The process of chemiosmotic hypothesis

Do you know how ATP is synthesised? Let us now learn and understand how ATP is synthesised in mitochondria. 

The process of the chemiosmotic hypothesis explains ATP synthesis. ATP stands for adenosine triphosphate, which provides energy to work out many processes in living cells and plants. Photosynthesis is how ATP is produced during the ‘photochemical’ phase or light reactions, generating the need for ATP synthesis. Thus, after cyclic and non-cyclic phosphorylation, the chemiosmotic hypothesis takes place. 

1961. Mitchell explained the chemiosmotic hypothesis in 1961. The mechanism explains the synthesis of ATP in the chloroplast. This whole process takes place in the chloroplast.

What is the process of the chemiosmotic hypothesis?

The chemiosmotic hypothesis is ATP synthesis through chemiosmosis.

This complete process compiles the movement of ions through an electrochemical gradient. A semipermeable membrane facilitates this movement. The process of the chemiosmotic hypothesis requires a membrane, ATPase enzyme, gradient of the proton, a pump of the proton.

The chemiosmotic hypothesis explains the complete process of synthesis ATP in the chloroplast. ATP synthesis is connected with the implementation of the proton gradient, i.e., in the lumen.

Three main events are found during the process of chemiosmotic hypothesis:

• Photolysis of water towards thylakoid membrane.
• Transfer of H+ from stroma to lumen as electrons move through ETS.
• NADP-reductase reaction occurs towards the stroma side of the membrane.

Step by step overview on the process of chemiosmotic hypothesis:

Photolysis of water towards thylakoid lumen:

The splitting of water molecules or photolysis occurs at the lumen side of the membrane. With this process, H+, produced in the internal lumen, gets accumulated.

Transfer of H+ from stroma to lumen as electrons move through photosystems:

The primary acceptor located towards the outer side of the membrane transfers its electron to an H+ carrier; after this electron, transportation takes place, which removes a proton. When this H+ carrier molecule passes on its electron to an electron carrier present on the inner side of the membrane, the H+ is then released into the lumen of the membrane.

NADPH reductase reaction occurs towards stroma:

• The NADP reductase enzyme is located on the stromal side of the membrane. Protons are necessary to reduce NADP+ to NADPH + H+, and these protons are removed from the stroma.

• So, within the chloroplast, protons in the stroma decrease in number, while protons accumulate in the lumen. It causes a decrease in the lumen pH and creates a proton gradient across the thylakoid membrane.

Why are we interested in the proton gradient?

This gradient is important because its breakdown triggers energy release.

The gradient is broken down. H+, a proton, moves from the lumen to the stroma by gradient channel ATP synthase. ATP synthase has two portions:

• F1: Present on the outer side, in the stroma. It converts the ADP (adenosine diphosphate) to ATP (adenosine triphosphate). It protrudes on the other surface of the thylakoid membrane.
• F0: Present on the inner side, in the lumen. It is embedded in the membrane and forms a channel that carries out the diffusion of protons across the membrane.

How does the chemiosmotic hypothesis explain the process?

The chemiosmotic hypothesis explains the process of ATP synthesis, as mentioned before.

The ATP generated is immediately used in the biosynthetic reaction. The synthesis of sugar and fixation of CO2 takes place in the stroma.

The chemiosmotic hypothesis claim ATP synthesis includes these steps:

Photosystem 2 is present in the membrane. When sunlight falls on photosystem 2, the antenna molecule absorbing the sunlight yields energy. Their electrons get excited and move to the reaction centre, chlorophyll a.

The splitting of water/photolysis of water occurs, generating H+, O2, e–(electron). The O2 diffuses out, the electrons run in the electron transport system, and H+ accumulates in the lumen.

The electron goes to photosystem 2 to the primary electron acceptor, then to the plastoquinone (Hydrogen carrier and electron carrier). Plastoquinone takes hydrogen from the stroma and releases it to the lumen. Stroma (outside) has less hydrogen, while lumen has high hydrogen ions. It also transfers electrons to cytochrome b & f; when it leaves cytochrome b & f, releasing sufficient energy in the lumen generates a proton motive force.

Electron Release energy comes to the ground state and moves to photosystem 1 via plastocyanin. Photosystem 1 again absorbs light; the electron moves to NADP+ via Fd to NADP+ (Present in the chloroplast).

It clearly shows the process of NADP+ reduction to NADPH at photosystem 1 in the case of non-cyclic phosphorylation.

Electro potential gradient is generated due to high H+ in the lumen. It is important to break the gradient to release the energy. The concentration gradient breakdown occurs. This is due to the proton movement. Proton movement occurs from lumen to stroma via ATP synthase. 

ATP synthase has two portions: one faces the stroma (outer membrane or head portion) side, while the other faces the lumen side (integral part F0). H+ comes outside with the help of F0; when it comes outside, it works as a motor and changes the structure of F1. ADP with inorganic phosphate makes ATP. These consequences complete the process of the chemiosmotic hypothesis.

Conclusion

This process of chemiosmotic hypothesis concludes the whole ATP formation with reference to photolysis of water, NADP reduction and electron transport system. It involves different electron and proton carriers to accomplish the activity. ATP forms as the product of this whole procedure utilized in the biosynthesis of sugar and provides energy to the proper functioning of the plants.