2013 HCI H2 Biology Prelims Paper 2 Answers

HWA CHONG INSTITUTION / 2013 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 1 HWA CHONG INSTITUTION 2013 JC2 H2 BIOLOGY PRELIMINARY EXAMINATION MARK SCHEME QUESTION 1 (a)(i) Identify the class of carbohydrate molecule of which agarose is an example. [1] polysaccharide (ii) Plants contain a carbohydrate called amylose. Amylose does not contain galactose. Describe one similarity and another difference in structure between agar ose and amylose. [2] similarity: 1. monosaccharides of both agarose and amylose are held together by glycosidic bonds 2. both agarose and amylose are unbranched difference: 1a. coiled/helical structures in amylose 1b. but linear/straight structures in agarose 2a. alternate glucose is not inverted/ same orientation in amylose 2b. but alternate galactose is inverted 180 o in agarose 3a. α(1,4) glycosidic bond is present in amylose 3b. different types of glycosidic bonds are present in agarose (iii) Suggest why bacteria are unable to metabolise agarose. [2] 1. lack of bacterial enzyme 2. to hydrolyse the type of glycosidic bonds present in agarose 3. substrate not complementary to active site of enzyme (b) (i) Describe how the structure of domain A of the cholera toxin is maintained. [3] 1. primary structure folds into secondary structures 2. maintained by hydrogen bonds formed between C=O and NH groups 3. further folding 4. into a unique 3D conformation 5. maintained by hydrogen bonds, ionic bonds, disu lfide bonds and hydrophobic interactions between R- groups of amino acid residues (ii) Explain how a globular protein like cholera toxin differs from a fibrous protein, such as collagen. [2] 1a. cholera toxin being a globular prot ein is soluble due to the presence of hydrophilic amino acids on the surface of the protein 1b. collagen being a fibrous protein is insoluble due to t he presence of mainly hydrophobic amino acids in the protein 2a. cholera toxin being a globular protein is more compact/ spherical in shape 2b. collagen being a fibrous protein is elongated in shape /forms multimolecular parallel filament to strands/ collagen fibrils and collagen fibres 3a. cholera toxin being a globular protein is made up of fi xed, non-repetitive specific sequence of amino acids 3b. collagen being a fibrous protein is ma de up of repetitive sequence of amino acids 4a. cholera toxin being a globular protein has a relati vely unstable structure ref. to weak non-covalent bonds 4b. collagen being a fibrous protein has a stable st ructure ref. to extensive intra- and inter-molecular hydrogen bonds/ covalent crosslinks 5a. cholera toxin being a globular protein perfor ms metabolic functions since they are soluble 5b. collagen being a fibrous protein performs structur al func