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functions, three ATP molecules are made. Again, be-cause
it happened twice, a total of six ATP are made in this
aerobic step.
Next, the two PGAs get broken down through a se-ries
of high-energy releasing enzymatic steps to two C 3
molecules of pyruvic acid. So much energy is given off in
these steps that four ADP and four PO 4 get added to form
four ATP molecules. The energy in the PGA molecules is
converted to the four high-energy ATP molecules. In this
step, we make four ATP but it is from these ATP that we
must pay back the two ATP used in the beginning of
glycolysis-. There fore, our net gain is only two ATP.
In summary, the glycolytic breakdown of one mol-
ecule of glucose produces two pyruvic acid molecules. It
took two ATP to start the sequence and four ATP were
produced. However, to pay back the two ATP, our net gain
is only two ATP. However, we also produced two NADH 2 ,
which are part of the electron transport sys-tem. When
oxygen is present, we produce six more ATP via electron
transport. Aerobic glycolysis produces six plus two or eight
ATP molecules. Anaerobic glycolysis -produces only two
ATP.
The Krebs Citric Acid Cycle
In the presence of O 2 , the two pyruvic acid molecules
formed as a result of glycolysis are further broken down in
the second step of biochemical respiration. This step is
named after its discoverer, a German-born British bio-
chemist, Sir Hans Krebs, who first postulated the scheme in
- This is the Krebs citric acid cycle (which takes place
in the mitochondria). We will explain this cycle using- only
one of the two pyruvic acid molecules pro-duced in
glycolysis. When finished, we will multiply all products by
The C 3 pyruvic acid is first converted to acetic acid
(ah-SEE-tic ASS-id) in a transition stage and then to the C 2
acetyl-CoA (ah-SEE-tal) by an enzyme called Co--
enzyme A. This causes the pyruvic acid molecule to lose
a carbon and two oxygens in the form of CO 2 gas as a
waste product. It also loses two hydrogens to NAD,
producing NADH 2 (thus, via electron transport three ATP
molecules are made in this step). The acetyl-CoA now
enters the Krebs citric acid cycle. This occurs on the cristae
of the mitochondria (Figure 4 - 2).
The C 2 acetyl-CoA reacts with a C 4 molecule
-oxaloacetic (ok-sah-low-ah-SEE-tik) acid to form the
C 6 molecule citric acid, hence the name of the cycle. No
ATP is produced in this step but an important event oc-
curs. CoA enzyme is regenerated to react with another
acetic acid to continue the cycle. Another enzyme now
Chapter 4converts the citric acid to the C 5 alpha-ketoglutaric
(AL-fah KEY-toh gluh-TAYR-ik) acid. This causes the
citric acid to lose a carbon and two oxygens as CO 2 gas
(waste product) and two hydrogens to NAD. Thus, NAD
gets reduced via electron transport to NADH 2 and three
ATP are made.
The C 5 alpha-ketoglutaric acid now gets broken down
into the first C 4 molecule succinic (suk-SIN-ik) acid. It
loses a carbon and two oxygens as CO 2 gas (waste product)
and two hydrogens twice to NAD. Thus, via electron
transport six more ATP molecules are made. Succinic acid
changes to another C 4 molecule, malic (MAH-lik) acid.
Finally, the malic acid loses two hydro-gens to flavin
adenine dinucleotide (FLAY-vin ADD-eh-neen dye-
NOO-klee-oh-tide), abbreviated as FAD. This is another
electron carrier of the electron transport system and two
more ATP molecules are made in this step. The malic acid
now is converted to the oxaloacetic acid. Also going from
alpha-ketoglutaric acid to oxaloacetic acid, another ATP
equivalent is made. This molecule is actu-ally guanosine
triphosphate (GTP).
In summary, for every pyruvic acid that enters the
Krebs citric acid cycle, three CO 2 , four NADH 2 , one
FADH 2 , and one ATP (GTP) are produced. Because two
pyruvic acids entered the cycle, we must multiply all of
these products by 2.The Electron Transport (Transfer) System..
Most of the ATP produced during biochemical respi-ration
is produced in the electron transport system (Figure4-3).
Two NADH 2 were produced in glycolysis. Two NADH 2
were produced during the acetyl-CoA for-mation. Then six
NADH 2 and two FADH 2 were produced in the citric acid
cycle. The NAD and FAD all donate the electrons of the
hydrogen atoms that they captured in these reactions to the
enzyme systems on the cristae of the mitochondria. Each of
these electron carriers has a slightly different electron
potential. As the electrons from the cofactor NADH 2 get
transferred from one elec-tron carrier to the next, they
slowly give up their energy. This energy is used in the
energy-requiring synthesis of ATP from ADP and
inorganic phosphate.
The electron transport system functions as a se-ries of
reduction/oxidation reactions. When NAD ac-cepts the two
hydrogens, it gets reduced to NADH 2. When it gives up the
two hydrogens to FAD, NAD gets oxidized while FAD
becomes FADH 2 and gets reduced. This -series of redox
reactions continues until the elec-trons of the -hydrogen
atoms get ultimately donated to -oxygen. -Several kinds of
electron carriers participate