A Level H2 Biology Plant Biology Quiz

Answer Key - A-Level Biology H2 Quiz: Plant Biology

1. Electrons are excited at PSII by light energy $\rightarrow$ move through the electron transport chain (ETC) $\rightarrow$ energy released is used by cytochrome $b_6f$ to pump $\text{H}^+$ into thylakoid lumen $\rightarrow$ creates proton gradient for ATP synthesis $\rightarrow$ electrons reach PSI for re-excitation to reduce NADP. [3]

2. Photolysis is the splitting of water using light energy $\rightarrow$ produces electrons, $\text{H}^+$ ions, and $\text{O}_2$ $\rightarrow$ essential to replace electrons lost by PSII to maintain the flow of the ETC. [3]

3. Cytochrome $b_6f$ uses energy from electron flow to pump protons from stroma to lumen $\rightarrow$ creates a high concentration of $\text{H}^+$ in lumen $\rightarrow$ protons flow back to stroma through ATP synthase (chemiosmosis) $\rightarrow$ drives phosphorylation of ADP to ATP. [3]

4. Palisade: tightly packed, columnar, many chloroplasts $\rightarrow$ maximizes light absorption. Spongy: loosely packed, large air spaces $\rightarrow$ facilitates rapid diffusion of $\text{CO}_2$ to cells. [3]

5. Chloroplasts are distributed around the periphery of the cell $\rightarrow$ reduces diffusion distance for $\text{CO}_2$ from the intercellular spaces to the chloroplast. [2]

6. Stomata open to allow $\text{CO}_2$ entry for photosynthesis $\rightarrow$ however, this leads to water loss via transpiration $\rightarrow$ closing stomata prevents wilting/desiccation but halts $\text{CO}_2$ fixation. [2]

7. Provides structural support to the leaf $\rightarrow$ ensures efficient transport of water (xylem) to mesophyll and export of sucrose (phloem) from source to sink. [2]

8. RuBisCO catalyses the carboxylation of Ribulose Bisphosphate (RuBP) $\rightarrow$ attaches $\text{CO}_2$ to RuBP to form two molecules of Glycerate-3-phosphate (GP). [2]

9. ATP provides energy and NADPH provides reducing power $\rightarrow$ used to convert GP to Triose Phosphate (TP) $\rightarrow$ some TP is then recycled using more ATP to regenerate RuBP, allowing the cycle to continue. [3]

10. RuBisCO acts as an oxygenase when $\text{O}_2$ levels are high $\rightarrow$ RuBP reacts with $\text{O}_2$ instead of $\text{CO}_2$ $\rightarrow$ produces 2-phosphoglycolate $\rightarrow$ wasteful because it consumes ATP and releases previously fixed $\text{CO}_2$ without producing sugar. [3]

11. High $\text{O}_2$ increases the rate of photorespiration $\rightarrow$ RuBisCO binds $\text{O}_2$ instead of $\text{CO}_2$ $\rightarrow$ reduces the efficiency of carbon fixation $\rightarrow$ net photosynthetic rate decreases. [3]

12. PEP carboxylase (C4) has a much higher affinity for $\text{CO}_2$ than RuBisCO $\rightarrow$ can fix $\text{CO}_2$ even at very low internal concentrations $\rightarrow$ RuBisCO (C3) is prone to oxygenation; PEP carboxylase is not. [3]

13. $\text{CO}_2$ is fixed into 4C compounds in mesophyll cells $\rightarrow$ transported to bundle sheath cells $\rightarrow$ $\text{CO}_2$ is released here $\rightarrow$ creates high $\text{CO}_2$ concentration around RuBisCO $\rightarrow$ outcompetes $\text{O}_2$ and minimizes photorespiration. [3]

14. C4 plants $\rightarrow$ better adapted to hot/dry conditions $\rightarrow$ can keep stomata partially closed to save water while still maintaining high $\text{CO}_2$ levels in bundle sheath cells $\rightarrow$ avoid photorespiration which increases at high temperatures. [3]

15. The factor that is in shortest supply/lowest concentration relative to the plant's needs $\rightarrow$ determines the overall rate of the process. [2]

16. At high light intensity, the light-dependent reactions are saturated $\rightarrow$ the rate is now limited by the Calvin cycle (e.g., RuBisCO concentration or $\text{CO}_2$ availability) $\rightarrow$ further light does not increase TP production. [3]

17. C3 plants may benefit more $\rightarrow$ higher $\text{CO}_2$ reduces the likelihood of photorespiration $\rightarrow$ increases efficiency of RuBisCO $\rightarrow$ may reduce the competitive advantage C4 plants currently have in high-$\text{CO}_2$ environments. [4]

18. Increased temperature increases kinetic energy $\rightarrow$ increases frequency of effective collisions between enzyme and substrate $\rightarrow$ increases rate of Calvin cycle (until denaturation). [2]

19. Transpiration at the leaf surface creates a negative pressure/tension $\rightarrow$ water is pulled up from the xylem in a continuous column (cohesion-tension) $\rightarrow$ maintains a gradient from soil $\rightarrow$ root $\rightarrow$ stem $\rightarrow$ leaf. [3]

20. CAM plants fix $\text{CO}_2$ at night when stomata are open (low transpiration) $\rightarrow$ store as organic acids $\rightarrow$ release $\text{CO}_2$ during the day for the Calvin cycle while stomata are closed $\rightarrow$ extreme water conservation in arid environments. [3]