PHOTORESPIRATION

 


INTRODUCTION TO PHOTORESPIRATION

Earlier the rate of breathing in the light was thought to have been almost the same as breathing in the dark. Light affects breathing and the light rate about 3 to 5 times more than breath in darkness, recently it has been found. This led to photorespiration being discovered. The C2 cycle is also known. Decker first showed in the years 1955 and 1959 the presence of photorespiration. He was the first to use the expression photorespiration for his colleagues.


Photorespiration in plants in the presence of light requires loss of fixed carbon as CO2. Chloroplasts are initiated. This is not a wasteful operation that produces either ATP or NADPH. Photorespiration normally occurs when the level of oxygen is high. RuBisCO, the enzyme which catalyses RuBP carboxylation in the first phase of the Calvin cycle, acts as oxygenase under these circumstances. Any O2 binds to RuBisCO and thus reduces CO2 fixation. In the direction of photorespiration, RuBP binds with O2 to form one PGA (3C compound) and phosphoglycolate (2C compound) molecule. The synthesis of sugar and the ATP are not present. Instead, it causes CO2 emissions through the use of ATP. The loss of fixed CO2 represents 25 percent.


In various plant plants such as Nicotiana, Phaseolus, Pisum, Petunia, Gossypium, Capsicum, Antirrhine, Oryza, Glycine, Helianthus, Chlorella and Nitella, photorespiration has been documented. In tropical grasses it has rarely been recorded. Light is always needed and its rates are maximum from 25°C to 35°C. Photo vacuum. The concentration of oxygen also depends. Photorespiration is very different from natural or soil breathing or dark breathing. Photorespiration results from the ribulose-1,5-bisphosphate carboxylase/oxygenase-catalyzed oxygenase reaction. Glycollate-2-phosphate is produced in this reaction and then transformed into intermediate glycerate-3-phosphate in the photorespiratory pathway for the Calvin cycle. CO2 and NH3 are created and photorespiration and reduced equivalents are consumed during this metabolic method, thus making photo breathing a wasteful operation. In warm areas, many plants, called C4 plants, use a separate carbon dioxide fixation technique. First, the carbon dioxide in specifically arranged mesophyl cells is fixed by a different cycle and then the carbon dioxide is transferred to cells where the usual Calvin cycle takes place.

It's a task of isolation from high levels of oxygen, which interfere with the carbon fixation enzyme, of the cells where the Calvin cycle takes place.


Photorespiration site


The site of photorespiration previously was thought to be chloroplaste, but the discovery of peroxisomes, which contain glycolate metabolism enzymes, indicated a connection among peroxisome and photorespiration. The site of photorespiration may thus be peroxisome. Khaki and Tolbert demonstrated in 1969 that peroxisoma was not the real C02 site during photo-photorespiration. The peroxisome offers clearly a C02 development substratum. In 1971 Tolbert offered that C02 development takes place in mitochondria during photorespiration. These results indicate a close connection between chloroplasts, peroxisomes and mitochondria. Today, the majority of the physiologists agree that the three cell organels take part in the photorespiration process, chloroplast, peroxisome and mitochondria. They shape the photorespiration website together.


Biochemicals


All three are active in photorespiration, with chloroplasts, mitochondrias and peroxisomes. These three cell organelles function closely together. Photosynthesis is used as the primary substrate for photorespiration in the chloroplast glycolate which is an early developed commodity. Glycolic acid is actually formed by ribulose diphosphate oxidation when the concentration in C02 is less than 1 percent in the outside atmosphere. In the presence of a RuDP Carboxylase enzyme, ribulosic diphosphate is initially oxidised into three-phosphoglyceric (PGA) and two-phosphogyl cyclic acid (also known as III oxygenase).

The rubisco (Ribulo-1,5 bisphosphate carboxylase/oxygenase). The rubisco (ribulo-1,5-bisphosphate carboxylate/oxygenase), which is present at large concentrations of chloroplast stroma, accounts for up to 30% of the total nitrogen in typical C3 leaves (RuBP). Both reactions include molecular CO2 competition with O2, which is formed first at the active location of the enzyme in the endiol form of RuBP. The division between carboxylation and oxygenation of RuBP is based upon Rubisco's kinetic parameters. The RuBP carboxylation leads to only glycerate-3-P formation. The remaining one sixth is exported either as triosphate phosphate for the synthesis of sucrose and other products in the cytoslic compartment or metabolised to form starch in chloroplasts, whereas five sixths of the glycerate-3-P molecules produced here are used to regenerate the RuBP. RuBP oxygenation results in the development of a single glycerate-3-P molecule and one glycolate-2-P molecule (a two carbon compound).

In the photo-resistant direction, glycolate-2-P is generated in highly high rates under ambient conditions ( C2cycle). Two glycolate-2-P molecules are metabolised to constitute a single molecule of Glycerate-3-P (PGA) & CO2, which are used immediately for the regeneration of RuBP using a reduction of pentoses without net phosphate synthesis. During this process, two molecules are metabolised. There is also a complete synchronisation between C3cycle and C2 cycle. Low intercellular concentrations of CO2 can result in even higher concentrations, for instance in water stress (for instance, when the stomata are closed). Conversely, doubling CO2 levels raises approximately two folds the Rubisco carboxylation/oxygenation ratio and reduces photo photorespiration by approximately 50%.


The pictorial pathway


Two 2-phosphoglycolate molecules are converted into a serines molecule (three carbons) and a CO2 molecule by the glycolate pathway.

A phosphatase transforms 2-phosphoglycolate into a glycolate in the chloroplast that is exported into peroxisome. Glycolate is oxidised by molecular oxygen and transamed to glycine for the resulting aldehyde (glyoxylate).

Hydrogen peroxide, which is produced as a side product of oxidation by glycolate, is harmless by peroxidoses.

Glycerin is transported from peroX to the mitochondrial matrix where glycine decarboxylase, the structurally and mechanically identical enzyme, undergoes oxidative Decarboxylation through two already encountered complex mitochondrial compounds: the dehydrogenase pyruvate complex and the dehydrogenase-ketoglucarate complex. ••

With the concomitant reduction of NAD to NH3, glycin decarboxylase complex oxidises glycins to CO2, with the conversion of glycine to the cofactor tetrahydrofolate.


• Tetrahydrofolate is transmitted by serine hydroxymethyltransferase to a second glycine that produces serine. The net reaction of the complex and serine hydroxymethyltransferase glycine decarboxylase is catalysed


• Serine is converted to hydroxypyruvate, glycerate, and ultimately to three-phosphoglycerate used to repair 1,5-bisphosphorous ribulose, and completes the long, costly cycle.

• The flux of glycolate salvage through the light sun can be very high, producing approximately five times as much CO2 as is normally emitted through all citric acid cycle oxidations.

For this massive flux, mitochondria produce prodigious quantities of Glycine Decarboxylase: in the mitochondrial matrix of the pea and spinach plants the four proteins of this complex comprise half of all protein.

Mitochondria has low concentrations of the glycine decarboxylase complex in non-photosynthetic sections of a plant, such as potato tubers.

Combined activity of the glycolate rescue pathway and rubisco oxygenase absorbs O2 and thus generates the term photorespiration.

This course could be best known as the photosynthetic oxidative carbon cycle (C2), names not inviting a connection with breathing in mitochondria.

• Unlike mitochondrial breathing, photo-repositive breathing is not energy efficient and could potentially impede the development of net biomass up to 50%.

This inefficiency led to evolutionary adaptations in the processes of carbon-assimilation, particularly in plants which developed in warm climates.



C4 plants and photoresiration


C4 plants largely occur in tropical areas because under warm and sunny conditions they develop quicker. On a hot, bright day, when photosynthesis reduces the chloroplast CO2 level and increases the O2 level, the photo-based breath rate is at the photosynthesis rate. However, C4 plants have a special leaf morphology as two photosynthetic cell types are present.

 Bundle-sheath cells

* T he cells of Mesophyll


In C4 plants many steps occur in the photosynthesis by which these plants maintain the CO2 concentration by which they can reduce the effect of photorespiration. The following steps occur as follows:

 Phosphoenol-pyruvate (PEP) converts into Oxaloacetate.

PEP carboxylase adds CO2 to PEP to produce Oxaloacetate.

this occurs in the mesophyll cells.

 The formation of malate from Oxaloacetate is catalysed by malate dehydrogenate reductase. NADPH is used during this step.

 Malate is transported from mesophyll cell into the bundle sheath cell.  Conversion of malate to py

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