cycle, also known as the citric acid cycle and the tricarboxylic acid (TCA) cycle,
is named for Sir Hans Krebs, a biochemist, who in the 1940s explained how
these reactions work. In the Krebs cycle, acetyl-CoA is split into carbon dioxide
and hydrogen atoms.
The carbon dioxide diffuses out of the mitochondria in eukaryotic cells and
eventually out of the cell itself. This series of reactions is called a cycle because
as one acetyl group enters the Krebs cycle and is metabolized, oxaloacetate com-
bines with another acetyl group to form citric acid and coenzyme A, which go
through the cycle again. As each acetyl group goes through the cycle, two mole-
cules of CO 2 are formed from the oxidation of its two carbon atoms.
Three pairs of electrons are transferred to NAD and one pair to FAD. These
coenzymes are important because they carry large amounts of energy. For every
molecule of acetyl-CoA that enters the Krebs Cycle, a molecule of ATP is pro-
duced. The Krebs cycle also provides substances for bacteria and other prokary-
otic cellular activities.
THE NEW CHAIN GANG:
THE ELECTRON TRANSPORT CHAIN
Glycolysis, the bridging reaction, and the Krebs cycle result in the synthesis of
only four ATP molecules when one glucose is oxidized to six CO 2 molecules.
Most of the ATP that is generated comes from the oxidation of NADH and
FADH 2 in the electron transport chain.
The electron transport chain, which occurs in the mitochondria in eukaryotic
cells and in the cytoplasm of prokaryotic cells, is composed of a series of elec-
tron carriers that transfer electrons from donor molecules, such as NADH and
FADH 2 to an acceptor atom like O 2. The electrons move down an energy gradi-
ent, like water flowing down a series of waterfalls in rapids.
The difference in free energy that occurs between O 2 and NADH releases
large amounts of energy. The energy changes that occur at several points in the
chain are very large and can provide the eventual production of large amounts
of ATP. The free energy that electrons have entering the electron transport chain
is greater in the beginning than at the end. It is this energy that enables the pro-
tons (H+) to be pumped out of the mitochondrial matrix.
When the electrons move through the chain they transfer this energy to the
pumps within the plasma membrane. The electron transport chain will separate
the energy that is released into smaller sections, or steps. The reactions of the
electron chain take place in the inner membrane of the mitochondria in eukary-
CHAPTER 5 The Chemical Metabolism^95