Membranes

Overview

  • Membranes are called a lipid bilayer

  • Basic composition   * Lipids - provide the foundation/support   * Proteins 

  • Percentage of lipid and protein composition depends on membrane location

  • Fluid Mosaic in which lipids and proteins are mobile - 2D fluid

  • Flexibility: membranes are able to bend

  • Functions may include   * Selective permeable barrier   * Protection   * Allows for compartmentalization   * Signals for compartmentalization   * Binding site for cytoskeleton   * Site of enzyme activity   * Transport and conductivity   * Cell to cell adhesion and attachment   * Antigenicity: has to do with recognizing cells or non-cells

  • Plasma membranes establish cell boundaries

  • Internal membranes form organelles   * Nucleus   * Lysosomes   * Mitochondria   * Peroxisomes   * Endoplasmic reticulum   * Chloroplasts   * Golgi apparatus

Membrane Structure

  • The framework of the membrane is the phospholipid bilayer
  • Phospholipids are amphipathic molecules   * Hydrophobic (water-fearing) region faces in   * Hydrophilic (water-loving) region faces out
  • Membranes also contain proteins and carbohydrates
  • The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component

Proteins Bound to Membranes

  • Integral or intrinsic membrane proteins    * Transmembrane proteins: region(s) are physically embedded in the hydrophobic portion of the phospholipid bilayer   * Lipid-anchored: an amino acid of the protein is covalently attached to a lipid
  • Peripheral or extrinsic membrane proteins: noncovalently bound either to integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids

Factors Affecting Fluidity

  • Length of fatty acyl tails: shorter acyl tails are less likely to interact, which makes the membrane more fluid
  • Presence of double bonds: a double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to \n interact and making the bilayer more fluid
  • Presence of cholesterol: cholesterol tends to stabilize membranes   * Effects vary depending on temperature

Membrane Phospholipids

  • Choline phospholipids   * Phosphatidylcholine (no net charge): most common lipid in cell membranes   * Sphingomyelin: only cell membrane not derived from glycerol
  • Non-choline phospholipids    * Phosphatidylserine (negatively charged)   * Phosphatidylethanolamine (neutral)   * Phosphatidylinositol (negatively charged)

Glycolipids

  • Glycolipids: lipids that are covalently bonded to monosaccharides or polysaccharides
  • Least common of membrane lipids 
  • Always found in the outer leaflet of the membrane (non-cytoplasmic side)   * Membrane glycolipids do not protrude into the cytoplasm
  • Functions of glycolipids   * Receptor binding    * Protection    * Self recognition 

Cholesterol

  • Cholesterol: Lipid soluble steroid 
  • Amphipathic
  • Found in both leaflets of the lipid bilayer 
  • Amount of cholesterol found is membrane-type and organism dependent   * Principle sterol in animal cells   * Not present at all in bacteria
  • Functions   * Regulating membrane fluidity and permeability      * Ie:  The higher the cholesterol concentration the greater the reduction in proton and sodium permeability   * Conductance (myelin sheaths are high in cholesterol)   * Cell signaling   * Intracellular transport

Microdomains

  • Microdomains: cholesterol and sphingomyelin enriched area of a membrane
  • Functions as a lipid “raft”   * As membranes are moved from one place to another in the cell     * endocytosis and exocytosis    * During signal transduction    * Viral and toxin entry   * Cell migration    * Site of calcium triggered membrane fusion

Cell Surface Carbohydrates

  • Membrane glycoproteins contain short oligosaccharides extending to the extracellular space
  • Proteoglycans  have one or more long polysaccharide attached
  • glycolipids + glycoproteins + proteoglycans = carbohydrate layer (external)   * Protects cell surface from mechanical and chemical damage   * Also important for cell-cell adhesion and recognition

Cell Cortex

  • Determines the shape of the cell and the mechanical properties of the plasma membrane
  • Fibrous network attached to the cytosolic surface of the plasma membrane 
  • Main component is spectrin    * Long thin flexible rod   * Intracellular attachment proteins attach spectrin to specific transmembrane proteins which serves to connect spectrin meshwork to the membrane

Membrane Protein Function

  • Transporters: Na+ pump actively pumps Na+ out of cells and K+ in
  • Anchors: integrins link intracellular actin filaments to extracellular matrix proteins
  • Receptors: PDGF receptor signals cause the cell to grow and divide
  • Enzymes: adenylyl cyclase catalyzes the production of intracellular cyclic AMP in response to extracellular signals

Transport Proteins

  • Transport proteins: transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes
  • Two classes based on type of movement   * Channels: form an open passageway for the direct diffusion of ions or molecules across the membrane     * Most are gated      * Example: Aquaporins   * Transporters: also known as carriers; conformational change transports solute across membrane     * Principal pathway for uptake of organic molecules, such as sugars, amino acids, and nucleotides 

Transporter Types

  • Uniporter: single molecule or ion
  • Symporter or cotransporter: two or more ions or molecules transported in same direction
  • Antiporter: two or more ions or molecules transported in opposite directions

Cells Can Restrict Movement of Proteins

  • Plasma membrane proteins are usually localized to specific areas within the bilayer = membrane domains 
  • Some proteins are linked to extracellular structures or tethered to the cell cortex
  • Cells can create diffusion barriers which restrict proteins to one area/domain

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