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Cross-bridges pull actin filament

Figure

According to the sliding filament theory, (a) ATP breakdown provides energy to "cock" the unattached myosin cross-bridge. When calcium ion concentration is low, the muscle remains relaxed. (b) When calcium ion concentration rises, binding sites on actin filaments open and cross-bridges attach. (c) Upon binding to actin, cross-bridges spring from the cocked position and pull on actin filaments. (d ) ATP binds to the cross-bridge (but is not yet broken down), causing it to release from the actin filament. As long as ATP and calcium ions are present, the cycle continues.

Energy from cellular respiration

Energy for muscle contraction

When cellular ATP is low

Energy from cellular respiration

Energy for muscle contraction

When cellular ATP is low

When cellular ATP is high

A muscle cell uses energy released in cellular respiration to synthesize ATP. ATP is then used to power muscle contraction or to synthesize creatine phosphate. Later, creatine phosphate may be used to synthesize ATP.

When cellular ATP is high

Figure

A muscle cell uses energy released in cellular respiration to synthesize ATP. ATP is then used to power muscle contraction or to synthesize creatine phosphate. Later, creatine phosphate may be used to synthesize ATP.

glucose as a source of energy for synthesizing ATP. Typically, a muscle stores glucose in the form of glycogen.

Oxygen Supply and Cellular Respiration

Recall from chapter 4 (page 116) that glycolysis, the early phase of cellular respiration, occurs in the cytoplasm and is anaerobic, not dependent on 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, produces many ATP molecules.

Blood carries the oxygen necessary to support aerobic respiration from the lungs to body cells. Oxygen is transported within the red blood cells 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 relatively low, oxygen is released from hemoglobin and becomes available for aerobic respiration.

Another pigment, myoglobin, is synthesized in muscle cells and imparts the reddish-brown color of skeletal muscle tissue. Like hemoglobin, myoglobin can loosely combine with oxygen, and in fact has a greater attraction for oxygen than does hemoglobin. Myoglobin can temporarily store oxygen in muscle tissue, which reduces a muscle's requirement for a continuous blood supply during contraction. This oxygen storage is important because blood flow may decrease during muscular contraction when contracting muscle fibers compress blood vessels (fig. 9.14).

Oxygen Debt

When a person is resting or moderately active, the respiratory and circulatory systems can usually supply sufficient oxygen to the skeletal muscles to support aerobic respiration. However, when skeletal muscles are used more strenuously, these systems may not be able to supply enough oxygen to sustain aerobic respiration. The muscle fibers must increasingly utilize the anaerobic phase of respiration for energy. This can lead to a rapid increase in blood levels of lactic acid, termed the lactic acid threshold (anaerobic threshold).

Chapter 4 (page 117) discussed how under anaerobic conditions, glycolysis breaks glucose down to pyruvic acid and converts it to lactic acid, which diffuses out of the muscle fibers and is carried in the bloodstream to the liver. Liver cells can react lactic acid to form glucose, but this requires energy from ATP (fig. 9.15). During strenuous exercise, available oxygen is primarily used to synthesize ATP for muscle contraction rather than to make ATP for converting lactic acid into glucose. Consequently, as lactic acid accumulates, a person develops an oxygen debt that must be repaid at a later time. The amount of oxygen debt roughly equals the amount of oxygen liver cells require to convert the accumulated lactic acid into glucose, plus the amount the muscle cells require to resynthesize ATP and creatine phosphate, and restore their original concentrations. It also reflects the oxygen needed to restore blood and tissue oxygen levels to preexercise levels.

The runners are on the starting line, their muscles primed for a sprint. Glycogen will be broken down to release glucose, and creatine phosphate will supply high-energy phosphate groups to replenish ATP stores by phosphorylating ADP. The starting gun fires. Energy comes first from residual ATP, but almost instantaneously, creatine phosphate begins donating high energy phosphates to ADP, regenerating ATP. Meanwhile, oxidation of glucose ultimately produces more ATP. But because the runner cannot take in enough oxygen to meet the high demand, most ATP is generated in glycolysis. Formation of lactic acid causes fatigue and possibly leg muscle cramps as the runner crosses the finish line. Already, her liver is actively converting lactic acid back to pyruvate, and storing glycogen. In her muscles, creatine phosphate begins to build again.

Shier-Butler-Lewis: Human Anatomy and Physiology, Ninth Edition

II. Support and Movement I S. Muscular System

© The McGraw-Hill Companies, 2001

Glycolysis and lactic acid-formation

Oxygen carried from lungs by hemoglobin in red blood cells is stored in muscle cells by myoglobin

Aerobic ' respiration

Glucose

Energy

Net gain of 2

Pyruvic acid H

Essentials of Human Physiology

Essentials of Human Physiology

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