PHOTORESPIRATION

 Negative effects of photorespiration on crop plants can be regulated and consequently, the photosynthetic productivity can be increased manifold. Possible measures would be : 1. By manipulating different atmospheric conditions i.e., by increasing atm. CO2 etc. 2. Use of inhibitors of glycolic acid oxidase, such as α- hydroxysulfonate . 3. Through genetic manipulation also, the process of photorespiration can be regulated. Positive Effects of Photorespiration Recently, the photorespiration process has been considered as a protective and supportive mechanism, which reduces O2 injury to chloroplasts.  Free radicals of O2 gas are very reactive which react with membrane components and destroy them. In the absence of photorespiration, the concentration of such O2 free radicals may reach very high and can attain a destructive level in the chloroplasts. Therefore, under such circumstances, efforts to reduce photorespiration may prove dangerous.

1. Often, the presence of photorespiration is considered a wasteful and energy-consuming process in crop plants which ultimately leads to a reduction in the final yield of crops. 2. It is estimated that during C3 photosynthesis, up to 50% of the CO2 fixed may have to pass through the photorespiratory process (glycolate pathway) to form carbohydrates such as sucrose thereby resulting in a considerable decrease in photosynthetic productivity. 3. Unlike usual mitochondrial respiration, assimilatory powers (ATP or NADPH2 ) are not generated in photorespiration. 4. However, photorespiration is considered as a metabolic adjunct to the Calvin cycle (i.e., it has been added to the Calvin cycle but essentially it is not a part of the C3 cycle). 5. Because the photorespiration process decreases the photosynthetic efficiency of crop plants, scientists are working to increase the efficiency of C3 plants by decreasing photorespiration. 5.1 Ways to reduce the effect of photorespiration would be...

 Because glycolate and some other metabolites of this glycolate cycle, viz., glyoxalate and glycine are 2-C compounds, the glycolate metabolism or glycolate cycle is also called as C2 Cycle. Difference between two types of plants :

 Like usual mitochondrial respiration, photorespiration is also an oxidative process where oxidation of glycolate occurs with subsequent release of CO2 ( post-illumination burst of CO2 ). The process of photorespiration takes place in three cell organelles viz., chloroplasts, peroxisomes, and mitochondria . Various steps of the glycolate metabolism synthesis of glycolate and its oxidation with subsequent release of CO2 (photorespiration). A. Reaction in Chloroplast 1.1. Glycolate is synthesized as a side product from some intermediates of photosynthesis in chloroplasts . It is probably derived from 1-C and 2-C of the ketose sugar phosphates of the Calvin cycle. 1.2. It is believed that O2 competes with CO2 for the enzyme RuBP carboxylase. When this enzyme reacts with O2 instead of CO2, it is called RuBP oxygenase. 1.3 In the second case, one molecule of phosphoglyceric acid (PGA) and one molecule of phosphoglycolic acid are formed from RuBP. 1.4. PGA then enters into the Calvin cy...

Photorespiration occurs only in chlorophyllous tissues of plants. The process of photorespiration occurs in three different organelles viz., 1. Chloroplasts 2. Peroxisomes and 3. Mitochondria. Substrates of Photorespiration Glycolate (glycolic acid) is the chief substrate of photorespiration. Other important metabolites are the two amino acids such as glycine and serine.

 Photorespiration is a special type of respiration shown by many green plants when they are exposed to light. The normal dark respiration (usual mitochondrial respiration) is independent of light and its rate is the same in both light and dark. The photorespiration process is carried on only in the presence of light. The term photorespiration is referred to as “ the release of CO2 in respiration in presence of light during photosynthesis”. Importance of Photorespiration a. Photorespiration is closely related to the CO2 compensation point. b. It usually occurs only in those plants, which have comparatively high CO2 compensation points. (eg. Tomato, wheat, oats, etc.) c. It is insignificant or rather absent in C4 plants, which have very low CO2 compensation points] (eg. maize, sugarcane, sorghum, pearl millet, amarantus, etc.).