Introduction

1. Introduction Elaborate, complex, intracellular and intercellular • Animal cells complex system of proteins e.g. • Receptor proteins (surface and intracellular) • Protein kinases • Protein phosphatases • GTP-binding proteins (G-proteins) Signal molecules include proteins, nucleotides, steroid hormones, fatty acid derivatives, dissolved gas. Target cell, receptor, response AH Cell Signalling

2. Directcommunication Junctions Animal Tight – membranes fused preventing leakage Desmosome – filaments protrude giving linkages between cell sheets Gap – cytoplasmic channels so molecules can flow between cells Plant Plasmodesmata – channels in adjacent cell walls with plasma membrane lining. Connection of cytoplasm AH Cell Signalling

3. Stages of cell signalling Reception – transduction – response • Signal reception • Specific • target protein(receptor) • Signal molecule called ligand • Change in shape of receptor protein • Local or distant effects • 3 main types of membrane receptors enzyme linked, G-protein linked and Ion-channel linked. AH Cell Signalling

4. Signal transduction (transfer of signal within cell) • Staged in signal transduction pathway • Allows amplification • Activates large numbers of molecules • Cascade of molecular events Molecules involved Protein kinases - phosphorylate proteins, cascade ATP into ADP to activate proteins. Cyclic AMP – second messenger small non protein molecules Ca2+ - second messengers increase in ions causes response AH Cell Signalling

5. Response • Cell may respond by: • opening/closing of ion channels in membrane • rearranging cytoskeleton • synthesis or degradation of enzymes or • other proteins • change in gene expression AH Cell Signalling

6. Extracellular hydrophobic signalling Signals can diffuse across membrane Examples include hormones e.g. thyroxine, sex hormones and cortisol Activate gene regulatory proteins Occur in animals and plants Ecdysone – promotes moulting in arthropods Thyroxine – increases metabolism Cortisol – increases release of sugars Sex hormones – stimulate secondary sexual characteristics etc. Slower than electrical signals but may persist for hrs/days AH Cell Signalling

7. Extracellular hydrophillic cell signalling Act on surface of plasma membrane to induce a response inside cells. • 3 main types of receptor: • enzyme linked receptors • G-protein-linked receptors • ion-channel-linked receptors AH Cell Signalling

8. Receptor proteins Enzyme linked Give enzyme linked response (kinase) on cytoplasmic side, activating intracellular proteins G-protein linked receptors Activate a GTP-binding protein (G-protein) made of seven alpha helices. G-protein acts as switch for enzymes in the cell depending on binding of guanine nucletides GDP (gives inactive form) or GTP (gives active form) Ion-channel linked receptors Transmembrane proteins open with a signal molecule Allows rapid ion movement e.g. Synapses AH Cell Signalling

9. Cell signalling in plants Include: Auxins (IAA) – promotes elongation of shoots Cytokinins – stimulate cytokinesis Gibberellins – stimulate stem elongation and germination Abscisic acid (ABA) – promotes abscission Ethylene – gas that inhibits fruit ripening AH Cell Signalling

10. Stages in plant cell signalling Reception – transduction - responses Signals involve light, gravity, touch, water Responses include tropisms, general growth, flower/fruit production, leaf/fruit abscission AH Cell Signalling