Chapter 3 Notes (Part 1)
Chapter 3 Notes (Part 1) ZOOL 4380
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This 9 page Class Notes was uploaded by Tiffany Schweda on Saturday January 23, 2016. The Class Notes belongs to ZOOL 4380 at University of Texas at El Paso taught by DR. ZAINEB AL-DAHWI in Spring 2016. Since its upload, it has received 64 views. For similar materials see Vertebrate Physiology in Animal Science and Zoology at University of Texas at El Paso.
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Date Created: 01/23/16
Chapter 3: The Cellular Basis of Animal Physiology (Part 1) Overview Everything that an animal does is due to the communication between the cells Communication happens when a signaling cell sends a signal to the target cell Chemical messengers can travel from signaling cells to nearby target cells by diffusion in a process called paracrine communication Messengers can even affect the signaling cell in a process called autocrine communication For long-distance cell to cell communication, animals use the endocrine system and nervous system In endocrine system the chemical messenger travels from the signaling cell to the target cell carried by the circulatory system Endocrine messengers are called hormones Types of cell signaling/cell communication o Direct cell signaling o Autocrine and paracrine cell signaling o Endocrine cell signaling o Neural signaling o Exocrine signaling Exocrine communication system is when animals send chemical messengers between individuals (pheromones) Biochemical Basis of Cell Signaling Cells are separated from their environment by a phospholipid membrane Most chemicals are either hydrophilic or hydrophobic which means that sending a chemical messenger from one cell to another can present a challenge General Features of Cell Signaling Gap junctions o composed of interlocking cylindrical proteins (for vertebrates: connexins) assembled in groups of 4 or 6 to form doughnut-like pores (for vertebrates: hemichannels or connexons) o specialized protein complexes hemichannels of two adjacent cells come together to form a hollow tube, connecting the two cells via an aqueous bridge gap junctions are between two adjacent cells in most physiological situations direct communication via gap junctions involves the movement of ions between cells Figure 1: (a) direct cell signaling (b) autocrine and paracrine signaling (c) endocrine signaling (d) neural signaling Figure 2: Gap junctions. Proteins called connexins (for vertebrates) or innexins (for invertebrates) form the structure of the gap functions Feature Autocrine/Paracrine Nervous Endocrine Exocrine Secretory Cell Various Neural Endocrine Various Target Cell Most cells in body Neuron, muscle, Most cells in Sensory and neural endocrine body Signal Type Chemical Electrical and chemical Chemical Chemical Maximum Short Long intracellularly, shortLong Very long Signaling Distance across synapse Transport Extracellular Synapse Circulatory External system environment Speed Rapid Rapid Slower Various Duration of Short Short Longer Various Response Chart 1: Comparison of systems 3 Major Characteristics 1: Release of a chemical messenger from the signaling cell into the extracellular environment 2: transport of chemical messenger through extracellular environment to the target cell 3: communication of the signal to the target cell via receptor binding Indirect Signaling Systems Autocrine and paracrine communication the messenger simple diffused through the extracellular fluid from the signaling cell to the target cell Intracellular signaling also occurs across short distances in the nervous system at a structure call the synapse, a region where the signaling cell and the target cell are very close together Endocrine system can regulate the activities of distant cells, tissues and organs by sending chemical signals through the blood in the form of hormones In exocrine communication chemicals called pheromones are released by one individual and travels through the external environment to exert its effects on a different individual Only neurons act as the secretory cells in nervous communication Some neurons can secrete neurotransmitters directly into the circulatory system Secretory cells of exocrine and endocrine tissues are often grouped into structures called gland Figure 3: Structure of exocrine and endocrine glands. Exocrine glands secrete chemicals into ducts that lead to the surface of the body. Endocrine glands secrete hormones directly into the circulatory system Structure of Messenger Determines Type of Signaling Mechanism Six main classes of chemicals that are known to participate in cellular signaling 1. Peptides 2. Steroids 3. Amines 4. Lipids 5. Purines 6. Gases Feature Hydrophilic Messengers Hydrophobic Messengers Storage Intracellular vesicles Synthesized on demand Secretion Exocytosis Diffusion across membrane Transport Dissolved in extracellular fluids Short distances: dissolved in extracellular fluid Long distances: bound to carrier proteins Receptor Transmembrane Intracellular or transmembrane Effects Rapid Slower or rapid Chart 2: Comparison of Hydrophilic and Hydrophobic Chemical Messengers Mechanisms for hydrophilic or hydrophobic messengers o Peptides Released by exocytosis Most peptide hormones and neurotransmitters are synthesized in advance and stored for later use Peptide hormones are often synthesized as large and inactive polypeptides known as preprohormones Signal sequence is cleaved from preprohormone prior to being packed into secretory vesicles (forming prohormone) Secretory vesicle contains proteolytic enzymes that cut prohormone into the active hormone(s) Paracrine peptides, such as cytokines, are synthesized only on demand Dissolve in extracellular fluids Rate of breakdown is measured as the messenger’s half-life Bind to transmembrane receptors Hydrophilic signaling molecules such as peptides and proteins can’t pass through the membrane of target cell Extracellular portion of transmembrane receptor contains the ligand binding domain Figure 4: Synthesis of peptide hormones. Peptide hormones synthesized by ribosomes on the rough endoplasmic reticulum Figure 5: Synthesis of arginine vasopressin (AVP). AVP is synthesized on the rough endoplasmic reticulum as a large polypeptide (preprovasopressin), which contains a signal peptide (SP), neurophysin (NHP), and a glycoprotein (GP). Provasopressin passes to the Gogli apparatus where it is packed into vesicles. In secretory vesicles the provasopressin is cleaved into three peptide: AVP, NPH and GP Figure 6: Structure of a transmembrane receptor (a) transmembrane receptors have extracellular ligand-binding domain, membrane-spanning domain and intracellular domain (b) when ligand binds to receptor the conformation of receptor changes o Steroids Derived from the molecule cholesterol Important hormones for both vertebrates and invertebrates Mineralocorticoids are involved in regulating sodium uptake by the kidneys Important for fluid and electrolyte balance in body Glucocorticoids have widespread actions Increasing glucose production Increasing breakdown of proteins into amino acids Increasing release of fatty acids from adipose tissue Regulating immune system Inflammatory responses Glucocorticoids are known as the stress hormones Bind to carrier proteins Steroids can easily pass through biological membranes and can’t be stored within the cell o Must be synthesized on demand Diffuse across short distances dissolved in extracellular fluids Long distances they are bound to carrier proteins Some steroids bind to specific proteins Others bind to generalized proteins such as albumin (principle carrier protein in blood) Bind to intracellular receptors Lipophilic steroids can easily cross the membrane of target cells o Can bind either to transmembrane receptors or receptors in cell Intracellular steroid receptors act as transcription factors Figure 7: Transport of hydrophobic chemical messengers Terminology from the Chapter Gap junctions: specialized protein complexes that create an aqueous pore between the cytoplasms of two adjacent cells Receptor: protein or cell that can detect an incoming stimulus Signal transduction pathways: biochemical pathways in which a change in conformation of a receptor protein in the target cell is converted to a change in the activity of that cell Paracrine: type of chemical messenger that is involved in local signaling between nearby cells; paracrine messengers move through the interstitial fluid diffusion Autocrine: a type of cell signaling in which a single cell signals another cell of the same type, including itself Endocrine system: the collective name for the group of glands and other tissues that secrete hormones into the circulatory system Nervous system: network of neurons and their supporting cells Neurotransmitter: chemical messenger released from a neuron into the synaptic cleft Hormones: type of chemical messenger that is carried in the blood and thus can act across long distances (classically defined as being a substance released from an endocrine gland and active at very low concentrations Synapse: junction between a neuron and another neuron or effector cell; consists of a presynaptic cell, synaptic cleft and a postsynaptic cell Pheromone: chemical messengers released by an animal into the environment that have an effect on another animals of the same species Neurohormone: chemical messenger released from a neuron into the blood Gland: specialized organ that secretes hormones Cytokines: hormones that trigger cell division Preprohormone: large inactive polypeptide that is a precursor to a peptide hormone Prohormone: polypeptide formed by the cleavage of a preprohormone; a precursor to the formation of a peptide hormone Half-life: period of time required for half of a population of molecules to be converted to another form; often applied to radioactive decay Transmembrane receptors: Ligand: chemical that specifically and reversibly binds to a receptor or enzyme Glucocorticoids: steroid hormones involved in the stress response that regulate carbohydrate, protein and lipid metabolism Mineralocorticoids: steroid hormones involved in water and ion balance Carrier proteins: blood proteins that help to transport hydrophobic molecules (such as steroid hormones) in the blood Albumin: binding globulin (carrier protein) that is one of the primary proteins of vertebrate plasma; makes a major contribution to blood osmotic pressure Amine: class of molecules based on ammonia, with a side group substituting for at least one N atom Biogenic amine: class of neurotransmitters derived from amino acids including the catecholamines and dopamine Catechholamines: the biogenic amines epinephrine and norepinephrine Dopamine: neurotransmitter (biogenic amine) produced in various regions of the vertebrate brain Norepinephrine: catecholamine neurotransmitter; in vertebrates, released by the sympathetic nervous system Epinephrine: catecholamie that can act as a hormone or neurotransmitter and is involved in the stress response; also known as adrenaline Serotonin: neurotransmitter (biogenic amine) involved in setting mood and regulating blood flow to the brain Melatonin: hormone found in all animal groups that regulates sleep-wake cycles Histamine: amino acid; a regulatory molecule that is released from mast cells in response to an immunological challenge Acetylcholine: neurotransmitter found in most animal species in many types of neurons, inkling motor neurons and the autonomic ganglia of vertebrates Nitric acid: a gaseous neurotransmitter and paracrine chemical signal that is involved in regulating many physiological processes; important vasodilator vertebrates Eicosanoid: type of short lived chemical signaling molecule Agonist: a substance that binds to a receptor and initiates a signaling event. May include both the natural endogenous ligand as well as pharmaceutical agents that mimic the natural substance Antagonist: substance that binds to a receptor but does not stimulate a signaling event. Antagonists interfere with the binding of the natural ligand Down-regulation: decrease in the amount or activity of a protein or process; e.g. a decrease in receptor number or activity on a target cell Up-regulation: increase in protein number or activity in a target cell Dissociation constant: a measure of the tendency of a complex to dissociate into its components; calculated as the ratio of the product of the concentrations of the dissociated components to the concentration of the complex once the reaction reaches equilibrium Affinity constant: reciprocal of the dissociation constant Ligand-gated ion channel: an ion channel that opens or closes in response to the binding of a specific chemical G-protein-coupled receptors: a transmembrane receptor that interacts with a G protein G protein: type of trimeric membrane protein, associated with specific transmembrane receptors, that plays a role in signal transduction. G proteins bind guanine nucleotides; when bound to GDP the G protein is inactive, but when bound to GTP it is active. The alpha subunit of the G protein of the G protein moves through the membrane and acts in subsequent steps in the signal transduction pathway Second messenger: short lived intracellular messenger that acts as an intermediate in a signal transduction pathway Calmodulin: calcium-sensing protein involved in many signal transduction pathways Cyclic adenosine monophosphate: (cAMP) cyclic adenosine monophosphate formed by the action of adenylate cyclase; a second messenger that activates kinase A Inositol triphosphate: second messenger in the phosphatidylinositol signaling system Diacylglycerol: second messenger in the phosphatidylinositol signaling system Set point: in a homeostatically controlled system, the level at which the regulated variable is maintained Tropic (trophic) hormones: hormones that cause the release of other hormones Insulin: peptide hormone that homeostatically regulates blood glucose levels Pancreas: vertebrate organ that produces endocrine hormones including insulin and glucagon and also produces exocrine secretions that are involved in digestion Islets of Langerhans: clusters of endocrine cells in the pancreas that produce the hormones glucagon and insulin Pancreatic β cells: cells within the vertebrate pancreas that secrete the hormone insulin Glucagon: hormone produced by the vertebrate pancreas that inhibits glycogen synthesis and stimulates glycogen breakdown, resulting in an increase in blood glucose Adrenal gland: a gland near the kidney, which in mammals is composed of an outermost layer (adrenal cortex) and an inner layer (adrenal cortex) Adrenal cortex: outermost layer of the adrenal gland Adrenal medulla: inner layer of the adrenal gland Chromaffin cells: cells that secrete the hormone epinephrine. In mammals they are located in the compact adrenal medulla, but in other vertebrates they are more dispersed
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