Mitochondrial Respiration For Losing Weight While Pregnancy (Oxidative Metabolism)
Mitochondrial respiration, also known as oxidative metabolism or aerobic metabolism, meets ATP demand at rest and during low-intensity exercise. Even during extremely high-intensity exercise, some cells (such as cardiac cells) can only use energy produced from aerobic metabolism. Aerobic metabolism completes the transfer of energy from the breakdown of carbohydrate, fats, and proteins from the foods we eat into the energy we need to maintain life. Before these macronutrients can be completely metabolized by the Krebs cycle and ETC, they must undergo preliminary metabolic steps. The following will give a brief overview of how each macronutrient is metabolized in a series of catabolic reactions that eventually produce energy and heat, as well as the metabolic by-products: water and carbon dioxide.
The pyruvate produced from glycolysis is converted to acetyl-CoA, and it enters the Krebs cycle (or TCA cycle or Citric acid cycle), an irreversible step. For each glucose molecule that enters glycolysis, two pyruvate molecules are formed, each can enter into the Krebs cycle given there is adequate oxygen. Ultimately, from the Krebs cycle, each molecule of glucose produces two ATP and ten hydrogen ions. Because these hydrogen ions are unstable, they are immediately bound to a nicotinamide adenine dinucleotide (NAD) or flavin adenine dinucleotide (FAD) molecule, thus becoming NADH and FADH2. The resulting six NADH and two FADH2 molecules then “carry” the electrons to the ETC within the mitochondrial membrane matrix. Once the hydrogen ions are removed, the NAD and FAD carriers can “pick up” additional electrons generated by the Krebs cycle. Within the ETC, hydrogen ions are sent through a series of oxida-tion-reduction, or redox reactions, within linked specialized complexes called cytochromes. As hydrogen molecules are “pumped” through the cytochromes, potential energy is stored in what can simply be thought of as a hydrogen reservoir. When the stored hydrogen molecules move down through the final cytochrome, there is an electrochemical gradient from which energy is captured and used to rephosphorylate ADP and Pi into ATP via the enzyme ATP synthase. Oxygen then combines with the free hydrogen ions to produce water (thus the requirement for oxygen). If oxygen is unavailable, the aforementioned metabolic reactions could not continue because the free hydrogen ions would quickly drop the pH of the cell and cause cessation of enzymatic activity (cellular death). The result is the production of 32 ATP from the ETC, and a total production of 36 ATP from the complete metabolism of one molecule of glucose.
Fats can also be used as a substrate for the oxidative energy system and, to a lesser degree, so can protein. Triglycerides in fat cells can be broken down through enzymatic action that releases free fatty acids into the blood, which can eventually be taken up by muscle cells. There are also limited amounts of triglycerides stored in the muscle that can be broken down into free fatty acids. Through a process called beta oxidation, free fatty acids that have entered the mitochondria are broken down. The result is the production of acetyl-CoA that enters the Krebs cycle, and hydrogen ions that are carried by NADH and FADH2 to the ETC. The energy yield from one triglyceride molecule is significantly greater than that of one molecule of glucose. The actual number of ATP produced depends upon the number of carbons in the triglyceride. As an example, a triglyceride containing three 18-carbon fatty acids will yield 463 molecules of ATP (22 ATP from the oxidation of glycerol, + 147 ATP per fatty acid x 3)! This is almost 13 times the amount of ATP produced from one glucose molecule. While the production takes much longer, the yield is significantly greater.
The oxidation of protein is a less-efficient process and therefore protein is not an ideal energy substrate. Some amino acids resulting from the breakdown of protein known as glucogenic amino acids can be converted into glucose via a process called gluconeogenesis, which then can be oxidized. There are ketogenic amino acids that can be broken down to acetyl-CoA and thus can be used in the Krebs cycle or can be used to produce ketones. While this process will be discussed in further detail in Chapter 4, it should be understood that protein is an inefficient source of energy and contributes minimally to short-term exercise, and about 5 to 10 percent of energy (and up to 18 percent) to prolonged exercise.
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The three energy systems, the phosphagen system, glycolysis, and mitochondrial respiration, are significantly more complex than what is explained in the aforementioned descriptions. To truly have a thorough understanding of energy metabolism, knowledge of exercise physiology or nutritional biochemistry or both is required. For the purpose of this text, this brief overview is sufficient to illustrate the overall concept that the energy demands of the body determine the energy system(s) that are called upon, as well as the need to consume the appropriate nutrients to supply substrates for these systems and how these three systems are interrelated.
The Three Energy Systems Work in Concert to Provide Energy for Activity
While the three energy systems appear to be distinct, it is important to understand they are interrelated and work together to contribute to energy required for exercise. Regardless of the type or intensity of the activity, all three energy systems produce ATP to meet energy demands. It is also important to consider that energy demands may differ throughout the body. For example, when performing maximal cycling sprints, ATP demand in the muscle cells of the quadriceps will be very high and primarily met through the phosphagen system and fast or incomplete glycolysis; however, energy demands of the biceps are probably much lower, therefore aerobic metabolism will be the primary ATP contributor.
Duration of the exercise activity is an important consideration for which energy system predominates. The phosphagen system is the immediate source of energy for high-intensity exercise, yet, is depleted quickly. Glycolysis does not provide ATP as rapidly at the phosphagen system, but it too is initiated at the onset of exercise, and after 10 seconds will be the primary contributor of energy for up to two minutes. Mitochondrial respiration is always being used at rest and rate of ATP production via this pathway increases at the onset of exercise, regardless of other contributors of ATP; however, it becomes the main energy source when exercise lasts longer than 75 seconds and continues to supply ATP indefinitely (albeit rate of ATP production can decrease and exercise capacity can diminish) (Baker, McCormick, and Robergs 2010). Even during long-duration exercise, the phosphagen system and fast glycolysis will continue to contribute small amounts of ATP. Ultimately, the production of ATP from all three systems allows the energy from food to fuel muscle contraction and exercise.
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