SAJC_2011_H2-Bio-TYS-ANS

2011 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2011 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ) 1 D 21 B 2 B 22 D 3 B 23 B 4 C 24 B 5 C 25 A 6 B 26 C 7 A 27 D 8 D 28 B 9 A 29 D 10 A 30 C 11 A 31 A 12 B 32 D 13 A 33 C 14 B 34 C 15 A 35 C 16 D 36 D 17 D 37 A 18 C 38 A 19 B 39 D 20 C 40 B

2011 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 (a) 1 Rate of reaction increases rapidly from 0 to 4 a.u. at low substrate concentration of 0 to 4 a.u. 2 Rate of reaction slows down as substrate concentration increases and eventually level-off to a plateau of 9.6 a.u. at a high substrate concentration of 80 a.u. Examiner’s comment: this required students to mention both the rapid initial increase followed by a levelling off to a plateau as well as quoting figures from both low and high substrate concentrations. (b) (i) (ii) 1 a competitive inhibitor has a close structural resemblance to the substrate, and therefore competes with the substrate for the same active site of enzyme 2 inhibitor binds to the active site, rate of reaction decreases 3 binding of the inhibitor to the active site is temporary and not permanent. At high substrate concentration, effect of inhibition is negligible / increasing the substrate concentration can decrease the effect of inhibition. (c) 1 toxin permanently binds to the active site of the enzyme and prevents hydrolysis of acetylcholine 2 acetylcholine accumulates and remains bound to receptors on the post-synaptic membrane 3 ligand-gated sodium ion channels would stay open, leading to repeated action potentials

2011 UCLES ‘A’ Level H2 Biology Mark Scheme 3 QUESTION 2 (a) A – (large ribosomal subunit ) ribosome B – polypeptide C – transfer RNA (tRNA) D – messenger RNA (mRNA) (b) 1 Attachment of an amino acid to its corresponding tRNA with specific anticodon by aminoacyl tRNA synthetases 2 which have an active site with 3D conformation complementary to the 3D conformations of the amino acid and the corresponding tRNA 3 catalyzes covalent attachment of the specific amino acid to its tRNA, 4 resulting in the formation of an aminoacyl-tRNA with 3D conformation that is no longer complementary to the conformation of the active site (and is released from the enzyme). (c) 1 The rRNA in ribosomes holds the tRNA and mRNA together in close proximity, via complementary base pairing using hydrogen bonds 2 positions the new amino acid for addition to the carboxyl end of the growing polypeptide 3 rRNA peptidyl transferase activity catalyzes formation of a peptide bond between the new amino acid and the polypeptide chain QUESTION 3 (a) Haemagglutinin 1 Binds to the sialic acid on host cell membrane 2 aids in the binding and entry into host cell Neuraminidase 3 Removes sialic acids on the envelopes of newly assemb