Name: Autotrophic Nutrition in Plants
Uploaded: 2019-07-19
Duration: 13 min 25 s
Description: Photosynthesis is a dye-sensitised redox process producing carbohydrate. Plants are self-reliant systems providing food to other organisms. Hence are autotro...

Question

What do you mean by carbon assimilation ? Describe the dark reaction phase of photosynthesis.

Accepted Answer

Photosynthesis is a physicochemical process in which glucose is formed from CO₂ and water in the presence of sunlight and chlorophyll. It occurs in two phases photochemical reaction phase (light reaction) and biochemical phase (dark phase). During the dark phase of photosynthesis, the carbon of carbon dioxide of the atmosphere is utilised in the preparation of glucose. The inclusion of carbon from the atmospheric carbon dioxide in the cell component is called carbon assimilation. Hence, photosynthesis is also called carbon assimilation.

The biosynthetic reaction which does not require light in which NADPH₂ and ATP are utilised to reduce carbon dioxide in different phases to produce triose phosphate is known as dark reaction. The dark reaction is the enzymatic reaction that takes place in the stroma of the chloroplasts. The reaction pathway that occurred during the dark phase of photosynthesis in most of the plants in which RuBP is the primary CO₂ acceptor is called C₃ cycle or Calvin cycle. The plants in which this cycle takes place are called C₃ plants. It is divided into three parts:

i) Carboxylation: It is the first step of C₃ cycle. The primary carbon dioxide acceptor is a 5C compound, RuBP that fix carbon dioxide to form two molecules of 3C phosphoglyceric acid (PGA). It takes place in the presence of an enzyme, RuBisCO. Six CO₂ molecules are required for the production of one molecule of glucose. Hence, 12 molecules of PGA are formed.

Reduction: The next step is the phosphoglyceric acid is combined with one ATP molecule to form 1, 3 - biphosphoglycerate. This reaction is catalysed by the triose phosphate dehydrogenase enzyme. 1,3-biphosphoglyceric acid accepts H⁺ ions and electrons from NADPH₂ and reduced to form phosphoglyceraldehyde (glyceraldehyde 3-phosphate). Out of 12 molecules of 3-Phosphoglyceraldehyde, only two molecules form one glucose molecule.

Regeneration of RuBP: Out of 12 molecules of 3-Phosphoglyceraldehyde, 10 molecules form 5 molecules of Ribulose 5-phosphate (RuMP) by different biochemical reactions. It undergoes phosphorylated to form RuBP.

What do you mean by carbon assimilation ? Describe the dark reaction phase of photosynthesis.

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