The glycolysis and fermentation are inefficient processes. Thus the end product of glycolysis (pyruvate) still contains large amount of energy. This energy can be released by further oxidation. Free oxygen accumulated in the atmosphere of the earth as a result of photosynthesis. Therefore, evolution of aerobic respiration took place in microorganisms and in the mitochondria. The evolution of aerobic respiration has made the Organism efficient. It provides them powerful and efficient way of removing energy from nutrient molecules. Life would become slow without mitochondrial ATP production. Thus most of animals could not have evolved.

The pyruvate is entered into Krebs cycle or citric acid cycle. The NADH goes to the electron transport chain. During aerobic metabolism, free oxygen accepts electrons and reduces to H2O as follows:

Pyruvare + 0, —3 CO2 + H20 + .34ATP + hex energy

34 molecules of ATP are produced during this reaction. The aerobic respiration is organized into a number of reactions. Each reaction is catalyzed by a specific enzyme. There are two steps of aerobic respiration:

  • Krebs cycle
  • Electron transport chain


Krebs cycle is a series of reactions in which the pyruvate from glycolysis is oxidized to CO2. The Krebs cycle was discovered by Hans Krebs in the 1930s. Two electron carriers act as hydrogen acceptors. These are

  • Nicotinamide adenine dinucleotide (NAD)


These are reduced to NADH and FADH2. Three molecules of CO2 are generated from ach pyruvate molecule during this phase of the cycle. Some energy is also produced in the form of ATP. Most of the remaining energy is in the form of NAD and FADH2. These two molecules are transferred into the electron transport chain. The pyruvate is first changed in Acetyl CoA. The Acetyl CoA then enters into the Krebs cycle. Following reactions occur during Krebs cycle:





1. Step 1: Acetyl CoA adds its two-carbon fragment to oxaloacetate. Oxaloacetae is a four-carbon compound. The unstable bond of acetyl CoA is broken and acetyl group is attached to oxaloacetate. The product is the SIY.



group has a different protein with a haem group. These are:

  • Cytochrome b
  • Cytochrome
  • Cytochrome c
  • Cytochrome a
  • Cytochrome a3

The last Cytochrome passes its electrons to oxygen. It also picks up a pair of hydrogen ions from aqueous medium to form water. Two molecules of ATP are produced during this transfer of electron through the cytochromes.


FA I H2 an alternate source of electron

An ther source of electrons for the transport chain is FADH2. It is produced du ng Krebs cycle. FADH2 adds its electrons to transport chain at a lower energy lev I than that of NADH. Therefore, the electron transport chain provides about orn third less energy from FADH2.

Th electron transport chain makes no ATP directly. Its function is to pass ele trons from food to oxygen. It breaks a large free energy into a series of sm Iler steps that release energy in manageable amounts. Mitochondria sy thesize ATP during a mechanism called chemiosmosis.




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