muscles and metabolism.doc

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Muscles and Metabolism
How does the body provide the energy needed for contraction?
-as muscles contract, ATP supplies the energy fro cross bridge movement and detachment
and for operation of the calcium pump in the sarcoplasmic reticulum
-muscles store only 4 to 6 seconds' worth of ATP reserves (usually just enough energy to
begin movement)
-since ATP is the only energy source used directly for contractile activities, it must be
regenerated as fast as it is broken down in order for contraction to continue
-after ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibres, it is
regenerated quickly by 1 or more of these pathways:
1) Direct phosphorylation of ADP to creatine phosphate
-during vigorous activity, the demand for ATP soars and the ATP stored in working
muscles is consumed within a few twitches
-creatine phosphate (CP), a unique high-energy molecule stored in muscles, is used to
regenerate ATP while the metabolic pathways are adjusting to the suddenly higher
demands for ATP
-the result of coupling CP with ADP is an almost instant transfer of energy and a phosphate
group from CP to ADP to form ATP
creatine phosphate + ADP -----creatine kinase-----> creatine + ADP
-muscle cells store 2 to 3 times as much CP as ATP and the CP-ADP reaction, catalyzed by
creatine kinase, is so efficient that the amount of ATP in muscle cells changes very little
during the initial period of contraction
-together, stored ATP and CP provide maximum muscle power for 14-16 seconds
-the coupled reaction is readily reversible and to keep CP readily available, CP reserves are
replenished during periods of rest or inactivity
2) Glycolysis (an anaerobic pathway) which converts glucose to lactic acid
-when stored ATP and CP become exhausted, more ATP is generated by breakdown
(catabolism) of glucose obtained from the blood or of glycogen stored in the muscle
-the initial phase of glucose breakdown is called glycolysis (sugar splitting)
-this pathway occurs in both the presence and absence of oxygen
-it does not use oxygen so it is anaerobic
-during glycolysis, glucose is broken down to two pyruvic acid molecules, releasing
enough energy to form small amounts of ATP (2 ATP per glucose)
-normally, pyruvic acid produced during glycolysis enters the mitochondria and reacts with
oxygen to produce more ATP (aerobic respiration)
-when muscles contract vigorously and contractile activity reaches about 70% of
the maximum possible, the bulging muscles compress the blood vessels within
them, impairing blood flow and oxygen delivery
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-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
-most of the lactic acid diffuses out of the muscles into the bloodstream and is
gone from the muscle tissue within 30 minutes after exercise stops
-the lactic acid is picked up by the liver, heart, or kidney cells which can
use it as an energy source
-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 storage
-the anaerobic pathway harvests only 5% as much ATP from each glucose molecule as the
aerobic pathway, but it produces ATP 2.5x faster
-when large amounts of ATP are needed for moderate periods (30-40 seconds of
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 support strenuous
muscle activity for nearly a minute
-in anaerobic glycolysis, huge amounts of glucose are used to produce relatively small
harvests of ATP and the accumulation of lactic acid is partially responsible for muscle
3) Aerobic Respiration
-during rest and light/moderate exercise (even if prolonged) 95% of ATP used for muscle
activity comes from aerobic respiration
-occurs in the mitochondria, requires oxygen, and involves a sequence of chemical
reactions in which the bonds of fuel molecules are broken and the energy released is used
to make ATP
-aerobic respiration includes glycolysis and the reactions that take place in the
-glucose is broken down entirely, yielding water, carbon dioxide, and large amounts of
-as exercise begins, muscle glycogen provides most of the fuel
-shortly after, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are the
major source of fuels
-after about 30 minutes, fatty acids become the major energy fuels
-aerobic respiration provides a high yield of ATP (32 ATP per glucose) but it is slow
because of its many steps and it requires continuous delivery of oxygen and nutrient fuels
to keep it going
Which pathways predominate during exercise?
-if there is enough oxygen, a muscle cell will form ATP by the aerobic pathway
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-when ATP demands are within the capacity of the aerobic pathway, light to moderate
muscular activity can continue for several hours in well-conditioned individuals
-when exercise demands begin to exceed the ability of the muscle cells to carry out the
necessary reactions quickly enough, glycolysis begins to contribute more and more of the
total ATP generated
aerobic endurance- the length of time a muscle can continue to contract using aerobic
anaerobic threshold- the point at which muscle metabolism converts to anaerobic
-activities that require a surge of power but last only a few seconds (weight lifting, diving,
and sprinting) rely entirely on ATP and CP stores
-on and off burst-like activities (tennis, soccer, 100 meter swimming) appear to be fuelled
almost entirely by anaerobic glycolysis
-prolonged activities such as marathon runs and jogging, where endurance is the goal,
depend mainly on aerobic respiration
-levels of CP and ATP don't change much during prolonged exercise since ATP is
generated at the same rate as it is used
-aerobic generation of ATP is relatively slow but the ATP harvest is enormous
Muscle Fatigue
-physiological inability to contract even though the muscle still may be receiving stimuli
-fatigue is due to a problem in excitation-contraction coupling or in more rare cases,
problems at the neuromuscular junction
-ATP is not a fatigue-producing factor in moderate exercise since ATP will not totally run
-a total lack of ATP results in contractures (state of continuous contraction because cross
bridges are unable to detach (cramps)
-severe ionic imbalances contribute to muscle fatigue
-as action potentials are transmitted, potassium is lost from the muscle cells and the Na+ -
K+ pumps are inadequate to reverse the ionic imbalances quickly, so K+ accumulates in
the fluids of the T tubules
-this ionic change disturbs the membrane potential of the muscle cell and halts
Ca2+ release from the sarcoplasmic reticulum
-in short duration exercise, an accumulation of inorganic phosphate from CP and TP
breakdown may interfere with calcium release from the SR or alternatively with the release
of Pi from myosin and thus hamper myosin's power strokes
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