66 Chapter 4 (^)
(^) Electron transfer system (^)
H 2 O (^)
NADH 2 FADH 2 Quinone H (^2)
2e– 2e– 2e– 2H+ (^)
2H+ 2H+ 2H+ 2e– 2e– (^)
NAD FAD Quinone (^)
Cytochromes (^)
O (^2)
ADP PO 4 ATP 2ADP 2PO 4 2ATP^
One ATP formed from NADH (^2) FAD
®^
Two ATP formed from FADH 2 through cytochromes to oxygen
Lear ni ng^
Cen gage^
(^)
©^
Figure 4- 3 The electron transport or transfer system and ATP production.
FADH 2 to quinone H 2 to the cytochrome system to O 2 (or
½ O 2 5 O), two more units of ATP are formed. You will
notice that the cytochrome system only accepts the two
electrons and then transfers them to oxygen (O). Therefore,
quinone H 2 must directly transfer the two hy-drogen
protons (2H^1 ) to oxygen (O), thus producing the waste
product water (H 2 O).
As we examine the electron transport system, we
observe that when electrons are donated to NAD, three
ATP units are formed during the entire electron transfer.
However, when the electrons are donated directly to FAD
and NAD is bypassed, only two ATP units are formed
during the electron transfer.
Summary of ATP Production
during Glycolysis, the Citric Acid
Cycle, and Electron Transport
The net products from glycolysis are two ATP units and
two NADH 2 per glucose molecule. Because each NADH 2
molecule produces three ATP during electron transport, a
total of eight ATP units result in glycolysis, which in-
cludes electron transport.
In the Krebs citric acid cycle and transition stage, four
NADH 2 , one FADH 2 , and one ATP (or GTP) are formed
during the breakdown of each pyruvic acid. However,
because each glucose molecule produces two pyruvic acid
molecules, we actually form eight NADH 2 , two FADH 2 ,
and two ATP (or GTP) units. The number of
ATP units formed during the citric acid cycle and elec-tron
transport then is 24 1 4 1 2 1 or 30 ATP or 24 1 4 5 28 ATP
and 2 GTP.
In total, 30 ATP from the citric acid cycle and elec-
tron transport plus 8 ATP from glycolysis and electron
transport produced a net gain of 38 ATP units per glu-cose
molecule or 36 ATP and 2 GTP. This represents a cellular
capture of about 60% of the energy available from the
breakdown of a single glucose molecule. This is very high
efficiency compared to that of any man-made machine.
It is important to remember that cellular or bio-
chemical respiration is a continuous process. Although we
tend to discuss it in three “steps,” these steps are not
separate events. We have seen that electron transport is part
of glycolysis when oxygen is available and that elec-tron
transport accounts for most of the ATP production in the
Krebs citric acid cycle.
Anaerobic Respiration
There are two situations when glucose is broken down in
the absence of oxygen. One is when yeast cells (a type of
fungus) feed on glucose, and this pro-cess is called
fermentation-. The other situation occurs in our muscle
cells when we overexercise and experi-ence muscle fatigue
and cannot get enough oxygen to the muscle cells. Then the
muscle cells begin to break down glucose in the absence- of
oxygen, a much less