A Level H2 Biology Practice Paper 2

TuitionGoWhere Practice Paper - Biology H2 A-Level

Answer Key (Version 2)

Q1 (Cells/Membrane) (a) 1. Binding of 3 $Na^+$ ions to the pump from the intracellular side. [1] 2. ATP hydrolysis occurs, phosphorylating the pump. [1] 3. Conformational change occurs, releasing $Na^+$ to the extracellular space. [1] 4. 2 $K^+$ ions bind from the extracellular side, triggering dephosphorylation and a return to original shape, releasing $K^+$ inside. [1] (b) Ouabain prevents $K^+$ influx. [1] This reduces the positive charge returning to the interior. [1] The membrane potential becomes more depolarized (less negative) or the resting potential cannot be maintained. [1]

Q2 (Biomolecules) (a) Primary structure is the linear sequence of amino acids. [1] This sequence determines the specific R-groups present. [1] Interactions between R-groups (e.g., hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) [1] cause the polypeptide to fold into a specific 3D shape. [1] (b) Misfolding exposes hydrophobic R-groups that are normally buried in the protein core. [1] These exposed hydrophobic regions seek to avoid the aqueous environment. [1] They interact with similar hydrophobic regions of other misfolded proteins, leading to aggregation. [1]

Q3 (Enzymes) (a) $V_{max}$ remains unchanged. [1] $K_m$ increases. [1] (b) Competitive inhibitors bind to the active site, competing with the substrate. [1] This reduces the affinity of the enzyme for the substrate, increasing $K_m$. [1] However, at very high substrate concentrations, the substrate outcompetes the inhibitor, allowing the enzyme to reach its maximum velocity ($V_{max}$). [1]

Q4 (Mitochondria) (a) Electrons from NADH/FADH2 pass through a series of protein complexes. [1] As electrons move, energy is released. [1] This energy is used to pump $H^+$ ions from the matrix into the intermembrane space. [1] This creates a high concentration of $H^+$ (proton gradient/electrochemical gradient). [1] (b) Sodium azide blocks the final electron acceptor (oxygen). [1] Electron flow stops, and the proton gradient is no longer maintained. [1] Without the proton motive force, ATP synthase cannot phosphorylate ADP to ATP. [1]

Q5 (Gel Electrophoresis) (a) An electric field is applied across the gel. [1] DNA/proteins are negatively charged and migrate toward the anode. [1] The gel matrix acts as a sieve. [1] Smaller fragments move faster/further than larger fragments, separating them by size. [1] (b) Two distinct bands will be observed. [1] Because the individual has two different alleles, each allele produces a fragment of a different size/length. [1]

Q6 (Operons) (a) Prevents the waste of energy and resources. [1] Enzymes for lactose metabolism are only produced when lactose is actually present. [1] This allows the cell to adapt to changing nutrient availability. [1] (b) The repressor protein binds to the operator, blocking RNA polymerase from transcribing the genes. [1] When allolactose (inducer) is present, it binds to the repressor. [1] This changes the repressor's shape, preventing it from binding to the operator. [1] RNA polymerase can then transcribe the operon. [1]

Q7 (Epistasis) (a) Recessive Epistasis. [1] The 9:3:4 ratio is characteristic of recessive epistasis. [1] Gene A is epistatic to Gene B; if the genotype is 'aa', the phenotype is white regardless of the B allele. [1] If A is present, B determines red or pink. [1] (b) Precursor $\xrightarrow{Gene A}$ Intermediate $\xrightarrow{Gene B}$ Final Pigment. [1] Gene A converts white precursor to pink intermediate. [1] Gene B converts pink intermediate to red pigment. [1] (Accept similar logical pathways).

Q8 (Cancer/Genetics) (a) Translocation between chromosomes 9 and 22. [1] A piece of chromosome 9 swaps with a piece of chromosome 22. [1] (b) The translocation creates a BCR-ABL fusion gene. [1] This gene produces a fusion protein with constitutive (always active) tyrosine kinase activity. [1] This leads to continuous phosphorylation of signaling proteins. [1] This signals the cell to divide uncontrollably, leading to leukemia. [1]

Q9 (PCR) (a) Denaturation ($\sim 95^\circ\text{C}$), Annealing ($\sim 55^\circ\text{C}$), Extension ($\sim 72^\circ\text{C}$). [3] (b) The denaturation step occurs at very high temperatures which would denature normal human DNA polymerase. [1] Taq polymerase is heat-stable and remains functional across multiple cycles. [1]

Q10 (Molecular Bio) (a) Transcription: Template = DNA; Product = mRNA. [1.5] Translation: Template = mRNA; Product = Polypeptide/Protein. [1.5] (b) A substitution could create a premature stop codon (nonsense mutation). [1] This truncates the protein. [1] The resulting protein is too short to fold correctly or lack essential active sites, rendering it non-functional. [1]

Q11 (Plant Bio) (a) C4 plants use PEP carboxylase to fix $\text{CO}_2$ into a 4-carbon compound in mesophyll cells. [1] This allows $\text{CO}_2$ to be concentrated in bundle sheath cells. [1] This reduces photorespiration by ensuring RuBisCO is saturated with $\text{CO}_2$ even when stomata are partially closed to save water. [1] Higher efficiency in hot/dry conditions. [1] (b) Bundle sheath cells are the site of the Calvin cycle. [1] They are arranged in a ring around the vascular bundle. [1] They maintain a high $\text{CO}_2$ concentration to minimize oxygenation of RuBP. [1]

Q12 (Plant Bio) (a) RuBisCO can act as both a carboxylase and an oxygenase. [1] High $\text{O}_2$ concentrations favor the oxygenation of RuBP (photorespiration). [1] This process consumes ATP and releases $\text{CO}_2$ without producing sugar. [1] Consequently, the net rate of carbon fixation/photosynthesis decreases. [1] (b) The light intensity at which the rate of photosynthesis equals the rate of respiration. [1] At this point, net gas exchange is zero. [1] If a plant stays below this point for too long, it will consume its energy reserves and die. [1]

Q13 (Physiology) (a) High blood glucose detected by $\beta$-cells of pancreas. [1] Insulin is secreted. [1] Insulin increases glucose uptake by muscle/fat cells and stimulates glycogenesis in the liver. [1] Blood glucose levels fall. [1] This removes the stimulus for insulin secretion (negative feedback). [1] (b) Hypothalamus detects high blood osmotic pressure (low water potential). [1] It stimulates the pituitary gland to release Antidiuretic Hormone (ADH). [1] ADH increases the permeability of collecting ducts in the kidney to water. [1] More water is reabsorbed into the blood, increasing water potential. [1]

Q14 (Physiology) (a) Resting potential is maintained. [1] Stimulus reaches threshold, $\text{Na}^+$ channels open. [1] $\text{Na}^+$ influx causes depolarization (potential becomes positive). [1] $\text{Na}^+$ channels close and $\text{K}^+$ channels open, $\text{K}^+$ efflux causes repolarization (potential returns to negative). [1] (b) Action potential triggers $\text{Ca}^{2+}$ influx into the presynaptic terminal. [1] This causes exocytosis of vesicles containing ACh into the synaptic cleft. [1] ACh binds to receptors on the postsynaptic membrane. [1] This opens $\text{Na}^+$ channels, triggering a new action potential in the postsynaptic neuron. [1]

Q15 (Evolution) (a) A population is split by a physical barrier (e.g., mountain range/river). [1] The two groups are reproductively isolated. [1] Each group faces different selective pressures/environmental conditions. [1] Mutations accumulate independently in each group. [1] Over time, they diverge so much that they can no longer interbreed even if the barrier is removed. [1] (b) Comparing base sequences of a conserved gene (e.g., Cytochrome c). [1] Fewer differences indicate a more recent common ancestor. [1] More differences indicate a more distant relationship. [1] It provides quantitative evidence for phylogeny. [1]

Q16 (Ecology) (a) Resources (food, space, light) become limiting. [1] Competition increases. [1] The birth rate decreases and/or death rate increases until they are equal. [1] (b) Carrying capacity ($K$) will decrease. [1] The predator increases the death rate of the prey population. [1] The environment can support fewer individuals of the prey species due to higher predation pressure. [1]

Q17 (Plant Bio) (a) A network of membranes (thylakoids) stacked into grana. [1] It provides a large surface area for photosystems and electron carriers. [1] It allows for the establishment of a proton gradient. [1] (b) Light excites electrons in PSII. [1] Electrons move through the ETC (plastoquinone, cytochrome complex). [1] Energy released is used to pump $H^+$ into the thylakoid lumen. [1] Electrons are then re-excited at PSI to eventually reduce $\text{NADP}^+$ to $\text{NADPH}$. [1]

Q18 (Molecular Bio) (a) DNA: Double-stranded, deoxyribose, thymine, stores genetic info long-term. [2] RNA: Single-stranded, ribose, uracil, acts as a messenger/catalyst for protein synthesis. [2] (b) Restriction endonucleases cut DNA at specific recognition sites (often leaving sticky ends). [2] DNA ligase joins the complementary sticky ends of the target DNA and the plasmid together by forming phosphodiester bonds. [2]

Q19 (Cells/Membrane) (a) Movement of molecules via a transmembrane protein (channel or carrier) down a concentration gradient. [2] Facilitated diffusion is passive (no ATP); active transport requires ATP to move molecules against a gradient. [2] (b) There are a limited number of carrier proteins in the membrane. [1] Once all binding sites are occupied, the rate cannot increase further (saturation). [1]

Q20 (Cell Cycle) (a) DNA replication occurs. [1] Each chromosome is duplicated to form two sister chromatids. [1] This ensures that each daughter cell receives a complete set of genetic information. [1] (b) Spindle fibres attach to the centromeres of sister chromatids. [1] They pull the chromatids apart toward opposite poles of the cell. [1] This ensures an equal distribution of chromosomes. [1] The result is two genetically identical nuclei. [1]