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 () into an organic molecule during the first stage of the Calvin cycle. Rubisco is so important to plants that it makes up or more of the soluble protein in a typical plant leaf. But rubisco also has a major flaw: instead of always using as a substrate, it sometimes picks up instead.
This side reaction initiates a pathway called photorespiration, which, rather than fixing carbon, actually leads to the loss of already-fixed carbon as . 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 or
As we saw in the introduction, the enzyme rubisco can use either or as a substrate. Rubisco adds whichever molecule it binds to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP). The reaction that uses is the first step of the Calvin cycle and leads to the production of sugar. The reaction that uses is the first step of the photorespiration pathway, which wastes energy and "undoes" the work of the Calvin cycle.
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 and and the temperature.
When a plant has its stomata, or leaf pores, open diffuses in, and water vapor diffuse out, and photorespiration is minimized. However, when a plant closes its stomata—for instance, to reduce water loss by evaporation— from photosynthesis builds up inside the leaf. Under these conditions, photorespiration increases due to the higher ratio of to .
In addition, Rubisco has a higher affinity for when temperatures increase. At mild temperatures, rubisco's affinity for (tendency to bind to) is about times higher than its affinity for . At high temperatures, however, rubisco is less able to tell the molecules apart and grabs oxygen more often.
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 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 cycle.
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 .
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 or molecules are captured by rubisco. Photorespiration results in a loss of fixed carbon atoms under these conditions, while the Calvin cycle results in a gain of 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 defenses.
Want to join the conversation?
- can we say that global warmimg will result with more photorespiration in c3 plants if precautions are not taken?(8 votes)
- I'm not sure about this, because CO2 levels in the atmosphere are increasing... which might indicate that there would actually be LESS photorespiration because of increased CO2 levels. Curious to hear what others think.(6 votes)
- what are plant pigments?(3 votes)
- There are many types such as Chlorophyll A, Chlorophyll B, Xanthophyll, Carotenoids, etc. They function to lengthen the absorption spectrum. Some, like the last two pigments listed, also function to increase photoprotection. The Chlorophyll A is what is most prevalent in the leaf, and therefore, gives the leaf its color.(17 votes)
- In the last paragraph. How can photorespiration support plant immune defenses?(5 votes)
- Plant defense mechanisms include different kinds of cell signaling pathways and cascades. Certain membrane-spanning receptors react to pathogens and activate gene expression. However, in order to proceed with those mechanisms, photosynthesis has to be disrupted.
As we know, photorespiration is that 'salvage' way which occurs once photosynthesis is disrupted.(4 votes)
- When C3 plans are in a hot environment what do they first die of? Because their ability to produce glucose is significantly reduced and they will eventually use up all their H2O ?(3 votes)
- I think of take in CO2 during day whicc CAM plants doesn't and en up losing (not use up) H2O.(2 votes)
- Why do the numbers of carbon in the diagram for Photorespiration not match up(3 votes)
- They do, There are 6 5-carbon RuBP molecules (30C), which are broken into 6 3-carbon PGA molecules (18 C) and 6 2-carbon phosphoglycolate molecules (12C). These 2-carbon phosphoglycolate molecules are further broken up into 3 3-carbon PGA molecules (9C) and 3 1-carbon CO2 molecules (3C). The CO2 is released (3C), the 9 total PGA (9x3=27C) are used to regenerate some, but not all of the RuBP. Total product being 9 PGA and 3 CO2 (27C+3C=30C) which is the same amount as we started with in the 6 RuBP(2 votes)
- How photorespiration occurs in C3 plants which related in giving negative effects to yields ?(3 votes)
- Can you rephrase your question?
How photorespiration happens in described in the text above...(1 vote)
- Concerning the last part of the text above... How exactly can plants get light-induced damage, how does this damage the plant and how does the redox balance help against this (Does this have something to do with reactive oxygen species?)?(2 votes)
- Yes, exactly! It has to do with reactive oxygen species! It was also mentioned in the videos prior to this.
Photorespiration is some kind of 'salvage' and protective pathway which avoids the accumulation of unnecessary 02 which could potentially elicit great damage in cells in the form of reactive oxygen species.(2 votes)
- why is photorespiration taking place in three different places chloroplasts, leaf peroxisomes and mitochondria?(2 votes)
- This is because the different enzymes needed for amination and oxidation of different intermediates are present in the above said 3 organelles. The detailed diagram of the cycle ought to clear your doubt.(2 votes)
- If alternative systems exist to solve this problem, why don't all plants use those systems? Is it less efficient?(2 votes)
- It isn't really such a big enough deal to completely wipe out RuBisCo. Remember, back when plants were first starting, there was WAY more CO2 than oxygen. Plus, scientists are beginning to research if there are hidden benefits that haven't been identified yet.(1 vote)
- Why photorespiration occur in three organells?(2 votes)
- Because of its complexity.
Usually, when something is super complex requires specialization. Specialization means different compartments.(1 vote)