biological membranes

• Fluid mosaic model


  Current model for structure of plasma membrane and other cell membranes
• Fluid mosaic model

  • Fluid - phospholipid bilayer
  • Mosaic - protein molecules
• Functions of Membranes

  • Transport
  • Enzymatic activity
  • Signal transduction
  • Cell-cell recognition
  • Intercellular joining
  • Attachment to cytoskeleton and ECM
• Phospholipid Bilayer


  • Nonpolar, hydrophobic fatty acid chains of phospholipids project into interior of double-layered membrane
  • Polar, hydrophilic heads located on 2 surfaces of double-layered membrane
• How is lipid asymmetry created

  1. Enzymes in SER join fatty acids, glycerol, phosphate, and head groups to make phospholipids

  2. Phospholipid is inserted into a monolayer

  3. FLIPPASES flip some lipids into other bilayer for membrane growth

  4. Only transfer specific lipids

  5. Makes membrane asymmetrical
• Phospholipids in plasma membrane can move within bilayer

  • Most lipids and some proteins drift laterally
  • Lipid may flip-flop transversely (although rare because of energy use)
• Membrane Proteins and Their Functions

  • Membrane - collage of different proteins often grouped together, embedded in fluid matrix of lipid bilayer
  • Proteins determine most of membrane specific functions
• Why is fluidity important


  Allows membrane proteins to move rapidly in bilayer plane

  • Permits membrane lipids and proteins to diffuse from sites where they are inserted into bilayer
  • Enables membranes to fuse and mix molecules
  • Ensures membrane molecules are distributed evenly between daughter cells during division
• Peripheral proteins


  Bound to surface of membrane
• Integral proteins


  Penetrate hydrophobic core
• Transmembrane proteins

  Integral proteins that span membrane
• Hydrophobic region of integral protein


  Consists of 1+ stretches of nonpolar amino acids
• Membrane structure


  Results in selective permeability
• Plasma membrane

  Selectively permeable; regulates cell molecular traffic
• The permeability of the lipid bilayer

  Hydrophobic molecules (ex: hydrocarbons) can dissolve in lipid bilayer and pass thru membrane rapidly

  • Hydrophilic molecules including ions and polar molecules do not cross membrane easily
• Transport proteins

  Allows passage of hydrophilic substances across membrane
• Types of Transport Proteins

  • Channel proteins - hydrophilic channel that some molecules/ions can use as tunnel
  • Aquaporins - facilitate passage of water
  • Ion channel - facilitate diffusion of ions
  • Gated channels - open or close in response to stimulus
  • Carrier proteins - bind to molecules and change shape to shuttle across membrane
• Carrier proteins


  Specific for the substance it moves
• Passive transport


  Diffusion of substance across membrane with no energy investment
• Active transport


  Uses energy (ATP) to diffuse substances
• Osmosis


  Diffusion of water across a selectively permeable membrane
• Tonicity

  • Isotonic - solute concentration is same as in cell
  • Hypertonic - solute concentration is more than cell; cell loses water
  • Hypotonic - solute concentration is less than that inside cell; cell gains water
• Cell walls help maintain water

  • Hypotonic solution makes cell turgid (FIRM) because too much water
  • If plant cell and surroundings are isotonic, no net movement of water in cell; cell becomes flaccid (LIMP)
  • Plant cells lose water in hypertonic environment
• Facilitated diffusion

  Passive transport aided by proteins
• Membrane potential

  Voltage difference across membrane
• Electrochemical gradient


  Drives diffusion of ions across membrane
• Electrogenic pump


  Transport protein that generates voltage across membrane
• Sodium-potassium pump


  Major electrogenic pump of animal cells
• Proton pump


  Main electrogenic pump of plants, fungi, and bacteria
• Cotransport


  Coupled transport by membrane protein
• Bulk transport across plasma membrane

  Occurs by exocytosis and endocytosis
• Exocytosis


  Transport vesicles migrate and fuse with membrane and releases contents out of cell
• Endocytosis


  Cell takes in macromolecules by forming vesicles from plasma membrane
• Types of Endocytosis

  • Phagocytosis - cellular eating
  • Pinocytosis - cellular drinking
  • Receptor-mediated
• Plasmolysis - lethal effect where membrane pulls away from cell