Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants

C₃ and C₄ pathways

• C₃ plants fix CO₂ directly via ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in mesophyll cells, producing two molecules of 3-phosphoglycerate (3-PGA) per CO₂

• C₄ plants initially fix CO₂ in mesophyll cells via phosphoenolpyruvate carboxylase (PEPCase), forming a four-carbon acid (oxaloacetate), which is transported to bundle sheath cells where CO₂ is released for the Calvin cycle

• C₃ plants lack a biochemical CO₂ pump and are susceptible to RuBisCO’s oxygenase activity (photorespiration), especially under high temperature or low CO₂

• C₄ plants concentrate CO₂ around RuBisCO, virtually eliminating photorespiration and improving water- and nitrogen-use efficiency

• energy and reducing power requirements per CO₂ fixed are 3 ATP + 2 NADPH for both, but C₄ cycle adds extra ATP cost for PEP regeneration

Cyclic and non-cyclic photophosphorylation

• non-cyclic photophosphorylation involves both PS II and PS I in series (Z-scheme), splitting water at PS II to replace electrons, evolving O₂, and producing both ATP and NADPH + H⁺

• cyclic photophosphorylation involves only PS I in the stroma lamellae (which lack PS II and NADP⁺ reductase), in which excited electrons cycle back through the electron transport chain, generating only ATP and no NADPH

• cyclic flow operates when light beyond 680 nm predominates or when extra ATP is needed to meet the demands of the Calvin and C₄ cycles

Anatomy of leaf in C₃ and C₄ plants

• C₃ leaves have a single type of photosynthetic cell (mesophyll) with chloroplast-laden palisade and spongy layers; bundle sheath cells lack chloroplast specialization and intercellular spaces allow free gas diffusion

• C₄ leaves exhibit Kranz anatomy: each vascular bundle is encircled by a wreath (“Kranz”) of large, chloroplast-rich bundle sheath cells with thick walls and no intercellular spaces, surrounded further by mesophyll cells

• high vein density in C₄ leaves minimizes diffusion distance between mesophyll and bundle sheath, facilitating rapid metabolite shuttling and gas exchange under high light and temperature conditions