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

Write about the outline concept of light and dark reaction phases 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. Photosynthesis is divided into two phases: light phase or photochemical reaction phase and dark phase or biosynthetic phase. The reactions that occur only in the presence of light is known as the light reaction phase that occurs in the thylakoids in grana and stroma lamellae. Dark reaction is the light-independent reaction that takes place in the stroma.

Light reaction phase: Light absorption by pigment molecules, water splitting, oxygen release and production of ATP and NADPH₂ are the different processes that take place in the light reaction phase. Light is absorbed by the different pigment molecules which are arranged as photosystems in the membrane of the grana and stroma lamellae. One molecule of chlorophyll a molecule in the photosystem acts as the reaction centre. Chlorophyll a absorbs light and getting excited and discharges electrons. These electrons carried by several electron carriers in the electron transport chain. The movement of electrons in the electron transport system leads to the formation of ATP and NADPH₂. After absorption of light, the photolysis of the water begins to meet the demand of electron of the chlorophyll molecule. This leads to the production of oxygen as a by-product of water. Hence, ATP, NADPH₂ and oxygen are the products of light reaction.

Dark phase: 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 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 Calvin cycle. The first step of this is carbon fixation and the enzyme catalysed for this reaction is RuBisCO. Here, CO₂combined with RuBP to produce two molecules of phosphoglyceric acid. 6 molecules of CO₂ are required for the production of one molecule of glucose.

The next step is the reduction in which phosphoglyceric 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. The remaining 10 molecules form 5 molecules of Ribulose 5-phosphate (RuMP) by different biochemical reactions. It undergoes phosphorylation to form RuBP.

Write about the outline concept of light and dark reaction phases of photosynthesis.

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