P a t h w a y s f o r m a k i n g g l u c o s e •
F ig u r e 1 4 . 1 3
The liver and skeletal muscles store about 500 g (1.1 lb) of glycogen,
which is made from glucose following stimulation by insulin. When
stimulated by glucagon and epinephrine, liver and skeletal muscle cells
can break glycogen into glucose (glycogenolysis) for release into the
blood and subsequent use by other cells. When these same cells are
stimulated by glucagon and cortisol, glucose can be made from other
molecules in a process called gluconeogenesis.
Key:
Synthesis of glycogen
Breakdown of glycogen
Gluconeogenesis
Catabolism of triglycerides
Triglycerides
Glucose is the primary source of energy for cells. As
a result, glucose must always be supplied. This glucose
is mostly acquired from the food we eat. If glucose is not
being absorbed from food being processed in the diges-
tive system, however, it must be created from other ma-
terials. Glucose can be made by breaking down glycogen
(
g ly c o g e n o ly sis
) stored in the liver and skeletal muscle
(
F igu re 1 4 .1 3
). Glucose can also
be made from pyruvic acid, glyc-
eraldehyde-3-phosphate
(G3P),
certain amino acids, lactic acid,
and
glycerol
(from
triglyceride
breakdown)
through
a
process
called
g lu c o n e o g e n e s is
. The liver
is capable of converting these mol-
ecules into glucose.
If glucose availability and glycogen stores become de-
pleted, your body breaks down fats to make glucose. Once
fat reserves have been exhausted, your body resorts to
breaking down proteins to meet its energy demands. Let’s
take a closer look at how fat and proteins can be broken
down to make ATP.
Lipids Are Broken Down into
Intermediates of Cellular Respiration
Like carbohydrates, lipids can be broken down to make
ATP, using some of the same pathways that are used for
glucose oxidation. Lipid catabolism involves a type of lipid
called a triglyceride and occurs in muscle, liver, and fat
cells. First, triglycerides are split into glycerol and three
fatty acids (
F igu re 1 4 .1 4
). The glycerol gets converted
to G3P, an intermediate in glycolysis. Depending on the
needs of the cell, G3P can be (1) made into glucose (via
gluconeogenesis) or (2) converted to carbon dioxide, wa-
ter, and ATP (via cellular respiration). The fatty acids are
converted to acetyl CoA (used in cellular respiration) or
k e to n e s
(or
ketone bodies)
for use as an energy source in
other cells. Ketones are transported from liver cells to oth-
er cells, where they can be easily changed back to acetyl
CoA and metabolized through cellular respiration.
If the body has no immediate need to use lipids to
make ATP, the triglycerides are stored for future use. Tri-
glycerides can also be created from
excess glucose, using these same
biochemical
pathways.
Whether
lipids are synthesized via
lip ogen -
e sis
or broken down via
lipolysis
depends on the cell’s energy needs
and stimulation by various hor-
mones.
Insulin
stimulates
lipid
formation,
while
epinephrine,
norepinephrine, and cortisol stimulate lipid breakdown.
Because most lipids do not dissolve in water, they must
be shuttled among various cells and tissues, wrapped in
water-soluble packages called
lip o p r o te in s
.
g lu c o n e o g e n e sis
(gloo'-ko-ne-o-JEN-e-
sis) The synthesis of
glucose from certain
amino acids, glycerol,
pyruvic acid, or lactic
acid; essentially, the
reverse of glycolysis.
lip o g e n e sis
(li-po-
GEN-e-sis) The synthe-
sis of triglycerides.
lipolysis
(lip-OL-i-sis)
The splitting of fatty
acids from a triglycer-
ide or phospholipids.
420 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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