BOTANY Higher Secondary Second Year - Textbooks Online

9. The malic acid is oxidized to oxaloacetic acid by the enzyme malic
dehydrogenase. Here, NAD + is reduced to NADH 2 .
Significance of Krebs cycle
2 molecules of acetyl CoA enter into Krebs cycle which on subsequent
oxidation generate 6NADH , 2FADH When 6NADH , 2FADH enter
2 2.. 2 2
into the electron transport system generate 22ATP molecules. In one step,
there is substrate level phosphorylation whch directly yield 2ATP molecules.
So, during Krebs cycle, every 2 molecules of acetyl CoA enter into Krebs
cycle 24 ATP molecules are generated. So, primarily it is a energy
producing system. Since, Krebs cycle involves with both anabolic and
catabolic processes, it is also described as amphibolic process.
Electron transport chain
Electron transport system (ETS) is a chain of electron carriers consisting
of NAD + , FAD + , CoQ and cytochromes (cyt. b, cyt. c, cyt. a and cyt.
a ). The glucose molecule is completely oxidized by the end of the citric
3
acid cycle. But, energy is not released, unless NADH and FADH are
2 2
oxidized through electron transport system. Transfer of electrons and
protons from NADH and FADH to oxygen through a series of components
2 2
like flavoprotein, cytochrome is called electron transport chain. This
process leads to coupling of electrons to form high-energy phosphate
bonds in the form of ATP from ADP is called oxidative phosphorylation.
The electron transport components are arranged in the inner membrane of
mitochondria.
According to modern concept, the electron carriers in the electron
transport system are arranged in four complexes – complex I, complex II,
complex III and complex IV. When NAD + is a primary acceptor of
electrons, the electrons are transported from complex I to II, II to III and
then to complex IV. When electrons are transported from one complex to
next complex, an ATP is produced. Thus, one molecule of NADH 2
generates three ATPs. When FAD + is a primary acceptor of electrons,
the electrons are transported from complex II to III and then to complex
IV. Thus, one molecule of FADH 2 generates two ATPs.
The molecular oxygen forms the terminal constituent of the electron
transport system. It is the ultimate recipient of electrons and picks up the
protons from the substrate to form water.
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