Q.Write four differences between the C3 and C4 cycles. OR Briefly describe the Z scheme of the light reaction of photosynthesis.

C3 and C4 plants differ in their first CO2-fixation product, the enzymes and leaf anatomy involved, and their photosynthetic efficiency (photorespiration) under hot, bright, dry conditions.

Four key differences between C3 and C4 plants:

1. First stable product of carbon fixation: In C3 plants, CO2 combines directly with RuBP (via RuBisCO) to form two molecules of the 3-carbon compound 3-phosphoglyceric acid (3-PGA) — hence the name C3 pathway. In C4 plants, CO2 first combines with PEP (phosphoenolpyruvate) to form the 4-carbon compound oxaloacetic acid (OAA) — hence the name C4 pathway.

2. Primary carboxylating enzyme and site: In C3 plants, CO2 fixation is carried out entirely by the enzyme RuBisCO in the mesophyll cell chloroplasts. In C4 plants, CO2 is first fixed by PEP carboxylase in the mesophyll cells; the resulting 4-carbon acid is then transported to bundle sheath cells, where CO2 is released and re-fixed by RuBisCO in the Calvin cycle.

3. Leaf anatomy: C3 plants have normal leaf anatomy without any special arrangement around the vascular bundles. C4 plants show a specialised leaf anatomy called Kranz anatomy, in which bundle sheath cells around the vascular bundles are large, thick-walled, and contain numerous chloroplasts, arranged in a wreath-like ring, with mesophyll cells radiating around them.

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C3 and C4 plants differ across at least eight features: first CO2 acceptor, first stable product, leaf anatomy, chloroplast dimorphism, primary carboxylating enzyme, occurrence of photorespiration, CO2/light saturation points, and water-use efficiency.

C3 and C4 photosynthesis differ in their first CO2-fixation product, fixing enzyme, leaf anatomy, and efficiency under hot/dry conditions.

1. First stable product of CO2 fixation — In the C3 pathway (Calvin cycle), CO2 combines with the 5-carbon RuBP to directly form two molecules of the 3-carbon compound 3-phosphoglyceric acid (PGA), hence the name 'C3'. In the C4 pathway, CO2 first combines with the 3-carbon PEP (phosphoenolpyruvate) to form the 4-carbon compound oxaloacetic acid (OAA), hence 'C4'.

2. Primary CO2-fixing enzyme — C3 plants use the enzyme RuBisCO for primary fixation, which has a dual affinity for both CO2 and O2 (leading to photorespiration when O2 binds instead of CO2). C4 plants use PEP carboxylase for the primary fixation step in mesophyll cells; this enzyme has a very high affinity for CO2 and does not bind O2 at all.

3. Leaf/cellular anatomy — In C3 plants, the entire Calvin cycle (both CO2 fixation and reduction) occurs within the chloroplasts of ordinary mesophyll cells. C4 plants show a specialised 'Kranz anatomy': CO2 is first fixed in the mesophyll cells (as OAA/malate), which is then transported to the bundle-sheath cells surrounding the vascular bundles, where the actual Calvin cycle (C3 pathway) takes place.

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