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

80 Chapter 2: Cell Chemistry and Bioenergetics
large glycogen
granules in
the cytoplasm
of a liver cell
branch point
glucose units
(A) (B)
1 µm
chloroplast envelope
vacuole
grana
thylakoid
starch
fat droplet
cell wall
grana
(C)
1 µm
(D)
50 µm
Figure 2–51 The storage of sugars and fats in animal and plant cells. (A) The structures of starch and glycogen, the
storage form of sugars in plants and animals, respectively. Both are storage polymers of the sugar glucose and differ only in the
frequency of branch points. There are many more branches in glycogen than in starch. (B) An electron micrograph of glycogen
granules in the cytoplasm of a liver cell. (C) A thin section of a chloroplast from a plant cell, showing the starch granules and lipid
(fat droplets) that have accumulated as a result of the biosyntheses occurring there. (D) Fat droplets (stained red) beginning to
accumulate in developing fat cells of an animal. (B, courtesy of Robert Fletterick and Daniel S. Friend; C, courtesy of K. Plaskitt;
D, courtesy of Ronald M. Evans and Peter Totonoz.)
for only about a day of normal activities, but enough fat to last for nearly a month.
If our main fuel reservoir had to be carried as glycogen instead of fat, body weight
would increase by an average of about 60 pounds.
The sugar and ATP needed by plant cells are largely produced in separate
organelles: sugars in chloroplasts (the organelles specialized for photosynthesis),
HOCH 2
O
HOCH 2
O
HO
OH
O
OH
O
glycogen polymer
OH
OH
P i
glycogen
phosphorylase
HOCH 2
OH
HO
O
O
P
HO
P
OCH 2
O
MBoC6 m2.75,e13.21/2.51 GLYCOLYSIS
OH
OH
HO
HOCH 2
OH
OH
OH
glucose 1-phosphate glucose 6-phosphate
O
O glycogen polymer
OH
Figure 2–52 How sugars are produced
from glycogen. Glucose subunits are
released from glycogen by the enzyme
glycogen phosphorylase. This produces
glucose 1-phosphate, which is rapidly
converted to glucose 6-phosphate for
glycolysis.