ATP is the energy coin of the cell. It is utilized to power cellular operations. We utilize food, the biomolecules described earlier to create ATP. Three distinct, but linked, metabolic pathways are utilized to transfer the energy released from this breakdown of fuel molecules into ATP.
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ATP Formation
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In the process of breaking down the food molecules, there are times when the energy released is much greater than the energy stored in ATP. Think of a bucket of water. You have an empty glass, and it is filled by poouring the bucket of water into the glass. Of course the bucket had more water that the glass would hold. The extra water is lost when the bucket is poured into the glass. Therefore, just as we have five and ten dollar bills, we have differnt denominations of energy storage. ATP is the one dollar bill, Flavine Adenine Dinucleotide (FAD) is a two dollar bill, and Nicotine Adenine Dinucleotide (NAD) is a three dollar bill.
The processes of producing ATP are three. The first is Glycolysis, or the breakdown of glucose and occurs in the cell cytoplasm. The second is the Krebs, Calvin, or Citric Acid cycle and occurs in the mitochondria. The Third is electron transport or oxidative phosphorylation, which also occurs in the mitochondria.
The first part of glycolysis requires an investment of energy. We invest a couple of molecule of ATP in order to put phosphate handles on glucose. Once we have done this we can pull the glucose molecule in half, which creates two three carbon fragments.
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Glycolytic Pathway 1
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These fragments are process to extract their energy. We regain are initial envestment of ATP twice over and also make a couple of NADHs. The net product, pyruvate, can then either be fed into the Krebs cycle or processed anaerobically.
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Glycolytic Pathway 2
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When we have sufficient oxygen the mitochondriacan process pyruvic acid. When we are lacking oxygen, they can not. Consider the case where you are exercising very hard. Your are burning energy faster than you can make it. You can not breath enough oxygen to keep up with the demand. You go into an anaerobic state. You use the NADH to make lactic acid NAD, which is then used to breakdown more glucose and produce more ATP.
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Anaerobic Conditions
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The net result is that you produce more ATP, and lactic acid. Later, after the exercize, you keep breathing hard, obtain the oxygen you deed, and then process the lactic acid back to pyruvate. All is fine except for the sore muscles.
The Krebs cycle is where we produce the carbon dioxide we exhale. We can take the pyruvate and add a handle creating a molecule called Acetyl Coenzyme A, in the process releasing carbon dioxide and producing NADH. (What happens to the carbon dioxide? NADH?) Acetyl is a two carbon fragment and is a basic building block for larger molecules such as fatty acids. Therefore, we can also break down fats and lipids and feed them into the Krebs cycle to produce energy.
The many steps of the Krebs cycle are designed to both degrade the acetyl carbons into carbon dioxide and to capture the energy of these reactions as NADH, FADH, and GTP. These are worth 3, 2, and 1 ATP eventually. We can also feed amino acids into the Krebs cycle to break then down for energy.
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Kreb Cycle
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NADH and FADH are fed into the electron transport mechinery found on the walls, cristae, of the mitochondria. Here the oxygen we breath is attached to the Hydrogen transported by NADH and FADH to make water. Note that the oxygen we breath does not get exhaled with the carbon dioxide we exhale. (Where does the carbon dioxide come from?)
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Phosphorylation
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During oxidative phosphoralation ATP is produced. There is enough energy in NAD to produce 3 ATP and there is enough energy in FADH to produce 2 ATP.
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Catabolism
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Altogether then, the aerobic catabolism of glucose can produce 38 molecules of ATP, six molecules of carbon dioxide, and 6 molecules of water. We also require 6 molecules of oxygen to acomplish this. All of the rest of the components of this process are recycled and used over and over again.
Plants store glucose as starch, animals store it as glycogen. In addition to being a good way to store glucose, the fact that each glycogen molecule can be made of hundreds of glucose can significantly reduce tonicity. Remember that it is not the size but the number of molecles that contribute to tonicity. Therefore, one glycogen molecutle composed of a hundred glucose molecules, has only one one-hundredth of the tonicity of the individual glucose molecules.
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Glycogen
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Glucose and glycogen are converted back and forth in our cells under the influence of several hormones. Two of these are insulin and glucogon; they play a major role in maintaining blood glucose levels within a narrow range, homeostasis, both when you have just eaten and when its has been a long time since your last meal.
When you have not eaten for a while, your blood sugar starts to drop. Glucogon levels increase, and you start to produce more glucose. Two substraits for this are fats and proteins. The body starts with the fats and, after they are used up, move on to the proteins.
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Gluconeogenesis
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Fats are high energy density storage molecules. They are used both by the mitochondria to produce ATP and as building blocks to produce glucose. Some organs, like the brain, require glucose as their fuel. therefore, in order to keep our brain functioning, we must supply it with glucose. After the glycogen has been used up, we have no more store of glucose and, thus, are required to synthesize it, gluconeogenesis.
Fats consist of one to three fatty acids attached to a glycerol backbone. These fatty acids are composed of even numbers of carbons attached to the carboxilic acid moity. By the process of beta oxidation, we slice off two carbon, acetyl, fragements from the fatty acid chain. These are then run through the Krebs cycle to produce NADH, FADH, and GTP, all of which can be converted into ATP.
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Fatty Acid Metabolism
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Amino acid are characterized by have an amine moity attached to it. Chains of amino acids are polypeptides (so called because of the peptide bond conecting the amino acids together) or proteins, if they are very large/long molecules. If we remove the amine group from an amino acid, we are left with a keto acid. The process of removing the amine group is called deamination. The amine moity is coupled with hydrogen to produce ammonia.
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Amination 1
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The use of keto acids is varied, we can, for example, use them in the Krebs cycle. They can also be used to produce amino acids. One transformation our cells can perform is to create an amino acid from a keto acid by using the amino group from a different amino acid. This process is termed transamination.
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Amination 2
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Once the amino group is removed from an amino acid, the remainder of the molecule can be metabolized by entering the glycolytic pathway or the Krebs cycle. The amino group, nitrogen, can be utilized to synthesize other molecules such as the purine and pyrimidine bases of the nucleic acids.
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Amino Acid Metabolism
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The amine moity removed from the amino acid, deamination, may be used to create other amino acids or nitrogen containing compounds. If the ammine/ammonia is not utilized in this manner, it is rapidly converted in the liver to urea. The kidneys filter the urea from the blood and excrete it in the urine.
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Pathways of AA Metabolism
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The major biologgical molecules, proteins, fats, and glycogen can easily be hydrolyzed by the enzymes of our bodies into their component parts. These pieces can be utilized to create ATP, or traded to produce other necessary molecules. The molecules that can not be synthesized de novo, or created from other components, must be obtained from your diet. These components are then called essential nutrients and are vital to good health.
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Interrelation
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