Photorespiration

Photosynthesis is the process by which plants produce nutrients through sunlight. The final product of Photosynthesis is formed during its second phase called the ‘Biosynthetic phase.’ This phase classifies plants as C3 and C4 plants. Another classification source comes from Photorespiration, also known as  C2 cycle. To grasp the concept of Photorespiration better, we should first understand the Calvin Cycle.

The Calvin Cycle

The Calvin Cycle is the first process in the Biosynthetic phase in C3 plants to obtain Carbohydrates. This cycle contains some steps, which are:

1. Carbon Fixation, in which the plant attaches atmospheric Carbon dioxide to RuBP.  After attachment, the six-carbon compound split into two molecules of 3-phosphoglyceric acid. This reaction is catalysed by the most abundant enzyme in the world, called RuBP carboxylase-oxygenase/RuBisCO.

2. Reduction: The 3-phosphoglyceric acid molecules convert into simple sugar molecules called glyceraldehyde-3 phosphate/G3P. This stage uses energy from ATP and NADPH created during photosynthesis. The Calvin cycle helps plants convert sunlight energy into storage molecules like sugars at this reduction stage. The energy from the ATP and NADPH is then transferred to these sugars.

This step is called ‘reduction’ because in chemistry, donating electrons is called ‘reduction and taking electrons is called ‘oxidation.’ In this process, the NADPH donates electrons to the 3-phosphoglyceric acid molecules to create glyceraldehyde-3 phosphate. Regeneration, where glyceraldehyde-3 phosphate molecules produce glucose, while others are recycled to regenerate the five-carbon RuBP compound used to accept new carbon molecules.

This regeneration process requires ATP and because it takes six-carbon molecules to make glucose, this cycle repeats six times. To complete this equation, five out of six glyceraldehyde-3 phosphate molecules created through the Calvin cycle are regenerated to form RuBP molecules.

Now that we know the stepwise process of the Calvin Cycle let us understand Photorespiration in C3 plants a little better.

Photorespiration in C3 Plants

Photorespiration is a counterproductive metabolic activity in C3 plants, where RuBisCO binds to oxygen molecules, deviating from standard processes. Because of this deviation, instead of producing two molecules of G3P, only one is made and a toxic phosphoglycolate (which the plant has to expel) is also formed. During Photorespiration, no sugar or ATP molecules are synthesized but carbon dioxide is released at the expense of ATP, making the whole process wasteful. As a result, C3 plants undergoing photorespiration often do not yield a decent yield. This process generally occurs on hot and dry days when the stomata close and the oxygen concentration in the leaf exceeds the concentration of carbon dioxide.

C4 plants have evolved some mechanisms to avoid photorespiration due to the unique leaf anatomy. C4 plants contain an efficient enzyme in their mesophyll cells called PEP Carboxylase, which catalyses the Carboxylation reaction, wherein the pyruvate accepts the carbon dioxide molecule and forms a four-carbon acid. The four-carbon acids are carried to the bundle sheath cells to release carbon dioxide, entering the Calvin cycle. The thick walls of the bundle sheath cells, impervious to gaseous exchange, also help maximise the concentration of Carbon Dioxide near RuBisCo. Therefore, the oxygenase activity of RuBisCo is minimized or barely present. As C4 plants successfully bypass Photorespiration, Photosynthesis becomes more productive and increases yield.

Definition of Photorespiration

We know about the process of photorespiration. Therefore we can now fully grasp the definition.

Simply put, photorespiration is the process wherein RuBisCO binds to oxygen molecules. One molecule of G3P is produced along with a (toxic) Phosphoglycolate. Sugar and ATP molecules are not synthesised and carbon dioxide is released at the expense of ATP.

It is important to emphasize that there is no synthesis of sugars, NADPH or ATP in this photorespiratory process. Instead, it helps the release of carbon dioxide through ATP. This is, therefore, a costly operation that the plant undergoes.

Importance of Photorespiration

Photorespiration is a necessary evil of plant metabolism. Photorespiration allows leaves to utilise excess light energy and reduce photo-oxidative damage when the stomata are closed. However, many plants do not undergo photorespiration. C4 plants are an example of these.

Conclusion

Photorespiration is a wasteful process in which one molecule of G3P is produced along with a Phosphoglycolate. As a result, sugar and ATP molecules are not synthesised, and Carbon Dioxide is released at the expense of ATP. This process generally occurs on dry, hot, and bright days, when the stomata close and the oxygen concentration in the leaf exceeds the concentration of carbon dioxide. Photorespiration definition allows leaves to utilise excess light energy and reduce Photo-oxidative damage when the stomata are closed.