Biology

1. 1.

   Explain why ATP is useful as an immediate energy-transfer molecule rather than simply calling it an energy store.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Cells use a recyclable coupling molecule instead of directly attaching every task to fuel oxidation. Hydrolysis and phosphorylation allow controlled transfer to transport, synthesis or movement.

   Marking points

   • ATP hydrolysis can be coupled to energy-requiring processes.
   • Phosphate transfer can alter/reactivate target molecules.
   • ATP is rapidly regenerated from ADP and phosphate.

   Examiner tip: Avoid saying that breaking a bond alone releases energy; refer to the overall hydrolysis reaction.

2. 2.

   State the roles of oxygen and reduced NAD in aerobic oxidative phosphorylation.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Electron flow begins with reduced carriers and ends with reduction of oxygen. Removing oxygen prevents sustained oxidation of those carriers, disrupting the proton-gradient-generating pathway.

   Marking points

   • Reduced NAD donates electrons to the electron transport chain.
   • Oxygen is the final electron acceptor.
   • Oxygen combines with electrons and protons to form water.

   Examiner tip: Oxygen does not directly phosphorylate ADP.

3. 3.

   A respirometer absorbs CO2 and records 3.6 cm^3 oxygen uptake by 2.0 g of seeds in 12 minutes. Calculate uptake rate per gram per minute.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Normalise the gas change by both exposure duration and biological mass: 3.6/(12 x 2.0). CO2 absorption means its production does not cancel the oxygen-driven gas volume decrease.

   Marking points

   • Divide uptake by time: 0.30 cm^3 min^-1.
   • Divide by seed mass.
   • Rate = 0.15 cm^3 g^-1 min^-1.

   Examiner tip: A temperature/pressure control is needed before attributing all volume change to respiration.

4. 4.

   Explain how ATP synthase produces ATP in a chloroplast, including the source and direction of the proton gradient.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The thylakoid membrane separates two compartments. Light reactions establish an electrochemical gradient; controlled return through ATP synthase couples that potential to ATP formation on the stromal side.

   Marking points

   • Light-driven electron transport and water splitting build proton concentration in the thylakoid lumen.
   • Protons flow from the lumen to the stroma through ATP synthase.
   • This flow drives ADP phosphorylation with inorganic phosphate.

   Examiner tip: Do not reverse lumen-to-stroma proton movement through ATP synthase.

5. 5.

   An uncoupler allows protons to cross the inner mitochondrial membrane without ATP synthase. Predict its effects on the proton gradient and ATP yield, and explain why electron flow may continue.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The uncoupler bypasses the energy-coupling route but does not necessarily block electron transport. This separates oxygen consumption from ATP production, showing that one cannot be inferred directly from the other under all conditions.

   Marking points

   • The proton gradient dissipates.
   • Oxidative ATP production falls.
   • Electron carriers may still transfer electrons to oxygen when substrates are available.
   • Energy is dissipated as heat rather than efficiently captured as ATP.

   Examiner tip: An uncoupler is not the same as an electron-transport inhibitor.

6. 6.

   A plant exposed to light but deprived of CO2 accumulates reduced NADP while carbon fixation slows. Explain the connection and why light alone cannot sustain carbohydrate synthesis.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Energy availability and material availability are different constraints. Without CO2, the cycle cannot sustain conversion of carbon into sugars, and reduced carriers are consumed more slowly despite illumination.

   Marking points

   • CO2 is the carbon source fixed in the Calvin cycle.
   • Reduced NADP supplies reducing power to reduce carbon intermediates.
   • Less fixation reduces use/reoxidation of reduced NADP.
   • Light supplies energy/reducing power but not the missing carbon substrate.

   Examiner tip: The light-independent reactions still depend on products of the light reactions.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.