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 somewhat 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.
What is anaerobic energy metabolism?
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Energy pathways in our cells occur in either the mitochondria or the intracellular fluid (cytoplasm). In the latter monosaccharides like 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 his pathway converts glucose to two molecules of pyruvate. In this process, two ATP and heat energy will be generated (see Glucose Fuel Figure). Since these ATP will be generated without the need for O2, 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 (see Glucose Fuel Figure). 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, which separate the blood from the cererbral spinal fluid bathing the nourishes the brain and spine, generate lactate which neurons in our brain can use. Perhaps the most famous source of lactic acid is muscle during intense exercise such as weight lifting or sprinting.
What is aerobic energy metabolism?
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Aerobic energy metabolism takes place in mitochondria and requires oxygen and produces water and carbon dioxide (see Electron Transport Chain Figure). 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 the need for oxygen energy generation in mitochondria is called aerobic. |
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In most cells the pyruvate generated by glycolysis enters mitochondria for combustion. (See Aerobic Energy Metabolism Figure) In addition, cells in certain tissue such as kidneys, liver, brain and muscle will convert circulating lactate to pyruvate which can enter the mitochondria. Meanwhile some amino acids are converted to pyruvate as well or enter mitochondria directly like fatty acids. Once inside the mitochondria, pyruvate can be converted to another molecule called acetyl CoA. Acetyl CoA can then enter another energy pathway called the Kreb’s cycle (see Aerobic Energy Metabolism Figure). |
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 (see Aerobic Energy Metabolism Figure). The electrons are passed from the carrier molecules to 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 probably contains thousands of electron-transport chains
As shown in Electron Transport Chain Figure, oxygen is needed to receive the electrons reaching the end of the electron-transport chain. Subsequently, the oxygen and electrons are coupled with hydrogen to make water. This serves to generate water in our body on a daily basis.
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