LAISHA Explains
Plant physiology and botany

Photorespiration: why Rubisco wastes carbon

How Rubisco's reaction with oxygen starts the C2 cycle, where the carbon and energy go, why heat makes it worse, and efforts to engineer a shortcut.

Exam essential5 min readUpdated
On this page
  1. Rubisco's two reactions
  2. The C2 cycle
  3. The cost
  4. What increases photorespiration
  5. How plants avoid it
  6. Engineering a shortcut
  7. Key points
  8. What the exams ask
  9. Questions people ask
  10. Sources
  11. Related on Laisha
  12. Discussion

Rubisco, the enzyme that fixes carbon dioxide in the Calvin cycle, is probably the most abundant protein on Earth. It is also surprisingly imperfect: it cannot fully tell CO2 from oxygen. When it grabs an oxygen molecule instead, the plant has to run an expensive salvage pathway that releases some of the carbon it has already fixed. This process, photorespiration, costs crops such as wheat, rice and soybean a substantial share of their potential photosynthesis, more in hot, dry conditions.

Photorespiration explains why C4 and CAM plants evolved, why CO2 enrichment boosts C3 crops, and why scientists are trying to engineer around it. It is a standard topic in plant physiology exams.

Rubisco's two reactions

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) acts on ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar:

  • Carboxylation: RuBP + CO2 → two molecules of 3-phosphoglycerate (3-PGA), which feed the Calvin cycle.
  • Oxygenation: RuBP + O2 → one 3-PGA + one 2-phosphoglycolate, a two-carbon compound that is useless to the Calvin cycle and inhibits some of its enzymes.

The two reactions compete at the same active site. Which one wins depends on the ratio of CO2 to O2 around the enzyme and on Rubisco's specificity factor (Sc/o), its relative preference for CO2.

Rubisco evolved more than two billion years ago, when the atmosphere had much more CO2 and almost no oxygen, so the oxygenation reaction did little harm. In today's air, with about 21 per cent oxygen and only about 0.04 per cent CO2, it matters a great deal.

The C2 cycle

The plant recovers most of the carbon in 2-phosphoglycolate through the photorespiratory or C2 cycle, which involves three organelles.

The photorespiratory (C2) cycle across three organelles: in the chloroplast Rubisco adds oxygen to RuBP, giving phosphoglycolate; glycolate goes to the peroxisome and becomes glycine; in the mitochondrion two glycines give serine, releasing carbon dioxide and ammonia; serine returns via the peroxisome as glycerate to the chloroplast, recovering three of every four carbon atoms Chloroplast Peroxisome Mitochondrion RuBP + O₂Rubisco oxygenase 2-phosphoglycolate+ 3-PGA to Calvin cycle glycolate glycerate → 3-PGAback to Calvin cycle glycolate → glyoxylateH₂O₂ made here glycine serine → glycerate 2 glycine → serinereleases CO₂ and NH₃ serine CO₂ lostNH₃ re-fixed
The photorespiratory cycle. For every two molecules of phosphoglycolate, one molecule of CO2 is lost in the mitochondrion and three of the four carbon atoms return to the Calvin cycle.Diagram: Laisha, original.
  1. Chloroplast: 2-phosphoglycolate is dephosphorylated to glycolate, which moves to the peroxisome.
  2. Peroxisome: glycolate is oxidised to glyoxylate, producing hydrogen peroxide (broken down by catalase); glyoxylate is converted to the amino acid glycine.
  3. Mitochondrion: two glycine molecules are converted into one serine, releasing one CO2 and one NH3. This is the step that loses carbon.
  4. Back through the peroxisome: serine becomes hydroxypyruvate, then glycerate.
  5. Chloroplast: glycerate is phosphorylated to 3-PGA and re-enters the Calvin cycle.

The ammonia released must be re-fixed through the glutamine synthetase–glutamate synthase (GS–GOGAT) cycle, which costs more ATP and reducing power. In many C3 leaves, the flow of photorespiratory ammonia is several times larger than the plant's uptake of nitrogen from the soil.

The name comes from the fact that the process uses O2 and releases CO2 in the light, like respiration, but it is a completely different pathway and produces no useful ATP.

The cost

  • Carbon: one CO2 released for every two oxygenations, so a quarter of the carbon in the phosphoglycolate is lost.
  • Energy: extra ATP and NADPH to re-fix ammonia and recycle glycerate, and to replace the lost carbon.
  • Yield: estimates suggest photorespiration reduces photosynthesis by roughly 20 to 50 per cent in C3 crops, depending on temperature, and costs major C3 crops a large amount of potential yield every year.
Worked example

How much carbon is lost?

At 25 °C, the ratio of the rates of carboxylation (vc) to oxygenation (vo) is:

vc ⁄ vo = Sc/o × [CO2] ⁄ [O2]

Take Sc/o ≈ 90 for a typical C3 crop Rubisco, a dissolved CO2 concentration in the chloroplast of about 8 µM and dissolved O2 of about 250 µM.

  • vc ⁄ vo = 90 × 8 ⁄ 250 ≈ 2.9: roughly three carboxylations for every oxygenation.
  • CO2 lost per carboxylation = 0.5 × (1 ÷ 2.9) ≈ 0.17.
  • So about 17 per cent of the carbon fixed is released again, before counting the extra energy cost.

On a hot day, specificity falls and CO2 becomes less soluble relative to O2, so the ratio drops and losses rise. Doubling the CO2 concentration roughly halves the oxygenation rate relative to carboxylation.

What increases photorespiration

  • High temperature: Rubisco's specificity for CO2 falls, and CO2 solubility falls faster than O2 solubility.
  • Low CO2 inside the leaf: when stomata close under drought, CO2 is used up and O2 from photosynthesis builds up.
  • High light with closed stomata.
  • High O2: in experiments, lowering O2 to 2 per cent largely abolishes photorespiration and raises C3 photosynthesis.

A related measure is the CO2 compensation point, the CO2 concentration at which photosynthetic uptake equals CO2 release. It is about 40 to 100 ppm in C3 plants, largely because of photorespiration, and below about 10 ppm in C4 plants.

How plants avoid it

  • C4 plants (maize, sorghum, sugarcane, millets) first fix CO2 with PEP carboxylase, which does not react with O2, and then concentrate CO2 around Rubisco in bundle-sheath cells, almost eliminating photorespiration.
  • CAM plants (pineapple, agave, cacti) fix CO2 at night and release it around Rubisco by day behind closed stomata.
  • Algae and cyanobacteria use carbon-concentrating mechanisms, such as carboxysomes and pyrenoids.

Photorespiration is not entirely wasteful: it removes toxic phosphoglycolate, may protect the photosynthetic machinery from excess light under stress, and is linked to nitrogen and sulphur metabolism. Mutants that cannot photorespire die in normal air but grow in high CO2.

Engineering a shortcut

Because photorespiration costs so much, researchers have tried to:

  • Improve Rubisco's specificity, so far with limited success, because faster and more specific seem to trade off.
  • Introduce C4 photosynthesis into rice, a long-term international effort.
  • Install photorespiratory bypasses: alternative pathways, built from algal and bacterial genes, that metabolise glycolate inside the chloroplast. In 2019, field-grown tobacco with one such bypass produced substantially more biomass, by up to about 40 per cent in some trials; work on food crops continues.
  • Raise CO2 in greenhouses, which is why CO2 enrichment is widely used for C3 greenhouse crops.

Key points

  • Rubisco can react with O2 instead of CO2, producing 2-phosphoglycolate.
  • The C2 cycle (chloroplast → peroxisome → mitochondrion → peroxisome → chloroplast) recovers three of every four carbons; CO2 and NH3 are released in the mitochondrion when two glycines make serine.
  • Photorespiration rises with temperature, low internal CO2 and high O2.
  • It can reduce C3 photosynthesis by about 20–50 per cent; C4 and CAM plants largely avoid it.
  • C3 plants have higher CO2 compensation points (about 40–100 ppm) than C4 plants (below 10 ppm).

What the exams ask

  • School-leaving and entrance papers: what photorespiration is, the organelles involved, and why C4 plants lack it.
  • Degree and research-fellowship entrances: the reactions of the C2 cycle, Rubisco's specificity factor, CO2 compensation point, effects of temperature, O2 and CO2, and the energy cost.
  • Research and professional examinations: photorespiratory bypasses, engineering C4 rice, and CO2 enrichment.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Why has evolution not produced a better Rubisco?

There appears to be a trade-off: versions that discriminate better against oxygen tend to work more slowly. Plants have instead evolved ways to concentrate CO2 around Rubisco, as in C4 and CAM plants.

Does photorespiration happen at night?

No. It needs RuBP and active Rubisco, which depend on light, so it occurs only in the light.

Will rising CO2 reduce photorespiration?

Yes, higher CO2 favours carboxylation, which is one reason C3 crops respond to CO2 enrichment. But rising temperatures work the other way, and other effects of climate change can outweigh the gains.

Sources

  1. Ogren, W. L. "Photorespiration: pathways, regulation, and modification." Annual Review of Plant Physiology 35, 415–442 (1984).
  2. Bauwe, H., Hagemann, M. and Fernie, A. R. "Photorespiration: players, partners and origin." Trends in Plant Science 15, 330–336 (2010).
  3. Walker, B. J., VanLoocke, A., Bernacchi, C. J. and Ort, D. R. "The costs of photorespiration to food production now and in the future." Annual Review of Plant Biology 67, 107–129 (2016).
  4. South, P. F., Cavanagh, A. P., Liu, H. W. and Ort, D. R. "Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field." Science 363, eaat9077 (2019).
  5. Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer Associates, 2015. Chapter 8.

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