What Is Photorespiration?

ByKen

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What is photorespiration? In plants, it is a pathway that begins when the enzyme Rubisco reacts with oxygen instead of carbon dioxide. The Georgia Tech Biological Principles page states that “Rubisco has oxygenase activity as well as carboxylase activity; it sometimes fixes O2 to RuBP instead of CO2.” The result is a toxic compound that the plant must process, and the process has a cost in carbon and energy. The name is misleading, and the sources say so directly. This article explains the mechanism and the conditions the sources link to it.

Quick answer

  • Photorespiration starts when Rubisco adds oxygen to RuBP, which the Georgia Tech page describes as the oxygenase activity of Rubisco.
  • The page says the oxygenase activity “becomes pronounced at high temperatures” and “occurs at low CO2, high O2 conditions.”
  • The product, 2-phosphoglycolate, is described as “a 2-carbon toxic compound” that releases CO2 and “resulting in loss of organic carbon and energy production.”
  • A research review in PMC describes photorespiration as “a key pathway in plant metabolism.”

The enzyme at the center

The Georgia Tech page calls Rubisco the enzyme “responsible for the vast bulk of organic carbon on the surface of the Earth.” The page then states the problem: “although rubisco is responsible for the vast bulk of organic carbon on the surface of the Earth, its oxygenase activity can severely reduce photosynthetic efficiency.”

The enzyme has two activities. The page names them as oxygenase and carboxylase activity. The carboxylase activity takes in carbon dioxide. The oxygenase activity takes in oxygen. The page states that the enzyme “sometimes fixes O2 to RuBP instead of CO2.” The choice between the two gases is the point at which photorespiration begins.

When the oxygenase activity increases

The page gives the conditions that favor oxygen use. It states that “The oxygenase activity occurs at low CO2, high O2 conditions, and becomes pronounced at high temperatures.” Two conditions are named there: low carbon dioxide and high oxygen, and high temperature. The page does not give a temperature threshold, so this article does not give one.

The research review in PMC adds a related point about light. It states that “Since photorespiration is intimately intertwined with photosynthesis, it is not surprising that light is an inducer of the expression of photorespiratory genes.” The review also states that “Similar to photosynthetic genes, genes encoding core enzymes of the photorespiratory pathway are up-regulated after exposure to light.” The sources therefore link photorespiration to light as well as to gas levels.

What happens to the toxic product

The Georgia Tech page explains the chemistry in general terms. It states that “The oxygenation of RuBP produces 2-phosphoglycolate, a 2-carbon toxic compound which undergoes a series of reactions in the peroxisome and mitochondria, releasing CO2 and resulting in loss of organic carbon and energy production.” The sentence has three parts. The product is toxic, it is processed through a series of steps in two cell structures, and the processing releases carbon dioxide while losing carbon and energy.

The page’s description of the loss is direct. It states that “As a result, organic carbon is oxidized, the opposite of photosynthesis, which reduces inorganic carbon to make organic carbon.” This is the core reason the sources treat photorespiration as costly for the plant.

Why the name is misleading

The Georgia Tech page explains the name directly. It states that “This process is called photorespiration – an awfully misleading name for students, because it has nothing to do with respiration and yields no ATP.” The name suggests a form of respiration. The page says the pathway has nothing to do with respiration and yields no ATP.

The research review gives an older view of the pathway and then revises it. It states that “Photorespiration has been considered a ‘futile’ cycle in C3 plants, necessary to detoxify and recycle the metabolites generated by the oxygenating activity of Rubisco.” It then notes that “several reports indicate that this metabolic route plays a fundamental role in plant metabolism.” The review concludes that “photorespiration is no longer seen as a simple pathway to detoxify metabolic intermediates or recycle carbons from 2PG into 3PGA.”

How plants limit the problem

The Georgia Tech page describes the basic trade-off that drives the problem. It explains that plants “have to open structures called stomata on their leaves” in order to take in CO2. The same openings let water escape. The page states that “Plants also lose water vapor through their stomata, which means that they can die from dehydration in dry conditions as they keep their stomata open for photosynthesis.” In response, “plants close their stomata to prevent dehydration.”

The page does not give the full sequence that links closed stomata to the oxygenase reaction, so this article does not extend that link.

The page also notes that some plants have evolved a fix. It states that “Some plants have evolved a way to minimize the oxygenase activity of rubisco.” The page’s title refers to C4 plants, and the sentence “C4 plants have evolved a mechanism to deliver CO2 to rubisco” appears as a section heading in the fetched text. The sources do not describe that mechanism in full in the fetched portions, so this article only names it.

Why plants have not removed the pathway

The Georgia Tech page explains that the enzyme has been hard to change. It states that “in over 2 billion years, neither nature nor human genetic engineering has been able to eliminate or even significantly reduce the oxygenase activity of rubisco without also affecting the carboxylase activity.” The sentence describes a trade-off inside the enzyme itself. Reducing the unwanted activity has also reduced the useful one.

The page also gives the history of the enzyme. It states that Rubisco “evolved even before oxygenic photosynthesis, when there was no oxygen in the atmosphere or in the ocean waters.”

What the research review adds

The PMC review is a research summary on the regulation of the pathway. It states that “Photorespiration is a key pathway in plant metabolism.” It also links the pathway to nitrogen and sulfur metabolism, and it calls the topic “a very interesting research topic.” The review identifies open questions, and it does not present a complete model of the pathway.

The review also describes plant stress. It notes that these stresses “often provoke a disequilibrium in redox cellular homeostasis, increasing the intracellular reactive oxygen species (ROS) level.” The sources do not give a numerical link between this stress and photorespiration rate.

What this means for gardeners

The sources describe a process inside leaf cells. They do not give growing advice for a bed, a pot or a lawn. The practical link for a grower is the set of conditions the sources name. The Georgia Tech page says that the oxygenase activity “becomes pronounced at high temperatures,” and it links dry weather to closed stomata. A grower who wants to manage heat and water stress can read how to measure soil temperature for planting and how to water a garden in drought. Those articles address soil and water, not the enzyme chemistry described here.

What we could not verify

The two sources explain the core chemistry and the stomata link. They do not give a full map of the pathway or measured rates for garden plants. The list below records what the fetched text does not say. No detail has been filled in from memory.

What the sources do not say

  • The sources do not give a temperature at which photorespiration becomes significant in a given crop or lawn grass.
  • The Georgia Tech page does not describe the full set of reactions in the peroxisome and mitochondria.
  • The PMC review is focused on regulation and does not give field rates of photorespiration.
  • The fetched text of the C4 section is limited, so the mechanism that reduces oxygenase activity is named but not explained here.

Frequently asked questions

Is photorespiration the same as respiration?

The Georgia Tech page says no. It states that photorespiration is “an awfully misleading name for students, because it has nothing to do with respiration and yields no ATP.”

Why does photorespiration happen?

The page explains that Rubisco “sometimes fixes O2 to RuBP instead of CO2,” and that the product is “a 2-carbon toxic compound.” The pathway processes that product.

When does photorespiration increase?

The page says the oxygenase activity “occurs at low CO2, high O2 conditions, and becomes pronounced at high temperatures.”

Why do plants close their stomata in dry weather?

The page states that “plants close their stomata to prevent dehydration,” because open stomata let water vapor escape and plants “can die from dehydration in dry conditions.”

Is photorespiration only a waste?

The PMC review says that the older view of the pathway as a “futile” cycle has changed, and that “several reports indicate that this metabolic route plays a fundamental role in plant metabolism.”

How we chose these sources

The PMC review is a research summary hosted by PubMed Central, and the Georgia Tech page is a biology teaching resource on plant metabolism. Both were opened on 2026-10-27 and returned HTTP 200 on a direct request. Each factual sentence in this article is tied to a quote from one of these two sources.

Sources

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