How do you get 36 ATP

Respiratory chain

The respiratory chain runs on the inner membrane of the mitochondria and is controlled by five protein complexes.

In order to release the energy conserved in the previous metabolic processes, a so-called potential gradient is created between the individual complexes: The electrons released from the reduction equivalents are sent down an electron transport chain together with the hydrogen, in which each subsequent stage has a lower redox potential than the previous one. At the end of the chain, the hydrogen is combined with oxygen, creating water. The special thing about it: A normal, uncontrolled reaction between oxygen and hydrogen (the so-called oxyhydrogen reaction) is strongly exothermic, so a lot of energy is released and the cell would immediately blow up. This energy is gradually released via this electron transport chain and used to generate energy in the form of ATP, so that the reaction at the end of the chain is less violent.

The energy released is used to produce ATP. This happens as follows: While the protein complexes adhere to the inner membrane, some of the hydrogen ions (protons) released from the oxidized (electron donating) reduction equivalents are "stuffed" into the space between the inner and outer membrane, the intermembrane space. This takes place with energy expenditure, since there are more protons in the intermembrane space than outside, so work must be done against the diffusion equilibrium. This energy comes from the electron transport chain. The protons trapped here cannot diffuse back by themselves, but only through a special enzyme, the ATP synthase, which sits as a "gatekeeper" in the inner cell membrane. It lets the protons pass in a controlled manner and uses the released energy to produce ATP. It is practically the cell's ’money printing machine’.

The respiratory chain works as follows:

  • At complex I (enzyme: NADH dehydrogenase), NADH is oxidized to NAD. Ubiquinol is formed. Here, four protons per NADH are channeled into the intermembrane space.
  • At complex II (enzyme: succinate dehydrogenase) electrons are transferred from FADH to ubiquinone. Ubiquinol is formed again. No protons are transferred here.
  • The ubiquinol is oxidized at complex III (enzyme: cytochrome c reductase). To do this, cytochrome C is reduced. Ubiquinone is produced again. Two protons migrate into the intermembrane space.
  • At complex VI (enzyme: cytochrome C oxidase) the cytochrome C is oxidized again and oxygen is reduced to water. Four protons migrate into the intermembrane space. The protons for water synthesis come from the matrix, the interior of the mitochondrion.
  • Complex V (enzyme: ATP synthase) is ultimately responsible for the synthesis of ATP. It is still unclear how many protons are required for an ATP: some authors assume three, others four.

The Overall balance from the respiratory chain reads.

10 NADH + 2 FADH + 32 ADP + 32 P + 6 O2 -> 12 H2O + 10 NAD + 2 FAD + 32 ATP

If you put all three metabolic pathways together, the cell gains 36 ATP per molecule of glucose: 2 from glycolysis, 2 from the citric acid cycle, 32 from the respiratory chain. “Only” CO2 and water remain of the glucose.