Photorespiration is a process of CO2 elimination and O2 absorption in the presenc of light which takes place simultaneously with the photosynthesis. As for the biochemical mechanism, this process differs substantially from photosynthesis as well as from respiration.
Photorespiration is more prominent in C3 photosynthetic plants under natural conditions, at an oxygen concentration of 21 % and a carbon dioxide concentration of 0.03 % (photorespiration constitutes 25–30 % of the photosynthetic gas excha-nge in leaves).
Under intense illumination and lower carbon dioxide concentration and higher oxygen concentration, the photorespiration process intensifies. High temperature has a similar effect on the intensity of photorespiration.
The essence of this process is that the basic enzyme in photosynthesis, RUBISCO, can act as both a carboxylase and an oxidase , catalyzing the decomposition of ribulose-1,5-diphosphate into phosphoglyceric acid and phosphoglycolic acid, which subsequently dephosphorylases into glycolic acid.
Since the primary product of the oxidation reaction is the glycolic acid, this path has taken the name of the glycolic pathway of carbon transformation. Originally the acceptor of carbon dioxide—ribulose-1,5-diphosphate forms a complex with the active center of the enzyme and, only after this, it fixates a molecule of CO2 or O2, performing the function of carboxylation or oxygenation.
In C4 plants, the CO2 released in the process of photorespiration is used in the carboxylation reaction of phosphoenolpyruvate from the mesophyll cells to form oxaloacetate and, then, by a reduction reaction—in malate, which is decarboxylated in the perivascular sheath cells, releasing CO2 in chloroplasts and then the latter is involved in the Calvin cycle again.
This peculiarity explains the high net photosynthesis. The physiological relevance of glycolic acid formation reactions in chloroplasts can be seen in several ways:
- Glycolic acid represents a mobile form of carbon transport from chloroplasts into the cytoplasm
- Amino acids glycine and serine are formed;
- NADPH+H+ is formed in peroxisomes;
- It is considered that photorespiration serves to protect the photosynthetic apparatus.
Factors determining the damaging of the photosynthetic apparatus are intense light at a relatively low concentration of carbon dioxide and oxygen. If in such conditions oxygen concentration increases, the glycolate formation mechanism is induced.
The glycolate carbon, undergoing a cycle of transformations until the formation phosphoglycolic acid, is removed partially as CO2, which is included in the Calvin cycle where it is reduced by the use of the “assimilation factor” of the chloroplasts—NADPH+H+ and ATP.
Consequently, the photosynthetic apparatus works in vain without absorption of external CO2, but ph-otosynthetic structures are protected in this manner by keeping the chloroplasts active. Such situations occur during drought conditions, which make the stomata close.
According to this hypothesis, photorespiration which is useful in drought conditions becomes a parasitic process under optimal conditions of life.
Faqs
What is the photorespiration process?
Photorespiration is a process of CO2 elimination and O2 absorption in the presenc of light which takes place simultaneously with the photosynthesis. As for the biochemical mechanism, this process differs substantially from photosynthesis as well as from respiration.
What is the function of photorespiration?
Photorespiration keeps photoinhibition from happening. When plants are under stress, like from drought, cold, or too much light, the amount of NADPH made in the light reactions of photosynthesis often exceeds what the Calvin cycle needs to reduce power.
Why do plants use photorespiration?
When it is too hot or dry, plants’ stomata often close to keep water from escaping. This keeps CO2 from getting into the leaf and stops O2 from getting out. Instead of the Calvin cycle, photosynthesis takes place when oxygen builds up inside the leaf.
Why do C4 plants avoid photorespiration?
C4 plants avoid photorespiration because, during the first step of carbon fixation, they possess the enzyme PEP. Therefore, carbon dioxide generated during photosynthesis is recycled via PEP
Why can C3 plants not avoid photorespiration?
C3 plants lack the anatomical structure (no bundle sheath cells) and abundance of PEP carboxylase required to avoid photorespiration, in contrast to C4 plants.