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Hormones and the Endocrine System

Hormones and the Endocrine System - Hormones and the...

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Hormones and the Endocrine System Hormones and other signaling molecules bind to target receptors, triggering specific response pathways o Types of Secreted Signaling Molecules Signaling molecules bind to specific receptor proteins on or in target cells. Endocrine cells secrete hormones, which travel through the bloodstream. Endocrine glands are ductless sescretory organs composed of ogroups of endocrine cells. Hormones regulate growth, development, and reproduction. They also maintain homeostasis. Local regulators are secreted molecules that reach their target cells by diffusion. Paracrine signaling acts on neighboring cells; Autocrine signaling acts on the secreting cell itself. Neurotransmitters are secreted by neurons at synapses with other neurons and muscles. Neurosecretory cells are specialized brain neurons that secrete molecules called neurohormones, which travel through the bloodstream to target cells. Pheromones are signaling molecules released into the environment that communicate between different individuals. o Chemical Classes of Hormones 3 groups of hormones include polypeptides (peptides & proteins), Amines, and steroid hormones. Polypeptides and many amines are water-soluble; steroids are lipid-soluble. o Hormone Receptor Localization Receptors for steroid and lipid-soluble hormones have been identified inside cells. Water-soluble hormones bind to cell membranes. o Cellular Response Pathways Water-soluble hormones are released by exocytosis, travel through the bloodstream, and bind to cell-surface receptors, thriggering a cytoplasmic response or change in gene expression. Lipid-soluble hormones travel in the bloodstream bound to transport proteins, enter target cells, and bind to a receptor in the cytoplasm or nucleus, triggering changes in gene transcription. Binding of H20-soluble hormones to plasma membrane receptors initiates signal transduction, which converts extracellular signals to specific intracellular responses. For instance, the binding of epinephrine, released in response to stress by the adrenal gland, to G-protein coupled receptors
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on liver cells initiates a signal transduction pathway that results in the release of glucose into the bloodstream. Steroid hormones bind to cytoplasmic receptors, and the hormone- receptor complex then moves into the nucleus, where it stimulates transcription of specific genes. Thyroxine, vitamin D, and other non-steroid lipid-soluble hormones bind with receptors inside the nucleus. o Multiple Effects of Hormones A given signal can have different effects on different target cells and can produce different effects in different species. These differences may result from the types of receptors For example, the α-type and β-type epinephrine receptors Differences may also result from specific signal transduction pathways present, or particular transcription factors activated.
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