Elementary plant Biochemistry and Biotechnology (2+1)

Electron transport chain and oxidative phosphorylation

• The mitochondrion is the aerobic organelle in which the final stage of the oxidation of food occurs.
  
• It is the site of the citric acid cycle, fatty acid oxidation and oxidative phosphorylation, processes that are responsible for the formation of ATP under aerobic condition.
  
• The two most important energy transductions in the biological systems are the oxidative phosphorylation (ATP synthesis driven by electron transfer to oxygen) and photophosphorylation (ATP synthesis driven by light).
  
• Oxidative phosphorylation is the process in which ATP molecules are formed as a result of the transfer of electrons from the reducing equivalents, NADH or FADH2 (produced by glycolysis, the citric acid cycle and fatty acid oxidation) to oxygen by a series of electron carriers in the form of a chain located in the inner membrane of mitochondria.This is the final reaction sequence of respiration.
• Since the electrons are transferred by a series of electron carriers in the form of a chain, it is known as electron transport chain (ETC).
  
• In plants, ATP is mainly derived through photosynthesis utilizing the energy derived from the sun. In non-photosynthetic tissues, ATPs are derived through respiration .
• The electrons are transferred along a set of cytochromes in the form of a chain in steps from the more electronegative components (NADH/FADH2) to the more electropositive oxygen.
• The respiratory chain consists of a number of protein complexes that are remarkably complicated in nature. They are known as NADH- ubiquinone reductase, succinate-ubiquinone reductase, ubiquinone-cytochrome c reductase and cytochrome c oxidase. These complexes are also called as NADH dehydrogenase, succinate dehydrogenase, cytochrome b-c complex and cytochrome c oxidase respectively or as complexes I - IV.
  
• All the three reductases are also known as iron-sulphur proteins since they contain Fe-S centres as their critical components. Iron in these enzyme complexes can exist in two forms as Fe2+ and Fe3+ .Each cytochrome in its oxidised form (Fe3+) accepts one electron and becomes reduced to Fe2+ form. Fe2+ donates electron to the next carrier.
  
• Oxidation of one molecule of NADH results in generation of 2.5 molecules of ATP whereas oxidation of one molecule of FADH2 generates 1.5 molecules of ATP