Photorespiration: Definition, Diagram, Process, Cycle, Topics, Examples

What Is Photorespiration?

Photorespiration is considered to be one of the key metabolic pathways of plants, which links in with the Calvin Cycle of photosynthesis. It is thought to be an energy-wasting route, as it brings down the net efficiency of photosynthesis with the consumption of energy and the evolution of carbon dioxide. Photorespiration is a very important process in understanding how plants adapt to different environmental settings, particularly in terms of carbon fixation.

Conditions Favouring Photorespiration

Stomatal Closure

Photorespiration primarily occurs on hot, dry days when the stomata of the plants close to prevent the excessive loss of water. The closing process diminishes the amount of carbon dioxide intake while oxygen is continuously produced and accumulated in the leaf.

Low Carbon Dioxide Concentration

When the concentration of carbon dioxide internally becomes less than the threshold of 5% or 50 ppm, RuBisCO starts fixing the oxygen; this would instead form a phosphoglycolate, which is a 2-carbon molecule.

Overview Of Photorespiration

Photorespiration comprises a few significant steps that add up to the entire process. These are:

Carbon Fixation

At low carbon dioxide, RuBisCO catalyzes the reaction of oxygen with ribulose bisphosphate, RuBP, to produce one molecule of 3-phosphoglycerate, PGA, and one molecule of phosphoglycolate.

Conversion Of Phosphoglycolate

To prevent the toxic accumulation of phosphoglycolate, it gets converted into glycolic acid by the plant. This occurs rather rapidly to avoid any possible harmful effects.

Transformation In Peroxisomes

Transport of glycolic acid to the peroxisomes for further conversion to glycine, a 2-carbon amino acid. This step is important in detoxifying the plant and making it ready for further metabolism.

Conversion In Mitochondria

Glycine is again transported to mitochondria where it undergoes a conversion into serine, a 3-carbon amino acid. This process is energy-consuming with carbon dioxide being formed as an end-product.

Impact Of Photorespiration On Plants

Photorespiration has various influences on the metabolism of plants:

Reduced Photosynthetic Efficiency

The energy used to convert the phosphoglycolate to serine is not returned for sugar production, so there is a net loss of carbon. Because of this inefficiency, it may limit the growth and productivity of plants.

Energy Loss

Photorespiration requires ATP and NADPH, the same substrates needed for photosynthesis. This depletes supplies for photosynthesis, and further impacts the general capacity of plants to capture energy efficiently.

Protective Role

Under certain conditions, photorespiration may serve as an energy sink, thus protecting plants from oxidative stress. Under high light intensity or conditions of drought, this protective mechanism may be of value.

Photosynthesis: Relation With Photorespiration

Photosynthesis and photorespiration can occur in a plant at the same time. During photosynthesis, oxygen is a by-product, but in photorespiration, carbon dioxide is produced. The gases formed in these processes are somewhat interrelated because the oxygen formed during photosynthesis may increase photorespiration when the concentration of carbon dioxide is low.

Adaptations To Reduce Photorespiration

To avoid or reduce the losses through photorespiration, some plants have evolved with alternate mechanisms of carbon fixation :

C4 Pathway

Plants like maize and sugarcane are of the C4 type that have evolved a means of concentrating carbon dioxide in the bundle sheath cells. This minimises the chance of photorespiration. They fix carbon dioxide into a 4-carbon compound—first oxaloacetic acid—before it enters the light-independent reactions.

CAM Pathway

Plants which follow Crassulacean Acid Metabolism such as cacti and succulents assimilate carbon dioxide in the dark. This prevents loss of water during the daytime and reduces the extent of photorespiration.

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