Biology · Photosynthesis in Higher Plants · NEET
Yes. In C3 plants the first stable product of CO2 fixation is 3-phosphoglyceric acid (PGA), a 3-carbon acid, so the pathway is called C3. In C4 plants the first stable product is oxaloacetic acid (OAA), a 4-carbon acid, so it is called C4. This one idea unlocks most of the differences, so learn it first.
In C3 plants CO2 is fixed by RuBisCO onto the 5-carbon acceptor RuBP. In C4 plants CO2 is first fixed by PEP carboxylase (PEPcase) onto the 3-carbon acceptor PEP in the mesophyll cells. Note: C4 mesophyll cells do not have RuBisCO; RuBisCO works only in the bundle sheath cells of C4 plants.
Yes, both use the Calvin (C3) cycle as the main biosynthetic pathway that actually makes sugar. In C3 plants the Calvin cycle runs in the mesophyll. In C4 plants the C4 (Hatch and Slack) pathway just concentrates CO2, and the Calvin cycle runs only in the bundle sheath cells. So the Calvin cycle is NOT the difference between them.
In C3 plants, when O2 is high and CO2 is low, O2 binds RuBisCO and starts photorespiration, wasting fixed carbon. C4 plants use their C4 pathway to pump CO2 into the bundle sheath cells, keeping CO2 very high around RuBisCO. High CO2 makes RuBisCO carboxylate (not oxygenate), so photorespiration does not occur. This is why NCERT calls photorespiration the process that makes the major difference between C3 and C4 plants.
Kranz anatomy is a leaf structure where large bundle sheath cells, rich in chloroplasts, form a ring (wreath) around the vascular bundles. Only C4 plants have Kranz anatomy; C3 plants do not. The bundle sheath cells have thick walls, no intercellular spaces, and hold the Calvin cycle in C4 plants.
C3 examples: rice, wheat, and most temperate plants. C4 examples: maize (corn), sugarcane, sorghum, and many plants of dry tropical regions. A quick tip: sorghum and maize appear in PYQs as C4, so their first product is OAA, not PGA.
The process which makes major difference between C3 and C4 plants is
A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilization. In which of the following physiological groups would you assign this plant?
Statement I: The primary CO2 acceptor in C4 plants is phosphoenolpyruvate and is found in the mesophyll cells. Statement II: Mesophyll cells of C4 plants lack RuBisCo enzyme. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The first stable product: C3 plants make a 3-carbon acid (PGA), C4 plants make a 4-carbon acid (OAA). The process that makes the major difference is photorespiration, which happens in C3 plants but not in C4 plants.
In C3 plants the first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon acid. In C4 plants it is oxaloacetic acid (OAA), a 4-carbon acid.
Only C4 plants have Kranz anatomy, with large chloroplast-rich bundle sheath cells arranged in a ring around the vascular bundles. C3 plants do not have Kranz anatomy.
Yes. C4 plants use the C4 (Hatch and Slack) pathway to concentrate CO2, but the Calvin cycle is still the main sugar-making pathway, running only in the bundle sheath cells.
C3: rice, wheat. C4: maize (corn), sugarcane, sorghum. C4 plants are common in dry tropical regions.
