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Click for larger image	Overview of Protein Metabolism Introduction: Nitrogen metabolism is no less important than carbohydrate and lipid metabolism. Proteins make up the structural tissue for muscles and tendons, transport oxygen or hemoglobin, catalyze all biochemical reactions as enzymes, and regulate reactions as hormones. Our bodies must be able to synthesize the many proteins, amino acids, and other non-protein nitrogen containing compounds needed for growth, replacement, and repair. Proteins in excess are used to supply energy or build reserves of glucose, glycogen, or lipids.
Click for larger image	Nitrogen Cycle: Nitrogen is recycled just as carbon and oxygen are recycled in nature. Various microorganisms have the appropriate enzymes to convert elemental nitrogen from the air into ammonia, nitrate, and nitrites. Green plants use the ammonia or nitrate as raw materials for the synthesis of amino acids and proteins. Animals and humans in turn use the plants to supply nitrogen to make amino acids and proteins. We humans are not as versatile as plants since we are unable to synthesize eight amino acids which must be included in the diet. Finally, the nitrogen cycle is completed when plant and animal residues are decayed by microorganisms back to nitrates, nitrites and finally nitrogen gas for the air.
Click for larger image	Nitrogen Pool: The "nitrogen or amino acid pool" is a grand mixture of amino acids available in the cell derived from dietary sources or the degradation of protein. Since proteins and amino acids are not stored in the body, there is a constant turnover of protein. Some protein is constantly being synthesized while other protein is being degraded. For example, liver and plasma proteins have a half-life of 180 days or more, while enzymes and hormones may be recycled in a matter of minutes or hours. Each day, some of the amino acids are catabolized producing energy and ammonia. The ammonia is converted to urea and excreted from the body and represents a drain on the nitrogen pool. A nitrogen balance is achieved by a healthy person when the dietary intake is balanced by the excretion of urea wastes. If nitrogen excretion is greater than the nitrogen content of the diet, the person is said to be in negative nitrogen balance. This is usually interpreted as an indication of tissue destruction. If the nitrogen excretion is less than the content of the diet, a positive nitrogen balance indicates the formation of protein. The dynamic balance of the nitrogen pool is summarized in the graphic on the left. Quiz: According to properties studied earlier, are lipids generally soluble or insoluble in the aqueous portion of blood? Explain.   Answer Lipids are generally nonpolar, therefore not very soluble in blood. There is a polar end which makes them slightly polar and soluble.