Q.32 Which of the following is(are) TRUE about photosynthesis?
(A) In C3 plants the first organic product of carbon fixation is 3-phosphoglycerate
(B) In C4 plants the first organic product of carbon fixation is oxaloacetate
(C) Crassulacean acid metabolism occurs in succulent plants living in arid conditions
(D) Oxygen is generated from carbon dioxide
Option Analysis
Option A: Correct. In C3 plants, CO₂ fixation occurs via the Calvin cycle where RuBisCO catalyzes the reaction between CO₂ and RuBP, producing 3-phosphoglycerate (3-PGA) as the first stable organic product.
Option B: Correct. C4 plants use PEP carboxylase in mesophyll cells to fix CO₂ with phosphoenolpyruvate (PEP), forming the four-carbon compound oxaloacetate as the initial product before transfer to bundle sheath cells.
Option C: Correct. Crassulacean acid metabolism (CAM) plants, such as succulents, open stomata at night in arid conditions to fix CO₂ into malate, minimizing daytime water loss while enabling photosynthesis.
Option D: Incorrect. Oxygen in photosynthesis originates from water molecules split during photolysis in photosystem II, not from carbon dioxide, which is reduced to form carbohydrates.
Photosynthesis C3 C4 CAM plants represent adaptive carbon fixation strategies crucial for competitive exams like CSIR NET Life Sciences. Understanding the first organic product of carbon fixation in each pathway clarifies plant adaptations to environmental stresses.
C3 Pathway Basics
C3 plants, dominant in temperate regions, rely on the Calvin cycle for CO₂ fixation. RuBisCO binds CO₂ to RuBP, yielding 3-phosphoglycerate immediately. This three-carbon compound marks C3 plants but risks photorespiration in hot conditions.
C4 Pathway Efficiency
C4 plants like maize combat photorespiration through spatial separation. PEP carboxylase in mesophyll forms oxaloacetate first, concentrating CO₂ for Calvin cycle in bundle sheath cells. This boosts efficiency in high-light, warm tropics.
CAM Adaptation in Arid Zones
Succulent CAM plants, including cacti, fix CO₂ nocturnally into malate using PEP carboxylase, decarboxylating it daytime. This temporal separation suits arid conditions, conserving water.
| Pathway | First Product | Key Enzyme | Adaptation |
|---|---|---|---|
| C3 | 3-Phosphoglycerate | RuBisCO | Cool, moist climates |
| C4 | Oxaloacetate | PEP carboxylase | Hot, sunny tropics |
| CAM | Malate | PEP carboxylase | Arid, dry deserts |
Oxygen evolves from water photolysis, not CO₂, supporting atmospheric O₂ production. These distinctions aid exam success in plant physiology.
