Class 11 Biology Photosynthesis In Higher Plants Photorespiration

Photorespiration 

  • The first step of the Calvin pathway is the first CO2 fixation step.
  • Respiration is the reaction where RuBP combines with CO2 to form 2 molecules of 3PGA, that is catalysed by RuBisCO.

RuBP + CO2 -> 2 X 3PGA

  • RuBisCO has a much greater affinity for CO2 than for O2.
  • In C3 plants, some O2 do bind to RuBisCO, and hence CO2 fixation is decreased.
  • The RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule and phosphoglycolate in a pathway called photorespiration.
  • In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP.
  • In C3 plants, some O2 do bind to RuBisCO, and hence CO2 fixation is decreased.
  • In the photorespiratory pathway, there is no synthesis of ATP or NADPH; therefore, photorespiration is a wasteful process.
  • In C4 plants, photorespiration does not occur 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, which results in increasing the intracellular concentration of CO2.

Class_11_Biology_Photosynthesis_in_Plants_PhotoRespiration

Fig. Photorespiration

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