The last step in the aerobic respiration of plant cells is the oxidation of re­duced co-enzymes by molecular oxygen.

The oxidation of one of the re­duced co-enzymes in Krebs cycle viz NADH2 takes place by the stepwise transfer of this electron to a series of electron acceptors forming ATP along the way. Finally, the electrons along with protons from Hydrogen ions are transferred to molecular oxygen, forming water.

The metabolic pathway along which the electrons of reduced co-enzyme are channelled is known as the electron transport system or the respiratory chain. For explaining the mechanism of terminal oxidation we will use the oxida­tion of malic acid to oxaloacetic acid as an example. In this reaction two electrons from malic acid are transferred to NAD which acts as a hydrogen acceptor and it is reduced to NADH2 The latter reacts with second electron acceptor FAD. As a result of this, electrons are transferred from NADH to’ FAD resulting in the oxidation of former and reduction of latter.

In this process electrons step down by one step and a part of their energy released. This energy is stored in ATP.

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The electrons are then transferred stepwise to a group of coloured compounds called cytochromes. There are 4 types viz. cyt b, cyt c, cyt a and cyt a3.

FADH2 is stable in air but is readily oxidised to FAD by cyt b whose iron gets reduced to ferrous state. Then electrons are transferred through cyt c, to a. Finally cyt a3 called cytochrome oxidase accepts the electrons. In the meantime, the energy stored in electrons is released into ATP.

Cytochrome oxidase is most important because this atom is capable of oxidation of molecular oxygen. 95% of oxygen utilized by cells is via cytochrome oxidase.

In the mechanisms outlined above the oxidation of NADH2 by the molecular oxygen is linked with the phosphorylation formation of ATP. This is called oxidative phosphorylation because oxygen is consumed in this process. All these processes occur in the mitochondrion which is the power house and store house of energy.