Sources of ATP for muscle contraction

Skeletal muscle uses only ATP as a source of energy for contraction. However, intracellular stores of ATP are quite limited. In fact, the amount of ATP normally found in skeletal muscle is enough to sustain only a few seconds of contraction. Therefore, metabolic pathways to form additional ATP are needed. These pathways include:

• Creatine phosphate

• Oxidative phosphorylation

• Glycolysis

Energy may be transferred from creatine phosphate to ADP by way of the following reaction:

Creatine phosphate + ADP < CK > Creatine + ATP

The enzyme creatine kinase (CK) facilitates the transfer of phosphate and energy to a molecule of ADP to form ATP. Stores of creatine phosphate are sufficient to sustain approximately 15 more seconds of muscle contraction. Because this is a single-step process, it provides ATP very rapidly and is the first pathway for formation of ATP to be accessed.

The second pathway to be utilized in the formation of ATP is oxidative phosphorylation. This process involves the metabolic breakdown of glucose and fatty acids. Because it requires oxygen, oxidative phosphorylation provides energy at rest and under conditions of mild (walking) to moderate (jogging) exercise. This pathway is advantageous because it produces a large amount of energy (36 molecules of ATP) from each molecule of glucose; however, oxidative phosphorylation is comparatively slow due to the number of steps involved. Furthermore, this process requires enhanced blood flow to the active muscles for continuous delivery of oxygen as well as nutrient molecules. Although glucose may be obtained by way of glyco-genolysis within the skeletal muscle fibers, these glycogen stores are limited. Glycogenolysis in the liver and lipolysis in the adipose tissues yield additional molecules of glucose and fatty acids for energy formation. As exercise is sustained, skeletal muscle relies more upon fatty acids as a source of fuel for the oxidative phosphorylation pathway. In this way, glucose is spared for the brain.

During intense exercise, when the oxygen supply cannot keep pace with the oxygen demand, skeletal muscle produces ATP anaerobically by way of glycolysis. Although this pathway provides ATP more rapidly, it produces much less energy (2 molecules of ATP) from each molecule of glucose. Furthermore, glycolysis results in production of lactic acid in the muscle tissue. The accumulation of lactic acid may lead to pain as well as to muscle fatigue.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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