Do We Generate the Same Amount of Energy When Using Energy Nutrients in Our Body as Generated in a B

We combust energy nutrients in our cells and in the process generate the same amount of energy as in the bomb calorimeter. In fact, the reason we bring oxygen into our body is so that it can be used in the combustion of energy nutrients within our cells. Furthermore, carbon dioxide is produced during the combustion of these energy nutrients in our cells and we must breathe it out.

Despite several similarities between the combustion of energy nutrients in a bomb calorimeter and in our cells, there are a couple of fundamental differences. First, when amino acids and proteins are combusted in a bomb calorimeter, nitrogen-containing gases are produced. Contrarily, when amino acids are used for energy in our cells, most of the nitrogen is ultimately used to make urea. Second, the combustion of energy nutrients in a bomb calorimeter is for the most part an instantaneous process, while the combustion of energy nutrients occurring within our cells happens over a series of many chemical reactions (energy pathways). Last, unlike a bomb calorimeter, when we combust energy nutrients in our cells, we capture roughly 40 percent of the energy released in the formation of ATP and to a lesser degree guanosine triphosphate (GTP) (see Chapter 1). Meanwhile, the remainder of the energy released in the breakdown of energy nutrients is converted to heat.

• It is common to use calories (lower case "c") to express energy in relation to the body. To comply with common use, this book uses calorie generally to imply kilocalorie. However it is recognized that a calorie is a thousandth of a kilocalorie or Calorie (capital "C"). Food labels correctly use Calories.

160 Energy Metabolism and Body Weight How Are Energy Nutrients Used by Our Cells?

Carbohydrates, amino acids, fat, and alcohol can all be used by our cells to make ATP. Although the energy pathways involved in the metabolism of these substances are unique, they are indeed interconnected at various points. This allows us to convert glucose and certain amino acids to fatty acids and also to convert amino acids, glycerol, and lactate to glucose. However, only certain tissue will engage in these conversion activities.

Carbohydrate use for fuel begins with an anaerobic pathway and becomes aerobic like fat and amino acids.

What Is Anaerobic Energy Metabolism?

Energy pathways in our cells occur in either the mitochondria or the intracellular fluid (cytoplasm). In the latter, monosaccharides such as glucose become engaged in an energy pathway called glycolysis. All cells can use glucose for energy; meanwhile fructose and galactose are used by the liver mainly. Glycolysis converts glucose to two molecules of pyruvate. In this process, two ATP molecules and heat energy will be generated (Figure 8.2). Since these ATP will be generated without the need for oxygen, glycolysis is often referred to as anaerobic energy metabolism.

Pyruvate has several options, depending on the type of cell and what is going on inside of that cell (Figure 8.3). If the cell lacks mitochondria, such as in RBCs, pyruvate is converted to lactic acid (lactate). This lactate enters the blood and can serve as fuel for certain other organs such as the kidneys. Meanwhile, astrocytes that create the blood-brain barrier produce lactate which neurons in our brain can use. The blood-brain barrier is a special molecular fence that separates the cerebral spinal fluid, which nourishes the brain and spine, from the general circulation. Perhaps the most famous source of lactic acid is muscle during intense exercise such as weight lifting or sprinting.

What Is Aerobic Energy Metabolism?

In order for pyruvate and lactate from glycolysis or fatty acids and amino acids to be used for energy in cells there need to be two things—mitochondria and ample oxygen. Because of the need for oxygen, energy generation in mitochondria is called aerobic. In most cells the pyruvate generated by glycolysis enters mitochondria for combustion. In addition, cells in certain tissue such as kidneys, liver, brain, and muscle will convert circulating lactate to pyruvate which can enter the mitochondria.

Figure 8.2 In our cells pyruvate can enter mitochondria where it is broken down further to produce energy (ATP). Because oxygen is needed for our mitochondria to produce ATP the processes are called "aerobic." If oxygen is not abundant in that cell or if the cell does not have mitochondria—a red blood cell, for example—then pyruvate is converted to lactic acid.

Figure 8.2 In our cells pyruvate can enter mitochondria where it is broken down further to produce energy (ATP). Because oxygen is needed for our mitochondria to produce ATP the processes are called "aerobic." If oxygen is not abundant in that cell or if the cell does not have mitochondria—a red blood cell, for example—then pyruvate is converted to lactic acid.

Meanwhile some amino acids are converted to pyruvate as well or enter mitochondria directly like fatty acids.

Aerobic energy metabolism takes place in mitochondria and requires oxygen, and produces water and carbon dioxide.

Once inside the mitochondria, pyruvate can be converted to another molecule called acetyl CoA. Acetyl CoA can then enter another energy pathway called the Krebs' cycle (Figure 8.4).

During several of the chemical reactions that take place in our mitochondria, electrons are removed by carrier molecules and transported to special links of proteins embedded in the inner membrane of mitochondria. These special links of protein are called the electron-transport chain (Figure 8.5). The electrons are passed from the carrier molecules to

162 Energy Metabolism and Body Weight Mitochondria

Pyruvate e- e e"

Fatty (3-oxidation acids

Pyruvate e- e e"

Electron transport chains

Electron transport chains

ATP ATP ATP ATP ATP

ATP ATP ATP ATP ATP ATP ATP ATP ATP ATP

Figure 8.3 In the mitochondria of our cells, pyruvate and fatty acids are broken down to acetyl CoA which then is broken down in a series of chemical reactions called the Krebs' cycle. During the breakdown of fatty acids, pyruvate and acetyl CoA, electrons are removed and carried to the electron transport chains that are stitched into the inner membrane. The electrons then become important in the making of ATP, which can be used by that cell to power an operation! It should also be mentioned that these processes also produce carbon dioxide and water.

the electron-transport chain and then, like a bucket brigade, are passed along its length. As electrons are passed along the electron-transport chain, energy is released which drives the formation of ATP. Each of our mitochondria contains thousands of electron-transport chains.

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