1/29/2015 Hormones and the Body | Principles of Biology from Nature Education contents Principles of Biology 138 Hormones and the Body Hormones are chemical messengers that coordinate functions in the body in response to changes in an animal's internal environment or stimuli from the outside world. Hormones regulate growth and development. From fertilization through adulthood, hormones mediate growth and development. Hormones even play a role in formation of the bond between mother and child. Picture Partners/Science Source. Topics Covered in this Module The Chemical Structure of Hormones Hormones and Homeostasis When Hormone Regulation Fails Major Objectives of this Module Classify hormones based on their chemical structure and solubility. Describe how hormones trigger responses in target cells. Distinguish between negative and positive feedback loops and give an example of each. Explain the role of hormones in maintaining homeostasis. page 704 of 986 5 pages left in this module http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145728 1/1 1/29/2015 Hormones and the Body | Principles of Biology from Nature Education contents Principles of Biology 138 Hormones and the Body Most hormones can be classified based on their chemical composition and structure as either amine hormones, peptide hormones, or steroid hormones. Additionally, hormones can be classified as either water soluble or lipid soluble. The chemical properties and solubility of a hormone have consequences on how it is secreted, how it is transported through the blood, and how it signals to its target cell. The Chemical Structure of Hormones Hormones are chemical messengers produced in endocrine cells in one part of the body and transported through the blood to target cells in other parts of the body. Hormones carry out numerous regulatory functions in response to stimuli from both outside and inside the body. Hormones also regulate reproduction, growth, and the changes that occur during puberty. Hormones can be classified based on chemical structure. Hormones are a diverse group of organic compounds with a variety of chemical structures. The major hormones can be classified into three categories — amine hormones, peptide hormones, and steroid hormones. Amine hormones are all derived from single amino acids. Several hormones fall into this category, including thyroxine, dopamine, epinephrine, and norepinephrine, which are derived from the amino acid tyrosine, and melatonin and serotonin, which are derived from the amino acid tryptophan. Peptide hormones contain two or more amino acids joined by peptide bonds or are derivatives of such molecules. Some peptide hormones are relatively small molecules; for example, the hormones oxytocin and vasopressin are composed of only nine amino acids. Other peptide hormones are much larger; human growth hormone, for instance, is 191 amino acids in length. Steroid hormones are derived from cholesterol, a hydrophobic molecule containing four fused rings. Cholesterol is converted into steroid hormones in several endocrine glands, including the adrenal cortex, the testes (in males), and the ovaries (in females). Examples of steroid hormones include cortisol, testosterone, and progesterone. The solubility of a hormone, which is a function of its chemical structure, is important in determining how it is secreted, how it is transported through the body, and how it communicates its message once it reaches its target cell. All peptide hormones and most amine hormones are water soluble. Some amine hormones, including the thyroid hormones thyroxine and triiodothyronine, are lipid soluble. Steroid hormones, like their parent compound cholesterol, are lipid soluble. Water­soluble hormones cannot cross the lipid bilayer of plasma membranes and are typically released from endocrine cells by exocytosis. These hormones diffuse into the bloodstream and travel to target cells, which have cell surface receptors for the hormones associated with the plasma membrane. Hormone binding induces a conformational change in the receptor that activates a signal transduction pathway, triggering a response in the cell (Figure 1a). Most lipid­soluble hormones can freely diffuse through the plasma membranes of both the endocrine cell and the target cell. However, the lipid­soluble thyroid hormones thyroxine and triiodothyronine must be carried across the plasma membrane by transport proteins. Lipid­soluble hormones travel through the aqueous environment of the blood associated with transport proteins. Some receptors for lipid­soluble hormones are on the cell surface, and others are located inside the cell. To reach intracellular receptors, lipid­soluble hormones diffuse across the plasma membrane. The intracellular receptor­hormone complex moves into the nucleus, where it binds DNA and activates or inhibits expression of particular genes (Figure 1b). http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145728/1 1/2 1/29/2015 Hormones and the Body | Principles of Biology from Nature Education Figure 1: Transmission pathways of water­soluble and lipid­soluble hormones. Water­soluble hormones (left), which cannot pass through the plasma membrane, bind to a receptor on the outside of the target cell. Signal transduction transfers the signal from the outside of the cell to the inside. Lipid­soluble hormones (right), which can diffuse through cell membranes, often bind intracellular receptors that directly mediate a cellular response. © 2014 Nature Education All rights reserved. The Hypothalamus Integrates Nervous System Input and Endocrine System Output The nervous system senses external stimuli such as the scent of a predator and internal stimuli such as an increase in blood pressure. The endocrine system mediates a response to these stimuli. Thus, these two systems must be closely coordinated. In vertebrates, the hypothalamus, an endocrine gland located in the brain (Figure 1b), is the primary site at which sensory input is converted to endocrine output. The hypothalamus receives sensory input from neurons and sends this information to the pituitary gland, which extends from the bottom of the hypothalamus. The pituitary has anterior and posterior lobes. The posterior pituitary is an extension of the hypothalamus that contains axons from specialized neurons originating in the hypothalamus. The specialized neurons, called neurosecretory cells, secrete neurohormones that circulate in the blood and act on distant cells. The neurosecretory cells of the posterior pituitary produce two types of neurohormones: oxytocin and vasopressin (ADH, also known as antidiuretic hormone, or ADP). Vasopressin regulates water and salt balance, and oxytocin moderates behavior and stimulates mammary glands and uterine contractions. The anterior pituitary, which is a separate organ from the posterior pituitary, synthesizes and secretes hormones based on hormonal signals from the hypothalamus. Hormones secreted from the anterior pituitary include growth hormone (GH), thyroid­stimulating hormone (TSH), follicle­stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and prolactin (PRL). GH stimulates growth and metabolism. TSH stimulates the thyroid gland, which is involved in maintaining metabolic balance. FSH stimulates egg and sperm production. LH regulates ovaries and testes. ACTH causes the adrenal cortex to release glucocorticoids in response to long­term stress. Prolactin stimulates the production and secretion of milk. The hypothalamus regulates activity of other endocrine glands. For example, in response to stress, the hypothalamus stimulates secretions of the adrenal gland, which sits above the kidneys. The hypothalamus may activate two different stress­response pathways in the adrenal gland, depending on whether the stress is acute or long term. In an acute stress response, which is activated by a scary event such as encountering a tiger in the woods, a nerve signal is sent from the hypothalamus to the adrenal gland. In response, the middle part of the adrenal gland, called the adrenal medulla, releases the hormones epinephrine and norepinephrine into the blood. Epinephrine and norepinephrine stimulate the "fight­or­flight" response, which results in the breakdown of glycogen by the liver and an increase in breathing and heart rate. Glycogen breakdown frees up glucose, an energy source the body will need for fight or flight; the increased breathing rate enables greater intake of oxygen, which will be needed for the additional cellular respiration cells will be performing during fight or flight to generate energy; and the increased heart rate speeds the delivery of that oxygen to the cells that will be performing cellular respiration. In long­term stress, which is activated by a stressful event such as loss of a job, the hormone corticotropin­releasing hormone (CRH) is released from the hypothalamus and travels through blood to anterior pituitary. CRH stimulates the anterior pituitary to secrete adrenocorticotropin hormone (ACTH), which travels to the adrenal gland through the bloodstream. In response, the outer part of the adrenal gland, called the adrenal cortex, releases glucocorticoids. Glucocorticoids promote breakdown of fats and proteins to increase blood glucose levels, and reduce immune function. Increased levels of glucocorticoids are associated with improved memory and vigilance, which are presumably needed to get an animal through a stressful situation. However, increased this vigilance takes an