How NEET Tests 'Photosynthesis' - High-Yield Traps

This is one of the highest-yield NEET chapters. The hard questions test exact sites, products, numbers and the C3/C4 distinction. Master these rank-splitters:

  • Where things happen: light reaction = thylakoid membrane; Calvin cycle = stroma; photolysis of water = PS II, on the lumen side; protons accumulate in the thylakoid lumen.
  • Cyclic vs non-cyclic photophosphorylation: non-cyclic = PS II + PS I -> ATP + NADPH + O2; cyclic = PS I only -> only ATP (no NADPH, no O2), in the stroma lamellae.
  • Order in the Z-scheme: electrons flow PS II (P680) -> PS I (P700); PS II is used first though PS I is numbered first. O2 comes from water, not CO2 (van Niel; confirmed by Ruben & Kamen with O-18).
  • Calvin cycle numbers: CO2 acceptor = RuBP (5-C); enzyme = RuBisCO; first product = 3-PGA (3-C); per CO2 = 3 ATP + 2 NADPH; per glucose (6 CO2) = 18 ATP + 12 NADPH; 6 turns of the cycle per glucose.
  • C3 vs C4: C4 first product = OAA (4-C) in the mesophyll (enzyme PEPcase, no O2 affinity); C3 first product = 3-PGA (3-C). In C4, RuBisCO is confined to the bundle-sheath cells (Kranz anatomy) and there is no photorespiration. C4 is more efficient.
  • Photorespiration: occurs in C3 plants when RuBisCO fixes O2 (RuBP + O2 -> PGA + phosphoglycolate); no sugar, no ATP, no NADPH; CO2 released. RuBisCO has a greater affinity for CO2 than O2, so it is the CO2/O2 ratio that decides which reaction wins.
  • Pigments/spectra: chlorophyll a = chief/reaction-centre pigment; accessory pigments (chl b, carotenoids) widen absorption and protect; absorption & action spectra both peak in blue and red.
  • Limiting factors (Blackman): the rate is set by the factor nearest its minimum; CO2 is usually the major limiting factor.

Below are challenging assertion-reason, match-the-column, multi-statement and sequence questions built on exactly these distinctions.