Photorespiration is a pathway of metabolism associated with photosynthesis that recycles compounds produced when Rubisco reacts with oxygen rather than carbon dioxide. In plants, its principal reactions span chloroplasts, peroxisomes, and mitochondria. The pathway recovers much of the carbon diverted by this reaction, but consumes energy and releases some previously assimilated carbon as CO₂. It is therefore both a constraint on photosynthetic efficiency and an essential system for processing potentially harmful metabolic intermediates. Related pathways occur in algae and cyanobacteria. (pmc.ncbi.nlm.nih.gov)
Biochemical origin
Rubisco is the enzyme responsible for the main entry step of carbon fixation in the Calvin–Benson cycle. Its substrate, ribulose-1,5-bisphosphate (RuBP), contains five carbon atoms. During carboxylation, addition of CO₂ ultimately produces two molecules of three-carbon 3-phosphoglycerate. During oxygenation, RuBP instead yields one molecule of 3-phosphoglycerate and one molecule of two-carbon 2-phosphoglycolate. The latter cannot directly enter the normal reactions of the Calvin–Benson cycle. (academic.oup.com)
The balance between these reactions depends on Rubisco’s catalytic properties, temperature, and the CO₂-to-O₂ ratio at the enzyme’s location. Photorespiration is associated with illumination because photosynthesis supplies RuBP and supports the recycling reactions; it is not a separate process that directly captures light. Unlike cellular respiration, which oxidizes organic substrates to support ATP production, the photorespiratory pathway as a whole imposes an energetic cost on photosynthetic carbon assimilation. (nph.onlinelibrary.wiley.com)
The plant recycling pathway
The canonical pathway moves carbon through three major cellular compartments, requiring coordinated enzyme activities and metabolite transport:
- Chloroplast. In the chloroplast, phosphoglycolate phosphatase removes the phosphate group from 2-phosphoglycolate, producing glycolate. Glycolate is then exported toward the peroxisome. (nph.onlinelibrary.wiley.com)
- Peroxisome. In the peroxisome, glycolate oxidase converts glycolate into glyoxylate, generating hydrogen peroxide. Catalase decomposes this peroxide. Transfer of an amino group to glyoxylate produces glycine. (nph.onlinelibrary.wiley.com)
- Mitochondrion. Glycine enters the mitochondrion, where glycine decarboxylase and serine hydroxymethyltransferase together convert two glycine molecules into one serine molecule, releasing CO₂ and ammonia. These reactions also generate NADH. (nph.onlinelibrary.wiley.com)
- Return to the chloroplast. Serine returns to the peroxisome and is converted through hydroxypyruvate into glycerate. Glycerate subsequently enters the chloroplast, where glycerate kinase uses ATP to produce 3-phosphoglycerate, restoring carbon to the Calvin–Benson cycle. (nph.onlinelibrary.wiley.com)
This sequence describes the core plant pathway rather than every possible route. Alternative reactions, including cytosolic hydroxypyruvate reduction, supplement the principal organellar pathway. Photorespiration also exchanges intermediates with other metabolic processes rather than functioning as an entirely closed cycle. (pmc.ncbi.nlm.nih.gov)
Carbon and energy balance
For every two molecules of 2-phosphoglycolate processed through the canonical pathway, four carbon atoms enter and three return as one molecule of 3-phosphoglycerate; the remaining carbon atom leaves as CO₂. Thus, 75% of the carbon entering this recycling branch is recovered. The corresponding 25% loss applies to carbon entering the branch, not to all carbon fixed by a leaf. Total photorespiratory loss depends on the relative rates of Rubisco oxygenation and carboxylation. (pmc.ncbi.nlm.nih.gov)
Additional costs include ATP-dependent glycerate phosphorylation and reassimilation of released ammonia. Chloroplastic glutamine synthetase and glutamate synthase return this nitrogen to organic compounds, consuming ATP and reducing power. Oxygenation also diverts Rubisco activity and RuBP away from productive CO₂ fixation. These combined effects lower net carbon assimilation even though individual photorespiratory reactions can generate reducing equivalents. (nph.onlinelibrary.wiley.com)
Environmental controls and photosynthetic strategies
Photorespiration is particularly important in plants using C3 photosynthesis. Higher temperatures generally increase oxygenation relative to carboxylation. Water shortage can also favor photorespiration when closure of stomata restricts CO₂ entry while photosynthesis continues consuming internal CO₂. Consequently, photorespiratory pressure depends on conditions inside the leaf, not simply on atmospheric gas concentrations. Increasing CO₂ around Rubisco suppresses its oxygenase reaction. (nph.onlinelibrary.wiley.com)
C4 photosynthesis concentrates CO₂ around Rubisco, usually through coordinated reactions in mesophyll and bundle-sheath cells. Crassulacean acid metabolism achieves carbon concentration through temporal separation, typically storing carbon acquired at night and releasing CO₂ internally during daylight. These mechanisms reduce photorespiration but do not necessarily eliminate the need for 2-phosphoglycolate metabolism. (pubmed.ncbi.nlm.nih.gov)
Physiological significance
Photorespiration prevents accumulation of 2-phosphoglycolate, which inhibits enzymes involved in photosynthetic carbon metabolism. Experiments with photorespiratory mutants show that disrupting essential pathway steps can severely impair growth in ordinary air, whereas elevated CO₂ can rescue many such defects by suppressing oxygenation. The pathway therefore cannot be characterized solely as carbon waste: it salvages carbon and maintains metabolic function. (pmc.ncbi.nlm.nih.gov)
Its intermediates also connect photosynthetic carbon processing with amino acid synthesis and nitrogen metabolism. In cyanobacteria, the arrangement differs from the multi-organelle plant pathway. Research on Synechocystis identified three routes for processing 2-phosphoglycolate; eliminating all three prevented growth under ambient conditions despite the organism’s carbon-concentrating mechanism. (nph.onlinelibrary.wiley.com)
Experimental modification
Researchers have investigated accelerating native reactions and introducing alternative glycolate-processing pathways. A 2019 field study engineered tobacco chloroplasts with synthetic glycolate pathways and reduced glycolate export into the native pathway. Selected engineered lines showed improved photosynthetic performance and increased biomass. These experiments demonstrate that modifying photorespiratory metabolism can improve plant growth under tested conditions; they do not establish equivalent yield gains across crop species or environments. (pubmed.ncbi.nlm.nih.gov)
References
- Photorespirationpmc.ncbi.nlm.nih.gov
- Photorespiration Revisitedpmc.ncbi.nlm.nih.gov
- C(4) photosynthesis: principles of CO(2) concentration and prospects for its introduction into C(3) plants.pubmed.ncbi.nlm.nih.gov
- The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plantspmc.ncbi.nlm.nih.gov
- Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the fieldpubmed.ncbi.nlm.nih.gov