In the year 1961, Peter Dennis Mitchell, a British biochemist postulated the Chemiosmotic hypothesis for which he was awarded the Nobel Prize. He explained the complete mechanism of ATP synthesis within the chloroplasts during the process of photosynthesis.
Both ATP and NADP are generated during the photochemical phase or light reaction. These are the key components and used in the dark reaction for the production of a final product of photosynthesis i.e. sugar molecules. Let us see how this ATP and NADPH are generated during the light reaction.
Chemiosmotic Hypothesis & Photosynthesis
According to the chemiosmotic hypothesis, ATP- Adenosine triphosphates are produced as the result of proton gradient that is developed across the thylakoids membrane. The fundamental components required for the chemiosmosis process are proton gradient, ATP synthase, and proton pump. ATP synthase is an enzyme which helps in the synthesis of ATP molecules.
The enzyme ATP synthase possesses two subunits -F0 and F1. The F0 subunit is involved in the protons transportation across the membrane as a transmembrane channel while configuration changes in F1 activate the enzymes. The enzyme phosphorylates ADP, by adding one more phosphorus group to the ADP and converting ADP molecules to ATP molecules. The proton gradient developed across the membrane is the driving force of ATP synthase.
During the light reaction of photosynthesis, photosystems help chlorophyll to absorb light. As a result, hydrolysis process occurs which releases electrons and protons. The released electrons get moved to higher energy level and are carried by electron transport system. While the released protons from stroma begin accumulating inside the membrane. This generates a proton gradient a product of the electron transport chain. The small quantity of the generated protons is used by photosystem I to reduce NAHP+ to NADPH by electrons, from the photolysis of water. When the proton gradient is collapsed it releases energy and protons that are carried out back to stroma via F0 the transmembrane channel of ATP synthase. This released energy causes changes in F1 configuration and triggers the ATP synthase to convert ADP to ATP.
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