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

HOW CELLS OBTAIN ENERGY FROM FOOD
85
– O
O
P
O
O
P
O
O
P
O
O
guanine
N
C
C NH
HC
C C
N N NH
CH 2
2 O
Figure 2–58 The structure of GTP. GTP
and GDP are close relatives of ATP and
ADP, respectively.
O –
O –
O –
OH
OH
ribose
GDP
GTP
At the end of this series of electron transfers, the electrons are passed to molecules
of oxygen gas (O 2 ) that have diffused into the mitochondrion, which simultaneously
combine with protons (H + ) from the surrounding solution to produce
water. The electrons have now reached a low energy level, and all the available
energy has been extracted from the oxidized food molecule. This process, termed
oxidative phosphorylation (Figure 2–61), also occurs in the plasma membrane
of bacteria. As one of the most remarkable achievements of cell evolution, it is a
central topic of Chapter 14.
In total, the complete oxidation of a molecule of glucose to H 2 O and CO 2 is
used by the cell to produce about 30 molecules of ATP. In contrast, only 2 molecules
of ATP are produced per molecule of glucose by glycolysis alone.
Amino Acids and Nucleotides Are Part of the Nitrogen Cycle
So far we have concentrated mainly on carbohydrate metabolism MBoC6 m2.83/2.58 and have not yet
considered the metabolism of nitrogen or sulfur. These two elements are important
constituents of biological macromolecules. Nitrogen and sulfur atoms pass
GLUCOSE
glucose 6-phosphate
fructose 6-phosphate
nucleotides
amino sugars
glycolipids
glycoproteins
serine
GLYCOLYSIS
3-phosphoglycerate
dihydroxyacetone
phosphate
lipids
amino acids
pyrimidines
alanine
aspartate
other amino acids
purines
pyrimidines
heme
chlorophyll
oxaloacetate
succinyl CoA
phosphoenolpyruvate
pyruvate
CITRIC
ACID
CYCLE
citrate
α-ketoglutarate
cholesterol
fatty acids
glutamate
other amino acids
purines
Figure 2–59 Glycolysis and the citric
acid cycle provide the precursors
needed to synthesize many important
biological molecules. The amino acids,
nucleotides, lipids, sugars, and other
molecules—shown here as products—in
turn serve as the precursors for the many
macromolecules of the cell. Each black
arrow in this diagram denotes a single
enzyme-catalyzed reaction; the red arrows
generally represent pathways with many
steps that are required to produce the
indicated products.