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bioa02 chapter 41

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Biological Sciences
Mary Olaveson

Chapter 41 41.1 What are hormones and how do they work? - the chemical signals are hormones, secreted by cells of the endocrine system into the extracellular fluid - the cells that secrete hormones are called endocrine cells, and the cells that have receptors for those hormones are called target cells - hormones secreted in the extracellular fluid can diffuse into the blood, so they can activate target cells far from the site of release o this is called circulating hormones - these hormones affect only target cells near their release site, they are called paracrine hormones - when a hormone influences the cell that releases it, it is said to have an autocrine function o can provide negative feedback to control rates of secretion - many hormones are secreted by aggregations of endocrine cells forming secretory organs called endocrine glands o they do not have ducts - exocrine glands have ducts that carry their produces to the surface of the skin o sweat gland - plants do not have nervous systems, but they do have hormones - the largest group of arthropods are insects and they have rigid exoskeleton o their growth is episodic; punctuated with molts o each growth stage between two molts is called in instar - the two hormones working in sequence regulate molting o prothoracictropic hormone (PTTH) (brain hormone) o ecdysone - PTTH is transported to and stored in a pair of structures called corpora cardiaca attached to the brain o After stimulation, the PTTH released from these structures and it diffuses in the extracellular fluid to an endocrine gland, the prothoracic gland o PTTH stimulates the prothoracic gland to release the hormone ecdysone Ecdysone diffuses to target tissues and stimulate molting - Ecdysone is a lipid-soluble steroid molecule that readily enters its target cells o In the target cells, it binds to a receptor that is probably ancestral to the vertebrate testosterone receptor o The hormone-receptor complex acts as a transcription factor and induces expression of the genes for enzymes involved in digesting the old cuticle and secreting a new one o The control of molting by PTTH and ecdysone is a general arthropod hormonal control mechanism and is an example of how a hormonal system works with the nervous system to integrate diverse information that help determine the optimal timing for growth and development o The nervous system (the brain) then controls the endocrine gland (the prothoracic gland) producing the hormone (ecdysone) that orchestrates the physiological processes involved in development and molting - Because the head of Rhodnius is long, it is possible to remove just the front part of the head, which contains the brain, while leaving the rear part of the head, which contains the brain, while leaving the rear part intact - The substance responsible for preventing maturation is juvenile hormone, which is released continuously from the same structures that release PTTH in response to feeding o As long as this is present, Rhodnius molts into another juvenile instar o Usually stop after the fifth instar - As long as juvenile hormone is present in high concentrations, larvae molt into larvae - When the level of juvenile hormone falls, larvae spin cocoons and molt into pupae o Because no juvenile hormone is produced in pupae, they molt into adults - The majority of hormones are peptide or polypeptide o These hormones are water-soluble and are thus easily transported in the blood, but they cannot pass readily through lipid-rich cell membranes o Therefore, peptide and protein hormones are packaged in vesicles in the cell that make them and are released by exocytosis - Steroid hormones such as testosterone and estrogen are derivatives of the steroid cholesterol o Are lipid-soluble and easily dissolved in and pass through cell membranes o They diffuse out of the cells that make them as they are synthesized o Because steroid hormones are not soluble in blood, they much be bound to carrier proteins in order to be transported to their target cells - Amine hormones are mostly derivatives of the amino acid tyrosine o Some amine hormones are water-soluble and others are lipid-soluble o Their modes of release differ accordingly - Lipid-soluble hormones can diffuse through plasma membranes, and therefore their receptors are inside the cell, in their the cytoplasm or the nucleus - Water-soluble hormones cannot readily pass through plasma membranes, so their receptors are on the cell surface o These receptors are large glycoprotein complexes with three domains Binding domain projects outside the plasma membrane Transmembrane domain that anchors the receptor in the membrane Cytoplasmic domain extends into the cytoplasm of the cell o The cytoplasmic domain initiates the target cells response by activating protein kinases or protein phosphatases
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