Chapter 9
Muscles and Muscle Tissue
299
9
of ATP are needed for moderate periods (30–40 seconds) of strenu-
ous muscle activity, glycolysis can provide most of the ATP needed
as long as the required fuels and enzymes are available. Together,
stored ATP and CP and the glycolysis–lactic acid pathway can sup-
port strenuous muscle activity for nearly a minute.
Although anaerobic glycolysis readily fuels spurts of vigor-
ous exercise, it has shortcomings. Huge amounts of glucose are
used to produce relatively small harvests of ATP, and the accu-
mulating lactic acid is partially responsible for muscle soreness
during intense exercise.
Aerobic Respiration (Figure 9.19c)
Because the amount of
creatine phosphate is limited, muscles must metabolize nutri-
ents to transfer energy from foodstuffs to ATP. During rest and
light to moderate exercise, even if prolonged, 95% of the ATP
used for muscle activity comes from aerobic respiration.
Aero-
bic respiration
occurs in the mitochondria, requires oxygen,
and involves a sequence of chemical reactions that break the
bonds of fuel molecules and release energy to make ATP.
Aerobic respiration, which includes glycolysis and the reac-
tions that take place in the mitochondria, breaks down glucose
entirely. Water, carbon dioxide, and large amounts of ATP are
its final products.
Glucose
1
oxygen
carbon dioxide
1
water
1
ATP
±e carbon dioxide released diffuses out of the muscle tissue
into the blood, to be removed from the body by the lungs.
(a) Direct phosphorylation
Coupled reaction of creatine phosphate
(CP) and ADP
Energy source:
CP
Energy source:
glucose
Energy source:
glucose; pyruvic acid; free
fatty acids from adipose tissue; amino
acids from protein catabolism
(b) Anaerobic pathway
Glycolysis and lactic acid formation
(c) Aerobic pathway
Aerobic cellular respiration
Oxygen use:
None
Products:
1 ATP per CP, creatine
Duration of energy provided:
15 seconds
Oxygen use:
None
Products:
2 ATP per glucose, lactic acid
Duration of energy provided:
30-40 seconds,
or slightly more
Oxygen use:
Required
Products:
32 ATP per glucose, CO
2
, H
2
O
Duration of energy provided:
Hours
Creatine
kinase
ADP
CP
Creatine
Glucose (from
glycogen breakdown or
delivered from blood)
Glucose (from
glycogen breakdown or
delivered from blood)
Glycolysis
in cytosol
Pyruvic acid
Released
to blood
net gain
2
32
Lactic acid
O
2
O
2
O
2
O
2
H
2
O
CO
2
Pyruvic acid
Fatty
acids
Amino
acids
Aerobic respiration
in mitochondria
ATP
ATP
ATP
net gain per
glucose
Figure 9.19
Pathways for regenerating ATP during muscle activity.
The fastest pathway
is direct phosphorylation
(a)
, and the slowest is aerobic respiration
(c)
.
“sugar splitting”). ±is pathway occurs in both the presence and
the absence of oxygen, but because it does not use oxygen, it is
an anaerobic (an-a
9
er-ōb-ik; “without oxygen”) pathway. Dur-
ing glycolysis, glucose is broken down to two
pyruvic acid
mol-
ecules, releasing enough energy to form small amounts of ATP
(2 ATP per glucose).
Ordinarily, pyruvic acid produced during glycolysis then en-
ters the mitochondria and reacts with oxygen to produce still
more ATP in the oxygen-using pathway called aerobic respira-
tion, described shortly. But when muscles contract vigorously
and contractile activity reaches about 70% of the maximum
possible (for example, when you run 600 meters with maximal
effort), the bulging muscles compress the blood vessels within
them, impairing blood flow and oxygen delivery. Under these
anaerobic conditions, most of the pyruvic acid produced during
glycolysis is converted into
lactic acid
, and the overall process is
referred to as
anaerobic glycolysis
. ±us, during oxygen deficit,
lactic acid is the end product of cellular metabolism of glucose.
Most of the lactic acid diffuses out of the muscles into the
bloodstream. Subsequently, the liver, heart, or kidney cells pick
up the lactic acid and use it as an energy source. Additionally,
liver cells can reconvert it to pyruvic acid or glucose and release
it back into the bloodstream for muscle use, or convert it to
glycogen for storage.
±e anaerobic pathway harvests only about 5% as much ATP
from each glucose molecule as the aerobic pathway, but it produces
ATP about 2½ times faster. For this reason, when large amounts
previous page 333 Human Anatomy and Physiology (9th ed ) 2012 read online next page 335 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off