BL1011 - Cell Structure - Lecture 4 - Exocytosis.pptx

THE ENDOMEMBRANE  Series of membrane-bound organelles evolved from plasma membrane by invagination  As cells increased in size, maintains membrane surface area to volume ratio THE SECRETORY PATHWAY The part of the endomembrane system involved in the synthesis, modification and sorting...

THE ENDOMEMBRANE  Series of membrane-bound organelles evolved from plasma membrane by invagination  As cells increased in size, maintains membrane surface area to volume ratio THE SECRETORY PATHWAY The part of the endomembrane system involved in the synthesis, modification and sorting of secreted proteins and transmembrane proteins. THE SECRETORY PATHWAY  Comprises:  Endoplasmic Reticulum (ER)  Golgi Apparatus  Secretory and Transport Vesicles  Membrane-bound organelles THE ENDOPLASMIC RETICULUM  A series of hollow tubules, vesicles and sacs (cisternae)  Extends throughout cell  Continuous lumen  Up to 50% of cell’s membrane content  THE ENDOPLASMIC RETICULUM Structure  Phospholipid bilayer  Dense cisternae towards cell centre  Tubules towards periphery  Extends throughout cell  Continuous with nuclear envelope THE ROUGH ER  Cytoplasmic surface studded with ribosomes  Site for protein Synthesis  Secretory  Transmembrane  Resident Electron Micrograph of rER  PROTEIN TRANSLOCATION Proteins made by ribosomes on rER are translocated into the ER Signal sequence of nascent protein recognised Threaded into the translocon Soluble proteins released into ER lumen Transmembrane proteins released sideways into membrane PROTEIN TRANSLOCATION - SOLUBLE Ribosome Newly synthesised protein Cytosol Translocon Chaperone ER lumen THE ROUGH ER  Once in the ER lumen:  The Protein folds (aided by chaperones)  Assembly of subunits  Glycosylation  Quality Control  Mis-folded proteins are detected by the chaperones  Unfolded protein response clears mis-folded proteins THE SMOOTH ER No ribosomes Generally tubular Periphery of the cell Functions: Synthesis of lipids (FAs) and steroids Calcium homeostasis Detoxification (Liver) SARCOPLASMIC RETICULUM  Specialised form of sER  Found in muscle cells  Stores and Releases Ca2+ to trigger contraction ER EXIT  Proteins leave the ER via COP-coated vesicles  Vesicles moved to the Golgi Apparatus along microtubules THE GOLGI APPARATUS  Series of flattened sacs (cisternae)  ER side = Cis Golgi  Middle = Medial Golgi  Cell membrane side = Trans Golgi THE GOLGI APPARATUS Transmission EM THE GOLGI APPARATUS Green: Golgi Red: Mitochondria  THE GOLGI APPARATUS Functions: Post-translational Modifications Modification of carbohydrates Addition of new carbohydrates Sorting into vesicles for transport Plasma membrane Endosomes/Lysosomes VESICLE TRAFFICKING  SECRETION Via exocytosis Two forms: 1. Constitutive secretion 2. Regulated secretion  CONSTITUTIVE SECRETION  Continuous secretion  Uncontrolled secretion  Examples:  Serum proteins, e.g. albumin, from hepatocytes  Extracellular matrix proteins, e.g. collagen, from fibroblasts REGULATED SECRETION  Proteins are packaged into dense secretory granules  Granules stored in the cytoplasm  Released only upon stimulation  Hormones  Depolarisation  E.g. mucus, insulin, neurotransmitters