Energy from Glucose

In fast white fibers, glycolysis catabolizes glucose. The relative lack of mitochondria in these fibers causes the white appearance. The rapid breakdown of glucose by anaerobic metabolism means that ATP is made rapidly. These muscles are used in rapid, short-duration movement and exhibit a fast twitch when electrically stimulated. The flight muscles of birds are of this type—remember that you find the white meat of a chicken on the breast. 

In slow red fibers, glucose metabolism leads into the TCA cycle and metabolism is aerobic. The red appearance of these muscles comes from the large number of mitochondria in them—the iron-containing cytochromes and myoglobin give the tissue its red appearance. The leg muscles (dark meat) of birds are of this type. 

The same distinctions hold in humans. Sprinters and marathon runners have different proportions of muscle fibers, and therefore different metabolisms. Sprinters have relatively more fast white fibers, and can run very rapidly, but not for long distances. Marathon runners, on the other hand, have more slow red fibers and can carry out aerobic metabolism for very long periods of time, although they can t go as fast. Well-trained, world-class runners may have as much as 90 percent of their leg muscle of one type or the other, depending on their sport. Some sports, such as basketball and soccer, involve both aerobic endurance and anaerobic sprinting these athletes tend to have both types of muscle fiber. Untrained individuals have about 50 percent of each type. The relative contributions of training and heredity to each type of metabolism remain unknown, although both play some part. 

The brain relies on the circulation for nutrients and is a chief consumer of glucose. The brain uses about 15 percent of the energy required for minimal maintenance of body functions (called the basal metabolic rate). Brain tissue doesn t store energy. Instead, the brain must rely on the circulation for its fuel supply. Not all molecules can be transported across the blood-brain barrier to be used for energy. One molecule that can cross the blood-brain barrier is glucose, the preferred fuel source for the brain. Brain tissue can also adapt to ketone bodies such as acetoacetate as a source of fuel. 

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Recall that during respiration, animals gain energy from glucose by oxidizing it—that is, by transferring... 

This reaction is a key step in glycolysis, a key pathway for extracting energy from glucose (Section 16.1.3). Dehydrations to form double bonds, such as the formation of phosphoenolpyruvate (Table 14,1) from 2-phosphogly cerate (reaction 8), are important reactions of this type. 

The glucose metabolism in aerotolerant anaerobes like trichomonad flagellates, Amoeba and Giardia spp. is somewhat different than in trypanosomes. These parasites carry out anaerobic production of energy from glucose, which may be stored in the form of glycogen upto 30% of their dry weight. 

To harness the energy from glucose, cells must break the bonds in which the energy is stored. The net exothermic reaction that takes place when the bonds in glucose are broken is similar to the combustion of hydrocarbons. 

Human red blood cells contain no mitochondria so they derive their energy from glucose purely on the basis of anaerobic glycolysis. Thus, from Prob. 10.33, it might be expected that each glucose molecule would generate... 

Energy transfer from ATP involves hydrolysis to ADP (11.11), which is thai converted into ATP using fresh energy from glucose oxidation. The simpUlied schane is indicated in Hgure 11.17. The conversion of ADP to ATP utilises the energy made available (by light and chlorophyll) in the complex photosynthetic and subsequent carbohydrate oxidation processes, and is usually referred to as photophosphorylation. 

Cells derive their energy from glucose oxidation which as an overall process can be represented by the reverse of Equation 11.54. As in other biochemical schemes, the process actually occurs in a series of steps, thus avoiding the sudden release of a large amount of energy as heat which would disrupt cell structure and render enzymes inoperative (Figure 11.20). 

Oxidation and Reduction Redox reactions are common in nature, in industry, and in many everyday processes. Batteries use redox reactions to generate electrical current. Our bodies use redox reactions to obtain energy from glucose. In addition, the bleaching of hair, the rusting of iron, and the electroplating of metals all involve redox reactions. 

Oxygen is needed for respiration by which cells extract energy from glucose. 

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