Our body is in a constant need for energy in order to maintain homeostasis, or to keep a constant internal environment inside our body. This homeostasis must be done through the constant supply of energy in the form of ATP and GTP which are produced in metabolism of glucose and fatty acids. In addition there must be a constant supply of building blocks for the synthesis of proteins and DNA molecules.
These building blocks are amino acids and DNA bases. Amino acids are obtained through food rich in proteins which is degraded to amino acids in the stomach. DNA bases are synthesized in the cells or are obtained from the diet in the form of DNA molecues.
Proteins are synthesized from amino acids as was mentioned earlier. There are 20 amino acids which constitute the building blocks of all proteins. about half of them are synthesized in the liver. These amino acids that are synthesized in the liver are called non-essential amino acids because our body is capable of making them.
Other amino acids are not possible for the liver to make. These are called essential amino acids. They also participate in the synthesis of proteins in the body. These essential amino acids therefore must be obtained in the diet in the form of food rich in proteins. Otherwise there will be a metabolic disease that is manifested as the lack of certain enzymes which catalyze reactions in the cell.
Glucose is an important compound for the generation of energy. This energy that is generated in processes called glycolysis and krebs cycle and oxidative phosphorylation. Glycolysis is not an efficient reaction to generate ATP. In this reaction few ATP molecules are generated.
In krebs cycle and oxidative phosphorylation much more ATP molecules are generated. Glycolysis is important in particular for generating energy for the brain cells. The products of glycolysis enter the krebs cycle to generate more ATP molecules. Glycolysis does not require oxygen but krebs cycle and oxidative phosphorylation require oxygen for its reaction to generate ATP.
Energy for our body can also be obtained not only from the combustion of glucose, but also from the oxidation of fatty acids. Fatty acids are obtained in the diet in the form of tri-glycerdes in which are hydrolyzed to glycerol and fatty acids. The oxidation of fatty acids which are stored in fat cells or adipocytes gives a much more energy in the form of ATP than the combustion of glucose. Thats why fats predisposes to obesity.
Glucose is the ultimate source of energy for the brain cells through glycolysis and krebs cycle. In hypoxia or shortage of oxygen supply the brain is the first organ in the body to be affected. The damage to the brain as a result of the hypoxia is irreversible and this is so because the brain cells are not capable of renewing themselves.
Enzymes, which are a special group of proteins are an important group of macromolecules because the participate in the catalysis of metabolic many reactions if not all of them. A deficiency of an enzyme which can be genetic or acquired can cause a metaboic disease which has distinct clinical signs. Therefore a constant supply of essential amino acids is required for building these important group of proteins.
Amino acids in the liver are metabolised to ammonia. Ammonia is a toxic base in high concentration. This condition is found in liver disease, in which the liver is unable to dispose of the ammonia. Thus its concentration in the blood increase the thing that can affect the brain cells due to the accumulation of ammonia there. This condition is called hepatic encephalopathy.
Normal metabolism of the body can be achieved through internal sources of proteins and glycogen which are used by the body cels in the case of shortage in food in the diet. Glycogen is a polymer of glucose and is stored in the liver. It starts polymerization when it receives a signal from glucagon ( a hormone secreted by the islet cells of the pancreas).
High levels of glucagon stimulates the degradation of glycogen. Insulin is an antagonist to glucagon. Its high concentration stimulates the synthesis of glycogen from glucose.
Normal metabolism requires certain metals and vitamins which are essential for the cellular activity and function of enzymes, especially the metals. Our body has limited amount of these metals which are obtained through the diet. Therfore they must be given with food in the diet. Deficiency of any of these metals will affect the function of certain enzymes which use these metals as cofactors. This deficiency will be manifested as a metabolic disease.
DNA molecules are built from DNA bases which are synthesized in the cells. This synthesis requires a vitamin that is called folic acid. Folic acid is not synthesized in the body but must be given in the diet in the form of Tablets. Its deficiency means impaired bases synthesis and hence less DNA molecules.
Food that we ingest is degraded in the stomach and the intestine using special enzymes. These enzymes are found in the mouth and the stomach and are secreted by the pancreas. Any deficiecy of one of these digestive enzymes means less efficient digestion of the food. This has clinical manifestations such as constipation due to bad digestion and fats in the stool, in addition to weight loss due to lack of nutrients.
The building of macromoecules in the body such as proteins and carbohydrates is called anabolism, while the degradation of macromolecules is called catabolism.