Question 1
In the Z-scheme of photosynthesis, the reaction centre of Photosystem I (P700) is oxidised and electrons are ultimately used to reduce:
AO₂ to form water
BNADP⁺ to form NADPH
CCO₂ to form glucose directly
DADP to form ATP
Question 2
The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) in the Calvin cycle:
ACatalyses the light-dependent splitting of water
BFixes CO₂ onto RuBP to form two molecules of 3-PGA (the first stable product)
CReduces NADP⁺ to NADPH in the stroma
DPumps H⁺ across the thylakoid membrane to generate ATP
Question 3
C₄ plants (e.g., maize, sugarcane) are more efficient photosynthetically than C₃ plants in hot, arid conditions because:
AThey use a different form of RuBisCO that is not inhibited by O₂
BCO₂ is first fixed into a C₄ acid in mesophyll cells and then concentrated around RuBisCO in bundle sheath cells, suppressing photorespiration
CThey perform photosynthesis only at night to reduce water loss
DTheir stomata remain permanently open to maximise CO₂ uptake