Types of Metabolism in Humans
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Metabolism is the physical and chemical processes that occur inside the cells of the body to maintain life by generating, storing, and expending energy. It is the combination of the constructive phase known as anabolism and the destructive phase known as catabolism (Metabolism). Anabolism is the process of building up molecules, and catabolism is the process of breaking down molecules. (Hill 392). Anabolic reactions require the use of energy to build up compounds that the body needs, while catabolic reactions break down food to provide energy. Cellular metabolism is the chemical change of molecules in a cell, and enzymes can be used as catalysts to speed up the chemical changes. The major form of energy used for cellular metabolism is ATP (adinosene triphosphate). Adinosene triphosphate is a molecule that stores energy until it is needed (McGraw-Hill). The transformation of carbohydrates, proteins, and fats into food, energy, and other physiological processes are part of the metabolic process (Metabolism).
Carbohydrates are made up of carbon, hydrogen, and oxygen and depending on the number of sugar units they have, can be monosaccharides, disaccharides, or polysaccharides. Glucose, galactose, and fructose are monosaccharides attained from foods that are digested in the small intestine, where carbohydrate metabolism begins (Metabolism). These monosaccharides are then absorbed into the blood stream and moved from the intestinal mucosa to the liver (Overview Metabolism). Here, they are either stored as glycogen in the liver or muscle, made into energy for all the tissues of the body, or converted to fat, amino acids, and other compounds (Metabolism).
Also, carbohydrate metabolism plays an important role in blood sugar. Blood sugar concentrations are controlled by insulin, glucagon and epinephrine. If the blood sugar concentration is too high, the pancreas secretes insulin to stimulate the cells to absorb the glucose. Therefore, the glucose concentration in the blood would in turn decrease because the entry of glucose into most tissues is dependent on the presence of the hormone insulin (Overview Metabolism). Glucose is converted into glycogen in the liver and muscles by the anabolism process called glycogenesis. When blood sugar levels drop, epinephrine and glucagon are secrete to stimulate liver cells to break down glycogen to glucose in the catabolism process called glycogenolysis (Taber's Cyclopedic Medical Dictionary 1452).
When glucose is needed immediately and does not need to be stored in the liver or muscles, a catabolism process begins, called glycolysis (Overview Metabolism). Glycolysis is a series of steps that oxidize glucose into pyruvate. The oxidation of pyruvate forms Acetyl CoA whitch then allows for entry into the Krebs cycle, which releases energy as an end result (Glycolysis). The energy released is used to form ATP. At the beginning of glycolysis, two ATP molecules are used to add two phosphates to the six-carbon glucose molecule. The product of this reaction is a six-carbon diphosphate molecule and two molecules of ADP (adenosine diphosphate). Next, the six-carbon sugar diphosphate molecule is then broken in half resulting in two three-carbon molecules. Through a series of steps, known as the Krebs Cycle or the citric acid cycle, and the conversion of pyruvate to Acetyl CoA, ATP is formed in the process (McGraw-Hill).
During anaerobic conditions, such as swimming or running, pyruvate is converted into lactic acid, causing muscle fatigue. During aerobic conditions, such as resting, lactic acid is converted back to pyruvic acid. As a result, the pyruvic acid is converted back to glucose by an anabolism process known as gluconeogenesis. If then the glucose is not needed, it is converted into glycogen by glycogenesis to be stored in the liver and muscles. (Taber's Cyclopedic Medical Dictionary 1452).
In summary, carbohydrate metabolism can also be described as a series of cellular respiration reactions (Dupree). Carbohydrate metabolism of food is virtually the same as glucose metabolism because these carbohydrates are eventually broken down into glucose. Glucose metabolism produces energy that is stored in the form of ATP. The process of energy being released from molecules is a process known as the oxidation process. Cellular respiration is when the energy that has been released from those molecules, is transferred to other molecules. "This happens in every cell of the body and is the cell's source of energy (Dupree)." The series of cellular respiration reactions involve glycolysis, aerobic and anaerobic respiration, and oxidative phosphorylation (Overview Metabolism).
Proteins are made up of carbon, hydrogen, oxygen, nitrogen, and other atoms (Metabolism). "Proteins help to transport hemoglobin, regulate hormones, make up the structural tissue for muscles and tendons, and catalyze all biochemical reactions" (Overview Metabolism). Proteins are broken down to amino acids during digestion. If the body already has enough amino acids, and no more are needed, then they are metabolized to glycogen or fat. Therefore, they will eventually be used for energy metabolism. If the body is in need of amino acids to be used for energy, they are broken down even further into acetyl CoA. Acetyl CoA can then enter the Krebs cycle for oxidation, which will eventually produce ATP (Metabolism).
Proteins in the body are broken down when a person's dietary supply of energy is low during sickness or starvation. The body will begin to break down proteins in the liver first, before breaking down the proteins in other organs (Metabolism).
Protein metabolism is mainly producing amino acids needed for synthesis of protein molecules in the body. During this process, energy is released and byproducts such as ammonia and keto acid are produced. The liver converts the ammonia into urea, which the blood carries to the kidneys for elimination in the urine. The keto acid enters the Krebs cycle and is converted into pyruvic acids to produce ATP (Dupree).
Fats are made up of carbon, hydrogen, oxygen, and other trace elements. Lipids contain almost all of our body's stored energy and they are usually stored in adipose tissue (Dupree). The main forms of fat in the foods we eat are triglycerides, phospholipids, and sterols. The major pathways of lipid (fat) metabolism are lipolysis, ketosis, lipogenesis, and betaoxidation (Metabolism). Lipolysis and betaoxidation are carried in the cell's mitochondria for the removal of two carbons to form acetyl CoA from the fatty acid cycle. Later, this enters the Krebs cycle and produces energy in the form of ATP. Water and carbon dioxide are also output products of this. In other words, lipolysis and betaoxidation are the catabolic process of breaking down stored fat. In prolonged starvation or when large amounts of fatty foods are ingested, the ketosis pathway is used. This is when the body has an excessive breakdown of fats. The lipogenesis pathway happens in the cytosol of the cell membrane, not the mitochondria. Lipogenesis is the production of fat from the conversion of carbohydrates or proteins to fat. This pathway causes triglycerides to synthesize in the liver, intestinal mucosa and muscle tissues for long term energy use (Lipid Metabolism Pathways) (Medical Dictionary).
When the body is ready to metabolize fats, a process of catabolic reactions start to break down fatty acid chains to form acetyl CoA. Acetyl CoA then enters the Krebs cycle to produce ATP. This process happens over and over again until the entire fatty acid chain is converted into acetyl CoA (Dupree).
Overall, most tissues in the body are able to convert glucose, amino acids, and fatty acids to acetyl CoA to produce energy from the Krebs cycle. However, the brain and the rest of the nervous system depend almost entirely on glucose. Since metabolism maintains life by generating, storing, and expending energy, it is important that all the processes along with it be working properly. Therefore, seeing as how carbohydrate metabolism, protein metabolism, and fat metabolism all interrelate, it is critical for one to have a balanced diet in order to maintain these processes.
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