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Introduction to Endocrine System
In order for multicellular organisms to survive, the individual cells within that organism must be able to communicate with one another. This is known as intracellular communication. Cells communicate with one another using special types of molecules. The nervous system is one type of system that uses a chemical called the neurotransmitter in intracellular communication. The neurotransmitter travels only very short distances, is very specific to the type of cell it binds to and creates a very rapid but short-lived response. Another system that our body uses to communicate is the endocrine system. The endocrine system uses glands to create and release chemicals called hormones. The endocrine glands release the hormone into the blood stream or lymph system, which means the hormone circulates throughout the body and travels a long distance before it locates its target cell. Unlike neurotransmitters, hormones do not only bind to specific cells but rather bind to a wide range of different types of cells. Hormones travel a long distance, are slow-acting and can affect the organism in the long-term. Endocrine glands should not be confused with exocrine glands, which are glands that release chemicals through a duct and into some external environment. The sudoriferous gland is an example of an exocrine gland. The endocrine system uses hormones and hormones come in three different types. Peptide hormones are synthesized in the rough ER, packaged and modified in the Golgi complex and released into the blood stream. They dissolve in the blood because they are peptide-based and do not need any carrier proteins. Once they arrive at the target cell, they cannot pass across the cell membrane because the membrane is mostly hydrophobic. Therefore peptide hormones bind to receptor proteins found in the membrane of the target cell. Once bound, they usually use some sort of secondary messenger system to induce a change in the cell. Steroid hormones are made from cholesterol or other lipids in the smooth ER or the mitochondria. They are released into the blood stream and use a protein carrier because they are lipid-soluble. Once they arrive at the target cell, they can easily pass across the cell membrane and bind to the receptor protein in the cytosol of the cell. They then enter the nucleus, where the steroid hormone will induce some sort of transcriptional change. Tyrosine-derivative hormones come in two types - water-soluble and lipid-soluble. Water-soluble tyrosine hormones bind to receptor proteins on the plasma membrane while fat-soluble go directly to the nucleus of the target cell to induce a response.
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