A Level H1 Biology Plant Biology Quiz

Answer Key - A-Level Biology H1 Quiz: Plant Biology

Section A: Light-Dependent Reactions

1. Chlorophyll (1); Chloroplast (1).
2. Water is split via photolysis (1). This provides electrons to replace those lost by PSII (1) and releases protons (H⁺) and oxygen (1).
3. Light excites electrons in PSII (1). Electrons move through the ETC to PSI (1). Energy released during this transfer is used to actively pump protons from the stroma into the thylakoid lumen (1), creating a high concentration of H⁺ inside the lumen (1).
4. ATP synthase provides a channel for protons to diffuse down their electrochemical gradient (1) from the lumen to the stroma (1). This flow of protons (chemiosmosis) provides energy to phosphorylate ADP into ATP (1) via the rotation of the enzyme's catalytic head (1).
5. PSII absorbs light to split water and initiate the ETC (1); PSI absorbs light to further energize electrons (1). PSII primarily contributes to the proton gradient for ATP (1), while PSI facilitates the reduction of NADP⁺ to NADPH (1).
6. Non-cyclic: Electrons flow from water $\rightarrow$ PSII $\rightarrow$ ETC $\rightarrow$ PSI $\rightarrow$ NADP⁺, producing both ATP and NADPH (3). Cyclic: Electrons from PSI return to the ETC, producing only ATP (2).
7. The light-dependent reactions provide ATP (1) for the phosphorylation of GP and regeneration of RuBP (1), and reduced NADP (1) for the reduction of GP to TP (1).

Section B: The Calvin Cycle

8. Rubisco (Ribulose bisphosphate carboxylase/oxygenase) (1); RuBP (Ribulose bisphosphate) (1).
9. Carbon Fixation: CO₂ combines with RuBP via Rubisco to form an unstable 6C compound, which splits into two 3C molecules of GP (2). Reduction: GP is reduced to TP using ATP and NADPH (2). Regeneration: Most TP is used to regenerate RuBP using ATP (2).
10. ATP provides the energy (phosphate group) (1) and NADPH provides the reducing power (electrons/hydrogen) (1) to convert the acid GP into the sugar TP (1). This is a reduction reaction (1).
11. 12 molecules of TP are produced (1). 10 molecules are used to regenerate RuBP (1), leaving 2 for organic synthesis (1).
12. Lower light intensity reduces the production of ATP and NADPH (1). This slows the reduction of GP to TP (1). Consequently, GP levels increase and TP levels decrease (1), slowing the overall rate of carbon fixation (1).
13. It does not directly require photons to proceed (1). However, it depends on the products of the light reactions (ATP/NADPH) (1), which are short-lived and produced during the day (1).
14. TP can be converted into glucose/starch via gluconeogenesis (2) or converted into glycerol and fatty acids to form lipids (2).

Section C: Plant Transport

15. Xylem consists of vessels and tracheids (1) made of dead cells with lignified walls (1). Function: Transport water and minerals (1) from roots to leaves via a continuous column (1).
16. Transpiration at the leaf creates a tension (negative pressure) (1). Water molecules are held together by hydrogen bonding (cohesion) (1). This pulls the entire water column upwards (1). Adhesion to xylem walls prevents the column from breaking (1). The process is passive (1).
17. Loading: Sucrose is actively transported into sieve tube elements (1), lowering water potential (1). Water enters by osmosis from xylem, creating high hydrostatic pressure (1). Movement: Sucrose moves by bulk flow to "sinks" (1) where it is unloaded (1), increasing water potential and allowing water to return to xylem (1).
18. K⁺ ions are actively pumped into guard cells (1). This lowers the water potential inside the guard cells (1). Water enters by osmosis, causing the cells to become turgid (1). The thin outer walls stretch more than the thick inner walls, causing the stoma to open (1).
19. Xylem: Unidirectional (upwards) (1), transports water/minerals (1), passive process (1). Phloem: Bidirectional (source to sink) (1), transports organic solutes (1), requires energy for loading/unloading (1).
20. Increased CO₂ increases the substrate concentration for Rubisco (1). This increases the rate of carbon fixation (1), provided other factors are not limiting (1). However, this may lead to partial stomatal closure to conserve water, which could eventually limit the rate (1).