DHS 2023 H2 Prelim P1-3 Mark Scheme

1 DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 Biology Preliminary Exam 2023 Mark Scheme Paper 1: Multiple Choice Question 1 D 11 D 21 A 2 D 12 C 22 C 3 C 13 C 23 B 4 A 14 D 24 D 5 A 15 A 25 B 6 D 16 B 26 C 7 A 17 A 27 D 8 B 18 B 28 D 9 B 19 B 29 B 10 C 20 C 30 A

2 Paper 2: Structured Question Question 1 (a) • Ref to Multiple cell types are found in the human capillary such as red blood cells and endothelial cell [name at least 2 cell types]; • which support the cell theory that living organisms are composed of cells; OR • Ref to the nucleus of white blood cell/endothelial cell contains DNA + presence of mitochondria for synthesis of ATP; • which support the cell theory that cells are the basic unit of life; [2] (b) • [Describe] The radioactivity level increased from 60 to 80 arbitrary unit from 5 to 10 minutes; • [Explain] Ref to ribosomes were synthesising new proteins using the radioactive amino acids which contributed to the initial high level of radioactivity; • [Describe] The radioactivity level decreased from 80 to 5 arbitrary unit from 10 to 40 minutes; • [Explain] Ref to the newly synthesised proteins entered the ER lumen and subsequently moved to Golgi apparatus; [4]

3 (c) • Some radioactive proteins synthesised are not secretory proteins hence synthesized by free ribosomes; • some radioactive proteins are rER enzymes and remained in rER; • Some of the newly synthesised radioactive proteins have not reached Golgi apparatus at 40 minutes; Any 2 [2] (d) (i) • Peptidoglycan cell wall; • Circular DNA; • 70S ribosomes; Any 2 [2] (ii) • Primary lysosomes fuse with phagosomes /vesicles containing bacteria taken in via phagocytosis; • Autophagy in which primary lysosomes fuse with vesicles containing worn-out organelles; • Lysosomes contain hydrolytic enzymes to hydrolyse DNA/ protein/ carbohydrates in bacteria and worn-out organelles; [3] Question 2 (a) • Unique number and sequence of amino acids determines bonds and interactions of R groups; • Segments of polypeptide chain coiled into alpha-helices and folded into beta-pleated sheets + stabilized by hydrogen bonds between C=O and N- H groups of polypeptide backbone / peptide bonds; • Further folding into globular structure, stabilised by bonds between R groups – hydrogen bonds, disul fide bridges, hydrophobic interactions and ionic bonds (any 3); [3] (b) • An enzyme has an active site w ith a 3D conformation that is complementary to the protein / polypeptide / peptide bonds it binds and acts on; • The spatial arrangement / charge of the binding and catalytic residues restricts the type of substrates it can catalyse; • As only certain substrates have chemical groups orientated in a manner that would allow formation of temporary bonds with the contact residues to form ES complex; • (Ref. to ES complex) Substrates must also have chemical groups that are orientated near catalytic residues to facilitate the breaking and reforming of bonds / conversion of substrate to product; Any 2 [2]

4 (c) (i) • Cooked protein is more digestible than raw protein + (Quote data) Macia: 60% digestibility in raw while 70% in cooked OR NK8828: 56% digestibility in raw while 70% in cooked; [1] (ii) • Cooked protein with acid is more digestible + (Quote data) Macia: 70% digestibility in cooked while 84% when cooked with acid OR NK8828: 70% digestibility in cooked while 80% when cooked with acid; [1] (d) • Cooking involves heating at high temperatures  High kinetic energy may break the weak bonds eg. Hydrogen / hydrophobic interaction / ionic bonds (Any 1) present in kafirin; • Presence of high concentration of H+ in acid  neutralize negative charges and hence disrupt ionic bonds; • This results in protein denaturation / a change in 3D conformation; • Protease can now easily access and bind to the polypeptides  increased digestibility; [4]

5 Question 3 (a) • Ions are charged and water molecules are polar / both molecules are hydrophilic; • Unable to transverse the hydrophobic core of the phospholipid bilayer; • Hence, the interior of channels are lined with hydrophilic residues for the transport of water molecules and ions across; [3] (b) • Gated ion channel requires change in conformation to open it for facilitated diffusion while water channel is constantly open and doesn’t require conformation change; [1] (c) (i) [1] (ii) • Maximum rate of transport is reached; • At high ion concentrati on, there is a maximum number of molecules which can pass through the carrier protein / limited protein channels which become saturated with ions / OWTTE; [2] (d) • Hydrolysis of ATP provides energy for the transport of molecules; • Molecules transported against concentration gradient does not require consideration of the concentration gradient; • Bulk transport of molecules into or out of the cell does not take into account the concentration gradient; Any 2 [2]

6 Question 4 (a) (i) • Ref to most species of organisms use the same deoxyribonucleotides; R: ref to genetic code being universal • 3D conformation of active site of Taq polymerase is complementary to the 3’ OH group of existing nucleotide; [2] (ii) • Ref to specificity e.g. Allows for selective amplification of a segment of DNA / flanking target sequence; • via complementary base pairing to 3’ ends of the template strands; [2] (b) Feature PCR DNA replication Nature of primer DNA primer s provide free 3’ OH for polymerase. RNA primers used. How primer is synthesized Primers synthesized artificially with knowledge of the target sequence. Primers synthesized by RNA primase. Enzyme for polymerisation Taq polymerase used for polymerisation / catalysis of phosphodiester bonds. Human DNA polymerase used. Separation of template strand Heat to 95°C to denature dsDNA. Helicase used. Replication Amplification of only the target DNA sequence. Replication of entire DNA molecule. Location Occurs artificially in a thermal cycler. Occurs within nucleus of cells. Any 2 [2] (c) (i) • Male 3; • Only Male 3 with genotype (29, 31) would be able to contribute the chromosome with 29 repeats at locus E to the child; [2] (ii) • Ref to only 5 gene loci were examined, (paternity testing requires at least 13 different VNTRs – given in preamble); • Ref to other males may have the same number of repeats at these 5 gene loci; [1]

7 Question 5 (a) (i) • Glucose can be used directly as a respiratory substrate to produce ATP; • …for binary fission to occur at a faster rate; • However, lactose needs to be first hydrolysed into glucose and galactose / monosaccharides, which takes time; [3] (ii) • The allolactose in the cell binds to the active lac repressor, inducing a conformational change and inactivate repressor; • Inactive repressor can no longer binds to the operator region of the lac operon; • hence RNA polymerase can access the promoter to initiate transcription of the lacZ gene to synthesise β-galactosidase; [3] (b) • Only phage A can bind to the bacterial cells due to its tail fibre being complementary in shape to the receptors on the bacterial cell surface therefore can infect; • Hence can infect and cause osmotic lysis of the bacterial cell (which shows up as clear zones); [2]

8 (c) (i) • In the experimental group, death rate was lower between 7-10% [accept: average 8.25%] for all injection Day 0 to Day 3 than control group, with higher death rate between 50-55% [accept: average 52%] for all the injection days; • For injection Day 4, death rate in the experimental group increased sharply to 50%; • Bacteriophage treatment is effective in reducing death rate for only 3 days, after which