Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

82 Chapter 2: Cell Chemistry and Bioenergetics
stored fat
FAT CELL
hydrolysis
glycerol
fatty acids
bloodstream
MUSCLE CELL
Figure 2–54 How stored fats are
mobilized for energy production in
animals. Low glucose levels in the blood
trigger the hydrolysis of the triacylglycerol
molecules in fat droplets to free fatty
acids and glycerol. These fatty acids enter
the bloodstream, where they bind to the
abundant blood protein, serum albumin.
Special fatty acid transporters in the
plasma membrane of cells that oxidize fatty
acids, such as muscle cells, then pass
these fatty acids into the cytosol, from
which they are moved into mitochondria for
energy production.
fatty acids
oxidation in
mitochondria
CO 2
ATP
rapidly decarboxylated by a giant complex of three enzymes, called the pyruvate
dehydrogenase complex. The products of pyruvate decarboxylation are a molecule
of CO 2 (a waste product), a molecule of NADH, and acetyl CoA (see Panel 2–9).
The fatty acids imported from the bloodstream are moved into mitochondria,
where all of their oxidation takes place (Figure 2–55). Each molecule of fatty acid
(as the activated molecule fatty acyl CoA) is broken down completely by a cycle of
reactions that trims two carbons MBoC6 m2.78/2.54 at a time from its carboxyl end, generating one
molecule of acetyl CoA for each turn of the cycle. A molecule of NADH and a molecule
of FADH 2 are also produced in this process (Figure 2–56).
Sugars and fats are the major energy sources for most nonphotosynthetic
organisms, including humans. However, most of the useful energy that can be
extracted from the oxidation of both types of foodstuffs remains stored in the acetyl
CoA molecules that are produced by the two types of reactions just described.
The citric acid cycle of reactions, in which the acetyl group (–COCH 3 ) in acetyl
CoA is oxidized to CO 2 and H 2 O, is therefore central to the energy metabolism
of aerobic organisms. In eukaryotes, these reactions all take place in mitochondria.
We should therefore not be surprised to discover that the mitochondrion is
the place where most of the ATP is produced in animal cells. In contrast, aerobic
bacteria carry out all of their reactions, including the citric acid cycle, in a single
compartment, the cytosol.
The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups
to CO 2
In the nineteenth century, biologists noticed that in the absence of air cells produce
lactic acid (for example, in muscle) or ethanol (for example, in yeast), while
in its presence they consume O 2 and produce CO 2 and H 2 O. Efforts to define the
pathways of aerobic metabolism eventually focused on the oxidation of pyruvate
and led in 1937 to the discovery of the citric acid cycle, also known as the
Sugars and
polysaccharides
plasma membrane
sugars glucose pyruvate pyruvate
Fats fatty acids fatty acids fatty acids
acetyl CoA
MITOCHONDRION
CYTOSOL
Figure 2–55 Pathways for
the production of acetyl CoA
from sugars and fats. The
mitochondrion in eukaryotic cells
is where acetyl CoA is produced
from both types of major food
molecules. It is therefore the
place where most of the cell’s
oxidation reactions occur and
where most of its ATP is made.
Amino acids (not shown) can
also enter the mitochondria, to
be converted there into acetyl
CoA or another intermediate of
the citric acid cycle. The structure
and function of mitochondria are
discussed in detail in Chapter 14.