Plant Photosynthesis

Earlier it was believed that respiration in light was approximately equal to respiration in dark but work with isotopes of CO₂ and O₂ has shown that respiration may indeed be very high in light. Photorespiration is the uptake of O₂ and release of CO₂ in light and results from the biosynthesis of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the same leaf cell. Biochemical mechanism for photorespiration is also called glycolate metabolism. Microbodies found near the chloroplasts (peroxisomes) are considered to be cellular site of photorespiration. The process of photorespiration involves the involvement of chloroplasts, peroxisomes and mitochondria. RuBP carboxylase also catalyses another reaction which interferes with the successful functioning of Calvin cycle. In the presence of high concentration of oxygen the enzyme acts as oxygenase and results in the formation of 3 phosphoglyceric acid and 2 phosphoglycolate. The 2 phosphoglycolate loses phosphate group to form glycolate. The glycolate synthesized in chloroplast is an early product which moves to peroxisomes. The glycolate is oxidized to glycolate and hydrogen peroxide with the help of enzyme oxidase. Glycolate is converted into an amino acid glycine with the help of enzyme glutamate-glycoxylate transaminase. The two molecules of glycine then interact to form a molecule of serine and a molecule of CO₂ in mitochondria without the production of ATP and NADH. Finally serine passes out of mitochondria and is converted to carbohydrate.

Photorespiration works to undo the act of photosynthesis, as no energy rich compound is produced in this process. When temperature increases, more and more photosynthetically fixed carbon is lost by photorespiration. This is due to special ability of RuBP carboxylase enzyme that with increase in temperature and oxygen concentration, affinity of the enzyme with CO₂ decreases and for oxygen increases.

Photorespiration generally occurs in temperate plants. Few photorespiring plants are: Rice, beans, wheat, barley, etc. In C₄ plants, this process is insignificant.

During photorespiration loss of carbon takes place in the form of CO₂. It is considered to be an integral and inseparable aspect of photosynthesis. Process is considered to nullify the results of photosynthesis as no ATP and NADPH₂ is produced during this process. Moreover affinity of RuBP carboxylase for CO₂ decreases and for O₂ increases in high temperature and oxygen concentrations, thus this process poses serious challenge to plants found in tropical areas.