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