Question:

Read the given statements and select the correct option. Carboxylation is the most crucial step of Calvin cycle where $CO_{2}$ is utilised for the carboxylation of $RuBP$. Carboxylation is catalysed by the enzyme $RuBisCO$ which results in the formation of two molecules of $3PGA$.

Updated On: Jul 7, 2022
  • Both statements $1$ and $2$ are correct
  • Statement $1$ is correct but statement $2$ is incorrect
  • Statement $1$ is incorrect but statement $2$ is correct
  • Both statements $1$ and $2$ are incorrect
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The Correct Option is A

Solution and Explanation

Carboxylation is the fixation of $CO_{2}$ into a stable organic intermediate. It is the most crucial step of Calvin cycle where $CO_{2}$ is utilised for the carboxylation of $RuBP$ through the use of $ATP$ and $NADPH$ generated by the light reactions. This reaction is catalysed by enzyme $RuBisCO$ which results in the formation of two molecules of $3$-phosphoglyceric acid or $PGA$, which is the first stable product of photosynthesis.
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Concepts Used:

C4 Pathway

  • Every photosynthetic plant moves according to the Calvin cycle, but in some plants, there is a primary stage to the Calvin Cycle called the C4 pathway. Plants in tropical desert regions commonly move accordingly to the C4 pathway. Here, a 4-carbon compound called oxaloacetic acid (OAA) is the first product of carbon fixation. Such plants are special and have specific adaptations as well.
  • The C4 pathway commences with a molecule called phosphoenolpyruvate (PEP) which is a three-carbon molecule. This is the primary CO2 acceptor and the carboxylation, that takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule known as oxaloacetic acid (OAA).
  • In due course, it is converted into another 4-carbon compound called malic acid. Later, they are shifted from mesophyll cells to bundle sheath cells. Here, OAA is crumbled to yield carbon dioxide and a 3-C molecule.
  • The CO2 thus formed, is utilized in the Calvin cycle, whereas the 3-C molecule is shifted back to mesophyll cells for regeneration of PEP.
  • Sugarcane, corn, and shrubs are some examples of plants that follow the C4 pathway. Calvin pathway is a very common pathway in both C3 plants and C4 plants, but it takes place only in the mesophyll cells of the C3 Plants and not in the C4 Plants.