Photorespiration (the C2 cycle) happens when the enzyme RuBisCO binds oxygen (O2) instead of carbon dioxide (CO2). Instead of making 2 molecules of 3-PGA, RuBP splits into one 3-carbon phosphoglycerate and one 2-carbon phosphoglycolate. It makes NO sugar, NO ATP and NO NADPH, and it wastes energy while releasing CO2. Memory hook: "C2 = Costly and CO2-losing" — RuBisCO gets confused between O2 and CO2 in C3 plants.
RuBisCO can bind CO2 (carboxylation, makes sugar) or O2 (photorespiration, C2 cycle). Binding O2 forms a 3-C and a 2-C phosphoglycolate, giving no sugar, no ATP and no NADPH while releasing CO2.
Your doubts, answered
Does photorespiration make ATP or NADPH like normal respiration?
No. This is the most tested point. In the photorespiratory pathway there is NO synthesis of sugar, NO ATP and NO NADPH. In fact it USES up ATP and releases CO2. So it is a purely wasteful pathway for the plant, unlike normal cellular respiration which produces ATP.
Why does RuBisCO bind O2 instead of CO2?
RuBisCO's active site can bind BOTH CO2 and O2 (that is why it is called carboxylase-oxygenase). The binding is competitive, so it depends on the relative amounts of the two gases. In C3 plants, when O2 is high compared to CO2, some O2 binds RuBisCO and CO2 fixation drops, starting photorespiration.
What is the 2-carbon product formed in photorespiration?
When RuBP (5C) binds O2, it forms one molecule of 3-carbon phosphoglycerate (3-PGA) and one molecule of 2-carbon phosphoglycolate. Because a 2-carbon molecule is formed, the pathway is called the C2 cycle. Phosphoglycolate is the marker product NEET loves to ask about.
Does photorespiration occur in C3 or C4 plants?
It occurs in C3 plants. It does NOT occur in C4 plants, because C4 plants concentrate CO2 at the RuBisCO site inside bundle sheath cells, forcing RuBisCO to act as a carboxylase. This is why photorespiration is the key process that makes a difference between C3 and C4 plants.
Is photorespiration the same as normal respiration?
No. Normal respiration breaks down glucose to release ATP. Photorespiration happens only in light, is triggered by RuBisCO's oxygenase activity, makes no ATP, and its biological function is still not clearly known. Do not confuse the two in NEET.
⚠️ The NEET trap ✗ Photorespiration produces ATP and NADPH and fixes extra CO2, so it helps the C3 plant grow faster. ✓ Photorespiration produces NEITHER ATP nor NADPH nor sugar. It USES ATP and RELEASES CO2 (O2 is consumed, not generated). It is a wasteful pathway seen only in C3 plants. 🧠 NEET repeatedly flips whether photorespiration releases or consumes energy.
Real NEET questions
2020
The oxygenation activity of RuBisCO enzyme in photorespiration leads to the formation of:
A · 1 molecule of 6-C compound
B · 1 molecule of 4-C compound and 1 molecule of 2-C compound
C · 2 molecules of 3-C compound
D · 1 molecule of 3-C compound ✓
Solution: When RuBisCO acts as an oxygenase, RuBP binds O2 (instead of being converted to 2 molecules of PGA) and forms one molecule of 3-carbon phosphoglycerate and one molecule of 2-carbon phosphoglycolate. The 3-carbon phosphoglycerate re-enters the Calvin cycle; the official key marks the 3-C compound (option D). The full biochemical answer is one 3-C plus one 2-C molecule.
ReNEET 2026
The photorespiration reaction catalysed by RuBisCO is: RuBP + O2 -> 3-Phosphoglycerate + X. Identify X.
A · Phosphoenolpyruvate
B · 2-Phosphoglycolate ✓
C · Oxaloacetate
D · Malate
Solution: Acting as an oxygenase, RuBisCO makes RuBP bind O2 and split into one molecule of 3-phosphoglycerate (3C) and one molecule of 2-phosphoglycolate (2C). So X = 2-phosphoglycolate. Oxaloacetate and PEP belong to the C4 pathway, not photorespiration.
ReNEET 2026
Which of the following statements regarding photorespiration are correct? (a) Does not occur in C3 plants; (b) CO2 is consumed and O2 is generated; (c) Phosphoglycolate is formed; (d) No synthesis of ATP and NADPH.
A · (a) and (d) only
B · (c) and (d) only ✓
C · (b) and (d) only
D · (a) and (b) only
Solution: Photorespiration DOES occur in C3 plants, so (a) is false. In it O2 is consumed and CO2 is released, so (b) is false. Its product is 2-phosphoglycolate, so (c) is true. It makes no ATP or NADPH, so (d) is true. Correct answer: (c) and (d) only.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because the pathway begins with the formation of a 2-carbon molecule, phosphoglycolate, when RuBP binds oxygen. The '2' refers to this 2-carbon product.
What triggers photorespiration in C3 plants?
A high O2 to CO2 ratio. When CO2 is low (for example when stomata close in hot, dry conditions), O2 competes with CO2 for RuBisCO's active site and the oxygenase activity increases.
Is photorespiration useful to the plant?
For NEET, the answer is that it is a wasteful process with no known clear benefit. NCERT states that the biological function of photorespiration is not known yet, and it lowers net photosynthesis in C3 plants.
Which enzyme is responsible for photorespiration?
RuBisCO (Ribulose bisphosphate carboxylase-oxygenase), the most abundant enzyme in the world. Its oxygenase activity starts photorespiration.
Why do C4 plants avoid photorespiration?
C4 plants pump CO2 into bundle sheath cells (Kranz anatomy), raising CO2 concentration at the RuBisCO site so it acts as a carboxylase and its oxygenase activity is minimised.
