Page 981 - Fundamentals of anatomy physiology

968 Unit 5 Environmental Exchange
Several of the steps in a turn of the citric acid cycle involve reactions, but for our purposes the most important are those
more than one reaction and require more than one enzyme.
Water molecules are tied up in two of those steps. We can sum-
marize the entire sequence as follows:
CH3CO − CoA + 3NAD + FAD+ GDP + Pi + 2H2O ¡
CoA + 2CO2 + 3NADH + FADH2 + 2H+ + GTP
The only immediate energy benefit of one turn of the citric
acid cycle is the formation of a single molecule of GTP (guano-
sine triphosphate) from GDP (guanosine diphosphate) and Pi. In
practical terms, GTP is the equivalent of ATP, because GTP read-
ily transfers a phosphate group to ADP, producing ATP:
GTP + ADP ¡ GDP + ATP
The formation of GTP from GDP in the citric acid cycle
is an example of substrate-level phosphorylation. In this
process, an enzyme uses the energy released by a chemical
reaction to transfer a phosphate group to a suitable acceptor
molecule. GTP is formed in the citric acid cycle, but many reac-
tion pathways in the cytosol phosphorylate ADP and form ATP
directly. For example, the ATP produced during glycolysis is
generated through substrate-level phosphorylation. Normally,
however, substrate-level phosphorylation provides a relatively
small amount of energy compared with oxidative phosphoryla-
tion, which we discuss next.
Oxidative Phosphorylation and the ETS
Oxidative phosphorylation is the generation of ATP within
mitochondria in a reaction sequence that requires coenzymes
and consumes oxygen. The process produces more than 90 per-
cent of the ATP used by body cells. The key reactions take place
in the electron transport system (ETS), a series of integral and
peripheral proteins in the inner mitochondrial membrane. The
basis of oxidative phosphorylation is the formation of water, a
very simple reaction:
25 2H2 + O2 ¡ 2H2O
Cells can easily obtain the ingredients for this reaction.
Hydrogen is a component of all organic molecules, and oxy-
gen is an atmospheric gas. The only problem is that the reac-
tion releases a tremendous amount of energy all at once. In
fact, this reaction releases so much energy that it is used to
launch space shuttles into orbit. Cells cannot handle energy
explosions. Instead, energy release must be gradual, as it is in
oxidative phosphorylation. This powerful reaction proceeds in
a series of small, enzymatically controlled steps. Under these
controlled conditions, energy can be captured safely, and ATP
generated.
Oxidation, Reduction, and Energy Transfer. The enzymatic
steps of oxidative phosphorylation involve oxidation and re-
duction. (There are different types of oxidation and reduction
involving the transfer of electrons.) The loss of electrons is a
form of oxidation. The gain of electrons is a form of reduc-
tion. The two reactions are always paired. When electrons pass
from one molecule to another, the electron donor is oxidized
and the electron recipient is reduced. Oxidation and reduction
are important because electrons carry chemical energy. In a
typical oxidation–reduction reaction, the reduced molecule gains
energy at the expense of the oxidized molecule.
&T i p s T r i c k s
To remember what occurs with electrons in oxidation and
reduction reactions, think OIL RIG for oxidation is loss and
reduction is gain.
In such an exchange, the reduced molecule does not gain
all the energy released by the oxidized molecule. Some energy
is always released as heat. The remaining energy may be used
to do physical or chemical work, such as forming ATP. By pass-
ing electrons through a series of oxidation–reduction reactions,
cells can capture and use much of the energy that is released as
water is formed. Water is formed as these electrons ultimately
combine with hydrogen ions and oxygen atoms.
Coenzymes play a key role in this process. A coenzyme acts
as an intermediary that accepts electrons from one molecule
and transfers them to another molecule. In the citric acid cycle,
the coenzymes NAD and FAD remove hydrogen atoms from
organic molecules. Each hydrogen atom consists of an electron
(e2) and a proton (a hydrogen ion, H1). Thus, when a coen-
zyme accepts hydrogen atoms, the coenzyme is reduced and
gains energy. The donor molecule loses electrons and energy as
it gives up its hydrogen atoms.
Note that NADH and FADH2 are the reduced forms of
NAD and FAD. As indicated in Figure 25–4a, they then transfer
their newly acquired hydrogen atoms to other coenzymes. The
protons are subsequently released. The electrons, which carry
the chemical energy, enter a sequence of oxidation–reduction
reactions known as the electron transport system. This sequence
ends with the transfer of electrons to oxygen and the forma-
tion of a water molecule. At several steps along the oxidation–
reduction sequence, enough energy is released to support the
synthesis of ATP from ADP. Now let’s consider that reaction
sequence in greater detail.
The coenzyme FAD accepts two hydrogen atoms from the
citric acid cycle. In doing so, FAD gains 2 electrons, forming
FADH2. The oxidized form of the coenzyme NAD has a positive
charge (NAD1). This coenzyme also gains 2 electrons as 2 hy-
drogen atoms are removed from the donor molecule, resulting
in the formation of NADH and the release of a proton (H1).
For this reason, the reduced form of NAD is often described as
“NADH 1 H1.”