PHOTORESPIRATION

PHOTORESPIRATION:

·         In 1920s, German biochemist Otto Warburg (1883-1970) discovered that plants consumed oxygen at a higher rate when they were illuminated. He also found that this increased rate of O2 consumption inhibited photosynthesis. Stimulation of O2 consumption by light is now referred as photorespiration.

·         Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high energy molecule made by the light reaction. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.

·         Photorespiration is a biochemical process in plants especially under conditions of water stress and oxygen inhibits the Calvin cycle, the carbon fixation process of photosynthesis. In Calvin cycle, first CO2 fixation step, this is the reaction where RuBP combines with CO2 to form two molecules of 3PGA i.e. catalyzed by RuBisCo.

               RuBP +CO2 – 2*3PGA

RuBisCO is the most abundant enzyme in the world which fixes CO2 during the calvin cycle. It is characterized by the fact that its active site can bind to both CO2 and O2. Hence RuBisCO has a much greater affinity for CO2 than for O2. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme. Thus, fixation of CO2 typically exceeds fixation of O2, even though atmospheric CO2 levels are about 0.035 % whereas O2 is about 2%.

·         In C3 plants some O2 does bind to RuBisCO and hence CO2 fixation is decreased. Hence, the RuBP instead of being converted to two molecule of PGA binds with oxygen to form one molecule of phosphoglycolate in a pathway called photorespiration. Rather it results in the release of CO2 with the utilization of ATP. In the photorespiratory pathway there is no synthesis of ATP and NADPH. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2 and this result in increasing the intracellular concentration of CO2 in turn, this ensures that the RuBisCO function as a carboxylase minimizing the oxygenase activity. C4 plants lack photorespiration so productivity and yields are better in these plants. These plants show tolerance to higher temperature.

·         Many plant physiologists believe that photorespiration is an artifact of the ancient evolutionary history of photosynthesis. RuBisCO is originated in bacteria several years ago, when there was very little atmospheric O2 present. Thus, there was little selection pressure for the ancient RuBisCO to discriminate between CO2 and O2.

·         Photorespiration observed in all C3 plants which have been examined but non-existent in C4 plants. This is because C4 plants segregate their RuBisCO enzyme in bundle sheath cells deep within the leaf and the CO2 concentration in these cells maintained at very high levels.  C4 plants have higher growth rates than C3 plants because they do not waste their ATP and NADPH in photorespiration.

·         It results in light dependent release of oxygen and carbon dioxide which is associated with metabolism and synthesis of small molecule named Glycolate. The process takes place in green plants along with photosynthesis. Its end result decreases the amount of CO2 and both photosynthesis and photorespiration which works opposite to each other.

·         Plants especially C3 plants face the problem of photorespiration in hot dry days. These plants tend to close their stomata to prevent excessive loss of water (from transpiration). CO2 cannot enter the leaves (via the stomata) resulting in the levels of CO2 within the leaves to become low. Since there are few CO2 molecule to fix the O2 molecule used as a substrate to produce G3P (Glyceraldehyde 3 Phosphate) because of photorespiration instead of about two molecule of G3P is produced and a toxic phosphoglycolate (which plants gets rid off) is also formed. Some plants such as CAM plants and C4 plants have evolved mechanism to avoid photorespiration.

Factors affecting Photorespiration:

1)      The rate of photorespiration increases at any time when the level of CO2 is low and O2 is high. Such conditions occur when stomata remain partially closed or completely closed and photosynthesis is underway.

2)      The stomata of plants are open, resulting in lowering down the rate of photorespiration. But when plants become water stressed, they close stomata to prevent loss of water via transpiration. Thus, on the other hand it restricts the exchange of gases. The level of CO2 gradually rises as water splits during light reaction.

3)      In desert and dry tropical areas, photorespiration is reduced due to water stress and on the other hand, results in lowering down the potential of plant growth. Some plants have adapted to this problem by modifying the way they carry out photosynthesis. One of the common adaptations is called CO2 metabolism in which plants develop different leaf anatomy called Kranz anatomy.

Important Points:

·         Photorespiration occurs in green plants in the presence of sunlight.

·         The primary substrate in glycolate formed from RuBP.

·         Photorespiration occurs in most of the C3 plants.

·         It occurs intracellularly, the process occurs in peroxisomes in association with chloroplasts and mitochondria.

·         The process increases with increasing concentration of O2 and decreasing concentration of CO2.

·         Hydrogen peroxide is formed during the process.

·         Phosphorylation does not occur in photorespiration.

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