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Most students can say that photorespiration is "wasteful". Fewer can show where the carbon goes, which is what assertion-reason and statement questions test. This post works through the carbon bookkeeping, puts numbers on the cost from published research, and lists the statements examiners like to twist.
Where is it in the NEET syllabus and NCERT?
The NEET (UG) 2026 syllabus lists "Photorespiration C3 and C4 pathways" under plant physiology. The topic is section 11.9 of the NCERT Class 11 chapter Photosynthesis in Higher Plants (2026-27 reprint). Both are current; nothing on photorespiration has been dropped.
What goes wrong at RuBisCO?
NCERT explains that RuBisCO's active site "can bind to both CO₂ and O₂", that the binding is competitive, and that the relative concentration of the two gases decides which one binds. The two reactions are:
Carboxylation: RuBP (5 carbons) + CO₂ gives two molecules of 3-PGA (3 carbons each).
Oxygenation: RuBP + O₂ gives one 3-PGA and one phosphoglycolate (2 carbons).
NCERT adds that the photorespiratory pathway makes neither sugars nor ATP nor NADPH, and instead releases CO₂ while using ATP.
Worked example: following the carbon
A review of the pathway by Peterhansel and colleagues (2010) states that the cycle "rescues ¾ of the carbon in phosphoglycolate" and releases 0.5 CO₂ per phosphoglycolate formed. Use that to follow two oxygenation events.
Start: 2 RuBP = 2 × 5 = 10 carbons.
After oxygenation: 2 × 3-PGA (6 carbons) + 2 phosphoglycolate (4 carbons).
Photorespiration: the 4 carbons of phosphoglycolate return as one 3-PGA (3 carbons), and 1 carbon leaves as CO₂.
End: 3 × 3-PGA = 9 carbons. One carbon that the plant had already fixed is lost.
Compare two carboxylation events: 10 carbons + 2 CO₂ gives 4 × 3-PGA = 12 carbons, a gain of 2. The same enzyme doing the same number of reactions gives +2 carbons one way and −1 the other.
Now a hypothetical leaf, with sample numbers: suppose RuBisCO did 3 carboxylations for every oxygenation. It fixes 3 CO₂ and loses 0.5 CO₂, so the net gain is 2.5, and 0.5 ÷ 3 ≈ 17% of the fixed carbon is lost again. The same review estimates that undoing one oxygenation costs 4.75 ATP and 3 reducing equivalents.
How much does this cost real crops?
Walker and colleagues (2016) modelled US crops and reported that photorespiration currently decreases soybean yields by 36% and wheat yields by 20%. In field trials described by South and colleagues (2019), tobacco plants given an engineered pathway that recycles the by-product with less energy lost were about 40% more productive than unmodified plants. These numbers are for exam background only; NEET questions come from NCERT.
How do C4 plants avoid it?
NCERT states that photorespiration does not occur in C4 plants. The four-carbon acid made in mesophyll cells is broken down in bundle sheath cells, releasing CO₂ there and raising its concentration around RuBisCO, so the enzyme acts as a carboxylase. NCERT links this to better productivity and tolerance of higher temperatures in C4 plants. Our post on Hatch and Slack and the C4 pathway explains how that shuttle was found.
Is photorespiration pure waste?
NCERT says "The biological function of photorespiration is not known yet." Research reviews take a more mixed view. Walker and colleagues call it "essential for C3 plants", since it recycles the toxic compound formed when RuBisCO reacts with oxygen, and Peterhansel and colleagues discuss positive roles alongside the cost. For the exam, write NCERT's line; for understanding, see it as the plant cleaning up after RuBisCO's mistake.
Which statements are commonly twisted?
"Photorespiration produces ATP." False: it uses ATP and makes none.
"Phosphoglycolate has three carbons." False: it has two.
"C4 plants show high photorespiration." False: NCERT says it does not occur in them.
"RuBisCO prefers O₂." NCERT says RuBisCO has much greater affinity for CO₂ when the CO₂ to O₂ ratio is nearly equal; the ratio decides.
"Photorespiration is the same as the respiration that makes ATP from glucose." It is a separate pathway with no ATP yield.
A practice routine
Redraw the carbon count from memory: 10 carbons in, 9 out as 3-PGA, 1 as CO₂.
Fill NCERT's Table 11.1 (C3 versus C4) without the book, then check it.
Write five true-or-false statements of your own about photorespiration and swap them with a friend.
If the link between the Calvin cycle and photorespiration is still unclear, the Fermi Concept Library covers every concept in biology with its own lesson, so you can revise RuBisCO and the Calvin cycle separately.
Tonight, explain to someone in three sentences why a C4 plant does not lose carbon this way.
