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However, it is available immediately, and it enables fast-twitch fibers to generate a lot of force quickly. The immediate system is exhausted in only a few seconds.

The glycolytic system is able to come on line within a few seconds to replace the ATP depleted at the onset of muscle activity. The glycolytic enzymes are located in the cytoplasm of the muscle fiber, and therefore the ATP they generate is rapidly available to the myosin filaments. However, as we saw in Chapter 7, glycolysis alone is an inefficient way to produce ATP, and it rapidly leads to the accumulation of lactic acid, which slows the process. Thus, the glycolytic system and the immediate system together can provide most of the energy for active muscles for less than a minute (see Figure 47.10).

Oxidative metabolism can come on line fully in about a minute, producing relatively huge amounts of ATP due to its ability to completely metabolize carbohydrates and fats. However, it requires many reactions (see Chapter 7), and it takes place in the mitochondria, so the ATP has to diffuse to the myosin filaments in the muscle. Therefore, the rate at which oxidative metabolism can make ATP available to do work is slower than the rate at which the other two systems can supply ATP.

Constriction of circular muscles elongates the segments, pushing them forward.

Constriction of circular muscles elongates the segments, pushing them forward.

Bristles anchor the segments to prevent backward sliding.

Bristles anchor the segments to prevent backward sliding.

Constriction of longitudinal muscles shortens the segments, pulling the trailing segments forward.

Constriction of longitudinal muscles shortens the segments, pulling the trailing segments forward.

47.11 A Hydrostatic Skeleton Alternating waves of muscle contraction move the earthworm through the soil.The red dot enables you to follow the changes in one segment as the worm moves to the right.

47.11 A Hydrostatic Skeleton Alternating waves of muscle contraction move the earthworm through the soil.The red dot enables you to follow the changes in one segment as the worm moves to the right.

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