Question

8. The net reaction of oxidative phosphorylation is the reduction of molecular oxygen to form water. Reduced coenzymes are oxidized during this process. (15 points)

(Use the reduction potentials table)

1. Calculate the potential difference under standard conditions when NADH is oxidized by molecular oxygen across the mitochondrial membrane

2. Calculate the standard free energy change for this reaction ( Ignore membrane potential and transport energy)

3. How is the energy of this change coupled to ATP synthesis?

Answer 1

Oxidative phosphorylation is simply production of ATP in presence of molecular oxygen. It is also termed as electeon linked phosphorylation due to the transfer of electrons from NADH and FADH2 through a complex system across the membrane of mitochondria or chloroplasts, reducing molecular oxygen to water.

The various components of electron transport system through which oxidative phosphorylation takes place are arranged into 4 kinds of complexes-

• Complex I (NADH dehydrogenase complex)
• Complex II (Succinate dehydrogenase complex)
• Complex III (Cytochrome bc1 complez)
• Complex IV (Cytochrome oxidase complex)

Apart from these 4 complexes there is another complex, ATP synthase complex.

These complexes are arranged in the inner membrane of mitochondria.

​The net reaction during the electron transfer occurs as such:

• NADH from the Krebs cycle, transfers hydrogen to complex I. The coenzyme NADH gets oxidised. At the same time, a proton is pumped to the inter membrane space of mitochondria through complex I.

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• Reduced Complex I transfers the electrons to coenzyme Q.
• FADH2 gives its electrons to Complex II and gets oxidised.

• The electrons from complex II are now transferred to coenzyme Q.
• Being mobile, coenzyme Q passes through Complex III to cytochrome C, providing enough energy to complex III to pump large amount of H+ to the inteinter membrane space.
• From Cytochrome C the electrons pass through complex IV back into the mitochondrial matrix, where they combine with O2 forming H2O.
• Now as the concentration of H+ is higher in the inteinter membrane spacspace, they are pumped back in via Complex V or the ATP synthase complex. As the protons move in the convert ADP into ATP in presence of inorganic phosphate, thus completing the oxidative phosphorylation.

Oxidation of NADH

Free energy change

One molecule of NADH produces 3 ATP.