Question

Number of carboxylation in $C_4$ cycle is

• A. 1
• B. 2
• C. 5
• D. 3

Answer 1

The correct option is B 2

$C_4$ cycle has two carboxylations in it. In C$C_4$ plants, after entering through stomata, CO₂diffuses into a mesophyll cell. Being close to the leaf surface, these cells are exposed to high levels of O₂, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). Instead, the CO₂is inserted into a 3-carboncompound (C₃) called phosphoenolpyruvic acid forming the 4-carbon compound oxaloacetic acid. Oxaloacetic acid is converted into malic acid oraspartic acid (both have 4 carbons), which is transported into bundle sheath cells. Here the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch. Pyruvic acid is transported back to a mesophyll cell where it is converted back into PEP.