ii) Name the type of molecule forming the central core of the glycogen granule, in Fig. 1.1. [1] Protein / polypeptide (iii) Suggest the role of the molecule forming the central core of the glycogen granule. [1] 1. binds / holds 2 glycogen molecules together; 2. enzymes that catalyse the synthesis of glycogen (b) Explain how the structure of glycogen is related to its role in living organisms. [4] 1. Large molecule/long chain so insoluble in water ; 2. Hence does not affect water potential of cells / no osmotic effect ; 3. Helical molecule so more glucose* units per unit volume, thus compact structure for storage ; 4. Extensively branched so provides many ends for hydrolysis of glycosidic bonds ; 5. which releases more glucose* molecules per unit time ; (c) Describe how the structure of cellulose is different from the structure of glycogen. [3] Cellulose Glycogen monomer β-glucose* α-glucose* bonds β-1,4 glycosidic bonds* α-1,4 and α-1,6 glycosidic bonds* Orientation of monomer Alternate glucose units are rotated 180 0/inverted with respect to each other All glucose units in the chain have same orientation Structure of each molecule forming straight / linear unbranched chains ; Forms helical coil which is extensively branched ; Bonds between molecules Hydroxyl groups projecting outwards in both directions allow intermolecular/ interchain hydrogen bonding  leading to microfibril formation for cellulose No interchain hydrogen bonding in glycogen Note: must be written in full sentences

Raffles Institution Nov 2017 (H2 Biology) Paper 2 2023 N17P2Q2 (a) (i) Explain why proteins are required for the transport of glucose across the cell surface membrane. [3] 1. Glucose is polar* and thus, hydrophilic*; 2. Glucose is too large to pass through the transient pores of the phospholipid bilayer ; 3. The hydrophobic core* of the phospholipid bilayer would repel glucose ; 4. Transport proteins / protein channel provides a hydrophilic* channel/pore through the membrane for the passage of the solutes; (ii) Describe the structural features of a protein that enable it to transport glucose into a cell. [2] 1. The protein is a transmembrane protein that; 2. has amino acid* residues with R groups that is non-polar / uncharged, allowing it to form hydrophobic interactions* with the hydrophobic core* of the phospholipid bilayer*; 3. The protein has a hydrophilic* pore/channel that is lined with amino acids* with polar / charged R groups that allows hydrophilic glucose molecules to pass through the membrane; (iii) State the letter of the protein in Fig 2.1 that could transport glucose into a cell. [1] Q (b) Using the information shown in Fig 2.2, describe and explain the effect of increasing the external glucose concentration on the rate of glucose uptake into a cell. [4] Describe: 1. When external concentration of glucose increase from 0 - 1 mmol dm -3, rate of glucose uptake increases sharply from 0 – 180 arbitrary units; 2. When external concentration of glucose increases from 1 – 11 mmol dm-3, rate of glucose uptake increases from 180 – 395 arbitrary units, but rate of glucose uptake increase less with each unit increase in external glucose concentration; 3. Rate of glucose uptake plateaus at 395 arbitrary units when external glucose concentration increase from 11 - 12 mmol dm-3; [A if no other pts awarded: Increase in external concentration of glucose from 0 - 11 mmol dm-3, increases the rate of glucose uptake from 0 to 395 arbitrary units.] Explain: 4. Increase in external glucose concentration increases concentration gradient of glucose across the membrane; 5. Maximum rate of glucose uptake is reached when all glucose transporters/transport proteins are saturated

Raffles Institution Nov 2017 (H2 Biology) Paper 2 2023 N17P2Q3 (a) Describe the main features of the molecular structures of haemoglobin and collagen, visible in Fig. 3.1. [5] Haemoglobin: 1. Haemoglobin is a globular protein; 2. It has a quaternary structure and comprises 2 α-globin* subunits and 2 β-globin* subunits; 3. Each subunit is associated with a haem* prosthetic group; Collagen: 4. Collagen is a fibrous protein; 5. It has both a secondary struc and a quaternary structure and comprises 3 polypeptide chains coiled around each other; 6. Each polypeptide is a loose helix; (b) Explain how the molecular structures of haemoglobin and collagen are related to their functions. [5] Haemoglobin Molecular structure Function 1. Each subunit is bound to a haem* prosthetic group; Allows for each subunit to bind to an oxygen molecule; 2. Hemoglobin has a quaternary structure as it is made up of 2 α-globin* subunits and 2 β-globin* subunits; 4 subunits held together by weak intermolecular interactions formed between R groups (hydrogen bonds, ionic bonds and hydrophobic interactions), allows movement tha t influences affinity for oxygen allowing for cooperative binding* of oxygen; Or As a result binding of one oxygen molecule to one haemoglobin subunit induces a conformational* change in remaining 3 subunits so that their affinity for oxygen increases; To highlight Idea: having multiple subunits , intermolecular bonds allow movement to influences affinity for oxygen  so when 1 subunit binds to oxygen, conformation changes in other subunits ( cooperativity) 3. Amino acids with hydrophilic R groups are found on the exterior of the protein and amino acids with hydrophobic R groups are buried in interior of protein; Allows molecule to be soluble in water so that it can be easily transported in blood;

Raffles Institution Nov 2017 (H2 Biology) Paper 2 2023 Collagen: Molecular structure Function 1. Numerous hydrogen bonds* between on different polypeptides Gives rise to high tensile strength * which means can stretch without breaking; 2. Staggered arrangement of collagen molecules within the fibrils Minimises point of weakness which means can stretch without breaking; 3. 3 polypeptide* chains wound together to form 1 tropocollagen* molecule; Gives rise to high tensile strength * which means can stretch without breaking;; 4. small glycine* residues allow formation of a very tight triple helical structure (so that hydrogen hydrogen bonds can then form OH groups on different polypeptides) Point 4 and 5 will be awarded if students mention point 3. Reason being this point is relevant only if structure is part of a tropocollagen. Gives rise to high tensile strength * which means can stretch without breaking; 5. bulky, inflexible proline and hydroxyproline confer rigidity on the molecule Point 4 and 5 will be awarded if students mention point 3. Reason being this point is relevant only if structure is part of a tropocollagen. Gives rise to rigidity; 6. covalent cross -links* form between lysine* residues at C and N ends of adjacent/parallel tropocollagen molecules; 7. collagen fibrils lie in parallel bundle to form collagen fibres; 8. staggered arrangement to minimises points of weaknesses along length of the fibrils; AVP Gives rise to high tensile strength*; For function of collagen: where appropriate + elaborated correctly, a