Elementary plant Biochemistry and Biotechnology (2+1)

Chemical hypothesis

• Many attempts have been made since 1920 to identify an energy-rich metabolite linking oxidation and phosphorylation.No such intermediates was isolated and in 1960, Peter Mitchell suggested that no possibility of existence of such an intermediate compound. So, the chemical hypothesis has become discredited

Chemiosmotic theory

• The chemiosmotic theory states that the coupling of oxidation to phosphorylation is indirect. According to this, the hydrogen ions (protons) generated by the oxidation of components in the respiratory chain are ejected to the outside (matrix) of the inner membrane.The electrochemical potential difference resulting from the asymmetric distribution of the hydrogen ions (protons or H+) is used to drive a membrane-located ATP synthase which in the presence of Pi + ADP forms ATP

Inhibitors of respiratory chain

1. Inhibitors, which inhibit respiratory chain, may be grouped as follows:
2. Inhibitors of electron transfer
3. Inhibitors of ATP synthase
4. Uncouplers of oxidative phosphorylation


• Inhibitors that arrest respiration by blocking the respiratory chain act at three sites.Compounds such as barbiturates, amytal, rotenone prevent the transfer of electron from FeS centre to ubiquinone.Carboxin specifically inhibits transfer of reducing equivalents from succinate dehydrogenase to ubiquinone.Antimycin A blocks electron transfer from cytochrome b to cytochrome c1.


• Substances such as cyanide (CN-), azide (N3-) and carbon monoxide inhibit cytochrome c oxidase by binding to heme group and are extremely poisonous. Oligomycin inhibits ATP synthase.In the presence of the uncouplers such as dicoumarol and 2,4-dinitrophenol, oxidation proceeds without phosphorylation (dissociation of oxidation in the respiratory chain from phosphorylation) releasing energy in the form of heat rather than in the form of ATP