A Level H1 Biology Plant Biology Quiz

Answer Key - A-Level Biology H1 Quiz (Plant Biology)

1. Chlorophyll a (or Chlorophyll). [1]

2. The splitting of water molecules using light energy [1] into protons, electrons, and oxygen. [1]

3. Stroma. [1]

4. Rubisco (Ribulose bisphosphate carboxylase/oxygenase). [1]

5. The light-independent reactions require ATP and reduced NADP (NADPH) [1], which are produced during the light-dependent reactions. [1]

6. To transport electrons from PSII to PSI [1], creating a proton gradient across the thylakoid membrane. [1]

7. Protons are pumped/accumulate in the thylakoid lumen [1] and flow down their electrochemical gradient into the stroma through ATP synthase. [1]

8. PSII absorbs light to split water and energize electrons [1]; PSI absorbs light to further energize electrons to reduce NADP to NADPH. [1]

9. (a) The rate of photosynthesis increases linearly [1] as more light energy is available to excite chlorophyll molecules. [1] (b) Other factors become limiting [1] (e.g., $\text{CO}_2$ concentration or temperature/enzyme activity). [1]

10. Grana increase the surface area [1] of the thylakoid membranes [1], allowing for a higher density of photosystems and electron transport chains to capture light. [1]

11. Light-dependent products: ATP and NADPH [1]. Calvin cycle requirements: ATP for phosphorylation and NADPH for reduction [1] of GP to TP. [1]

12. RuBP is the $\text{CO}_2$ acceptor [1]. Without its regeneration, the cycle would stop [1] as there would be no molecule to fix incoming $\text{CO}_2$. [1]

13. (a) No significant immediate effect (or slight decrease due to feedback). [1] (b) ATP is required for the reduction of GP to TP [1] and the regeneration of RuBP [1]. Without ATP, glucose synthesis ceases. [1]

14. Water provides electrons to replace those lost by PSII [1]. Without water, the electron flow stops [1], preventing the production of ATP and NADPH. [1]

15. $\text{CO}_2$ is the substrate for Rubisco [1]. Low $\text{CO}_2$ reduces the frequency of collisions between $\text{CO}_2$ and RuBP [1], slowing the rate of carbon fixation. [1]

16. The absorption spectrum shows wavelengths chlorophyll absorbs [1]. The action spectrum shows the rate of photosynthesis at those wavelengths [1]. They overlap closely, indicating chlorophyll is the primary driver of the process. [1]

17. (8 marks)

    • Light is absorbed by PSII, exciting electrons. [1]
    • Water is split (photolysis) to replace electrons, releasing $\text{H}^+$ and $\text{O}_2$. [1]
    • Electrons move through the ETC to PSI. [1]
    • Energy from electrons is used to pump protons from stroma into the thylakoid lumen. [1]
    • This creates a proton gradient (electrochemical gradient). [1]
    • Protons flow back to the stroma via ATP synthase (chemiosmosis). [1]
    • This movement provides energy to phosphorylate ADP to ATP. [1]
    • ATP is then available in the stroma for the Calvin cycle. [1]

18. (6 marks)

    • $\text{CO}_2$ must diffuse across the plasma membrane and chloroplast membrane to reach the stroma. [1]
    • Water must enter root cells via osmosis and move through the plant to reach the leaves. [1]
    • Active transport of minerals (e.g., Magnesium for chlorophyll) across membranes. [1]
    • The thylakoid membrane maintains the proton gradient essential for ATP synthesis. [1]
    • Export of synthesized sugars (triose phosphates) out of the chloroplast via transport proteins. [1]
    • Overall, membrane selectivity and transport regulate the availability of substrates and the efficiency of energy conversion. [1]

19. (6 marks)

    • $\text{CO}_2$ combines with RuBP (5C) catalyzed by Rubisco. [1]
    • This forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate (GP). [1]
    • ATP phosphorylates GP. [1]
    • NADPH reduces the phosphorylated GP to glyceraldehyde-3-phosphate (TP). [1]
    • This process uses the chemical energy stored in ATP and NADPH. [1]
    • Some TP is then used to synthesize glucose/starch. [1]

20. (6 marks)

    • Low temperatures: Low kinetic energy, fewer collisions between enzymes (Rubisco) and substrates, slow rate. [1]
    • Optimal temperature: Maximum enzyme-substrate complex formation, peak rate. [1]
    • High temperatures: Thermal denaturation of enzymes (Rubisco) [1], altering the active site and stopping carbon fixation. [1]
    • High temperatures also increase membrane fluidity/leakiness [1], disrupting the proton gradient in thylakoids. [1]
    • This leads to a sharp decline in ATP production and overall photosynthetic rate. [1]