Membrane Structure & Function The Plasma Membrane 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

Endocytosis Continued Receptor-mediated endocytosis = process of importing specific macromolecules into the cell by inward budding of vesicles from coated pits; response of ligands binding to surface receptors More discriminating than pinocytosis Ligand = a molecule that binds to a specific receptor site of another molecule Coated pits = membrane-embedded proteins with specific receptor sites exposed to the cell’s exterior Clathrin = layer of fibrous protein; lines and reinforces the coated pit on the cytoplasmic side; helps form vesicle

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