Chemiosmotic hypothesis of ATP formation in mitochondria and chloroplast.

Chemiosmotic hypothesis of ATP formation

The photosynthesis does not occur in etiolated and leaves without chlorophyll. In the variegated leaves, photosynthesis occurs only at the places which possess chlorophyll.

Willstatter studied the effect of the amount of chlorophyll on the rate of photosynthesis. He studies photosynthesis/chlorophyll unit in leaves of different ages of plants. This is known as the assimilatory number. The assimilatory number decreases with the age of the leaf.

The chemiosmotic hypothesis of ATP formation was propounded by Peter Mitchell in the United Kingdom in the year 1961 in the case of mitochondria and chloroplast. G.Hind and Andre Jagendorf confirmed this chemiosmotic theory at Cornell University in 1963.

According to this chemiosmotic hypothesis, electron transport, both in respiration and photosynthesis produces a proton gradient (pH gradient). The gradient develops inside the thylakoid lumen in chloroplasts. The lumen of thylakoid becomes enriched with H+ ion due to the photolytic splitting of water.

The primary acceptor of the electron is found on the outer side of the thylakoid membrane. This acceptor transfers its electrons to a Hydrogen carrier. The carrier removes a proton from matrix while transporting electron to the inner side of the membrane.

Then the proton is released inside the lumen while the electron crosses to the next carrier. The NADP reductase is located on the outside of the thylakoid membrane. It obtains electrons from PS I and protons from the matrix to reduce NADP+ to NADP+H+ state.

Because of these events, the concentration of protons decreases in matrix or stroma region while their concentration in thylakoid lumen rises resulting in the decrease in pH.

A proton gradient develops across the thylakoid. The proton gradient is broken due to movement of protons through transmembrane channels, CF0 of APTase (CF0 of ATPase (CF0-F1 particle). As protons pass to the other side of ATPase, they bring about conformational changes in CF1 particles of ATPase enzyme and form ATP from ADP and inorganic phosphate.

Therefore, ATP synthesis through chemiosmosis needs a membrane a proton pump, a proton gradient and CF0-CF1 particle or ATP-ase. One molecule of ATP is formed when the ATP synthesis uses 3H+.

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