Chemiosmotic Hypothesis

Chemiosmotic hypothesis

In 1961, Peter Mitchell postulated the Chemiosmotic hypothesis. It explains the mechanism of ATP synthesis within chloroplast during photosynthesis. During photochemical phase or light reaction ATP and NADP are generated. These are the key components and used in the dark reaction for the production of the final product of photosynthesis i.e. sugar molecules. Let us see how this ATP and NADPH are generated during the light reaction.

According to the chemiosmotic hypothesis, ATP production is the result of proton gradient developed across the membrane of thylakoids. The essential components required for chemiosmosis are proton pump, proton gradient, and ATP synthase. ATP synthase is an enzyme which helps in ATP synthesis. The enzyme ATP synthase has two parts-F0 and F1. F0 is a transmembrane channel while configuration changes in F1 activate the enzymes. They phosphorylate ADP. One of the driving factors of ATP Synthase is the protein gradient that is developed across a membrane. 

In plants, during the light reaction, photosystems help chlorophyll to absorb light. As a result, hydrolysis (splitting of water) takes place and releases electrons and protons. The electrons get excited to higher energy level and are transported by the electron transport system while protons (hydrogen ions) from the stroma starts to accumulate inside the membrane. This creates a proton gradient. Some protons are used by photosystem I for reduction to NADPH. When the proton gradient is collapsed, it releases energy and protons 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.

The Chemiosmotic Theory

According to this theory, molecules like glucose are metabolized to develop acetyl CoA in the form of an energy-rich intermediate. The proper oxidation of acetyl CoA occurs in the mitochondrial matrix and coupled to the reduced form of a carrier molecule such as FAD and NAD. The carriers then pass electrons to the transport chain of the electron in the inner membrane of mitochondria, which pass them to different other proteins present in the ETC. The energy present in the electrons is basically used to pump out protons from the matrix in the inner mitochondrial membrane. It is used to store the energy in the form of a transmembrane electrochemical gradient.

The protons return to the inner membrane by the ATP enzyme synthase. The flow of protons travels into the matrix of mitochondria through ATP synthase, which gets the good amount of energy for ADP to integrate with inorganic phosphate to produce ATP.

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