Animals make ATP in two ways:

  1. Substrate level phosphorylation

The synthesis of ATP during reaction of glycolysis and Krebs cycle is called substrate level phosphorylation. ATP formation from ADP and phosphate requires input of energy. The reaction is coupled with an exergonic reaction. So it releases energy. An enzyme transfers a phosphate group to ADP from a substrate. So ATP molecule is formed. The energy from the exergonic reaction is greater than the energy input necessary to drive ATP synthesis. The generation of ATP by coupling is strongly exergonic reactions. Substrate-level phosphorylation appeared very early in the history of organisms. Following factors support this view:

1. Organisms initially use carbohydrates as an energy source. Energy is removed from carbohydrates by substrate level phosphorylation.

  1. The mechanism for substrate level phosphorylation is present in most living animal cells.
  2. Substrate-level phosphorylation is one of the most fundamental of all ATP-generating reactions.


2. Chemiosmosis

The coupling reaction in which synthesis of ATP molecule takes during movement of H+ across an H+ gradient is called chemiosmosis. Chemiosmosis generates more ATP then substrate level ATP. Chemiosmosis has following mechanism:

  1. The mitochondrial Membranes have transmembrane channels. These

channels can pump protons. The flow of electrons induce a change of shape in the protein. Thus the protons move out of the inner compartment of a mitochdindrion. Therefore, the proton (I-I) concentration in the outer compartment of the mitochondrion becomes greater than that of the inside compartment.



  1. Electrical-chemical proton gradient is established between outer and inner membranes. This gradient derives the outer protons across the membrane. Thus the protons move down this gradient between outer and inner mitochondria! compartments. Their movement induces the formation of ATP from ADP and phosphate. An enzyme ATP synthetase controls the synthesis of ATP during this process.3. T        electrons are obtained from chemical bonds of food molecules in all

    or anisms.. This electron removing process need free oxygen. So it is called c Ilular respiration, or aerobic respiration. Aerobic respiration is the o dation of food molecules to obtain energy.



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