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Muscle Metabolism

Pearson
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The potential or stored energy in ATP is released when the terminal high-energy bond is broken by a hydrolytic enzyme. The end products of the hydrolysis of ATP are ADP (adenosine diphosphate), inorganic phosphate, and energy. The end products of ATP hydrolysis are not discarded as waste, but can be recombined to form a new ATP molecule. Rebuilding ADP into ATP requires a synthetic enzyme to carry out dehydration synthesis and a new source of energy to “rebuild” the high-energy bond. The immediate source of energy for synthesizing ATP comes from creatine phosphate. The second source of energy for ATP synthesis comes from glycogen and glucose during the process called glycolysis. The end products of gycolysis are two ATP molecules and pyruvic acid. If oxygen is not available, pyruvic acid is converted to lactic acid in the anaerobic pathway. In the presence of oxygen, the aerobic pathway takes place. Pyruvic acid is converted into acetyl CoA which enters the Krebs cycle. In oxidated phosphoralation, energy is transferred to ATP. The aerobic pathway yields the greatest amount of ATP; 36 molecules of ATP per molecule of glucose. Byproducts of the aerobic process include water and carbon dioxide. Recall that ATP is used by the muscle cell for the power stroke of the myasin crossbridge, for disconnecting the crossbridge from the binding site on actin and for transporting calcium ions back into the sarcoplasmic reticulum. The mechanisms that produce ATP can be compared to a cellular factory. We have now examined the three processes for synthesizing ATP: hydrolysis of creatine phosphate, glycolysis, and the Krebs cycle and oxidative phosphorylation. Let’s see which process produces the most ATP. Hydrolysis of creatine phosphate produces only one ATP per creatine phosphate molecule. Glycolosis produces two ATP per glucose molecule. The Krebs cycle and oxidative phosphorylation produces 36 ATP per glucose molecule. After the exercise period is concluded, the muscle restores depleted energy reserves used earlier in the exercise. These processes are usually referred to as repaying the "oxygen debt." 1. Lactic acid present in the cytosol is converted back into pyruvic acid, which is converted to acetyl CoA that enters the Krebs cycle. Aerobic respiration produces ATP, water and carbon dioxide. 2. The ATP is used to rephosphorylate creatine into creatine phosphate. 3. Glycogen is synthesized from glucose molecules. 4. Additional oxygen re-binds to myoglobin. White muscle fibers are large in diameter and light in color due to reduced or absent myoglobin. These types of muscle cells are generally surrounded by only a few capillaries and have relatively few mitochondria. However, they have a high glycogen content. Given these characteristics, would you expect white muscle fibers to synthesize ATP mainly by glycolysis or by the Krebs cycle and oxidative phosphorylation? White muscle fibers mainly use glycolysis to synthesize ATP. Since these cells have little myoglobin and few capillaries, only a small amount of oxygen is available for metabolism. Recall that glycolysis does not require oxygen, but the Krebs cycle and oxidative phosphorylation do. Having relatively few mitochondria, these cells lack the cellular machinery, as well as the oxygen supply, for carrying out adequate aerobic respiration. However, with their high glycogen content they have a ready supply of glucose for glycolysis. Red muscle fibers are about half the diameter of white muscle fibers. They are dark red in color due to their large quantity of myoglobin. Red muscle fibers are surrounded by many capillaries and contain numerous mitochondria. However, they have a low glycogen content. Given these characteristics, would you expect red muscle fibers to synthesize ATP mainly by glycolysis or the Krebs cycle and oxidative phosphorylation? To synthesize ATP, red muscle fibers mainly use the Krebs cycle and oxidative phosphorylation, which require mitochondria and oxygen. Oxygen comes from the abundant myoglobin and capillaries. Oxygen diffuses rapidly throughout these small cells. Being deficient in glycogen, red muscle cells do not rely solely on glucose for energy. Instead, they also metabolize fatty acids, which are broken down into acetyl CoA that enters the Krebs cycle.
The potential or stored energy in ATP is released when the terminal high-energy bond is broken by a hydrolytic enzyme. The end products of the hydrolysis of ATP are ADP (adenosine diphosphate), inorganic phosphate, and energy. The end products of ATP hydrolysis are not discarded as waste, but can be recombined to form a new ATP molecule. Rebuilding ADP into ATP requires a synthetic enzyme to carry out dehydration synthesis and a new source of energy to “rebuild” the high-energy bond. The immediate source of energy for synthesizing ATP comes from creatine phosphate. The second source of energy for ATP synthesis comes from glycogen and glucose during the process called glycolysis. The end products of gycolysis are two ATP molecules and pyruvic acid. If oxygen is not available, pyruvic acid is converted to lactic acid in the anaerobic pathway. In the presence of oxygen, the aerobic pathway takes place. Pyruvic acid is converted into acetyl CoA which enters the Krebs cycle. In oxidated phosphoralation, energy is transferred to ATP. The aerobic pathway yields the greatest amount of ATP; 36 molecules of ATP per molecule of glucose. Byproducts of the aerobic process include water and carbon dioxide. Recall that ATP is used by the muscle cell for the power stroke of the myasin crossbridge, for disconnecting the crossbridge from the binding site on actin and for transporting calcium ions back into the sarcoplasmic reticulum. The mechanisms that produce ATP can be compared to a cellular factory. We have now examined the three processes for synthesizing ATP: hydrolysis of creatine phosphate, glycolysis, and the Krebs cycle and oxidative phosphorylation. Let’s see which process produces the most ATP. Hydrolysis of creatine phosphate produces only one ATP per creatine phosphate molecule. Glycolosis produces two ATP per glucose molecule. The Krebs cycle and oxidative phosphorylation produces 36 ATP per glucose molecule. After the exercise period is concluded, the muscle restores depleted energy reserves used earlier in the exercise. These processes are usually referred to as repaying the "oxygen debt." 1. Lactic acid present in the cytosol is converted back into pyruvic acid, which is converted to acetyl CoA that enters the Krebs cycle. Aerobic respiration produces ATP, water and carbon dioxide. 2. The ATP is used to rephosphorylate creatine into creatine phosphate. 3. Glycogen is synthesized from glucose molecules. 4. Additional oxygen re-binds to myoglobin. White muscle fibers are large in diameter and light in color due to reduced or absent myoglobin. These types of muscle cells are generally surrounded by only a few capillaries and have relatively few mitochondria. However, they have a high glycogen content. Given these characteristics, would you expect white muscle fibers to synthesize ATP mainly by glycolysis or by the Krebs cycle and oxidative phosphorylation? White muscle fibers mainly use glycolysis to synthesize ATP. Since these cells have little myoglobin and few capillaries, only a small amount of oxygen is available for metabolism. Recall that glycolysis does not require oxygen, but the Krebs cycle and oxidative phosphorylation do. Having relatively few mitochondria, these cells lack the cellular machinery, as well as the oxygen supply, for carrying out adequate aerobic respiration. However, with their high glycogen content they have a ready supply of glucose for glycolysis. Red muscle fibers are about half the diameter of white muscle fibers. They are dark red in color due to their large quantity of myoglobin. Red muscle fibers are surrounded by many capillaries and contain numerous mitochondria. However, they have a low glycogen content. Given these characteristics, would you expect red muscle fibers to synthesize ATP mainly by glycolysis or the Krebs cycle and oxidative phosphorylation? To synthesize ATP, red muscle fibers mainly use the Krebs cycle and oxidative phosphorylation, which require mitochondria and oxygen. Oxygen comes from the abundant myoglobin and capillaries. Oxygen diffuses rapidly throughout these small cells. Being deficient in glycogen, red muscle cells do not rely solely on glucose for energy. Instead, they also metabolize fatty acids, which are broken down into acetyl CoA that enters the Krebs cycle.