Cytochrome Pump

This mechanism of ion transport is based on electrochemical gradient generated by electron transport. How such an electrochemical gradient is produced is explained by chemiosmotic hypothesis put forward by British biochemist P. Mitchel. According to Mitchell’s hypothesis, when hydrogen is removed from a substrate in respiration and carried along an electron transport chain, it is changed into two charged species:

H (hydrogen atom) → H⁺ (hydrogen ion) + e^- (electron)

The H⁺ and e^- are separated on opposite sides of the mitochondrial membrane by the electron carrier enzymes, which are so arranged in the inner mitochondrial membrane that they carry hydrogen ion to the outside and electron to the inside.

The hydrogen atoms transported through the membrane must be derived from water by the following reaction:

H₂O → OH^- + H⁺

H⁺ + e^- (from electron carrier) → H

The OH^- ions of water remain inside the membrane. Since H⁺ on the outside and OH^- ions on the inside of the membrane, the outside of the membrane becomes positively charged and the inside is negatively charged. This also generates a pH gradient, because outside of the membrane is more acidic because of accumulation of H⁺ ions and inside of the membrane is basic on account of the presence of OH^- ions. Proton gradients are produced both in mitochondrial membrane and the thylakoid membrane of chloroplasts. In mitochondria H⁺ ions move outwards, while in chloroplast membranes they move inwards along electron transport chain.

Proton gradients as proposed by Mitchell’s chemiosmotic hypothesis must be generated in the plasmalemma of cells for active transport of solutes. Electric potential difference present in the plasmalemma suggests that charge separation occurs across this membrane also. Electron carriers are also required for charge separation, but it is not yet known what substances in plasmalemma act as electron carriers. ATPase has been found in this membrane, which may generate proton gradient.

On the establishment of proton gradient, the cations move actively in exchange for H⁺ ions in a direction opposite to that of H⁺ ions and anions move passively to satisfy the charge balance.

Similarly, when anions move actively in exchange for OH^- ions, the cations move passively to maintain the charge balance. H⁺ ions crossing the membrane react with OH^- ions to form H₂O. Similarly, OH^- ions transported through the membrane react with H⁺ ions to produce water.

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