Q.Give comparison between the following:

(a) C3 vs C4: C3 uses RuBP (5C) as the CO2 acceptor and RuBisCO, giving 3-PGA (3C) as the first product, in mesophyll cells, with no Kranz anatomy and high photorespiration. C4 first uses PEP (3C) and PEP carboxylase in the mesophyll to give OAA (4C), then runs the Calvin cycle in bundle sheath cells (Kranz anatomy present), with negligible photorespiration and higher productivity. (b) Cyclic vs non-cyclic photophosphorylation: cyclic involves only PS I, makes only ATP, no water splitting and no O2 or NADPH; non-cyclic involves PS II and PS I (Z-scheme), makes ATP and NADPH, splits water and evolves O2. (c) Leaf anatomy in C3 vs C4: C4 shows Kranz anatomy (large, thick-walled, chloroplast-rich bundle sheath cells with no intercellular spaces); C3 lacks it and has ordinary mesophyll with poorly developed bundle sheath. …

(a) C3 pathway vs C4 pathway

• Primary CO2 acceptor: C3 uses RuBP (ribulose-1,5-bisphosphate, 5-carbon); C4 uses PEP (phosphoenol pyruvate, 3-carbon).
• Fixing enzyme: C3 uses RuBisCO; C4 first uses PEP carboxylase in the mesophyll (the bundle sheath then uses RuBisCO).
• First stable product: C3 gives 3-PGA (3-carbon); C4 gives OAA (oxaloacetic acid, 4-carbon).
• Site of Calvin cycle: C3 — all mesophyll cells; C4 — only bundle sheath cells.
• Kranz anatomy: absent in C3, present in C4.
• Photorespiration: high in C3; negligible/absent in C4 (CO2 is concentrated around RuBisCO).
• Temperature optimum and productivity: lower in C3; higher in C4, which tolerates high temperatures and high light and gives greater biomass. Examples: C3 — wheat, rice; C4 — maize, sorghum.

(b) Cyclic vs non-cyclic photophosphorylation

• Photosystems involved: Cyclic — only PS I; Non-cyclic — both PS II and PS I (the Z-scheme).
• Path of electrons: Cyclic — the excited electron is passed through the electron transport chain and returns to PS I (a closed loop). Non-cyclic — electrons flow from water → PS II → PS I → NADP+, an open path.
• Products: Cyclic makes only ATP; Non-cyclic makes both ATP and NADPH.
• Splitting of water / O2: Cyclic — no water splitting, so no O2 evolved; Non-cyclic — water is split at PS II, so O2 is evolved.
• Location / when: Cyclic occurs mainly in the stroma lamellae (and when only PS I is active, e.g. light of wavelength beyond 680 nm, or when the cell needs extra ATP but not NADPH); Non-cyclic occurs in the grana thylakoids.

(c) Anatomy of leaf in C3 and C4 plants

• C4 leaf — Kranz anatomy present: each vascular bundle is surrounded by large bundle sheath cells that have many chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces, arranged in a wreath-like ring. The mesophyll cells surround this sheath. Both mesophyll and bundle sheath take part in CO2 fixation. …

Method 1: Step-by-step

1. (a) List C3 vs C4 side by side: CO2 acceptor (RuBP, 5C vs PEP, 3C), enzyme (RuBisCO vs PEP carboxylase), first stable product (3-PGA, 3C vs OAA, 4C), site of the Calvin cycle (all mesophyll cells vs only bundle sheath cells), Kranz anatomy (absent vs present), photorespiration (significant vs negligible), examples (wheat/rice vs maize/sorghum).
2. (b) List cyclic vs non-cyclic photophosphorylation: photosystems involved (PS I only vs PS II + PS I in series/Z-scheme), electron path (returns to PS I in a loop vs flows one-way from water to NADP+), products (ATP only vs ATP + NADPH), water splitting/O2 (none vs occurs, O2 evolved), typical site/condition (stroma lamellae, or light beyond 680 nm vs grana thylakoids). …