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Photorespiration is a wasteful pathway that competes with the Calvin cycle. It begins when rubisco acts on oxygen instead of carbon dioxide.


Do you have any friends who are awesome people, but who also have some kind of bad habit? Maybe they procrastinate a lot, forget your birthday, or never remember to brush their teeth. You wouldn't stop being friends with them for these reasons, yet from time to time, you might find yourself wishing they would clean up their act.
RuBP oxygenase-carboxylase (rubisco), a key enzyme in photosynthesis, is the molecular equivalent of a good friend with a bad habit. In the process of carbon fixation, rubisco incorporates carbon dioxide (CO2) into an organic molecule during the first stage of the Calvin cycle. Rubisco is so important to plants that it makes up 30% or more of the soluble protein in a typical plant leaf1. But rubisco also has a major flaw: instead of always using CO2 as a substrate, it sometimes picks up O2 instead.
This side reaction initiates a pathway called photorespiration, which, rather than fixing carbon, actually leads to the loss of already-fixed carbon as CO2. Photorespiration wastes energy and decreases sugar synthesis, so when rubisco initiates this pathway, it's committing a serious molecular faux pas.
In this article, we'll explore why photorespiration happens, when it's most likely to take place (hint: think hot and dry conditions), and how it actually works.

Rubisco binds to either CO2 or O2

As we saw in the introduction, the enzyme rubisco can use either CO2 or O2 as a substrate. Rubisco adds whichever molecule it binds to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP). The reaction that uses CO2 is the first step of the Calvin cycle and leads to the production of sugar. The reaction that uses O2 is the first step of the photorespiration pathway, which wastes energy and "undoes" the work of the Calvin cycle2.
Rubisco can bind to either carbon dioxide or oxygen depending on environmental conditions. Binding to carbon dioxide and initiation of the Clavin cycle is favored at low temperatures and at a high carbon dioxide-to-oxygen ratio. Binding to oxygen and the initiation of photorespiration is favored at high temperatures and a low carbon dioxide-to-oxygen ratio.
What determines how frequently each substrate gets "chosen"? Two key factors are the relative concentrations of O2 and CO2 and the temperature.
When a plant has its stomata, or leaf pores, open CO2 diffuses in, O2 and water vapor diffuse out, and photorespiration is minimized. However, when a plant closes its stomata—for instance, to reduce water loss by evaporation—O2 from photosynthesis builds up inside the leaf. Under these conditions, photorespiration increases due to the higher ratio of O2 to CO2.
In addition, Rubisco has a higher affinity for O2 when temperatures increase. At mild temperatures, rubisco's affinity for (tendency to bind to) CO2 is about 80 times higher than its affinity for O2.3 At high temperatures, however, rubisco is less able to tell the molecules apart and grabs oxygen more often4.
The bottom line is that hot, dry conditions tend to cause more photorespiration—unless plants have special features to minimize the problem. You can learn more about plant "workarounds" in the videos on C4 plants and CAM plants.

Photorespiration wastes energy and steals carbon

Photorespiration begins in the chloroplast, when rubisco attaches O2 to RuBP in its oxygenase reaction. Two molecules are produced: a three-carbon compound, 3-PGA, and a two-carbon compound, phosphoglycolate. 3-PGA is a normal intermediate of the Calvin cycle, but phosphoglycolate cannot enter the cycle, so its two carbons are removed, or "stolen," from the cycle5.
To recover some of the lost carbon, plants put phosphoglycolate through a series of reactions that involve transport between various organelles. Three-fourths of the carbon that enters this pathway as phosphoglycolate is recovered, while one-fourth is lost as CO2.5
In the diagram below, you can see a comparison between photorespiration and the normal Calvin cycle, showing how many fixed carbons are gained or lost when either 6 CO2 or 6 O2 molecules are captured by rubisco. Photorespiration results in a loss of 3 fixed carbon atoms under these conditions, while the Calvin cycle results in a gain of 6 fixed carbon atoms.
Comparison of Calvin cycle and photorespiration pathways.
In the Calvin cycle, 6 CO2 molecules combine with 6 RuBP acceptors, making 12 3-PGA molecules. These are converted into 12 G3P sugars. 2 leave the cycle to make 1 glucose, while 10 are recycled to make 6 RuBPs. The cycle can begin again.
In the photorespiration pathway, 6 O2 molecules combine with 6 RuBP acceptors, making 6 3-PGA molecules and 6 phosphoglycolate molecules. The 6 phosphoglycolate molecules enter a salvage pathway, which converts them into 3 3-PGA molecules and releases 3 carbons as CO2. This makes for a total of 9 3-PGA molecules. These can be converted into 9 G3P sugars. This is not enough for any to exit the cycle as glucose. In fact, it is not even enough to regenerate the 6 RuBP acceptors. Instead, only 5 RuBP acceptors can be regenerated, with 2 leftover carbon atoms. The 3 carbons released as CO2 have been "stolen" from the cycle.
Photorespiration is definitely not a win from a carbon fixation standpoint. However, it may have other benefits for plants. There's some evidence that photorespiration can have photoprotective effects (preventing light-induced damage to the molecules involved in photosynthesis), help maintain redox balance in cells, and support plant immune defenses8.

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