Hardest AQA A-Level Biology Questions & 25-Marker (Grade A*)

Define an ion (an atom or molecule with an electrical charge due to loss or gain of electrons). State that inorganic ions occur in solution in the cytoplasm and body fluids, acting as enzyme cofactors, electrochemical messengers, and osmotic regulators that sustain life.

Scientific Depth: Detail the electron transport chain in oxidative phosphorylation and photophosphorylation. Energy from electrons powers proton pumps in the inner mitochondrial/thylakoid membrane, pumping H+ from matrix to intermembrane space, creating an electrochemical proton gradient. H+ diffuses back through ATP synthase stalked granules via facilitated diffusion, rotating the catalytic head to phosphorylate ADP + Pi to ATP.

Importance: Provides universal chemical energy for metabolic reactions (active transport, protein synthesis, muscle contraction). Without H+ gradient, ATP yield drops from 32 to 2 per glucose (glycolysis only), leading to cellular death.

Scientific Depth: The sodium-potassium pump actively transports 3 Na+ out for every 2 K+ in, creating resting potential (-70 mV). Upon threshold stimulus, voltage-gated Na+ channels open, causing influx and depolarisation (+30 mV). Voltage-gated K+ channels open, causing K+ efflux and repolarisation/hyperpolarisation. Myelination causes saltatory conduction between nodes of Ranvier.

Importance: Enables rapid transmission of nervous impulses across long distances, allowing animals to detect danger, coordinate muscular locomotion, and maintain homeostasis.

Scientific Depth: Depolarisation of presynaptic knob opens voltage-gated Ca2+ channels. Ca2+ influx causes synaptic vesicles containing acetylcholine to fuse with presynaptic membrane (exocytosis). In muscle fibres, action potential travels down T-tubules to sarcoplasmic reticulum, releasing Ca2+ into myofibrils. Ca2+ binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin filaments, facilitating actomyosin cross-bridge cycling.

Importance: Unidirectional transmission of information across nervous systems and voluntary/involuntary mechanical contraction of cardiac, skeletal, and smooth muscles.

Scientific Depth: Haemoglobin is a quaternary protein with four polypeptide chains, each with a haem prosthetic group containing an Fe2+ ion. Each Fe2+ reversibly binds one molecule of oxygen (O2), enabling cooperative binding and the sigmoidal oxygen dissociation curve. Mention the Bohr effect where high CO2 (H+ ions) shifts the curve to the right.

Importance: High-affinity loading of oxygen at respiratory surfaces (alveoli) and efficient unloading at respiring tissues to maintain aerobic cellular respiration and prevent lactic acidosis.

Scientific Depth: Active transport of Na+ out of epithelial cells into capillary by Na+/K+ pump lowers intracellular Na+ concentration. Na+ diffuses down its concentration gradient from ileum lumen into cell via sodium-glucose co-transporter protein (symport), pulling glucose against its concentration gradient.

Importance: Maximises glucose absorption from digestion into the blood, ensuring substrates are continuously supplied for cellular respiration.

"There is no significant difference between the mean stomatal density of leaves from the upper canopy and lower canopy; any observed difference is due to chance."

Student's t-test.
Justification: The investigation compares the difference between two means of a continuous variable (stomatal density per mm²).

1. The calculated value of t (2.48) is greater than the critical value (2.05) at p = 0.05.
2. There is a probability of less than 0.05 (less than 5%) that the difference in mean stomatal density is due to chance.
3. Reject the null hypothesis (H0).
4. There is a statistically significant difference in the mean stomatal density between upper and lower canopy leaves.

• C_P_: Has functional Enzyme C and Enzyme P ⇒ Precursor → Pink → Purple.
• C_pp: Has functional Enzyme C, but non-functional Enzyme P ⇒ Precursor → Pink (stops here).
• ccP_: Non-functional Enzyme C ⇒ Stays as White Precursor.
• ccpp: Both enzymes non-functional ⇒ Stays as White Precursor.

From cross CcPp x CcPp:

The expected ratio of offspring is: 9 Purple : 3 Pink : 4 White (Recessive Epistasis).

At resting potential (-70 mV), Na+/K+ pump actively pumps 3 Na+ out for 2 K+ in. When a stimulus arrives, some Na+ channels open. If threshold potential (-55 mV) is exceeded, voltage-gated Na+ channels open, causing rapid influx of Na+ by facilitated diffusion down electrochemical gradient, depolarising the membrane to +30 mV.

At +30 mV, voltage-gated Na+ channels close/inactivate. Voltage-gated K+ channels open, allowing K+ ions to rapidly diffuse out of the axon down their electrochemical gradient, repolarising the membrane. Because voltage-gated K+ channels are slow to close, excess K+ diffuses out, hyperpolarising the membrane to approximately -80 mV before the resting potential is re-established by the Na+/K+ pump.

• Unidirectional: Action potentials travel in one direction only, because previous region is in absolute refractory state (Na+ channels inactivated).
• Discrete Impulses: Prevents impulses from merging together into a continuous wave.
• Frequency Limitation: Caps the maximum frequency of nerve impulse transmission.

Without light, the light-dependent reaction stops, halting the photolysis of water and stopping the synthesis of ATP and reduced NADP.
ATP is required to regenerate RuBP from triose phosphate (TP). In the absence of ATP, RuBP cannot be regenerated.
However, remaining RuBP continues to react with available CO2 catalyzed by RuBisCO to form GP. Therefore, RuBP levels drop rapidly to near zero.

RuBP continues to be converted to GP by RuBisCO as long as residual RuBP and CO2 are present.
However, the reduction of GP into TP requires ATP and reduced NADP from the light-dependent reaction.
Since neither ATP nor reduced NADP is available in the dark, GP cannot be converted into TP.
Therefore, GP accumulates and its concentration rises.

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group from a phosphorylated donor molecule to ADP to form ATP without an electron transport chain or proton gradient. Occurs in cytoplasm (during glycolysis) and mitochondrial matrix (during Krebs cycle).
• Oxidative Phosphorylation: Indirect synthesis of ATP from ADP + Pi powered by the flow of protons through ATP synthase down an electrochemical gradient established by an electron transport chain with oxygen as terminal electron acceptor. Occurs across the inner mitochondrial membrane (cristae).

1. In the absence of oxygen (terminal electron acceptor), the electron transport chain, Link reaction, and Krebs cycle cease operating because reduced NAD and reduced FAD cannot be re-oxidised.
2. Glycolysis is the only pathway that continues, producing a net yield of only 2 ATP per glucose (compared to ~30–32 ATP in aerobic respiration).
3. Role of Lactate Production: Pyruvate is reduced to lactate by lactate dehydrogenase, which simultaneously oxidises reduced NAD back to NAD.
4. This regenerated NAD+ is essential because it allows glycolysis to continue supplying emergency ATP to the cell.

Increased methylation involves adding methyl groups (CH3) to cytosine bases in CpG islands located within the promoter regions of tumour suppressor genes.
This prevents transcriptional factors and RNA polymerase from binding to the promoter.
Consequently, the gene is transcriptionally silenced; proteins that inhibit cell division or initiate apoptosis are not synthesised, leading to uncontrolled cell division (tumour formation).

Removing acetyl groups from histones increases the positive charge on histones, increasing their attraction to negatively charged phosphate groups on DNA.
DNA becomes tightly condensed into heterochromatin.
Promoter regions become inaccessible to RNA polymerase, inhibiting transcription.

Double-stranded RNA is cut into small interfering RNA (siRNA) by an enzyme (Dicer).
One strand of siRNA associates with an enzyme complex (RISC) and binds to complementary target mRNA by base-pairing.
The enzyme cuts the mRNA into fragments, preventing it from being translated into protein at ribosomes.

In the thick ascending limb of the loop of Henle, Na+ and Cl- ions are actively pumped out of the filtrate into the interstitial fluid of the renal medulla.
The ascending limb is impermeable to water, so water cannot follow by osmosis.
This creates a very low water potential (high osmotic pressure) in the interstitial fluid of the medulla.

The descending limb is permeable to water, but relatively impermeable to ions.
Water moves out of the descending limb filtrate into the medulla interstitial fluid by osmosis down the water potential gradient and is carried away by the vasa recta capillaries.
Because filtrate flows in opposite directions in the two limbs (counter-current), a progressive osmotic gradient is multiplied down the entire depth of the medulla.

When blood water potential drops, osmoreceptors in the hypothalamus detect shrinkage and stimulate the posterior pituitary gland to secrete ADH.
ADH binds to receptors on collecting duct cells, activating phosphorylase which causes vesicles containing aquaporin channel proteins to fuse with the cell surface membrane.
The collecting duct becomes highly permeable to water.
Water moves out of collecting duct into medulla by osmosis, producing a small volume of concentrated urine.

• 95°C (Denaturation): High temperature breaks hydrogen bonds between complementary base pairs, separating double-stranded DNA into single template strands.
• 55°C (Annealing): Cooled to allow short single-stranded DNA primers to bind (anneal) to complementary base sequences at the 3' ends of the target DNA fragment.
• 72°C (Extension): Heated to the optimum temperature for thermostable Taq DNA polymerase, which synthesises new complementary strands by adding free DNA nucleotides starting from primers.

1. Plasmids contain two antibiotic resistance genes (e.g. ampicillin and tetracycline resistance).
2. Target gene is inserted into the middle of the tetracycline resistance gene, disrupting it (insertional inactivation).
3. Bacteria are first grown on ampicillin agar: only bacteria that have taken up a plasmid (recombinant or non-recombinant) survive.
4. Colonies are transferred via a sterile velvet pad (replica plating) onto tetracycline agar.
5. Identification: Colonies that survive on ampicillin but die on tetracycline possess the recombinant plasmid containing the inserted target gene.