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7.2.2.2.3. Electron transport chain

Electron transport chain

Although the glucose molecule, after glycolysis and the Krebs cycle, has been completely oxidized and produced energy in the form of ATP, most of the energy is found in the electrons that accepted NAD+  and FAD, which they were reduced to NADH and FADH2.

The NADH and FADH2  have great reducing power, and transfer their electrons to molecular oxygen (O2) through an electron transport chain or respiratory chain. In the transport of these electrons, a large amount of energy is released that is used to form ATP (oxidative phosphorylation).

Electronic transport

The electrons of the NADH and FADH2 molecules, with a high energy level, pass through different transporter molecules in a gradient of redox potentials until they reach O2, which is the final electron acceptor.

These transporter molecules, in the inner mitochondrial membrane, are reduced and oxidized, accepting electrons and giving them to the next molecule, lowering the electrons from high energy levels to lower ones. When lowering to other levels, energy is released that will be used in the synthesis of ATP by oxidative phosphorylation.

The electron transporting molecules of the respiratory chain are grouped into four large supramolecular complexes located in the inner mitochondrial membrane:

  • Complex I or NADH- dehydrogenase Complex.
  • Complex II or Ubiquinone or Coenzyme Q reductase.
  • Complex III or complex cytochrome b-c1.
  • Complex IV or Cytochrome oxidase complex.

Oxidative phosphorylation: Mitchell's chemosmotic hypothesis

The respiratory chain, in the mitochondrial crests, is made up of a series of molecules, the proton transporters (H+) and the electron transporters (e-). Protons and electrons pass from one to another, from the substrate to O2, which is reduced to obtain water.

When the electrons of a molecule pass to the one with a lower energy level, the passage of  protons  (H+) from the mitochondrial matrix to the intermembrane space occurs, creating a large potential difference with respect to that of the matrix. The protons then return to the matrix through the oxysomes, activating ATP synthase and forming ATP. This process is called oxidative phosphorylation, and it allows ATP to be synthesized from the energy obtained in the NADH and FADH2 molecules released in glycolysis and in the Krebs cycle . Decade  NADH 3 ATP are obtained and from FADH2, only 2 ATP.

In the electronic transport chain:
ADP + 3Pi + NADH + O2 → 3 ATP + 2 NAD+ + H2O
ADP + 2Pi + FADH 2  + O2 → 2 ATP + 2 FAD+ + H2O

In order not to get lost in the respiratory process, we will remember that in glycolysis two molecules of NADH  had been produced, the oxidation of pyruvic acid to acetyl CoA produced two molecules of NADH, and the Krebs cycle produced two molecules of FADH2 and six molecules of NADH for each glucose molecule.

NADH Glycolysis
NADH Pyruvic acid oxidation to acetyl CoA
FADH 2
Krebs cycle
NADH

This mechanism by which ATP is obtained was explained by the chemosmotic hypothesis or chemosmotic coupling theory, proposed by Peter Mitchell in 1961.

According to this hypothesis, the energy released when the electrons pass to another molecule with a lower energy level is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space. This creates a difference in the concentration of protons and electrical charges between the intermembrane space  and the matrix , the electrochemical gradient.

The ATP synthase enzymes of the inner mitochondrial membrane have a channel inside through which protons return to the mitochondrial matrix, producing the phosphorylation of ADP to synthesize ATP.

Curiosity: Peter Mitchell, 1978 Nobel Prize Winner in Chemistry

Peter Mitchell (1920-1992) was an English biochemist who was awarded the Nobel Prize in Chemistry in 1978 for his studies on biological energy transfer explained in chemosmotic theory.

Fundamental ideas about the electron transport chain

Electron transport chain

  • The glucose, after glycolysis and the Krebs cycle, has been completely oxidized and produced ATP, but most of the energy is in the form of NADH and FADH2.
  • The NADH and FADH2 lose their electrons to oxygen through an electron transport chain, and in that transport occurs  ATP  ( oxidative phosphorylation ).
  • The electrons (and the H+) reach the oxygen obtaining water.
  • It is carried out in the  inner mitochondrial membrane,.
  • Electrons pass from one molecule to another with a lower energy level and  protons (H+) pass from the mitochondrial matrix to the intermembrane space, creating a large potential difference with respect to that of the matrix. 
  • The  protons (H + ) return to the matrix activating  ATP synthase  and forming ATP.
    • The ATP synthase :
      • It acts like a proton pump (H+).
      • It is located in the mitochondrial ridges of the inner mitochondrial membrane and in the thylakoid membrane (in chloroplasts). 
      • Function: synthesize ATP from ADP and Pi.
  • For each  NADH  3 ATP are obtained  and for each  FADH2, only 2 ATP.
  • Remembering what was obtained in the respiratory process of a glucose molecule :