Photorespiration is a biochemical process in plants in which, especially under conditions of water stress, oxygen inhibits the Calvin cycle, the carbon fixation portion of photosynthesis.
Photorespiration results in the light-dependent uptake of oxygen and release of carbon dioxide and is associated with the synthesis and metabolism of a small molecule called glycolate. Photorespiration takes place in green plants at the same time that photosynthesis does. Because in photosynthesis carbon dioxide is taken in, and in photorespiration carbon dioxide is given off, these two processes work against each other.
The end result is that photorespiration decreases the net amount of carbon dioxide which is converted into sugars by a photosynthesizing plant. By interfering with photosynthesis in this way, photorespiration may significantly limit the growth rate of some plants.
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Photosynthesis
In green plants, photosynthesis takes place in the special energy-storing molecules called chloroplasts. Photosynthesis can be divided into two parts: the light reactions and the dark reactions. In the light reactions, light energy from the sun is captured by the plant and converted into chemical energy in the form of chloroplasts.
An additional feature of the light reactions is that a molecule of water is split so that its oxygen is released. In the dark reactions, a series of steps called the Calvin cycle converts carbon dioxide from the air into organic molecules such as sugars and starch.
The Calvin cycle requires energy in order to operate, and this is provided by the energy-storing molecules (such as adenosine triphosphate) formed in the light reactions. The carbohydrates thus formed can serve as food for the plant or for an animal that eats the plant.
The overall pattern of gas exchange in photosynthesis, therefore, is the release of oxygen and the uptake of carbon dioxide. It has been found that, to a lesser extent, light can also cause plants to do just the opposite—that is, to consume oxygen and release carbon dioxide. This phenomenon was discovered in the 1950’s and is termed photorespiration.
If net photosynthesis is defined as the total amount of carbon dioxide taken in minus the amount given off, it is apparent that increasing the rate of photorespiration will decrease net photosynthesis. In terms of agricultural plants, this translates into adecrease in the productivity of the crop.
Rubisco and Glycolate
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| Photorespiration |
Each of the reactions of the Calvin cycle must be catalyzed by an enzyme. The first reaction of the cycle, in which carbon dioxide is taken up, utilizes an enzyme popularly known as Rubisco, an abbreviation for ribulose bisphosphate carboxylase/oxygenase.
The normal function of Rubisco is to take carbon dioxide from the atmosphere and combine it with another molecule in the chloroplast, ribulose bisphosphate (RuBP). The resulting compound is then acted upon by other enzymes which eventually convert it into the simple sugar glyceraldehydes 3-phosphate, which is used in the synthesis of more complex sugars and other compounds.
Rubisco, however, does not always behave in its normal fashion. It is sometimes unable to distinguish between molecules of carbon dioxide and oxygen.
Rubisco will sometimes “mistakenly” incorporate an oxygen molecule into RuBP rather than the carbon dioxide that would normally have been used. The oxygen may come from the atmosphere, or it may originate from the oxygen that is continually being produced by the splitting of water during the light reactions of photosynthesis.
The result of this metabolic error is that, rather than forming compounds that can be converted into sugar, the plant forms a substance known as glycolate. Understanding what happens to glycolate is the key to understanding the process of photorespiration.
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The utilization of oxygen in the formation of glycolate accounts for part of the oxygen uptake that is observed during photorespiration. Instead of being used in sugar synthesis, the glycolate enters a different metabolic pathway, where it is acted upon by a different series of enzymes with different consequences.
These sequential reactions, referred to as the glycolate pathway, result in the conversion of glycolate into a series of different compounds. This pathway constitutes the remainder of the process of photorespiration.
The Glycolate Pathway
An unusual feature of photorespiration is that it involves three separate cellular organelles: the chloroplast, the peroxisome, and the mitochondrion. The first stage of photorespiration involves the formation of glycolate in the chloroplast. The glycolate does not undergo further reactions in the chloroplast but instead is transported to the peroxisome.
Once inside the peroxisome, the glycolate enters a series of reactions, one of which causes oxygen to be converted into hydrogen peroxide. This represents a second point in the process at which oxygen is consumed.
If hydrogen peroxidewere present in large quantities it could have a toxic effect upon the cell, so the peroxisome also contains the enzyme catalase,which destroys most of the hydrogen peroxide thus formed.
In subsequent steps of the glycolate pathway, one of the compounds formed is glycine,which enters the mitochondrion and loses a carbon atom in the form of carbon dioxide.
This, then, accounts for the observed release of carbon dioxide during photorespiration. If the carbon dioxide is lost to the atmosphere, it represents a decrease in the net amount of carbon taken up by the plant and, therefore, a decrease in net photosynthesis.
Further reactions of the glycolate pathway occur in the mitochondrion and peroxisome, and eventually a compound is formed which is returned to the chloroplast, where the process began.
This compound is capable of reentering the Calvin cycle and can actually be used for the synthesis of sugars. The critical point, however, is that not all the carbon atoms which left the chloroplast in the form of glycolate are returning to it.
Part of the carbon was lost in the form of the carbon dioxide that was released in the mitochondrion. Furthermore, certain steps in the glycolate pathway require the expenditure of energy. The process is, therefore, doubly wasteful in that it results in the loss of both carbon dioxide and energy storage molecules.
To summarize the process, oxygen is utilized in the chloroplast to form the two-carbon compound glycolate. The glycolate then enters a series of reactions that occur in the peroxisome and mitochondrion and that take up additional oxygen and release a portion of the carbon in the form of carbon dioxide.
The remaining carbon is converted into 3-phosphoglycerate, which can be returned to the chloroplast and reenter the Calvin cycle, where it is one of the normal intermediate compounds. This accounts for the light-dependent uptake of oxygen and release of carbon dioxide, which constitute photorespiration.
Factors That Increase Photorespiration
Although some amount of photorespiration occurs in many plants regardless of conditions, photorespiratory rates increase any time that carbon dioxide levels are low and oxygen levels are high. Such conditions occur whenever stomata (specialized pores for gas exchange) remain closed, or partially closed, while photosynthesis is under way.
Undermost conditions plants are able to keep their stomata open, so photorespiratory rates remain low. When plants become water stressed, they close their stomata to prevent further water loss by transpiration. Water stress is most likely under hot, dry conditions.
Under these conditions, the stomata close as far as needed to conserve water, thus restricting normal gas exchange. Carbon dioxide levels slowly rise as water is split during the light reactions. Consequently, photorespiratory rates accelerate, and photosynthetic efficiency drops to as low as 50 percent of normal.
In dry tropical and desert environments water stress, and thus photorespiration, can significantly reduce plant growth potential. Some plants have evolved solutions to this problem by modifying the way they carry out photosynthesis.
One common adaptation is called C4 metabolism. This modification involves a different leaf anatomy, called Kranz anatomy, as well as a different enzyme pathway for initially fixing carbon dioxide that is not prone to problems with oxygen.
The Calvin cycle still functions in C4 plants, as they are called, but it is protected from photorespiration by the C4 adaptations. Many tropical grasses, including corn and sugarcane, use this approach. Unfortunately, many crop plants, including wheat, soybeans, spinach, and tomatoes, do not possess this adaptation.
The other major adaptation is called CAM metabolism, which is short for crassulacean acid metabolism. This adaptation is common in succulents (plants that store excess water in their stems and leaves, making them very juicy), such as pineapples, cacti, and stonecrops. Stonecrops are in the family Crassulaceae,which is the source of the name for this adaptation.
CAM plants, as they are called, only open their stomata at night, when transpiration rates are low and water loss is minimal. Carbon dioxide enters the leaf at night and is attached to organic molecules in a different pathway that does not require light as an energy source.
Then during the day, when the stomata are closed, this carbon dioxide is released and enters the Calvin cycle. Photorespiration is prevented because carbon dioxide levels can be maintained at appropriate levels, even though the stomata are closed.
Why Photorespiration?
Several theories have been proposed to explain why plants photorespire. One possibility is that when plants first evolved, conditions on the primitive earth were very different from what they are now. The early atmosphere contained little oxygen, so the inability of Rubisco to distinguish between oxygen and carbon dioxide was not a problem.
As the oxygen level in the atmosphere gradually increased, the formation of glycolate during photosynthesis began to occur, and this led to the problem of photorespiration. The glycolate pathway then developed as a mechanism for salvaging some of the material that leaves the Calvin cycle in the formof glycolate, ultimately returning a portion of it to the cycle.
Seen in this context, the real culprit in photorespiration is the formation of glycolate by Rubisco, while the glycolate pathway is an evolutionary adaptation formaking the best of a bad situation. Perhaps, millions of years in the future, plants will evolve a form of Rubisco that can more effectively distinguish between these two gases, and photorespiration will diminish or cease.
An alternative theory about why plants photorespire is that the process does, in fact, performan important function: protecting the plant from the harmful effects of very high internal concentrations of oxygen or energy storage molecules. This high concentration could occur when the plant is exposed to high light intensities, causing photosynthesis to generate these substances very rapidly.
Photorespiration would then consume some of the excess oxygen and energetic molecules, depleting them to levels that would not be harmful to the plant. It has not yet been conclusively shown, however, that photorespiration really does play such a protective role. Further research will be required before scientists know whether photorespiration is beneficial to the plant.
