Lecture 9: Biomembranes- Composition and Organization of Lipid Bilayerso Biological membranes Are impermeable to water-soluble molecules and ions Have a gluey consistency with fluid-like properties- Phospholipids spontaneously form lipid bilayerso Leaflet  structure of phospholipid  minimizes contact of hydrophobic chains with water by aligning themselves tightly togethero No membrane can have an “edge” with exposed hydrocarbon fatty acyl chainso Endocytosis: Segment of plasma membrane buds inward and eventually pinches off as a separate vesicle- Phospholipid is a generic termo Refers to any amphipathic lipid with a phosphate-based head and 2-chain hydrophobic tailo Phosphoglycerides = most abundant  Glycerol backbone, two esterified fatty chains make up tail Saturated or unsaturated  Head is polar group estrified to phosphate 4 major head groups: Plasmalogens: group of Phosphoglycerides- Contain one fatty chain attached to glycerol by ester linkage and long hydrocarbon chain- Abundant in heart/brain tissue- Greater chemical stabilityo Sphingolipids = derived from sphingosine Structure consists of amino alcohol with long HC chain and long fatty acid attached in amide linkage to the sphingosine amino group Some have phosphate-based polar head group, most abundant being sphingolipid Sphingomyelin contains a phosphocholine head group Some have polar head groups as sugars and are not linked to tails via phosphate groups  glycolipidso Sterols (cholesterols): Basic structure is four-ring isoprenoid-based HC for tail and single polar hydroxyl for head group Sterols differ for different types of organisms:- Animals = cholesterol- Plants = stigmasterol- Fungi = ergosterol- Fluidity of the Membraneo Individual phospholipids can mov laterally throughout bilayero Factors affecting speed of molecules moving within and across layers include: Temperature  increase can cause disorder in tails, decreasing thickness of bilayer and making it more fluid-like  Structure of HC tails  long tails pack tightly, decreasing fluidity  kinks disrupttight packing and cause increase fluidity Number of cholesterol molecules in bilayer  cholesterol restricts random movements of tails  lower cholesterol causes slight increase in fluidity- Composition and Organization of Lipid Bilayerso Biomembranes are classified by its composition of lipids and proteinso Movement of membranes from one compartment to another can selectively enrich certain membranes  responses to environmento Lipid type can dictate membrane thicknesso Lipid composition affects membrane curvature Large Head, long tail  flat bilayer Small head, long tail  curved bilayer Combination of lipids  combination of shapeso Exoplasmic and cytoplasmic leaflets are composed of different lipidso PS ad PI lipids carry negative charge and therefore interact on cytoplasmic face with positively charged a.a. PI face cytoplasm  involved in cell signaling PS face cytoplasm  activates blood clotting enzymeso Flippases: relocate lipid between faces  requires ATPo Lipid rafts: microdomains  more ordered, less fluid- Membrane Proteins: Structure and Basic Functionso 3 types: integral, lipid-anchored, and peripheral Integral membrane proteins contain at least one hydrophobic membrane-spanning domains Transmembrane proteins and glycolipids are asymmetrically oriented in bilayero Integral proteins: 3 domains: cytosolic (hydrophilic), exoplasmic (hydrophilic), and membrane spanning (hydrophobic) Hydrophobic residues interact with HC core of bilayer All TM proteins have one or more alpha helices or multiple beta sheetso Lipid-anchored Covalently bound to lipids  does not enter bilayer Acylation: fatty acid attached to glycine residue Prenylation: HC attached to cysteine residue GPI anchor: attach cell-surface proteoglycanso Peripheral Do not contact hydrophobic core Interacts with integral or lipid-bound proteins Inside  can be cytoskeletal proteins Outside  can be ECM or cell wall proteins- Multimeric Membrane Proteinso Alpha-helical protein segments often have polar or charged residueso Side chains guide assembly/stability of multimeric proteinso Beta-sheet protein segments: Hydrophilic, hydrophobic