Anaerobic Metabolism

Glycolysis is the process by which energy is liberated from glucose (Fig. 4-15). It is an important energy provider for cells that lack mitochondria, the cell organelle in which aerobic metabolism occurs. This process also provides energy in situations when delivery of oxygen to the cell is

 

FIGURE 4-15 Simplified version of (A) the glycolytic or anaerobic pathway that occurs in the cytoplasm, (B) the mitochondrial citric acid or aerobic pathway, and (C) the electron transport chain, in which the hydrogen (H+) ions and electrons (e−) from NADH+ H+ complexes generated in the glycolytic and citric acid pathways are converted to ATP and water in the presence of oxygen (O₂).

delayed or impaired. Glycolysis involves a sequence of re­actions that converts glucose to pyruvate, with the con­comitant production of ATP from ADP. The net gain of energy from the glycolysis of one molecule of glucose is two ATP molecules. Although comparatively inefficient as to energy yield, the glycolytic pathway is important dur­ing periods of decreased oxygen delivery, as occurs in skeletal muscle during the first few minutes of exercise.

Glycolysis requires the presence of nicotinamide-adenine dinucleotide (NAD⁺), a hydrogen (H⁺) carrier. Im­portant end products of glycolysis are pyruvate and NADH + H⁺. When oxygen is present, pyruvate moves into the aerobic mitochondrial pathway, and the NADH + H⁺ de­livers the H⁺ and its electron to the oxidative electron transport system (to be discussed). Transfer of hydrogen from NADH to the electron transport system allows the glycolytic process to continue by facilitating the regenera­tion of NAD+. Under anaerobic conditions, such as cardiac arrest or circulatory shock, pyruvate is converted to lactic acid, which diffuses out of the cells into the extracellular fluid. Conversion of pyruvate to lactic acid is reversible, and after the oxygen supply has been restored, lactic acid is reconverted back to pyruvate and used directly for energy or to synthesize glucose.

Much of the reconversion of lactic acid occurs in the liver, but a small amount can occur in other tissues. The liver removes lactic acid from the bloodstream and converts it to glucose in a process called gluconeogenesis. This glucose is released into the bloodstream to be used again by the muscles or by the central nervous system. This recycling of lactic acid is called the Cori cycle. Heart muscle is also effi­cient in converting lactic acid to pyruvic acid and then using the pyruvic acid for fuel. Pyruvic acid is a particularly important source of fuel for the heart during heavy exercise when the skeletal muscles are producing large amounts of lactic acid and releasing it into the bloodstream.