294
UNIT TWO
usually not sufF cient to support maximal muscle activity for
more than about ten seconds during an intense contraction.
As a result, the muscle F bers in an active muscle soon use
cellular respiration of glucose to synthesize ATP. Typically, a
muscle stores glucose in the form of glycogen.
Oxygen Supply and Cellular Respiration
Recall from chapter 4 (p. 120) that glycolysis, the early phase
of cellular respiration, occurs in the cytoplasm and is
anaero-
bic,
not requiring oxygen. This phase only partially breaks
down energy-supplying glucose and releases only a few
ATP molecules. The complete breakdown of glucose occurs
in the mitochondria and is
aerobic,
requiring oxygen. This
process, which includes the complex series of reactions of
the
citric acid cycle and electron transport chain,
produces
many ATP molecules.
Blood carries the oxygen necessary to support the aero-
bic reactions of cellular respiration from the lungs to body
cells. Oxygen is transported in red blood cells, where it is
loosely bound to molecules of hemoglobin, the pigment
responsible for the red color of blood. In regions of the body
where the oxygen concentration is low, oxygen is released
from hemoglobin and becomes available for the aerobic reac-
tions of cellular respiration.
Another pigment,
myoglobin,
is synthesized in mus-
cle cells and imparts the reddish brown color of skeletal
muscle tissue. Like hemoglobin, myoglobin can loosely
bind oxygen and, in fact, has a greater attraction for oxy-
gen than does hemoglobin. Myoglobin can temporarily
store oxygen in muscle tissue, which reduces a muscle’s
requirement for a continuous blood supply during contrac-
tion. This oxygen storage is important because blood fl
ow
Energy Sources for Contraction
The energy used to power the interaction between actin and
myosin F laments during muscle F
ber contraction comes from
ATP molecules. However, a muscle F ber has only enough
ATP to contract briefl y. Therefore, an active F ber requires
regeneration of ATP.
The initial source of energy available to regenerate ATP
from ADP and phosphate is
creatine phosphate.
Like ATP,
creatine phosphate includes a high-energy phosphate bond,
and this molecule is four to six times more abundant in
muscle F bers than ATP. Creatine phosphate, however, can-
not directly supply energy to a cell. Instead, it stores energy
released from mitochondria. Whenever sufficient ATP is
present, an enzyme in the mitochondria (creatine phospho-
kinase) promotes the synthesis of creatine phosphate, which
stores excess energy in its phosphate bond
(f
g. 9.12)
.
As ATP is decomposed to ADP, the energy from creatine
phosphate molecules is transferred to these ADP molecules,
quickly phosphorylating them back into ATP. The amount of
ATP and creatine phosphate in a skeletal muscle, however, is
TABLE
9.1
|
Major Events of Muscle Contraction and Relaxation
Muscle Fiber Contraction
Muscle Fiber Relaxation
1. A nerve impulse travels down a motor neuron axon.
1. Acetylcholinesterase decomposes acetylcholine, and the
muscle f
ber membrane is no longer stimulated.
2. The motor neuron terminal releases the neurotransmitter acetylcholine (ACh).
2. Calcium ions are actively transported into the sarcoplasmic
reticulum.
3. ACh binds to ACh receptors on the muscle f
ber.
3. ATP breaks linkages between actin and myosin f
laments
without breakdown oF the ATP itselF.
4. The sarcolemma is stimulated, and a muscle impulse travels over the surFace oF
the muscle f
ber and deep into the f
ber through the transverse tubules.
4. Breakdown oF ATP “cocks” the cross-bridges.
5. The muscle impulse reaches the sarcoplasmic reticulum, and calcium channels
open.
5. Troponin and tropomyosin molecules inhibit the
interaction between myosin and actin f
laments.
6. Calcium ions di±
use From the sarcoplasmic reticulum into the sarcoplasm and
bind to troponin molecules.
6. Muscle f
ber remains relaxed, yet ready until stimulated
again.
7. Tropomyosin molecules move and expose specif
c sites on actin.
8. Actin and myosin Form linkages.
9. Thin (actin) f
laments are pulled toward the center oF the sarcomere by myosin
cross-bridges increasing the overlap oF the thin and thick f
laments.
10. The muscle f
ber contracts.
ADP
ATP
ATP
P
When cellular
is high
Creatine
Creatine
ADP
ATP
ATP
P
When cellular
is low
Creatine
Creatine
FIGURE 9.12
Creatine phosphate may be used to replenish ATP
stores when ATP levels in a muscle cell are low.
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