MetadataPastPaper.sampleTitle

The null hypothesis (H0) for a Chi-squared test of independence states that there is no association between the two species. Because the calculated \(\chi^2\) value (5.24) is greater than the critical value (3.84), we reject the null hypothesis. Therefore, there is a statistically significant association between the two species.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options. No partial marks are available.

The accumulation of protons in the thylakoid lumen creates an electrochemical gradient (proton motive force). As protons flow down this concentration gradient through the ATP synthase complex back into the stroma, the energy released is used to synthesize ATP from ADP and inorganic phosphate.

Award 1 mark for the correct option (C). Award 0 marks for incorrect options. No partial marks are available.

Under anaerobic conditions, glycolysis is the only source of ATP. Glycolysis requires a continuous supply of oxidized NAD+ to act as an electron acceptor. The conversion of pyruvate to ethanol in fermentation oxidizes NADH back to NAD+, thereby regenerating the NAD+ needed for glycolysis to continue.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options. No partial marks are available.

Competitive inhibitors bind directly to the active site of the enzyme, competing with the substrate. Because the substrate can outcompete the inhibitor at extremely high concentrations, the maximum velocity (\(V_{\max}\)) is unchanged. However, more substrate is needed to reach half-maximal velocity, which means the Michaelis constant (\(K_m\)) increases.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options. No partial marks are available.

A mutation that changes a codon coding for an amino acid into a translation termination (stop) codon is a nonsense mutation. This leads to the early termination of translation, resulting in a shortened (truncated) polypeptide that is typically non-functional.

Award 1 mark for the correct option (C). Award 0 marks for incorrect options. No partial marks are available.

Sucrose is actively loaded into the companion cells and sieve tube elements via an apoplastic pathway. H+ ions (protons) are actively pumped out of the companion cell using ATP, creating an electrochemical gradient. Protons then diffuse back into the companion cell through a co-transport protein, carrying sucrose along with them against its concentration gradient.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options. No partial marks are available.

Repolarization occurs when the voltage-gated sodium channels close (inactivate), preventing further entry of positively charged Na+ ions, and voltage-gated potassium channels open, allowing K+ ions to flow rapidly out of the axon, restoring the negative membrane potential inside.

Award 1 mark for the correct option (C). Award 0 marks for incorrect options. No partial marks are available.

As a hydrophilic hormone, epinephrine cannot pass through the hydrophobic core of the plasma membrane. Instead, it binds to specific cell-surface G-protein coupled receptors. This binding activates a G-protein, which stimulates the enzyme adenylyl cyclase to produce cyclic AMP (cAMP) as an intracellular second messenger.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options. No partial marks are available.

The calculated chi-squared value of \( 4.12 \) is greater than the critical value of \( 3.84 \) at the 5% significance level. This means the probability of the observed differences occurring by chance is less than 5%. Therefore, we reject the null hypothesis (which states that the two species are independent/not associated) and conclude that there is a statistically significant association between the two species.

Award [1] mark for the correct option A. Reject other options because they either accept the null hypothesis despite the test statistic exceeding the critical value, or they mischaracterize the meaning of rejecting the null hypothesis.

The accumulation of protons inside the thylakoid lumen creates an electrochemical proton gradient. Protons diffuse down this gradient back into the stroma through the enzyme complex ATP synthase. This movement (chemiosmosis) drives the phosphorylation of ADP to generate ATP.

Award [1] mark for option B. Protons do not directly reduce \( \text{NADP}^+ \) (option A), provide electrons for photolysis (option C), or excite reaction center chlorophylls (option D).

Per molecule of glucose, glycolysis yields 2 molecules of pyruvate. In the link reaction, 2 pyruvate molecules yield \( 2\ \text{CO}_2 \) and \( 2\ \text{NADH} \). In the Krebs cycle, the 2 acetyl-CoA molecules undergo two turns of the cycle, yielding a total of \( 4\ \text{CO}_2 \) and \( 6\ \text{NADH} \). Combining these, the totals are \( 2 + 4 = 6\ \text{CO}_2 \) and \( 2 + 6 = 8\ \text{NADH} \).

Award [1] mark for option C. All other combinations represent incorrect counting or omitting either the link reaction or one turn of the Krebs cycle.

In competitive inhibition, the inhibitor binds to the active site, competing with the substrate. Increasing the substrate concentration can overcome this competitive inhibition, allowing the reaction to reach its original maximum velocity (\( V_{\max} \)). However, because the substrate has to compete with the inhibitor, a higher concentration of substrate is required to achieve half the maximum velocity, increasing the Michaelis constant (\( K_m \)).

Award [1] mark for option C. Non-competitive and allosteric inhibition decrease \( V_{\max} \), which contradicts the scenario.

A base substitution that converts a codon coding for an amino acid into a premature termination (stop) codon is classified as a nonsense mutation. This leads to the premature ending of translation, resulting in a truncated, and typically non-functional, polypeptide chain.

Award [1] mark for option B. Missense mutations (A) result in a different amino acid, silent mutations (C) result in the same amino acid, and frameshift mutations (D) involve insertions or deletions, not single-base substitutions.

Active loading of sucrose into the phloem relies on an electrochemical gradient of protons. Protons (\( \text{H}^+ \)) are actively pumped out of companion cells into the cell wall spaces by proton-pumping ATPases. The high external proton concentration drives protons back into the companion cells through a co-transporter protein, which simultaneously transports sucrose against its concentration gradient into the companion cell/sieve tube complex.

Award [1] mark for option C. Options A and B describe incorrect mechanisms, and option D describes the bulk flow mass transport within the sieve tube itself, not the active loading mechanism.

During repolarisation, voltage-gated sodium channels close or inactivate (stopping the inward movement of sodium ions), while voltage-gated potassium channels open, allowing potassium ions to diffuse out of the cell down their electrochemical gradient to restore the resting membrane potential.

Award [1] mark for option B. A corresponds to depolarisation, C represents a state that would prevent proper signal propagation, and D occurs during the return to resting potential.

Amylose is a structural component of starch made of \( \alpha \)-D-glucose units joined by 1,4-glycosidic bonds, giving it a helical conformation. Cellulose is a structural polysaccharide found in plant cell walls, made of \( \beta \)-D-glucose units linked by 1,4-glycosidic bonds, which alternate orientation by 180 degrees to form rigid, straight chains.

Award [1] mark for option A. Option B is incorrect because amylose is unbranched (amylopectin is branched). Option C reverses the glucose subunits used. Option D is incorrect because glycogen, not amylose, is stored in animal cells.

The Lincoln index formula is defined as \(N = \frac{n_1 \times n_2}{m_2}\), where \(n_1\) is the number of individuals marked in the first sample, \(n_2\) is the total number of individuals captured in the second sample, and \(m_2\) is the number of marked individuals recaptured. Immigration of unmarked individuals increases the proportion of unmarked individuals in the population. As a result, when the second sample is taken, the number of marked individuals recaptured (\(m_2\)) is lower than it would be in a closed population. Because \(m_2\) is in the denominator of the equation, a smaller value of \(m_2\) leads to an overestimation of the population size \(N\).

[1 mark] Award 1 mark for the correct option (A). Reject other options because immigration dilutes the marked population, lowering the recaptured count (m2) and resulting in an overestimate, not an underestimate.

Plastoquinone (PQ) is an electron carrier that moves electrons from Photosystem II (PSII) to the cytochrome complex. The energy released during this transfer is used to pump protons from the stroma into the thylakoid lumen, establishing a proton gradient necessary for ATP synthesis. If PQ is blocked, electron transport ceases, preventing the pumping of protons and leading to a failure to establish the proton gradient. Photolysis of water at PSII will also stop because PSII cannot pass its electrons to PQ and remains in a reduced state.

[1 mark] Award 1 mark for selecting C. Award 0 marks for any other option.

Glycolysis requires a continuous supply of oxidized \(\text{NAD}^+\) to accept electrons. Under aerobic conditions, NADH is oxidized back to \(\text{NAD}^+\) via the electron transport chain. In the absence of oxygen, the link reaction, Krebs cycle, and oxidative phosphorylation cannot occur. Therefore, cells must convert pyruvate to lactate or ethanol, a process that oxidizes NADH back to \(\text{NAD}^+\), ensuring glycolysis can continue to produce ATP.

[1 mark] Award 1 mark for B. Correctly identifying the regeneration of NAD+ as the critical outcome.

Competitive inhibitors (Inhibitor X) compete with substrate molecules for the active site. Increasing the substrate concentration can overcome competitive inhibition, allowing the reaction to reach the same \(V_{\max}\), but a higher concentration of substrate is needed to reach half-maximal velocity (increased \(K_m\)). Non-competitive inhibitors (Inhibitor Y) bind to an allosteric site and change the enzyme's conformation, decreasing the number of functional enzymes and thus decreasing \(V_{\max}\), without altering the substrate affinity of the remaining active enzymes (unchanged \(K_m\)).

[1 mark] Award 1 mark for option A.

The CRISPR-Cas9 system uses a single guide RNA (gRNA) containing a specific 20-nucleotide sequence that is complementary to the target genomic DNA. The gRNA binds to the Cas9 enzyme (an endonuclease) and guides it to the precise location in the genome where the double-strand break is to be introduced.

[1 mark] Award 1 mark for option C. Reject other options as Cas9 performs the cleavage (not gRNA) and repair is done by host enzymes, not gRNA.

At the source (e.g., green leaves), companion cells actively load sucrose into the sieve tube elements. This active accumulation of sucrose reduces the solute potential (makes water potential more negative) within the sieve tubes. Consequently, water moves from the nearby xylem vessels into the sieve tubes by osmosis. This rapid influx of water increases the hydrostatic pressure at the source, driving the mass flow of phloem sap toward the sink.

[1 mark] Award 1 mark for option C. Reject other options as active transport of protons is used indirectly for cotransport but does not itself create hydrostatic pressure.

Once the action potential reaches its peak positive potential of approximately \(+30\text{ mV}\), voltage-gated sodium channels become inactivated, halting the influx of sodium ions. Simultaneously, voltage-gated potassium channels open, allowing potassium ions to diffuse out of the axon down their electrochemical gradient. This efflux of positive charges repolarizes the membrane, restoring a negative internal potential.

[1 mark] Award 1 mark for option B.

Hydrophobic (lipid-soluble) hormones, like steroid hormones (such as estrogen or testosterone), can freely diffuse across the hydrophobic phospholipid bilayer of the plasma membrane. Once inside the cell, they bind to specific intracellular receptors in the cytoplasm or nucleus. This hormone-receptor complex then binds to specific DNA sequences to regulate (activate or repress) the transcription of specific genes, acting as a transcription factor.

[1 mark] Award 1 mark for option B.

In a sigmoidal population growth curve, the transition phase occurs after the exponential phase. Resource limitation (such as depletion of glucose or accumulation of waste products) begins to slow down the rate of population growth. During this phase, natality begins to decrease and mortality begins to increase, but natality still exceeds mortality, leading to a slower rate of population increase until the plateau phase is reached.

Award [1] for the correct answer (B). [0] for incorrect options: Option A describes the early exponential phase; Option C describes the decline/death phase; Option D describes the plateau phase.

Photosystem II contains a reaction center chlorophyll (P680) that loses excited electrons to the primary electron acceptor. These electrons are replaced by electrons derived from the photolysis of water molecules: \(2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4\text{e}^-\).

Award [1] for the correct answer (C). [0] for other options: NADPH is an electron donor in the light-independent reactions; PSI passes electrons to NADP+; protons do not provide electrons.

Anaerobic respiration (glycolysis and fermentation) occurs entirely in the cytoplasm. Yeast undergoes ethanol fermentation, producing ethanol and carbon dioxide. Humans undergo lactic acid fermentation, producing lactate in the cytoplasm.

Award [1] for the correct answer (B). [0] for incorrect options: Mitochondria (A and D) are locations of aerobic respiration only; Yeast does not produce lactate (C).

Non-competitive inhibitors bind to an allosteric site (away from the active site), decreasing the overall maximum rate of reaction (\(V_{\max}\)). Since they do not compete with the substrate for the active site, the affinity of the enzyme for the substrate (represented by \(K_m\)) is unaffected.

Award [1] for the correct answer (B). [0] for incorrect options: Options A and C describe competitive inhibition, which increases \(K_m\) but leaves \(V_{\max}\) unchanged; Option D incorrect as non-competitive inhibition does not prevent substrate binding entirely but prevents the catalytic step.

Deleting a single base shifts the triplet reading frame (a frameshift mutation). Since mRNA is read in consecutive triplets (codons) during translation, every codon downstream of the deletion is altered, leading to a completely different sequence of amino acids in the polypeptide.

Award [1] for the correct answer (C). [0] for incorrect options: Option A describes a substitution (missense) mutation; Option B describes a nonsense mutation (which only happens if a stop codon is immediately created); Option D describes a silent mutation.

Transpiration pull is generated by the evaporation of water from the wet cell walls of the spongy mesophyll into the air spaces of the leaf, followed by its diffusion out through the stomata. This loss of water creates tension (negative pressure) that is transmitted down the xylem column due to the cohesive properties of water.

Award [1] for the correct answer (C). [0] for incorrect options: Option A describes root pressure, which is positive; Option B is a necessary property for transmitting tension but does not generate the pulling force itself; Option D is factually incorrect as xylem transport is passive and does not use companion cells.

Depolarisation is caused by the rapid influx of sodium ions (\(\text{Na}^+\)) down their electrochemical gradient into the neurone when voltage-gated sodium channels open in response to reaching threshold potential.

Award [1] for the correct answer (C). [0] for incorrect options: Option A restores resting potential; Option B is responsible for repolarisation; Option D initiates the graded potential but does not describe the depolarization phase of the action potential itself.

Amylopectin is a branched polymer of \(\alpha\)-glucose linked by 1,4 and 1,6-glycosidic bonds and serves as a major component of starch for energy storage in plants. Cellulose is an unbranched polymer of \(\beta\)-glucose, and glycogen is a branched polymer of \(\alpha\)-glucose in animals.

Award [1] for the correct answer (C). [0] for incorrect options: Option A incorrectly states cellulose is a branched polymer of \(\alpha\)-glucose; Option B is a non-existent name; Option D incorrectly states glycogen is unbranched and made of \(\beta\)-glucose.

The expected frequency of both species being present in a quadrat is calculated using the formula: \(E = \frac{\text{Row Total} \times \text{Column Total}}{\text{Grand Total}}\). Here, the number of quadrats containing Plantago lanceolata is 50, and the number of quadrats containing Trifolium repens is 45. The grand total of sampled quadrats is 100. Therefore, the expected frequency of quadrats containing both species is: \(E = \frac{50 \times 45}{100} = 22.5\).

Award 1 mark for the correct answer (B). Award 0 marks for incorrect options.

The rate of photosynthesis under different wavelengths is represented by the action spectrum, which closely aligns with the absorption spectrum of photosynthetic pigments like chlorophyll a and b. These pigments absorb blue-violet and red light most efficiently, utilizing this energy to drive photolysis of water and the electron transport chain, resulting in oxygen production. Green light is mostly reflected rather than absorbed, leading to a very low rate of photosynthesis.

Award 1 mark for the correct answer (A). Award 0 marks for incorrect options.

One molecule of glucose is split into two molecules of pyruvate during glycolysis. Each pyruvate undergoes the link reaction to produce one acetyl-CoA, which then enters the citric acid (Krebs) cycle. Therefore, the Krebs cycle turns twice per glucose molecule. Each single turn of the cycle produces 1 \(\text{ATP}\), 3 \(\text{NADH}\), and 1 \(\text{FADH}_2\). For one glucose molecule (two turns), the net yield is 2 \(\text{ATP}\), 6 \(\text{NADH}\), and 2 \(\text{FADH}_2\).

Award 1 mark for the correct answer (C). Award 0 marks for incorrect options.

Competitive inhibitors (X) bind to the active site and can be outcompeted by high substrate concentrations, allowing the reaction to reach its original maximum velocity (\(V_{\text{max}}\)). Non-competitive inhibitors (Y) bind to an allosteric site, altering the shape of the active site. This prevents substrate binding or reaction catalysis regardless of substrate concentration, leading to a permanent decrease in \(V_{\text{max}}\).

Award 1 mark for the correct answer (D). Award 0 marks for incorrect options.

Sickle cell anemia is caused by a base substitution mutation in the HBB gene. On the DNA sense strand, the triplet GAG is mutated to GTG, which is transcribed into the mRNA codon GUG instead of GAG. This leads to the replacement of the polar amino acid glutamic acid with the non-polar amino acid valine at position 6 of the beta-globin polypeptide.

Award 1 mark for the correct answer (B). Award 0 marks for incorrect options.

Phloem loading is an active process. Companion cells use proton pumps to actively transport hydrogen ions (\(\text{H}^+\)) out of the cytoplasm into the cell wall using energy from ATP, establishing an electrochemical proton gradient. Protons then diffuse back down their gradient into the companion cells through a co-transport protein (symporter), carrying sucrose molecules with them against their concentration gradient into the phloem sieve tube system.

Award 1 mark for the correct answer (A). Award 0 marks for incorrect options.

During repolarization, the voltage-gated sodium channels close (or become inactivated), stopping the rapid influx of sodium ions into the neuron. At the same time, voltage-gated potassium channels open, allowing potassium ions to diffuse out of the neuron down their electrochemical gradient. This outflow of positive charge restores the negative membrane potential.

Award 1 mark for the correct answer (A). Award 0 marks for incorrect options.

Lipid-soluble steroid hormones (like progesterone and estrogen) can easily diffuse across the plasma membrane of target cells and bind to specific intracellular receptors in the cytoplasm or nucleus, where the receptor-hormone complex acts directly as a transcription factor to regulate gene expression. Water-soluble peptide hormones (like insulin or ADH) cannot cross the hydrophobic lipid bilayer, so they bind to extracellular receptors on the cell surface, triggering an intracellular signaling cascade mediated by second messengers.

Award 1 mark for the correct answer (A). Award 0 marks for incorrect options.

(i) There is a positive correlation between mean annual temperature and mean methane emission rates; as the mean annual temperature increases (becomes less negative/more positive), the mean methane emission rate increases.

(ii) The coldest year is 2015 (-1.2 °C) with an emission rate of 12.4 mg m^-2 day^-1. The warmest year is 2022 (0.3 °C) with an emission rate of 31.2 mg m^-2 day^-1. Percentage increase = ((31.2 - 12.4) / 12.4) * 100 = (18.8 / 12.4) * 100 = 151.61% = 151.6%.

(iii) Rising temperatures due to global warming lead to the thawing of peatlands and permafrost, which increases microbial decomposition of organic matter under anaerobic conditions, producing more methane. Methane is a potent greenhouse gas that traps infrared radiation (heat) in the atmosphere. This increased heat retention further raises global temperatures, causing more peatlands to thaw and releasing even more methane, perpetuating the cycle.

(i) [Max 2 marks]
- Award 1 mark for identifying the positive correlation between temperature and methane emission.
- Award 1 mark for citing data points supporting this trend (e.g., lowest emissions of 12.4 at -1.2 °C and highest emissions of 31.2 at 0.3 °C).

(ii) [Max 2.75 marks]
- Award 1 mark for identifying the correct years/values for coldest (2015: 12.4) and warmest (2022: 31.2).
- Award 1 mark for correct mathematical formula: ((31.2 - 12.4) / 12.4) * 100.
- Award 0.75 marks for correct final value of 151.6% (accept 152%).

(iii) [Max 4 marks]
- Award 1 mark for explaining that warming melts permafrost/peatlands, exposing organic matter.
- Award 1 mark for stating that anaerobic decomposition by methanogens releases methane.
- Award 1 mark for identifying methane as a powerful greenhouse gas that traps heat/longwave radiation.
- Award 1 mark for linking this heat retention back to further warming and increased thawing (closing the positive feedback loop).

(i) For myelinated fibers, there is a direct linear relationship between axon diameter and conduction velocity (as diameter increases, conduction velocity increases proportionally).

(ii) Velocity-to-diameter ratio is calculated as Conduction Velocity / Axon Diameter.
For A-delta (myelinated): 15.0 m s^-1 / 3.0 micrometers = 5.0 s^-1.
For C-fiber (unmyelinated): 1.0 m s^-1 / 1.0 micrometers = 1.0 s^-1.
Comparison: The myelinated A-delta fiber has a velocity-to-diameter ratio 5 times greater than that of the unmyelinated C-fiber, demonstrating that myelination significantly increases conduction efficiency per unit diameter.

(iii) Myelin acts as an electrical insulator, preventing ion leakage across the axon membrane. Action potentials can only occur at the unmyelinated gaps called nodes of Ranvier, where high densities of voltage-gated sodium channels are located. The electrical signal jumps rapidly from node to node via local currents through the cytoplasm, a process known as saltatory conduction, which is much faster than continuous propagation.

(i) [Max 1 mark]
- Award 1 mark for stating that larger diameter results in faster conduction velocity in myelinated axons.

(ii) [Max 3.75 marks]
- Award 1 mark for correct calculation of A-delta ratio: 5.0 s^-1 (accept without units if calculations are clear).
- Award 1 mark for correct calculation of C-fiber ratio: 1.0 s^-1.
- Award 1.75 marks for comparison (e.g., myelinated fiber is 5 times faster per unit of diameter than unmyelinated fiber, or myelination vastly enhances conduction velocity beyond what is achieved by diameter alone).

(iii) [Max 4 marks]
- Award 1 mark for describing myelin as an insulator (lipid bilayer) preventing ion flow.
- Award 1 mark for mentioning the nodes of Ranvier (gaps in myelination).
- Award 1 mark for explaining that action potentials occur only at these nodes.
- Award 1 mark for explaining saltatory conduction (the jumping of electrical signals/depolarization from node to node).

(i) The limiting factor at 10 a.u. is light intensity. The reason is that both concentrations have the same low rate of 5 bubbles/min, indicating that increasing the carbon dioxide source does not increase the rate of photosynthesis when light is highly restricted.

(ii) At 80 a.u.:
Rate at 0.04% NaHCO3 = 26 bubbles/min.
Rate at 0.15% NaHCO3 = 55 bubbles/min.
Increase = 55 - 26 = 29 bubbles/min.
Percentage increase = (29 / 26) * 100 = 111.538% = 111.5%.

(iii) Under 0.04% NaHCO3, carbon dioxide concentration is low and becomes the limiting factor at high light intensities. Even though more light is available to power the light-dependent reactions (producing ATP and NADPH), the Calvin cycle (light-independent reactions) is limited by the low availability of CO2 for fixation by Rubisco. Under 0.15% NaHCO3, there is more CO2 available to react with RuBP, allowing the Calvin cycle to proceed faster and utilize the abundant products of the light-dependent reactions, resulting in a higher plateau.

(i) [Max 2 marks]
- Award 1 mark for identifying light intensity as the limiting factor.
- Award 1 mark for the reason (the rate is identical at both CO2 concentrations, so CO2 is not limiting/only light increase would raise the rate).

(ii) [Max 2.75 marks]
- Award 1 mark for identifying correct rates: 26 and 55 bubbles/min.
- Award 1 mark for correct working: ((55 - 26) / 26) * 100.
- Award 0.75 marks for the correct final percentage: 111.5% (accept 111.5% to 112%).

(iii) [Max 4 marks]
- Award 1 mark for stating that at 0.04% NaHCO3, carbon dioxide concentration is the limiting factor at high light intensities.
- Award 1 mark for explaining that CO2 is needed for carbon fixation/Calvin cycle/reaction with RuBP catalyzed by Rubisco.
- Award 1 mark for explaining that under 0.15% NaHCO3, higher CO2 levels allow more rapid carbon fixation, utilizing the ATP/NADPH from light-dependent reactions.
- Award 1 mark for mentioning that enzymes (like Rubisco) eventually reach their maximum rate of reaction (Vmax), but this threshold is higher when more substrate (CO2) is present.

(i) As gRNA length increases from 17 to 20 nucleotides, both the on-target cleavage efficiency and the off-target mutation rate increase. Specifically, cleavage efficiency increases from 62% to 92%, while the off-target mutation rate increases exponentially from 12 to 210 per 10^6 cells.

(ii) At 18 nucleotides, the off-target rate is 35 per 10^6 cells.
At 21 nucleotides, the off-target rate is 480 per 10^6 cells.
Factor increase = 480 / 35 = 13.714... = 13.7 times (or 13.7-fold).

(iii) The 19-nucleotide gRNA likely provides the best balance. It achieves high on-target cleavage efficiency (88%), which is close to the maximum (92% at 20 nt), but maintains a substantially lower off-target mutation rate (84 per 10^6 cells) compared to 20 nt (210) or 21 nt (480). High off-target mutation rates present a severe biological risk in clinical therapeutics because unintended double-stranded breaks can disrupt tumor suppressor genes or activate proto-oncogenes, potentially leading to oncogenesis (cancer formation) or cell death.

(i) [Max 2 marks]
- Award 1 mark for stating that both parameters increase with gRNA length from 17 to 20 nt.
- Award 1 mark for noting the difference in rate of change (e.g., efficiency levels off near 20 nt, whereas off-target mutations rise exponentially/rapidly).

(ii) [Max 2.75 marks]
- Award 1 mark for identifying the correct values: 35 (for 18 nt) and 480 (for 21 nt).
- Award 1 mark for correct mathematical setup: 480 / 35.
- Award 0.75 marks for the correct final answer of 13.7 (accept 13.7-fold or 14-fold).

(iii) [Max 4 marks]
- Award 1 mark for selecting 19 nt (or 18 nt with reasonable justification) as the best balance.
- Award 1 mark for justifying the selection with data (e.g., 19 nt offers near-maximum efficacy of 88% while avoiding the massive surge in off-target mutations seen at 20 and 21 nt).
- Award 1 mark for explaining that off-target cleavage can disrupt functional non-target genes (e.g., tumor suppressor genes).
- Award 1 mark for linking these mutations to clinical risks like cancer/cell malfunction/cell death.

(a) Lincoln Index formula: \(N = \frac{n_1 \times n_2}{m_2}\). Given: \(n_1 = 120\), \(n_2 = 150\), \(m_2 = 30\). Therefore, \(N = \frac{120 \times 150}{30} = 600\). (b) Assumptions: 1. No immigration/emigration/births/deaths during the study period. 2. Marked individuals mix randomly back into the population and are equally likely to be captured. (c) Top-down factors refer to regulation by trophic levels above, such as predators (e.g., woodpeckers) eating the beetles. Bottom-up factors refer to resources at lower trophic levels, such as the availability of dead wood/food sources limiting beetle survival.

(a) Award [1] for correct substitution in the formula and [1] for the final correct answer of 600. (b) Award [1] per valid assumption up to [2]. (c) Award [1] for explaining top-down regulation with an example, [1] for explaining bottom-up regulation with an example, and [1] for distinguishing their directions of control.

(a) The independent variables are carbon dioxide concentration (and light intensity, which is manipulated at two levels), while the dependent variable is the rate of photosynthesis. (b) As CO2 concentration increases, the rate of photosynthesis increases because more substrate is available for Rubisco. At high concentrations, the rate plateaus because another factor (such as enzyme concentration or temperature) becomes limiting. (c) In the light-independent reactions, ATP and NADPH produced in the light-dependent reactions are used in the Calvin cycle. ATP provides the chemical energy required to convert glycerate 3-phosphate (GP) to triose phosphate (TP) and to regenerate RuBP. Reduced NADP (NADPH) provides the hydrogen/electrons to reduce GP to TP.

(a) Award [1] for identifying independent variables and [1] for dependent variable. (b) Award [1] for describing the initial increase and [1] for stating that it plateaus/reaches a maximum. (c) Award [1] for stating ATP provides energy, [1] for stating NADPH provides hydrogen/electrons/reducing power, and [1] for stating they are used to reduce GP to triose phosphate.

(a) Glycolysis occurs in the cytoplasm of the cell. It involves the phosphorylation of glucose (using 2 ATP) to form fructose-1,6-bisphosphate, which is then split into two triose phosphate molecules. These are oxidized, with NAD+ being reduced to NADH + H+, and ATP is produced via substrate-level phosphorylation, yielding a net of 2 ATP and 2 molecules of pyruvate. (b) In human muscle cells, anaerobic respiration (lactate fermentation) produces lactate (lactic acid) only. In yeast cells, anaerobic respiration (alcoholic fermentation) produces ethanol and carbon dioxide. (c) The electron transport chain (ETC) consists of a series of electron carriers in the inner mitochondrial membrane. As electrons from NADH and FADH2 pass along the chain, they lose energy, which is used by the carrier proteins to pump protons (H+ ions) from the mitochondrial matrix into the intermembrane space, generating a high concentration gradient.

(a) Award [1] for cytoplasm, [1] for phosphorylation/splitting of glucose, [1] for net yield of 2 ATP/NADH/pyruvate. (b) Award [1] for human product (lactate) and [1] for yeast products (ethanol + CO2). (c) Award [1] for electrons passing through carriers/releasing energy, and [1] for protons pumped into the intermembrane space.

(a) Activation energy is the minimum amount of energy required for reactants to undergo a chemical reaction. Enzymes speed up reactions by lowering this activation energy barrier, destabilizing bonds in the substrate when it binds to the active site. (b) Competitive inhibitors have a structure similar to the substrate and bind directly to the active site, blocking substrate binding. Their effect decreases at high substrate concentrations. Non-competitive inhibitors bind to an allosteric site (away from the active site), causing a conformational change in the enzyme's active site so the substrate can no longer bind. Their effect cannot be overcome by high substrate concentrations. (c) A change in pH alters the charge of amino acid residues in the enzyme, particularly at the active site. This disrupts ionic and hydrogen bonds, changing the tertiary structure of the enzyme, which can denature the active site and prevent substrate binding.

(a) Award [1] for definition of activation energy, [1] for explaining that enzymes lower it. (b) Award [1] for active site vs allosteric site binding, [1] for structural similarity to substrate, [1] for effect of increasing substrate concentration. (c) Award [1] for change in charge of amino acids/bonds, [1] for alteration of active site tertiary structure/denaturation.

(a) The Cas9 enzyme acts as an endonuclease (molecular scissors) that cuts both strands of the target DNA at a specific sequence specified by the guide RNA. (b) A base substitution mutation is the replacement of a single nucleotide with another, which may change only one amino acid in the polypeptide (e.g., glutamic acid to valine in sickle cell anemia) or have no effect (silent mutation). A frameshift mutation involves the insertion or deletion of nucleotides (not in multiples of three), which shifts the reading frame of the mRNA. This alters every amino acid downstream of the mutation, usually leading to a non-functional protein. (c) Somatic gene therapy targets body cells, meaning genetic changes are not passed on to future generations. It is generally ethically accepted as it treats the individual's disease. Germline gene therapy targets reproductive cells (sperm, egg, or zygote), meaning the genetic changes will be inherited by all future descendants. This is highly controversial due to long-term risks, lack of consent from future generations, and potential for eugenics.

(a) Award [1] for identifying Cas9 as an endonuclease / cutting DNA at target site. (b) Award [1] for describing base substitution, [1] for describing frameshift/reading frame shift, [1] for comparing their overall effect on the polypeptide. (c) Award [1] for inheritance difference (somatic not inherited vs germline inherited), [1] for somatic ethical acceptability, [1] for germline ethical concerns.

(a) Active loading of sucrose involves proton pumps actively transporting hydrogen ions (\(H^+\)) out of companion cells into the cell wall space. This creates an electrochemical proton gradient. Protons then diffuse back down their gradient into the companion cells through a co-transport protein, bringing sucrose along with them against its concentration gradient. Sucrose then diffuses into the sieve tube elements via plasmodesmata. (b) According to the pressure-flow hypothesis, high concentrations of sucrose at the source lower the water potential in the sieve tube, causing water to enter by osmosis from the xylem. This builds high hydrostatic pressure at the source. At the sink, sucrose is unloaded, increasing the water potential so water leaves the phloem. This creates a low hydrostatic pressure at the sink. The resulting pressure gradient drives the mass flow of phloem sap from source to sink. (c) Xylem transport is unidirectional (roots to leaves) and passive (transpiration pull), whereas phloem transport is bidirectional (source to sink) and relies on active processes for loading.

(a) Award [1] for proton pumping out of companion cells, [1] for co-transport of sucrose with protons, [1] for movement of sucrose into sieve tube elements. (b) Award [1] for osmosis/high pressure at source, [1] for unloading/low pressure at sink, [1] for mass flow along the pressure gradient. (c) Award [1] for any valid difference (e.g., direction, driving force, or substances transported).

(a) The resting potential of \(-70\text{ mV}\) is maintained primarily by the active transport of the sodium-potassium pump, which pumps three sodium ions (\(Na^+\)) out of the cell for every two potassium ions (\(K^+\)) pumped in. Additionally, potassium leak channels allow potassium ions to diffuse out of the neuron down their concentration gradient much more easily than sodium can enter, leaving a net negative charge inside the cell. Negatively charged proteins (anions) inside the cytoplasm also contribute. (b) During depolarization, the membrane potential reaches a threshold, causing voltage-gated sodium channels to open. Sodium ions rapidly diffuse into the axon, making the inside positive relative to the outside. During repolarization, voltage-gated sodium channels close and voltage-gated potassium channels open, allowing potassium ions to diffuse rapidly out of the axon, restoring the negative internal charge. (c) Neurotransmitters are chemical messengers released from the presynaptic neuron that diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane to propagate or inhibit the electrical signal.

(a) Award [1] for active transport by sodium-potassium pump (3 Na+ out / 2 K+ in), [1] for potassium leak channels, [1] for resulting net negative interior. (b) Award [1] for depolarization (opening of voltage-gated Na+ channels/influx of Na+), [1] for repolarization (opening of voltage-gated K+ channels/efflux of K+), [1] for stating sodium channels close during repolarization. (c) Award [1] for stating they diffuse across synaptic cleft to bind to postsynaptic receptors.

### Part a: Diagram of a Chloroplast
An ideal diagram should include:
- **Double Membrane**: Outer and inner membrane with a narrow intermembrane space.
- **Thylakoids**: Flattened disc-like membrane sacs stacked to form **grana** (singular: granum).
- **Stroma**: The gel-like fluid surrounding the thylakoids containing necessary enzymes.
- **Starch Granules / Lipid Droplets**: Visible internal nutrient storage bodies.
- **DNA/Ribosomes**: Circular DNA loop and 70S ribosomes shown in the stroma.

### Part b: Structural Adaptations
- **Thylakoid Membranes**: Provide an exceptionally large surface area to hold photosystems, chlorophyll pigments, and electron transport chain carriers to maximize light absorption.
- **Thylakoid Space (Lumen)**: Has a very small volume, which enables protons (\(\text{H}^+\)) to accumulate rapidly, establishing a steep electrochemical gradient for ATP synthesis via chemiosmosis.
- **Stroma**: Fluid environment containing a high concentration of specific enzymes (such as Rubisco) and substrates needed for the light-independent reactions (Calvin cycle).
- **Double Membrane**: Compartmentalizes the organelle, isolating intermediate reactants and photosynthetic enzymes from the surrounding cytoplasm.
- **Chloroplast DNA and 70S Ribosomes**: Allows the rapid, autonomous synthesis of key photosynthetic enzymes on-site.

### Part c: Light-Independent Reactions
- **Carbon Fixation**:
- Carbon dioxide (\(\text{CO}_2\)) is accepted by a 5-carbon compound, ribulose bisphosphate (RuBP).
- This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase (Rubisco).
- The resulting unstable 6-carbon compound immediately splits into two molecules of a 3-carbon compound called glycerate 3-phosphate (G3P).
- **Reduction**:
- Each G3P molecule is phosphorylated by ATP (using energy) and then reduced by reduced NADP (NADPH) (providing electrons and hydrogen).
- This converts G3P into another 3-carbon sugar called triose phosphate (TP).
- **Regeneration of RuBP**:
- For every six molecules of triose phosphate produced, only one is used to synthesize half a glucose molecule (or other organic compounds).
- The remaining five molecules of triose phosphate undergo a complex sequence of reactions to regenerate three molecules of RuBP.
- This regeneration step requires energy from ATP.

**Part a [Max 4 marks]**
- Award [1] for outer and inner double membrane drawn and labeled correctly.
- Award [1] for grana (stacks of thylakoids) shown and labeled.
- Award [1] for stroma correctly labeled as the background fluid.
- Award [1] for starch granule / lipid droplet or circular DNA / 70S ribosomes correctly drawn and labeled.

**Part b [Max 4 marks]**
- **Thylakoid membranes**: provide a large surface area for photosynthetic pigments/electron carriers/ATP synthase. [1]
- **Small thylakoid space (lumen)**: allows fast generation of a high proton concentration gradient. [1]
- **Stroma**: fluid environment containing high concentration of enzymes (e.g., Rubisco) for the Calvin cycle. [1]
- **Double membrane**: isolates/compartmentalizes photosynthetic processes. [1]
- **DNA/Ribosomes**: allows autonomous protein synthesis of Calvin cycle enzymes. [1]

**Part c [Max 7 marks]**
- RuBP (5C) reacts with carbon dioxide (\(\text{CO}_2\)). [1]
- Catalyzed by the enzyme Rubisco. [1]
- Forms an unstable 6C intermediate which splits into two molecules of glycerate 3-phosphate (G3P/GP). [1]
- G3P is reduced to triose phosphate (TP). [1]
- Reduction requires ATP and reduced NADP (NADPH) from the light-dependent reactions. [1]
- One out of every six triose phosphates is used to produce glucose/organic molecules. [1]
- The remaining five triose phosphates are recycled to regenerate RuBP. [1]
- Regeneration of RuBP requires ATP. [1]

**Clarity/Construction [1 mark]**
- Award [1] if the presentation is clear, structured logically, and written using precise biological vocabulary (e.g., correct usage of G3P, TP, RuBP, Rubisco, stroma, thylakoid).

### Part a: Propagation along an Unmyelinated Axon
- **Depolarization**: An influx of sodium ions (\(\text{Na}^+\)) through voltage-gated channels causes the inside of the axon to become positively charged relative to the outside (membrane potential rises to approximately \(+30\text{ mV}\)).
- **Local Currents**: The high concentration of \(\text{Na}^+\) ions inside the depolarized region of the axon diffuses sideways (laterally) to the adjacent resting region.
- **Reaching Threshold**: This lateral movement of charge (local currents) depolarizes the adjacent membrane region to the threshold potential (around \(-50\text{ mV}\)).
- **Sequential Opening**: Once threshold is reached, voltage-gated sodium channels in the adjacent section open, causing a new action potential.
- **Unidirectional Flow**: The action potential only travels in one direction because the region behind it is in a refractory period (repolarizing due to potassium efflux and inactivated sodium channels).

### Part b: Myelination and Saltatory Conduction
- **Myelin Sheath**: Schwann cells wrap around the axon, creating an insulating myelin sheath.
- **Prevention of Ion Flow**: The lipid-rich myelin sheath prevents the movement of ions (\(\text{Na}^+\) and \(\text{K}^+\)) across the axonal membrane.
- **Nodes of Ranvier**: There are small uninsulated gaps in the myelin sheath called Nodes of Ranvier where voltage-gated channels are highly concentrated.
- **Saltatory Conduction**: Depolarization and action potentials can only occur at these nodes. The action potential effectively 'jumps' from one Node of Ranvier to the next.
- **Speed and Efficiency**: This saltatory conduction dramatically increases transmission speed (up to 100 times faster than unmyelinated fibers) and requires less metabolic energy (ATP) because fewer ions must be pumped back across the membrane by the sodium-potassium pump.

### Part c: Synaptic Transmission
- **Arrival of Impulse**: An action potential arrives at the pre-synaptic knob, depolarizing the pre-synaptic membrane.
- **Calcium Influx**: This depolarization causes voltage-gated calcium channels to open, allowing calcium ions (\(\text{Ca}^{2+}\)) to diffuse into the pre-synaptic neuron.
- **Vesicle Fusion**: The high internal concentration of \(\text{Ca}^{2+}\) triggers synaptic vesicles containing neurotransmitters to migrate to and fuse with the pre-synaptic membrane.
- **Exocytosis**: Neurotransmitters are released into the synaptic cleft via exocytosis.
- **Diffusion**: The neurotransmitters diffuse across the narrow fluid-filled synaptic cleft.
- **Receptor Binding**: Neurotransmitters bind to specific ligand-gated receptors on the post-synaptic membrane.
- **Post-Synaptic Potential**: This binding opens ion channels (e.g., ligand-gated sodium channels), causing an influx of sodium ions which depolarizes the post-synaptic membrane, potentially starting a new action potential.
- **Deactivation/Removal**: To prevent continuous stimulation, the neurotransmitter is rapidly broken down by specific enzymes (such as acetylcholinesterase) or reabsorbed back into the pre-synaptic neuron by active reuptake pumps.

**Part a [Max 4 marks]**
- Depolarization occurs when voltage-gated sodium channels open and \(\text{Na}^+\) rushes into the axon. [1]
- Local currents are created as sodium ions diffuse sideways inside the axon. [1]
- This lateral movement depolarizes the adjacent membrane section to threshold potential (approx. \(-50\text{ mV}\)). [1]
- When threshold is reached, voltage-gated \(\text{Na}^+\) channels in the next section open. [1]
- Propagation is unidirectional due to the refractory period of preceding channels. [1]

**Part b [Max 4 marks]**
- Myelin acts as an insulator, made by Schwann cells. [1]
- Prevents ion exchange/depolarization along the insulated regions of the axon. [1]
- Nodes of Ranvier are gaps in the myelin sheath where voltage-gated channels are concentrated. [1]
- Action potential jumps from node to node (saltatory conduction). [1]
- Saltatory conduction drastically increases the speed of transmission and reduces the amount of energy (ATP) needed by the \(\text{Na}^+/\text{K}^+\) pump. [1]

**Part c [Max 7 marks]**
- Action potential depolarizes the pre-synaptic membrane. [1]
- Voltage-gated calcium channels open, and calcium (\(\text{Ca}^{2+}\)) enters the pre-synaptic bulb. [1]
- Calcium ions cause vesicles containing neurotransmitters to fuse with the pre-synaptic membrane. [1]
- Neurotransmitters are released into the synaptic cleft by exocytosis. [1]
- Neurotransmitters diffuse across the synaptic cleft. [1]
- Neurotransmitters bind to specific receptors on the post-synaptic membrane. [1]
- This opens ligand-gated sodium channels, depolarizing the post-synaptic membrane (to initiate a new nerve impulse). [1]
- Neurotransmitters are rapidly removed/degraded (e.g., by acetylcholinesterase) or recycled by reuptake. [1]

**Clarity/Construction [1 mark]**
- Award [1] if the response is well-structured, exhibits clear logical progression, and consistently uses correct scientific terminology (e.g., pre-synaptic, post-synaptic, calcium channels, local currents, saltatory conduction, Nodes of Ranvier).