RI N19 H2 P2 ans

2019 H2 A level Paper 2 2019 H2 Biology A Level Paper 3 [Turn over Section A Answer all the questions in this section. 1 (a) List three structural features of E. coli that are visible in Fig. 1.1 [3] 1. Ribosomes 2. Circular DNA 3. (Peptidoglycan) Cell wall 4. Cell surface membrane (b) For one of the structural features listed in (a), state a detail of this feature that is characteristic of a bacterial cell. [1] 1. Ribosomes – 70s ribosomes in bacteria 2. DNA - circular DNA that is double stranded OR DNA molecule is not associated with histone but other histone-like protein molecules. 3. Cell wall – Made up of peptidoglycan* polymers 4. Cell surface membrane - Phospholipid bilayer with electron transport chains and ATP synthase are embedded to produce ATP (c) Describe what happens to the genome of bacteria during binary fission. [2] 1. Bacterial genome which is made of DNA replicates by semi-conservative replication* 2. DNA is unzipped by breaking hydrogen bonds between bases of the 2 strands 3. each parental strand serves as a template for synthesis of daughter strands 4. free deoxyribonucleotides complementary base pair with bases on the parental /template strand 5. DNA polymerase* forms phosphodiester bonds* between adjacent deoxyribonucleotides (d) Describe how the generation time and mean mass of an individual bacterial cell changes with nutrient availability, as shown in Table 1.1. [2] 1. As nutrient availability increased from low to high, the generation time decreased from 100 to 24 minutes (quote data); 2. And mean mass of individual bacterial cells increased from 150 x 10-15 g to 870 x 10- 15 g (quote data). (e) Suggest an explanation for the effect of decreasing nutrient availability on the generation time of these bacteria, as shown in Table 1.1. [2] Name nutrient + Effect of a lack of these nutrients on named cell processes 1. With low or no glucose, the bacteria will be unable to carry out aerobic respiration to generate sufficient levels of ATP 2. Hence the bacterial cells will have insufficient energy required for cell division/binary fission and hence take a longer generation time OR 1. Certain essential amino acids, which cannot be produced by the bacterial cells, need to be taken up from the environment 2. Without which the bacterial cells will be unable to synthesize important bacterial proteins for growth, and hence require a longer generation time AVP [Total: 10]

2 2019 H2 Biology A Level Paper 3 2 (a) Name the monomers that join to form a cellulose molecule. [1] β-glucose monomers (b) Name the covalent bond between two of these monomers in a cellulose molecule and describe how this bond is formed. [3] 1. (1-4) glycosidic bond* 2. The glycosidic bond forms between C1 of one β glucose* monomer and C4 of another β glucose* monomer; 3. Condensation* reaction joining two β glucose residues with removal of one water* molecule; 4. Reaction is catalysed by enzymes*; (c) Explain how the molecular structure of cellulose is related to the high tensile strength of the cellulose cell walls of plants. [3] 1. Adjacent β glucose* monomers are rotated 180o* with respect to each other cellulose 2. Forming a straight* chain/molecule and is able to lie parallel* to other cellulose chains; 3. Hydroxyl groups* project outwards from each molecule, allowing extensive hydrogen bonding* between adjacent chains, forming microfibrils* with high tensile strength* that make up the cell wall (d) Suggest why cellulose must be synthesised at the cell surface membrane and not inside the cell. [3] 1. Cellulose is a macromolecule found outside the cell as part of the cell wall and therefore easier to simply deposit it there. 2. Cellulose molecule may be too large to pass through cell membrane if it has to be transported to the exterior of the cell. 3. Cellulose is insoluble in the hydrophobic core of the phospholipid, and is hence synthesized at the cell surface membrane and transported out of the cell [Total:10] 3 (a) Explain why transcription and translation are able to occur simultaneously in a bacterial cell such as E. coli. [2] 1. E.coli is a prokaryotic cell that does not contain a membrane bound nucleus/nuclear envelope. Hence as soon as the mRNA is transcribed the ribosomes can attach to the mRNA and translation can be carried out; 2. Also, unlike eukaryotic cells, DNA that is transcribed to mRNA in prokaryotic cells need not undergo post-transcriptional modification and hence transcription and translation can occur simultaneously; (b) Identify structures A, B and C, as shown in Fig. 3.2. [3] A: DNA double helix B: mRNA/messenger RNA C: polypeptide (c) Describe how structure D is different from structure E. [2] 1. Structure D, the ribosome is made of rRNA and protein while structure E, the RNA polymerase is only made up of protein; 2. Structure D is made up of 2 subunits while structure E is a single molecule (based on the diagram; 3. Structure D, has an active site that is made of rRNA while structure E, has an active site that is made up of protein;

3 2019 H2 Biology A level Paper 3 [Turn over (d) List three ways in which the process of transcription is different from the process of translation. [3] Transcription Translation Location Occurs in nucleus* of eukaryotes Occurs on ribosomes* in cytoplasm / rough endoplasmic reticulum Process DNA used as template* in synthesis of mRNA mRNA used as template* in synthesis of polypeptide Enzymes that joins monomers Enzyme RNA polymerase* links ribonucleotides Peptidyl transferase* on ribosome joins amino acids Linkages Ribonucleotides linked by phosphodiester bonds* to form mRNA Amino acids linked by peptide bonds* to produce polypeptide Product Single-stranded mRNA Polypeptide Direction read DNA template read from a 3’ to 5’ direction mRNA read from 5’ to 3’ direction [Total: 10] 4 (a) Identify structures X, Y and Z on Fig. 4.1. [3] X: (Icosahedral) Capsid head Y: (contractile) Tail sheath Z: Tail fibre (b) With reference to Fig. 4.1, outline how the T4 phage viral genome is replicated. [3] 1. Inside the cell, the T4 bacteriophage DNA is immediately transcribed to synthesise messenger RNA using the host RNA polymerase 2. Phage enzymes coded by the phage genome takes over the bacterium’s macromolecular (protein, RNA, DNA) synthesising machinery for its own use. 3. The phage uses the host cell’s nucleotides and DNA polymerase to synthesise many copies of phage DNA. (c) Describe the two main differences between the energy requirements of the reproductive cycle of a T4 phage and the energy requirements of the reproductive cycle of an influenza virus, as shown in Table 4.1. Suggest an explanation for each of the two differences. [4] difference 1 and explanation: 1. T4 phage requires much higher energy, of 200 a.u., than influenza virus, of 10 a.u. for genome replication 2. T4 could require more energy due to the larger size of the viral genome required to be replicated OR T4 viral DNA could require more energy to replicate due it being double stranded OR T4 requires more energy to degrade host cell DNA to synthesize viral DNA as opposed to influenza virus which synthesizes viral mRNA in animal host cell using existing ribonucleotides

4 2019 H2 Biology A Level Paper 3 difference 2 and explanation: 1. T4 phage requires lesser/no energy/0 a.u for viral exit, as compared to influenza virus, requiring 300 a.u. for viral exit. 2. T4 viral gene codes for lysozyme which causes host bacterial cell wall to lyse and release bacteriophages, thus not r