Q.Describe the CAM cycle (schematic pathway) in detail, and then present a comparative account contrasting C3, C4 and CAM plants across their principal distinguishing features.
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Start your 14-day free trial to unlock the full solution →CAM (Crassulacean Acid Metabolism), found in succulents such as cacti, Opuntia, Bryophyllum and pineapple, performs the same two carboxylation steps as the C4 pathway but separates them temporally within a single mesophyll cell rather than spatially across two cell types. At night, when transpirational water loss through open stomata is minimised, CO2 enters and is fixed by PEP carboxylase onto PEP, forming OAA, reduced to malic acid, and stored overnight in the cell's large central vacuole, measurably raising the acidity of the leaf by morning. During the day, stomata close to conserve water in the heat, while the stored malic acid is decarboxylated, releasing CO2 internally within the same cell; this CO2 is then fixed a second time by RuBisCO via the standard Calvin cycle, powered by the ATP and NADPH the daytime light reaction supplies. Comparing all three strategies: C3 plants (wheat, rice, most trees) use RuBisCO alone on RuBP, forming PGA as the first product, with no Kranz anatomy and no separation between steps, which leaves them vulnerable to significant photorespiration under hot, bright, dry conditions. C4 plants (maize, sugarcane, sorghum) add PEP carboxylase and Kranz anatomy, separating initial fixation (mesophyll, forming OAA) from the Calvin cycle (bundle sheath) spatially, across two cooperating cell types, which minimises photorespiration through a high, sustained local CO2 concentration. CAM plants achieve a similarly minimal level of photorespiration, but …
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