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PART 2 Support and Movement
Re s t
Aerobicrespiration
1
ATP
2
Aerobicrespiration
Exerci s e
Lactate Anaerobicrespiration
ADP + P
Creatinephosphate
3
Energy
4
ADP + P
5
6
ATP
Energy
Creatine
+ P
Creatine
+ P
Creatinephosphate
7
Activemusclecontraction
(b)
4 As exercise begins, ATP already in the cell is used first, butduring moderate exercise, aerobic respiration provides mostof the ATP necessary for muscle contraction.
5 During times of extreme exercise, anaerobic respirationprovides small amounts of ATP that can sustain musclecontraction for brief periods.
6 Energy stored in creatine phosphate can also be used toproduce ATP.
7 Throughout the time of exercise, ATP from all of these sources(4–6) provides energy for active muscle contraction.
Maintainsmuscletone andposture(a)
1 At rest, ATP is produced by aerobic respiration.
2 Small amounts of ATP are used in musclecontractions that maintain muscle tone andposture.
3 Excess ATP is used to produce creatinephosphate, an energy-storage molecule.
PROCESS FIGURE 9.23 ATP Production in Resting and Exercising Muscle
present in the cell provide enough energy to sustain maximumcontractions for about 8–10 seconds. Figure 9.23 summarizeshow aerobic and anaerobic respiration, lactate fermentation,and creatine phosphate production interact to produce a con-tinuous supply of ATP in resting and exercising muscle.When a muscle cell is working too strenuously for ATP storesand creatine phosphate to be able to provide enough ATP, anaero-bic respiration predominates. Typically, the type II fibers are theprimary anaerobic fibers. The type II fibers break down glucoseinto the intermediate lactate, which can be shuttled to adjacenttype I fibers to make ATP or be secreted into the blood for uptakeby other tissues, such as the liver, to make new glucose. Thus, inskeletal muscle, the type II fiber (anaerobic) pathways and thetype I fiber (aerobic) pathways are not mutually exclusive. Rather,they work together, with lactate being the product of the type IIfiber pathways, which then serves as the starting point of the typeI fiber pathways.
Na + /K + pump for the resting membrane potential maintenance,all of which are required for proper muscle functioning. If theATP:ADP ratio declines, an enzyme transfers one phosphate fromone ADP to another ADP, generating one ATP and one AMP(adenosine monophosphate). The presence of AMP triggers aswitch from anaerobic respiration to aerobic respiration of bloodglucose and fatty acids. If this switch did not occur, the musclescould not maintain their activity and could ultimately fail (see“Fatigue” in the next section). Table 9.4 provides a summary ofcreatine phosphate, anaerobic respiration, and aerobic respirationas sources of ATP in skeletal muscle.
Predict 8
A condition called McArdle disease is due to a deficiency of an enzymenecessary for the breakdown of the stored form of glucose, calledglycogen. Predict how the disease affects a person’s ability to exercise.
Recycling of ADP
Ultimately, if the use of ATP is greater than the production ofATP, the ATP:ADP ratio decreases, which interferes with thefunctioning of all the major ATP-dependent enzymes in themuscle fibers. The ATP-dependent enzymes include the myosin + head, the sarcoplasmic reticulum Ca 2 re-uptake pump, and the
Muscle Fatigue
Fatigue (f ă -t ē g ′ ) is a temporary state of reduced work capacity.Without fatigue, muscle fibers would be worked to the pointof structural damage to them and their supportive tissues. His-torically, it was thought that build-up of lactate and the corre-sponding drop in pH (acidosis) was the major cause of fatigue.