Photorespiration Explained: Why RuBisCO Wastes Energy (NEET Biology)
You have just learnt that RuBisCO is the most abundant enzyme on Earth and the heart of the Calvin cycle. Then your textbook says the same enzyme sometimes grabs oxygen instead of carbon dioxide and wastes the plant's hard-earned energy. Why would evolution leave such a flaw in the most important enzyme of photosynthesis? Photorespiration is the answer, and it is a favourite NEET topic because it links C3 plants, C4 plants, three organelles and a clean set of one-line facts.
This concept matters for NEET (Biology, Class 11 Plant Physiology).
Quick Summary
RuBisCO can act as a carboxylase (adds CO₂ to RuBP) or as an oxygenase (adds O₂ to RuBP). Photorespiration is the oxygenase route.
The oxygenase reaction gives one molecule of 3-phosphoglycerate (3-C) and one of phosphoglycolate (2-C), instead of two 3-PGA.
The pathway runs through three organelles: chloroplast, peroxisome and mitochondrion. It releases CO₂ and makes no ATP and no NADPH.
It is significant in C3 plants at high temperature and high O₂ to CO₂ ratio. C4 plants largely avoid it.
Start from what you already know
In the Calvin cycle, RuBisCO attaches CO₂ to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). The six-carbon product splits at once into two molecules of 3-phosphoglycerate (3-PGA), which the cycle turns into sugar. That is the carboxylase activity of RuBisCO.
The catch is that the active site of RuBisCO cannot discriminate perfectly between CO₂ and O₂. Both are small, both reach the same site. Which one wins depends on how much of each is present around the enzyme. The enzyme evolved when the atmosphere had far less oxygen and far more CO₂ than it does now, so the mistake was once rare. Today air has about 21% O₂ but only about 0.04% CO₂, so the oxygen reaction happens often.
What actually happens
When RuBisCO adds O₂ to RuBP, the products are one molecule of 3-PGA and one molecule of phosphoglycolate, a two-carbon compound that the cell cannot use directly. 3-PGA goes back into the Calvin cycle without trouble. The phosphoglycolate is a problem and must be salvaged. The salvage route is photorespiration:
Chloroplast: phosphoglycolate loses its phosphate and becomes glycolate, which leaves the chloroplast.
Peroxisome: glycolate is oxidised and converted to the amino acid glycine.
Mitochondrion: two glycine molecules combine to make one serine, and CO₂ and NH₃ (ammonia) are released.
Back to the peroxisome and chloroplast: serine is converted into glycerate, which re-enters the chloroplast and is phosphorylated to 3-PGA, rejoining the Calvin cycle.
Count the carbon. Two RuBP molecules react with O₂ and give two phosphoglycolate molecules, which contain 4 carbon atoms. These become two glycine molecules. Two glycine make one serine (3 carbon atoms) and one CO₂ (1 carbon atom). So of 4 carbon atoms, 3 are recovered and 1 is lost as CO₂.
The ammonia released in the mitochondrion also has to be re-fixed, which costs the cell more energy.
Why it is called wasteful
Normal photosynthesis stores energy: the light reaction makes ATP and NADPH, and the Calvin cycle uses them to build sugar. Photorespiration does the opposite. It uses ATP and reducing power, makes no ATP and no NADPH, and releases CO₂ that was just fixed. That is why it is also called the photosynthetic carbon oxidation cycle. The word "photo" appears because it needs light to supply RuBP and the energy to regenerate it, and because it follows the light-driven Calvin cycle.
Worked example: how the gas ratio decides the winner
Suppose, as a simplification, that in a leaf the carboxylase and oxygenase reactions compete and that the oxygenase fraction depends on the ratio of O₂ to CO₂ near the enzyme. Take a C3 leaf on a cool day with stomata open: the internal CO₂ is reasonably high, say the O₂ to CO₂ ratio is R₁. Now take the same leaf on a hot, dry afternoon. Stomata close to save water, so CO₂ entering falls while photosynthesis keeps using up the CO₂ inside and releasing O₂. The internal CO₂ drops, perhaps to half its earlier value, while O₂ rises. The ratio becomes more than 2 × R₁.
Step 1: halving CO₂ at fixed O₂ doubles the ratio O₂ to CO₂. Step 2: a higher ratio means more RuBP meets O₂ rather than CO₂. Step 3: so the oxygenase share rises, and so does the carbon lost as CO₂ in the mitochondrion. Step 4: higher temperature also tips RuBisCO's balance further towards the oxygenase reaction.
The exact numbers vary, but the direction is the exam point: high temperature, high O₂, low CO₂ and bright light all increase photorespiration.
Why C4 plants escape
In C4 plants, such as maize and sugarcane, CO₂ is first captured in mesophyll cells by PEP carboxylase, which has no affinity for O₂. The four-carbon acid is then passed to the bundle sheath cells, where it releases CO₂ right next to RuBisCO. That raises the local CO₂ concentration so high that oxygen rarely gets a chance. This is the real purpose of Kranz anatomy, as seen in the earlier post on the C4 pathway. As a result C4 plants show negligible photorespiration and are more productive under hot, bright conditions. Our Sukriti NeetJee notes treat the C3 versus C4 comparison as one table to memorise, but the logic behind it is simply the CO₂ concentration around RuBisCO.
Common mistakes
Saying photorespiration makes ATP. It makes none, and neither does it make NADPH.
Confusing it with respiration. It does not occur in the dark in the sense of mitochondrial respiration, it does not oxidise food to make ATP, and the only common link is that CO₂ is released and mitochondria take part.
Naming only two organelles. The correct set is chloroplast, peroxisome and mitochondrion.
Forgetting that the first products are 3-PGA and a 2-carbon compound, not two 3-PGA.
Thinking C4 plants have no RuBisCO oxygenase activity. They have the enzyme, but the high CO₂ around it suppresses the reaction.
How NEET asks it
"Photorespiration occurs in which organelles?" with options listing different combinations of three organelles.
Assertion-reason: "C4 plants show negligible photorespiration" because "CO₂ is concentrated in bundle sheath cells".
Statement-based questions asking which is true: ATP is produced, CO₂ is released, glycine is converted to serine.
Matching the compound (glycolate, glycine, serine) with the organelle where it is made.
Pro Tip: Remember the order of organelles with the story of the carbon: Chloroplast, Peroxisome, Mitochondrion (C-P-M). The amino acids follow the same order of appearance, glycolate first, then glycine, then serine. And the CO₂ is always lost at the mitochondrion step where two glycine become one serine. If a question asks where CO₂ is released in photorespiration, the answer is the mitochondrion.
Frequently Asked Questions
Is photorespiration the same as cellular respiration?
No. Cellular respiration oxidises glucose to make ATP in all living cells. Photorespiration is a side reaction of RuBisCO in green parts of plants, consumes energy and makes no ATP.
Which enzyme is responsible for photorespiration?
RuBisCO (ribulose bisphosphate carboxylase-oxygenase). Its oxygenase activity starts the pathway when O₂ binds to RuBP in place of CO₂.
Why do C3 plants suffer more than C4 plants?
In C3 plants RuBisCO sits in mesophyll cells where CO₂ is not concentrated, so O₂ competes well. C4 plants concentrate CO₂ in bundle sheath cells, which suppresses the oxygenase reaction.
Does photorespiration have any benefit?
It recovers three of every four carbon atoms from phosphoglycolate, which would otherwise be lost, and it may help protect the plant from damage when light is strong and CO₂ is low. For NEET, though, it is described as a wasteful process.
Does it need light?
Yes, indirectly. Light drives the supply of RuBP and ATP, and oxygen builds up in the chloroplast in light, so the process is tied to illumination.
Got a doubt about the C3 and C4 comparison or the three organelles? Ask in the comments and share this with a fellow NEET aspirant who confuses it with respiration.
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