Hormones are molecules that your body uses for communication, often between organs in different organ systems or over long distances in the body. There are two major types of hormones: steroid hormones and protein, or peptide, hormones. Peptide hormones tend to work more quickly than steroid hormones, and serve many vital roles in the body.
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Your body cells need to be able to communicate with one another in order for you, as an organism, to maintain cohesive function. There are many different mechanisms through which cells can communicate with one another, and hormones are one class of communication molecules. A hormone is a molecule that a group of cells releases into the bloodstream. The hormone then travels throughout the body, explains Dr. Lauralee Sherwood in her book "Human Physiology," and is sensed only by target cells with receptors.
Protein-based hormones, more commonly called peptide hormones, are made up of amino acids, which are the structural building blocks of all proteins, including the proteins you eat. Peptide hormones tend to be globular, meaning that they have a ball-like shape. They also tend to be water-soluble, meaning they can dissolve in and travel easily through the blood, explains Dr. Reginald Garrett and Dr. Charles Grisham in their book "Biochemistry."
Protein Hormone Function
Unlike steroid hormones, which can cross through a cell's membrane and enter the cell, peptide hormones can't pass through the membrane. This is because cell membranes consist of fatty compounds called phospholipids, and protein-based hormones are water-soluble, not fat-soluble. Instead, peptide hormones bind to receptors on the outside of cells, which triggers a response inside the cell. Depending upon the specific peptide hormone in question, cellular responses vary significantly, but cells generally respond very quickly to peptide hormones.
Examples of Peptide Hormones
Perhaps the most widely recognized of the peptide hormones is insulin, which helps you regulate blood sugar after a meal. Glucagon, another blood sugar-regulating hormone, is also a peptide, as are prolactin and oxytocin. Prolactin stimulates milk production, while oxytocin stimulates milk letdown in lactating women. Oxytocin also stimulates feelings of emotional closeness in both men and women, and is sometimes colloquially known as the "cuddle hormone."