Photosynthesis converts light energy into chemical energy by fixing carbon dioxide into sugars. This process is not always perfectly efficient. A side reaction called photorespiration occurs when the photosynthetic machinery makes a mistake, leading to a substantial loss of fixed carbon and energy. This multi-organelle system is designed to salvage the unusable two-carbon compound that results from this error. The plant must recover this lost carbon to prevent toxicity and maintain the flow of the main carbon-fixing cycle.
The Organelle Handling Phosphoglycolate
The organelle primarily responsible for the initial recycling of the lost carbon is the peroxisome, a small membrane-bound compartment. The error in photosynthesis first produces 2-phosphoglycolate inside the chloroplast, which is converted to glycolate and transported out. This glycolate then diffuses into the peroxisome, the central hub for the carbon salvage operation. Inside, glycolate is oxidized to glyoxylate by glycolate oxidase, a reaction that consumes oxygen and generates hydrogen peroxide. The hydrogen peroxide is immediately neutralized by the peroxisomal enzyme catalase, and the resulting glyoxylate is converted into the amino acid glycine through transamination.
Why Plants Initiate Photorespiration
Photorespiration begins due to an inherent flaw in the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase, known as RuBisCO. RuBisCO is the gateway to the Calvin cycle, fixing atmospheric carbon dioxide onto the five-carbon sugar ribulose-1,5-bisphosphate (RuBP). The “oxygenase” part of its name indicates its dual nature: it can also bind to oxygen.
When RuBisCO binds carbon dioxide, it yields two molecules of 3-phosphoglycerate (3-PGA). When it binds oxygen, it performs the oxygenation reaction, resulting in one molecule of 3-PGA and one molecule of the two-carbon compound, 2-phosphoglycolate. This wasteful side reaction is favored under specific environmental conditions.
High temperatures and low concentrations of carbon dioxide relative to oxygen increase the likelihood of RuBisCO choosing oxygen as its substrate. As temperatures rise, the solubility of carbon dioxide decreases more rapidly than oxygen, lowering the CO2 to O2 ratio near the enzyme. Furthermore, when plants close their stomata to conserve water in hot, dry conditions, internal CO2 concentration drops while oxygen builds up, directly promoting the oxygenation reaction.
Tracking Carbon Through the Salvage Cycle
The complete carbon salvage pathway involves a coordinated effort across three organelles: the chloroplast, the peroxisome, and the mitochondrion. The process begins in the chloroplast when 2-phosphoglycolate is converted to glycolate, which moves into the peroxisome and is transformed into the amino acid glycine.
Two molecules of glycine then travel from the peroxisome to the mitochondrion, where the most metabolically costly step occurs. Within the mitochondrion, the two glycines are converted into a single molecule of the three-carbon amino acid serine, releasing one molecule of carbon dioxide and one molecule of ammonia. This release of already-fixed carbon is the source of photorespiration’s inefficiency.
The serine molecule then moves back to the peroxisome, where it is converted into glycerate. This glycerate travels back to the chloroplast. Once inside, the glycerate is phosphorylated, requiring ATP, to form 3-phosphoglycerate. This compound is a standard intermediate of the Calvin cycle, allowing the salvaged carbon to re-enter the main photosynthetic pathway and regenerate RuBP, effectively recovering 75% of the carbon.
The High Price of Photorespiration
Although the salvage cycle reclaims most lost carbon, photorespiration imposes a metabolic burden on the plant cell. The process consumes previously fixed carbon by releasing carbon dioxide in the mitochondrion, decreasing the net rate of carbon fixation. Furthermore, the numerous conversions across the three organelles require energy and reducing power.
Each time the cycle runs, the plant expends ATP and NADPH to drive enzymatic reactions and transport molecules. The ammonia released in the mitochondrion is also toxic and must be immediately detoxified and re-assimilated, which demands additional ATP and NADPH.
To cope with this inefficiency, some plants have evolved specialized mechanisms, such as C4 and Crassulacean Acid Metabolism (CAM) photosynthesis. These adaptations concentrate carbon dioxide around the RuBisCO enzyme, minimizing contact with oxygen and allowing them to bypass photorespiration almost entirely.