Photorespiration is a seemingly wasteful metabolic process in plants that occurs in the light, appearing to reverse the intended outcome of photosynthesis. This pathway begins when the primary enzyme responsible for fixing atmospheric carbon dioxide mistakenly binds to oxygen instead. The resulting diversion of chemical resources from sugar production represents a significant inefficiency in plant metabolism, particularly for the most common plant type globally. This metabolic detour is a consequence of the enzyme’s ancient structure and changing atmospheric conditions within the leaf.
The Dual Nature of Rubisco
The initial step of carbon fixation in nearly all plants is catalyzed by the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase, known as Rubisco. This enzyme has a fundamental flaw: its active site cannot perfectly distinguish between its intended substrate, carbon dioxide (\(text{CO}_2\)), and oxygen (\(text{O}_2\)). When Rubisco successfully binds \(text{CO}_2\), it initiates the Calvin cycle, combining the five-carbon sugar ribulose-1,5-bisphosphate (RuBP) with \(text{CO}_2\) to produce two molecules of 3-phosphoglycerate (3-PGA).
Under typical conditions, Rubisco’s affinity for \(text{CO}_2\) is about 80 times greater than its affinity for \(text{O}_2\), favoring the productive photosynthetic pathway. However, when a plant closes its stomata to conserve water on hot, dry days, internal \(text{CO}_2\) levels drop as they are consumed by photosynthesis, while \(text{O}_2\) levels rise. This shift in the \(text{CO}_2/text{O}_2\) ratio forces Rubisco to act as an oxygenase, initiating photorespiration. Rising temperatures further exacerbate this problem because the solubility of \(text{CO}_2\) decreases more rapidly than that of \(text{O}_2\), effectively lowering the \(text{CO}_2/text{O}_2\) ratio at the active site.
The Carbon Recovery Pathway
The oxygenase activity of Rubisco cleaves the five-carbon RuBP molecule into one molecule of 3-PGA, which proceeds through the Calvin cycle, and one molecule of the two-carbon compound, 2-phosphoglycolate (2PG). This 2PG cannot be used in the Calvin cycle and inhibits several photosynthetic enzymes, requiring a complex, multi-organelle recovery process to salvage its carbon atoms. This recovery pathway, often called the \(text{C}_2\) cycle, begins in the chloroplast where 2PG is dephosphorylated into glycolate.
The glycolate then leaves the chloroplast and enters the peroxisome, where it is oxidized and converted to the amino acid glycine. Glycine is transported into the mitochondrion. Inside the mitochondrion, two molecules of glycine are combined to form one molecule of the amino acid serine, a process that releases a molecule of \(text{CO}_2\) and ammonia (\(text{NH}_3\)).
The newly formed serine then returns to the peroxisome, where it is converted to glycerate. Finally, the glycerate is shuttled back into the chloroplast and phosphorylated using ATP to regenerate 3-PGA, which can then re-enter the main carbon fixation cycle.
The Energetic Cost of Photorespiration
The elaborate pathway required to recover carbon from 2-phosphoglycolate is metabolically expensive. For every two oxygenation reactions initiated by Rubisco, one molecule of \(text{CO}_2\) is released in the mitochondrion, effectively undoing a portion of the carbon fixation. This net loss means approximately \(25%\) of the carbon that enters the photorespiratory pathway is lost back to the atmosphere.
The process also demands a substantial expenditure of energy that could otherwise be used for growth and sugar synthesis. The complete photorespiratory cycle requires the consumption of ATP and NADPH equivalents generated by the light-dependent reactions. Specifically, the reaction consumes about 3.5 molecules of ATP and 2 molecules of NADPH equivalents per oxygenation event. Furthermore, the ammonia released in the mitochondrion is toxic and must be re-assimilated into amino acids, requiring additional ATP.
Plant Adaptations to Minimize Photorespiration
In response to the inefficiency of photorespiration, some plants have evolved specialized metabolic and anatomical mechanisms to suppress the oxygenase activity of Rubisco. \(text{C}_4\) photosynthesis, utilized by plants like maize and sugarcane, achieves a spatial separation of initial \(text{CO}_2\) fixation from the Calvin cycle. They use an enzyme called PEP carboxylase, which has no affinity for \(text{O}_2\), to initially fix \(text{CO}_2\) into a four-carbon compound within the outer mesophyll cells.
This four-carbon compound is then actively transported into specialized bundle sheath cells, which are rich in Rubisco and relatively isolated from atmospheric oxygen. Inside these cells, the compound is broken down, or decarboxylated, to release a high concentration of \(text{CO}_2\) around Rubisco. This effectively saturates the enzyme and prevents it from binding \(text{O}_2\).
Crassulacean Acid Metabolism (CAM), common in desert succulents like cacti, employs a temporal separation. They fix \(text{CO}_2\) at night when temperatures are cool and water loss is minimized by opening their stomata. The \(text{CO}_2\) is fixed into malic acid and stored in the vacuole until the day, when the stomata close. The stored malate is then released and decarboxylated to concentrate \(text{CO}_2\) around Rubisco.