NCERT grounding
Section 11.9 of NCERT Class 11 Biology (Photosynthesis in Higher Plants) introduces photorespiration as the process responsible for a critical difference between C3 and C4 plants. The text anchors it directly to the first step of the Calvin cycle — the carboxylation of RuBP by RuBisCO — and states that the enzyme can bind O₂ competitively at the same active site that normally binds CO₂.
"In C3 plants some O₂ does bind to RuBisCO, and hence CO₂ fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O₂ to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon) in a pathway called photorespiration."
— NCERT Class 11 Biology, Section 11.9
The NCERT summary reinforces this: "RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration." This phrasing — wasteful oxygenation reaction — is the textbook's own verdict and appears in NEET answer-key explanations.
RuBisCO's dual activity
RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is the most abundant enzyme on Earth. Its full name encodes the problem: it is both a carboxylase (binds CO₂) and an oxygenase (binds O₂). Both gases compete for the same active site.
Competitive inhibition
At current atmospheric O₂ (~21%) and CO₂ (~0.04%) levels, the O₂ : CO₂ ratio is roughly 500 : 1, making oxygenase activity thermodynamically significant in C3 plants. RuBisCO has a much greater affinity for CO₂ when concentrations are equal, but atmosphere is never equal.
The relative binding of CO₂ versus O₂ is entirely concentration-dependent. NCERT states: "It is the relative concentration of O₂ and CO₂ that determines which of the two will bind to the enzyme." When the CO₂ : O₂ ratio falls — due to high temperatures, stomatal closure, or intense light without adequate CO₂ supply — the oxygenase pathway dominates.
Oxygenation reaction and products
The carboxylase reaction (normal Calvin cycle) produces two molecules of 3-PGA (3C each) from one RuBP (5C) + one CO₂:
Figure 1. When O₂ outcompetes CO₂ at the RuBisCO active site, one RuBP (5C) + O₂ yields one 3-PGA (3C) and one 2-phosphoglycolate (2C). Contrast this with the carboxylase reaction, which yields two molecules of 3-PGA, both feeding directly into the Calvin cycle.
The key stoichiometric distinction: carboxylation of RuBP produces two 3-C compounds; oxygenation of RuBP produces one 3-C compound (3-PGA) and one 2-C compound (2-phosphoglycolate). The 3-PGA from oxygenation can still re-enter the Calvin cycle, but 2-phosphoglycolate cannot — it is dephosphorylated to glycolate and exported to peroxisomes, initiating the three-organelle photorespiratory route.
Three-organelle pathway
The photorespiratory carbon oxidation (PCO) cycle is unique in biology because a single metabolic pathway requires three distinct organelles working in sequence. All three are physically clustered in mesophyll cells, facilitating rapid metabolite shuttling.
-
Step 1
Chloroplast
RuBisCO binds O₂; RuBP (5C) → 3-PGA (3C) + 2-phosphoglycolate (2C). Phosphatase converts 2-phosphoglycolate to glycolate, which exits to peroxisome.
Oxygenase reaction -
Step 2
Peroxisome
Glycolate is oxidised by glycolate oxidase to glyoxylate; H₂O₂ produced is destroyed by catalase. Glyoxylate is transaminated to glycine, which moves to mitochondrion.
Oxidation + transamination -
Step 3
Mitochondrion
Two glycine (2C + 2C) → serine (3C) + CO₂ released + NH₃. This is where photorespiratory CO₂ loss occurs. No ATP is synthesised. Serine returns to peroxisome → glycerate → re-enters chloroplast.
CO₂ release · no ATP
The net result of the complete photorespiratory cycle is the release of one CO₂ per two glycolate molecules processed, with consumption of ATP but no production of sugar. NCERT states explicitly: "In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO₂ with the utilisation of ATP."
"The biological function of photorespiration is not known yet."
NCERT Class 11 Biology, Section 11.9 — directly quotable in NEET answers
Why C3 plants are more susceptible
In C3 plants, RuBisCO is present in all mesophyll cells and is directly exposed to atmospheric O₂ and CO₂. At midday, when stomata partially close to conserve water, internal CO₂ levels fall while O₂ from ongoing photosynthesis accumulates. This environment strongly favours oxygenase activity. Estimates suggest photorespiration in C3 plants may cause the loss of 25–50% of fixed carbon under field conditions at high temperatures — a substantial energetic penalty.
C4 plants: suppression mechanism
C4 plants effectively eliminate photorespiration through a CO₂-concentrating mechanism embedded in their Kranz anatomy. The mechanism operates as follows:
Core principle: In C4 plants, CO₂ is first fixed by PEP carboxylase in mesophyll cells into a C4 acid (OAA → malate/aspartate), which is transported to bundle sheath cells. Decarboxylation there releases CO₂ directly around RuBisCO, saturating its carboxylase site and preventing O₂ from competing.
Mesophyll cell role
Primary fixation: PEP carboxylase fixes CO₂ into OAA (C4 acid).
No RuBisCO here: mesophyll cells of C4 plants lack RuBisCO — no photorespiration possible at this site.
C4 acid transport: malate/aspartate exported to bundle sheath via plasmodesmata.
Bundle sheath cell role
Decarboxylation: C4 acid broken down, CO₂ released into bundle sheath compartment.
High [CO₂]: local CO₂ concentration far exceeds atmospheric; RuBisCO acts exclusively as carboxylase.
Calvin cycle here: 3-PGA formed normally; no 2-phosphoglycolate.
C3 Plants
High
photorespiration rate
- RuBisCO in all mesophyll cells, exposed to atmospheric O₂
- No CO₂-concentrating mechanism
- Oxygenase activity at ambient O₂ : CO₂ ratio
- 25–50% fixed carbon potentially lost
- Examples: wheat, rice, sunflower, tobacco
C4 Plants
Negligible
photorespiration rate
- RuBisCO confined to bundle sheath cells only
- C4 acid pump concentrates CO₂ at RuBisCO site
- High [CO₂] in bundle sheath suppresses oxygenase
- ~40% more efficient photosynthesis than C3 in hot/bright conditions
- Examples: maize, sugarcane, sorghum
NCERT states: "In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO₂ at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle sheath cells to release CO₂ — this results in increasing the intracellular concentration of CO₂. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity."
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| RuBisCO location | All mesophyll cells | Bundle sheath cells only |
| Photorespiration | Significant — present | Negligible — absent |
| CO₂ at RuBisCO | Atmospheric (~0.04%) | Concentrated (pumped in by C4 cycle) |
| 2-phosphoglycolate produced | Yes | No |
| Three-organelle PCO cycle active | Yes (chloroplast + peroxisome + mitochondrion) | No |
| Photosynthetic efficiency | Lower (energy lost to photorespiration) | Higher (~40% better under high light/temp) |
| Biological function known? | Not known yet (NCERT explicit statement) | |
Worked examples
A C3 plant is placed in a high-light, high-temperature environment with ambient CO₂. Explain why net carbon gain is lower than expected from the Calvin cycle alone.
Answer: Under high light and high temperature, stomata partially close to reduce water loss, decreasing internal CO₂. Simultaneously, the Calvin cycle produces O₂, raising internal O₂ levels. The resulting low CO₂ : O₂ ratio at the RuBisCO active site shifts the enzyme toward its oxygenase activity. RuBP then reacts with O₂ to form 3-PGA + 2-phosphoglycolate. 2-phosphoglycolate enters the photorespiratory pathway across chloroplast, peroxisome, and mitochondrion, ultimately releasing CO₂ without synthesising any ATP or sugar. This loss of fixed carbon — photorespiration — reduces net photosynthetic output below what the Calvin cycle alone would predict.
Why do C4 plants such as maize show higher productivity than wheat under identical tropical field conditions?
Answer: Maize is a C4 plant; wheat is C3. In maize, CO₂ is first captured by PEP carboxylase in mesophyll cells and stored as C4 acids (malate/aspartate). These are transported to bundle sheath cells, where decarboxylation releases CO₂ directly at the RuBisCO site. This CO₂ concentration mechanism keeps [CO₂] high around RuBisCO in bundle sheath cells, preventing O₂ from competing at the active site. Consequently, maize shows negligible photorespiration. In wheat, RuBisCO is exposed to atmospheric O₂ without any CO₂-concentrating mechanism, leading to significant photorespiration and loss of fixed carbon. Under tropical conditions (high temperature, high light), the difference is amplified because these conditions favour photorespiration in C3 plants — explaining the yield gap.
Identify the organelle in which CO₂ is actually released during the photorespiratory pathway, and explain the reaction involved.
Answer: CO₂ is released in the mitochondrion. After glycolate (2C) is produced in the chloroplast and oxidised to glycine (2C) in the peroxisome, two glycine molecules enter the mitochondrion. There, the glycine decarboxylase complex converts 2 glycine → 1 serine (3C) + 1 CO₂ + 1 NH₃. This decarboxylation step is the sole point of CO₂ release in the entire photorespiratory pathway, and it occurs without any coupled ATP synthesis — distinguishing it from mitochondrial respiration which is linked to the ETS and oxidative phosphorylation.
Common confusion & NEET traps
Photorespiration
0
ATP molecules produced
- Light-dependent (needs RuBP from Calvin cycle)
- Substrate: RuBP (with O₂)
- Organelles: chloroplast + peroxisome + mitochondrion
- No ETS involvement
- CO₂ released in mitochondrion via glycine decarboxylase
- Function: not known
Dark (mitochondrial) Respiration
36–38
ATP molecules produced (per glucose)
- Light-independent; occurs in dark and light
- Substrate: glucose (and other organic molecules)
- Organelles: cytoplasm + mitochondrion
- Uses ETS (oxidative phosphorylation)
- CO₂ released via Krebs cycle
- Function: ATP production for cellular work