Uses of Different Energy Sources

The blood serves as a common trough from which all the cells in the body are fed. If all cells used the same energy source, such as glucose, this source would quickly be depleted and cellular starvation would occur. Normally however, the blood contains

■ Figure 5.16 Pathways by which amino acids can be catabolized for energy. These pathways are indirect for some amino acids, which first must be transaminated into other amino acids before being converted into keto acids by deamination.

Cell Respiration and Metabolism a variety of energy sources from which to draw: glucose and ke-tone bodies that come from the liver, fatty acids from adipose tissue, and lactic acid and amino acids from muscles. Some organs preferentially use one energy source more than the others, so that each energy source is "spared" for organs with strict energy needs.

The brain uses blood glucose as its major energy source. Under fasting conditions, blood glucose is supplied primarily by the liver through glycogenolysis and gluconeogenesis. In addition, the blood glucose concentration is maintained because many organs spare glucose by using fatty acids, ketone bodies, and lactic acid as energy sources (table 5.3). During severe starvation, the brain also gains some ability to metabolize ketone bodies for energy.

As mentioned earlier, lactic acid produced anaerobically during exercise can be used for energy following the cessation of exercise. The lactic acid, under aerobic conditions, is reconverted to pyruvic acid, which then enters the aerobic respiratory pathway. The extra oxygen required to metabolize lactic acid contributes to the oxygen debt following exercise (see chapter 12).

Clinical Investigation Clues

Remember that Brenda found herself gasping and panting for air more than her teammates.

What is the term for the extra oxygen she needs following exercise? What function does it serve?

What would cause her to need less, and thus to gasp and pant less following exercise?

Table 5.3 Relative Importance of Different Molecules in the Blood with Respect to the Energy Requirements of Different Organs

Fatty Ketone Lactic Organ Glucose Acids Bodies Acid

Test Yourself Before You Continue

1. Construct a flowchart to show the metabolic pathway by which glucose can be converted to fat. Indicate only the major intermediates involved (not all of the steps of glycolysis).

2. Define the terms lipolysis P-oxidation and explain, in general terms, how fat can be used for energy.

3. Describe transamination and deamination and explain their functional significance.

4. List five blood-borne energy carriers and explain, in general terms, how these are used as sources of energy.

Glycogen

Glucose

■ Figure 5.17 The interconversion of glycogen, fat, and protein. These simplified metabolic pathways show how glycogen, fat, and protein can be interconverted. Note that while most reactions are reversible, the reaction from pyruvic acid to acetyl CoA is not. This is because a CO2 is removed in the process. (Only plants, in a phase of photosynthesis called the dark reaction, can use CO2 to produce glucose.)

■ Figure 5.17 The interconversion of glycogen, fat, and protein. These simplified metabolic pathways show how glycogen, fat, and protein can be interconverted. Note that while most reactions are reversible, the reaction from pyruvic acid to acetyl CoA is not. This is because a CO2 is removed in the process. (Only plants, in a phase of photosynthesis called the dark reaction, can use CO2 to produce glucose.)

HPer Links of Metabolism Concepts to the Body Systems

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