A Level H2 Biology Plant Biology Quiz

Answer Key - A-Level Biology H2 Quiz (Plant Biology)

Section A: Light-Dependent Reactions & Photosystems

1. Role of OEC: The OEC catalyzes the photolysis of water (splitting of $\text{H}_2\text{O}$ into $\frac{1}{2}\text{O}_2$, $2\text{H}^+$, and $2\text{e}^-$). [1] It provides a continuous source of electrons to replace those lost by the reaction center (P680) of PSII. [1]
2. Excitation of Electrons: Light energy is absorbed by antenna pigments and transferred to the reaction center. [1] This excites electrons to a higher energy level. [1] This high energy is required to drive the electrons through the ETC against the electrochemical gradient/to the next carrier. [1]
3. ATP Synthesis: As electrons move from PSII to PSI via the ETC, energy is released. [1] This energy is used by cytochrome complexes to pump protons ($\text{H}^+$) from the stroma into the thylakoid lumen. [1] This creates a proton gradient; protons flow back to the stroma through ATP synthase, driving the phosphorylation of ADP to ATP. [1] (Note: 4 marks total; award for clarity on gradient and ATP synthase).
4. PSI vs PSII: PSII absorbs shorter wavelengths (approx. 680nm) [1] and passes electrons to the ETC/PSI. PSI absorbs longer wavelengths (approx. 700nm) [1] and passes electrons to NADP+ reductase to form NADPH. [1]
5. Inhibitor Effect: NADPH production would decrease/stop. [1] Without the OEC, there are no electrons to replace those excited in PSII. [1] This halts the flow of electrons through the ETC to PSI, meaning $\text{NADP}^+$ cannot be reduced to NADPH. [1]
6. Cyclic Photophosphorylation: Electrons from PSI are recycled back to the ETC instead of going to $\text{NADP}^+$. [1] This generates ATP without producing NADPH or $\text{O}_2$. [1] Reason: The Calvin cycle requires more ATP than NADPH; this process balances the ATP:NADPH ratio. [1]
7. Proton Gradient: The accumulation of $\text{H}^+$ in the lumen creates a high concentration of protons relative to the stroma. [1] This represents potential energy (electrochemical gradient). [1] The flow of these protons down their gradient through ATP synthase provides the energy for ATP synthesis. [1]

Section B: The Calvin Cycle & Carbon Fixation

8. RuBisCO: It catalyzes the fixation of $\text{CO}_2$ by attaching it to the 5-carbon sugar Ribulose Bisphosphate (RuBP). [2]
9. Sequence after Fixation: An unstable 6-carbon intermediate is formed. [1] This immediately splits into two molecules of 3-phosphoglycerate (3-PG). [1] 3-PG is then phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P/TP). [1]
10. Regeneration of RuBP: RuBP is the $\text{CO}_2$ acceptor. [1] Without its regeneration, the plant cannot fix more $\text{CO}_2$, and the cycle stops. [1]
11. Photorespiration: Occurs when RuBisCO acts as an oxygenase, binding $\text{O}_2$ instead of $\text{CO}_2$ to RuBP. [1] This produces 3-phosphoglycerate and 2-phosphoglycolate. [1] It is wasteful because it consumes ATP and releases previously fixed $\text{CO}_2$ without producing sugar. [2]
12. $\text{O}_2$ Concentration: Increased $\text{O}_2$ increases the rate of photorespiration. [1] RuBisCO has a competitive affinity for $\text{O}_2$ and $\text{CO}_2$. [1] This reduces the net rate of carbon fixation/photosynthesis. [1]
13. C3 vs C4 Fixation: C3 plants use RuBisCO to fix $\text{CO}_2$ directly into a 3-carbon compound. [1] C4 plants use PEP carboxylase to fix $\text{CO}_2$ into a 4-carbon oxaloacetate. [1] PEP carboxylase has a much higher affinity for $\text{CO}_2$ and does not bind $\text{O}_2$. [1]
14. Kranz Anatomy: C4 plants have a distinct arrangement of mesophyll cells surrounding bundle sheath cells. [1] $\text{CO}_2$ is fixed in mesophyll cells as a 4C compound, then transported to bundle sheath cells. [1] $\text{CO}_2$ is released in the bundle sheath cells, creating a high $\text{CO}_2$ concentration. [1] This ensures RuBisCO operates under high $\text{CO}_2$ conditions, effectively eliminating photorespiration. [1]

Section C: Environmental Factors & Plant Anatomy

15. Light Intensity: Rate increases linearly with light intensity at low levels (light is limiting). [1] As intensity increases, the rate levels off at the light saturation point. [1] At this point, another factor (e.g., $\text{CO}_2$ or temperature) becomes limiting. [1]
16. Temperature: RuBisCO activity increases with temperature due to higher kinetic energy. [1] However, at high temperatures, the affinity of RuBisCO for $\text{O}_2$ increases relative to $\text{CO}_2$. [1] This increases photorespiration and decreases the net photosynthetic rate. [1]
17. Low $\text{CO}_2$: The rate of the Calvin cycle slows down. [1] This leads to a decrease in the regeneration of $\text{ADP}$ and $\text{NADP}^+$. [1] Consequently, the consumption of ATP and NADPH by the Calvin cycle decreases. [1]
18. Mesophyll: Palisade cells are columnar, tightly packed, and contain many chloroplasts to maximize light absorption. [2] Spongy cells are irregular with large air spaces to facilitate the diffusion of gases ($\text{CO}_2$ and $\text{O}_2$) throughout the leaf. [2]
19. Stomata Regulation: Guard cells take up $\text{K}^+$ ions, lowering water potential. [1] Water enters via osmosis, making cells turgid and bowing the cells open. [1] When water is scarce, $\text{K}^+$ leaves, cells become flaccid, and stomata close to prevent transpiration. [1]
20. C4 Energy Trade-off: C4 plants spend more ATP to pump $\text{CO}_2$ into bundle sheath cells. [1] In low temperatures, photorespiration in C3 plants is naturally low. [1] Therefore, the extra energy cost of the C4 pathway provides no competitive advantage and may be a disadvantage compared to C3 plants. [1]