84
Chapter 3
Table 3–10
summarizes the key features of oxidative
phosphorylation.
Reactive Oxygen Species
As we have just seen, the formation of ATP by oxidative phos-
phorylation involves the transfer of electrons and hydrogen
to molecular oxygen. Several highly reactive transient oxygen
derivatives can also be formed during this process—
hydro-
gen peroxide
and the free radicals
superoxide anion
and
hydroxyl radical:
2 H
+
O
2
H
2
O
2
O
2
Superoxide
anion
Hydrogen
peroxide
2 H
+
OH
2 OH
2 H
2
O
OH
+
Hydroxyl
radical
e
e
e
e
Although most of the electrons transferred along the
electron transport chain go into the formation of water, small
amounts can combine with oxygen to form reactive oxygen
species. As described in Chapter 2, these species can react
with and damage proteins, membrane phospholipids, and
nucleic acids. Such damage has been implicated in the aging
process and in infl ammatory reactions to tissue injury. Some
cells use these reactive molecules to kill invading bacteria.
Reactive oxygen molecules are also formed by the action
of ionizing radiation on oxygen and by reactions of oxygen
with heavy metals such as iron. Cells contain several enzy-
matic mechanisms for removing these reactive oxygen species
and thus providing protection from their damaging effects.
Carbohydrate, Fat, and Protein
Metabolism
Now that we have described the three pathways by which
energy is transferred to ATP, let’s consider how each of the
three classes of energy-yielding nutrient molecules—car-
bohydrates, fats, and proteins—enters the ATP-generating
pathways. We will also consider the synthesis of these fuel
molecules and the pathways and restrictions governing their
conversion from one class to another. These anabolic path-
ways are also used to synthesize molecules that have functions
other than the storage and release of energy. For example,
with the addition of a few enzymes, the pathway for fat syn-
thesis is also used for synthesis of the phospholipids found in
membranes.
The material presented in this section should serve as
a foundation for understanding how the body copes with
changes in fuel availability. The physiological mechanisms that
regulate appetite, digestion, and absorption of food, transport
of fuel sources in the blood and across cell membranes, and
the body’s responses to fasting and starvation are covered in
Chapters 15 and 16.
Cytochromes in electron transport chain
NADH + H
+
FADH
2
NAD
+
+ 2H
+
FAD + 2H
+
Matrix
H
2
O
H
+
2
e
2
e
2
e
Inner mitochondrial
membrane
Outer mitochondrial
membrane
1
2
O
2
+2
ADP
P
i
H
+
ATP
ADP
P
i
H
+
ATP
H
+
H
+
H
+
ADP
P
i
H
+
ATP
Figure 3–45
ATP is formed during oxidative phosphorylation by the fl ow of hydrogen ions across the inner mitochondrial membrane. A maximum of two or
three molecules of ATP are produced per pair of electrons donated, depending on the point at which a particular coenzyme enters the electron
transport chain.
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