BIOLOGY CH 11 Signal transduction pathway series of steps converting signals from cell surface to cellular response o Exchange of mating factors mating new cells made Concentration of signaling molecules allows bacteria to sense local population density in QUORUM SENSING ex Dental plaque Local signaling very direct o Embryonic development o Immune response o Cell junctions o Cell cell recognition local signals o Plasmodesmata between plant cells Local signaling by messanger molecules across a short distance o Paracrine signaling growth factors your skin cells are not dividing because they recognize they are surrounded by other skin cells If you are wounded and adjacent cells are removed then remaining cells can divide o Synaptic signaling electrical signal triggers release of neurotransmitter Neurotransmitter diffuses across a synapse Long distance signaling o Endocrine hormonal signaling Hormones can affect cells at a great distance in multicellular organisms Plant vs animal hormones o Auxins ethylene some cytokinins plant growth factors plant hormones o Testosterone estrogen adrenalin insulin animal hormones Signaling molecule ligands o Reception transduction response Most signaling molecules LIGANDS are too big or too polar or both to diffuse across the cell membrane They bind to RECEPTOR PROTEINS on the CELL SURFACE and cause a conformation change that generates a signal inside the cell Most receptors are plasma membrane proteins 3 main types of MEMBRANE RECEPTORS o Gprotein coupled receptors GPCRs Cell surface transmembrane receptors that work with the help of a G protein G proteins bind to energy rich GTP guanosine Triphosphate all very similar in structure GPCR systems extremely widespread and diverse in their functions Many extracellular signals are transmitted through a pathway that involves GTP GDP is inactive GTP is active o When GTP stays active cholera pest control 60 of all drugs interfere with cell signaling through G proteins Receptors for Leutinizing hormone LH Follicle stimulating hormone FSH Serotonin dopamine etc Attach phosphate to Tyrosines another major class and triggers many signal transduction pathways AT ONCE Ex Receptor for growth hormones insulin Inactive monomers come together for an active dimer fully activated when the dimer is phosphorylated o Receptor tyrosine kinases o Ion channel receptors Ligand gated ion channel receptor Many neurotransmitters such as o Intracellular receptors acetylcholine bind to gated ion channel proteins and open them to allow lots of ions to pour down their electrochemical gradient IN OR OUT of the cell Ligand opens the gate In cytoplasm or nucleus of the target cell Hydrophobic hormones steroid and thyroid diffuse across the plasma membrane and bind to these receptors This binding causes a change in conformation Activated receptor hormone complex is transported into the nucleus where the protein binds DN and activates specific hormone responsive genes SIMPLIST PATHWAY In many pathways the signal transmitted by a cascade of protein phosphorylaions Protein kinases transfer phosphates PROTEIN PHOSPHATASES remove the phosphates from proteins a process called DEPHOSPHORYLATION A molecular switch Many signaling pathways involve SECOND MESSENGERS o Small non protein water soluble molecules or ions that spread throughout the cell by diffusion Ex cAMP cGMP active eg Viagara Ca2 IP3 inositol phosphate DAG diacyl glycerol Regulation of the response o A response to a signal may not be simply on or off FOUR aspects of signal regulation Amplification of the signal enzyme cascades the number of activated products is much greater in the preceding step Specificity of the response different cells different protein Overall efficiency of response enhanced by scaffolding proteins Termination of the signal ligand concentration falls unbound receptors o How do signaling molecules affect cells differently and specifically Different cells have different proteins that are targeted Scaffolding proteins hold relay proteins in place to allow RAPID COMMUNICATION o Apoptosis programmed cell death Important for normal growth and development Integrates multiple signaling pathway CASPASES are the main proteases enzymes that cut up proteins to carry out apoptosis Apoptosis can be triggered by Extracellular death signaling ligand Dna damage in the nucleus Protein misfolding in the endoplasmic reticulum
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