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Receptors & Cellular Transport Mechanisms. James Peerless May 2011. Objectives. Cellular Transport Mechanisms Types Mechanisms Receptors What are receptors for? Types Messenger systems. Membrane Transport Mechanisms. Membrane Transport of Substances.
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Receptors & Cellular Transport Mechanisms James Peerless May 2011
Objectives • Cellular Transport Mechanisms • Types • Mechanisms • Receptors • What are receptors for? • Types • Messenger systems
Membrane Transport of Substances • Membranes control movement of many types of particle between intra- and extracellular compartments • Phospholipidbilayer allows diffusion of water, small molecules and lipid-soluble substances • Passive and active mechanisms
Simple Diffusion • Random movement of molecules • Passive process • Net movement occurs down a concentration gradient • kp dependent upon: • Temperature, membrane permeability • molecular properties (lipophilicity, charge, MW) Q = kpA(C1-C2)/T
Facilitated Diffusion • Molecule moves down concentration gradient • Rate determined by carrier and solute concentration • Rate of transport is ‘facilitated’ by a carrier molecule • Carrier • Channel
Active Transport • Specialized facilitated diffusion • Mediated by membrane carrier proteins & requires energy • Transport of substances against their concentration gradient • Affecting transport: • Carrier saturation and density of carriers • Speed of carrier conformational change
Secondary Active Transport • Transport of a substance and an ion together • No direct energy input
Exo- & Endocytosis • No transport of substances through membrane • Vesicles formed by invagination of the membrane • Pinocytosis is the specialized uptake of water
Receptors “A molecule that recognises specifically a second molecule whose binding brings about the regulation of a cellular process.” Lambert DG (2004). Drugs & Receptors. Continuing Education in Anaesthesia, Critical Care & Pain; 4 (6): 181-4
Role of Receptors • Cell communication • Chemical messengers can be local or widespread • Regulation, mediation and amplification of signals • Allows homeostatic control
Properties Ligands Receptors Specificity Sensitivity Saturation Down- & Up-regulation • Affinity • Competition • Activity (agonist/antagonist) • Half-life • Lipid solubility
Signal Transduction • Membrane permeability • Membrane potential • Membrane transport • Contractile activity • Secretory activity • Protein synthesis
Clinically important Receptor Types • Ligand-gated ion channels • Acetylcholine receptors • G-protein coupled receptors • Adrenergic receptors • Tyrosine kinase coupled receptors • Insulin • Intracellular receptors • Steroids
The Ach Receptor • Pentameric, transmembrane structure • 2 α and β, γ, δ subunits • Ion channel opens when 2x Ach binds to α-subunits
G-protein Coupled Receptors • Extensive and important • Adrenergic • Muscarinic • Opioid • Act via second messengers • cAMP increased or decreased • Activation of protein kinases • Protein phosphorylation • Inactivated by phosphodiestereases
MCQ 1 • The rate of diffusion of a gas (Fick’s Law) across a membrane: (a) is directly proportional to the area (b) is directly proportional to the partial pressure gradient (c) is inversely proportional to thickness of the membrane (d) is directly proportional to the molecular weight (e) is inversely proportional to the density of the gas T T T F F
MCQ 2 The following receptors are part of a ligand-gated ion channel: • A) opioid mu receptor • B) muscarinic cholinergic receptors • C) nicotinic cholinergic receptors • D) GABAA receptors • E) GABAB receptors F F T T F
Summary • Cells have passive and active mechanisms for controlling passage of molecules across boundaries • Intercellular communication allows homeostatic control via the specific use of chemical messengers and cellular receptors