4. Kreb’s cycle or citric acid cycle.
The processes of breakdown of pyruvic acid into CO2 and H2O in
presence of oxygen with the releases of assimilatory products
NADH2 and FADH2 is called Kreb’s cycle.
It takes place in the matrix of mitochondria as follows,
• The pyruvic acid formed during glycolysis enters the matrix of
mitochondria. During this processes pyruvic acid converts into
acetyl co-A by dehydrogenation and decarboxylation.
– Here NAD is reduced to NADH2 and a molecule of CO2 is released.
• The acetyl co-A on condensation with oxalo acetic acid forms
citric acid.
• The citric acid on dehydration converts into cis aconitic acid,
which on hydration forms isocitric acid.
• The isocitric acid on dehydrogenation and decaroxylation
converts into
–ketoglutaric acid. This reaction yields a
molecule of NADH2 and CO2.
5. • The α–ketoglutaric acid on dehydrogenation and
decaroxylation converts into succinic acid. This reaction
yields a molecule of NADH2, GTP and CO2.
• The succinic acid on dehydrogenation converts into
fumaric acid. Here the FAD [flavin adenine dinucleotide] is
reduced to FADH2.
• The fumaric acid on hydration converts into malic acid.
• The malic acid on dehydrogenation converts into oxalo
acetic acid. During which NAD is reduced to NADH2.
• The oxalo acetic acid again enters the kreb’s cycle.
• Thus a pyruvic acid in Kreb’s cycle yields 4 NADH2, 1
FADH2, 1 GTP and 3 CO2 molecules.
• Hence one molecule of glucose yields 8 NADH2, 2 FADH2,
2 GTP and 6 CO2 molecules.
