Phosphoglycolate: The 2-Carbon Product of Photorespiration
Biology · Photosynthesis in Higher Plants · NEET
In photorespiration, RuBisCO acts as an oxygenase: it adds oxygen (O2) to RuBP. Instead of making two molecules of 3-carbon PGA, it makes one molecule of 3-C PGA (3-phosphoglycerate) and one molecule of 2-C phosphoglycolate. Memory hook: "Oxygen splits the cycle into a 3 and a 2" - phosphoglyco-LATE is the 2-carbon late arrival that leaves the Calvin cycle.
RuBisCO with CO2 gives two 3-carbon PGA (Calvin cycle). With O2 it gives one 3-carbon PGA plus one 2-carbon phosphoglycolate, starting the wasteful photorespiration pathway.
Your doubts, answered
What exactly does RuBisCO produce when it uses O2 instead of CO2?
When RuBisCO binds O2 to RuBP (5-carbon), the RuBP is NOT split into two 3-carbon PGA molecules. Instead it forms ONE molecule of 3-carbon phosphoglycerate (PGA) and ONE molecule of 2-carbon phosphoglycolate. This is the start of photorespiration (the C2 cycle).
Is phosphoglycolate 2-carbon or 3-carbon? I keep confusing it with PGA.
Phosphoglycolate is the 2-CARBON (2-C) product. PGA (3-phosphoglycerate) is the 3-CARBON product. NCERT clearly states RuBP binds O2 to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon). The names look similar but the carbon count is the exam trap: glycoLATE = 2C, glycerate = 3C.
Phosphoglycolate vs phosphoglycerate - how do I tell them apart?
PhosphoGLYCERATE (PGA) = 3 carbons, it is the normal first stable product of the Calvin cycle and re-enters the cycle. PhosphoGLYCOLATE = 2 carbons, it is the wasteful product formed ONLY when RuBisCO uses O2 (photorespiration). Trick: the shorter word 'glycolate' is the shorter (2C) molecule.
Why does phosphoglycolate form at all - isn't RuBisCO for CO2?
RuBisCO has a much greater affinity for CO2, but its active site cannot perfectly reject O2. When O2 concentration is high and CO2 is low, O2 competes and binds RuBP. Because O2 has no carbon to add, the RuBP splits unevenly into 3-C PGA + 2-C phosphoglycolate instead of 2 x 3-C PGA. This is why the enzyme is called carboxylase-OXYGENASE.
Does photorespiration make any sugar, ATP or NADPH from phosphoglycolate?
No. Photorespiration does NOT produce sugar, does NOT produce ATP, and does NOT produce NADPH. Phosphoglycolate leads to a pathway that releases CO2 and is a wasteful process for C3 plants. This 'no synthesis' point is a common NEET question.
⚠️ The NEET trap ✗ RuBP + O2 gives two molecules of 3-carbon PGA (same as normal fixation). ✓ RuBP + O2 gives ONE 3-C PGA (3-phosphoglycerate) + ONE 2-C phosphoglycolate. Only carboxylation (RuBP + CO2) gives two 3-C PGA. 🧠 CO2 fixation = two 3-C PGA. O2 fixation = one 3-C + one 2-C (phosphoglycolate). Count the carbons to avoid the trap.
Real NEET questions
NEET 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 and 1 molecule of 2-C compound ✓
Solution: When RuBisCO acts as an oxygenase, RuBP binds O2 and forms ONE molecule of 3-carbon phosphoglycerate (PGA) and ONE molecule of 2-carbon phosphoglycolate. So the products are a 3-C compound and a 2-C compound. NCERT Ch 11: RuBP instead of forming 2 molecules of PGA binds O2 to form phosphoglycerate and phosphoglycolate (2 Carbon).
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: In photorespiration RuBisCO acts as an oxygenase: RuBP binds O2 and forms one molecule of 3-phosphoglycerate (3-C) and one molecule of 2-phosphoglycolate (2-C). So X = 2-phosphoglycolate.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Phosphoglycolate is a 2-carbon compound formed when RuBisCO adds O2 to RuBP during photorespiration. It is produced along with one 3-carbon PGA molecule and marks the start of the wasteful C2 (photorespiratory) pathway.
How many carbons does phosphoglycolate have?
Phosphoglycolate has 2 carbons. Do not confuse it with phosphoglycerate (PGA), which has 3 carbons.
What are the products of RuBisCO oxygenase activity?
One molecule of 3-carbon phosphoglycerate (PGA) and one molecule of 2-carbon phosphoglycolate. No sugar, ATP or NADPH is formed.
Does phosphoglycolate form in C4 plants?
Practically no. C4 plants concentrate CO2 around RuBisCO in bundle sheath cells, so O2 rarely competes and phosphoglycolate (photorespiration) is largely avoided.
Why is photorespiration called wasteful?
Because it uses O2, releases CO2, and produces no sugar, no ATP and no NADPH. The 2-carbon phosphoglycolate leaves the Calvin cycle instead of building glucose.
