Membrane Structure & Function Lecture #2 Membrane Structure & Function Cell Communication
Membrane structure, I Selective permeability Amphipathic~ hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model
Membrane structure, II Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization “Mosaic” Structure~ Integral proteins~ transmembrane proteins Peripheral proteins~ surface of membrane Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins
Membrane structure, III Membrane protein function: •transport •enzymatic activity •signal transduction •intercellular joining •cell-cell recognition •ECM attachment
Membrane traffic Diffusion~ tendency of any molecule to spread out into available space Concentration gradient Passive transport~ diffusion of a substance across a biological membrane Osmosis~ the diffusion of water across a selectively permeable membrane
Water balance Osmoregulation~ control of water balance Hypertonic~ higher concentration of solutes Hypotonic~ lower concentration of solutes Isotonic~ equal concentrations of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis~ plasma membrane pulls away from cell wall
Specialized Transport Transport proteins Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Active transport~ movement of a substance against its concentration gradient with the help of cellular energy
Types of Active Transport Sodium-potassium pump Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane •phagocytosis •pinocytosis •receptor-mediated endocytosis (ligands)
Exocytosis secretes macromolecules by the fusion of vesicles with the membrane Transport vesicles leave the trans face of the Golgi apparatus Secretory Cells Pancreas cells – Insulin Neurons – Neurotransmitters Plant Cells – Carbohydrates for cell wall
Endocytosis new vesicles from the exterior of the plasma membrane form Almost the reverse of exocytosis Three types: Phagocytosis – “cellular eating” Pinocytosis – “cellular drinking” Receptor mediated – acquire specific substances Human cells: Used to take in cholesterol for membrane synthesis
Protein Synthesis All cell made proteins start in the nucleus as DNA DNA is copied into an mRNA strand that leaves the nucleus mRNA, rRNA and tRNA combine to synthesize a protein Different proteins are synthesize on the ER and Free Ribosomes in Eukaryotes
Proteins Free ribosomes will synthesize proteins that are used within the cell Ribosomes on the ER synthesize proteins that leave the cell and are used in signaling Glycoproteins (polysaccharides + amino acids)
A side note to signaling Smooth ER is responsible for the synthesis of lipids and hormones Hormones are large molecules composed of lipids and modified amino acids These molecules are used as signal molecules in both short distance and long distance signaling in the endocrine system
Signal-transduction pathway Def: Signal on a cell’s surface is converted into a specific cellular response Local signaling (short distance): √ Paracrine (growth factors) √ Synaptic (neurotransmitters) Long distance: hormones
Cell Signaling Earl W. Sutherland, Jr. Vanderbilt University Nobel Prize, Physiology, 1971 Epinephrine’s ability to convert blood glycogen to glucose in the liver
Stages of cell signaling Glycogen depolymerization by epinephrine 3 steps: •Reception: target cell detection •Transduction: single-step or series of changes •Response: triggering of a specific cellular response
Receptors Monomers – single receptor molecules that initiate signal with recognized binding; can activate relay proteins or enzymes Dimers – both monomers must have a recognized signal molecule which causes them to form a dimer, which can activate relay proteins
Protein phosphorylation Protein activity regulation Adding phosphate from ATP to a protein (activates proteins) Enzyme: protein kinases (1% of all our genes) Example: cell reproduction Reversal enzyme: protein phosphatases
Second messengers Non-protein signaling pathway ( Example: cyclic AMP (cAMP) Ex: Glycogen breakdown with epinephrine Enzyme: adenylyl cyclase G-protein-linked receptor in membrane (guanosine di- or tri- phosphate)
Cellular responses to signals Cytoplasmic activity regulation Cell metabolism regulation Nuclear transcription regulation