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
79
O –
O
enol phosphate
bond
anhydride
bond to carbon
phosphate
bond in
creatine
phosphate
– O
C O
H 2 C C O P O –
H CH 3 H O –
H 2 O
O –
O O
C C O P O –
H 2 O
O –
O H
+ NH 2 O
C C N C N P O –
H 2 O
phosphoenolpyruvate
(see Panel 2–8, pp. 104–105)
for example,
1,3-bisphosphoglycerate
(see Panel 2–8)
creatine phosphate
(activated carrier that
stores energy in muscle)
–61.9 kJ
–49.0 kJ
–43.0 kJ
ΔG o FOR HYDROLYSIS
–60
–40
anhydride
bond to
phosphate
(phosphoanhydride
bond)
C
O
O
P
O –
O
O
P O
O –
H 2 O
O
P O –
O –
for example,
ATP when hydrolyzed
to ADP
–30.6 kJ
–20
phosphoester
bond
C
H
C
H
O
O P O –
O –
for example,
glucose 6-phosphate
(see Panel 2–8)
–17.5 kJ
H 2 O
type of phosphate bond
specific examples showing the
standard free-energy change (ΔG ˚)
for hydrolysis of phosphate bond
0
Figure 2–50 Phosphate bonds have different energies. Examples of different types of phosphate bonds with
their sites of hydrolysis are shown in the molecules depicted on the left. Those starting with a gray carbon atom
show only part of a molecule. Examples of molecules containing such bonds are given on the right, with the
standard free-energy change for hydrolysis in kilojoules. The transfer of a phosphate group from one molecule
to another is energetically favorable if the free-energy change (ΔG) for hydrolysis of the phosphate bond of
the first molecule is more negative than that for hydrolysis of the phosphate bond in the second. Thus, under
standard conditions, a phosphate group is readily transferred from 1,3-bisphosphoglycerate to ADP to form
ATP. (Standard conditions MBoC6 often do m2.74/2.50 not pertain to living cells, where the relative concentrations of reactants and
products will influence the actual change in free energy.) The hydrolysis reaction can be viewed as the transfer
of the phosphate group to water.
subunits in the large branched polysaccharide glycogen, which is present as
small granules in the cytoplasm of many cells, including liver and muscle. The
synthesis and degradation of glycogen are rapidly regulated according to need.
When cells need more ATP than they can generate from the food molecules taken
in from the bloodstream, they break down glycogen in a reaction that produces
glucose 1-phosphate, which is rapidly converted to glucose 6-phosphate for glycolysis
(Figure 2–52).
Quantitatively, fat is far more important than glycogen as an energy store for
animals, presumably because it provides for more efficient storage. The oxidation
of a gram of fat releases about twice as much energy as the oxidation of a gram
of glycogen. Moreover, glycogen differs from fat in binding a great deal of water,
producing a sixfold difference in the actual mass of glycogen required to store the
same amount of energy as fat. An average adult human stores enough glycogen