UnitC 4

• Cell membranes


  Composed of specialized transport proteins

  
• Protein-free membranes (artificial lipid bilayer)


  • Impermeable to most water-soluble molecules

  
• Cell membranes that contain specific membrane transport proteins

  • Can facilitate passive diffusion of specific molecules
  • Can actively pump larger molecules/ions through the plasma membrane

  
• Lipid bilayers


  Impermeable to ions and most uncharged polar molecules

  
• Cell membranes are composed of specialized transport proteins
• Protein-free membranes (artificial lipid bilayer)


  • Impermeable to most water-soluble molecules

  
• Cell membranes that contain specific membrane transport proteins

  • Can facilitate passive diffusion of specific molecules
  • Can actively pump larger molecules/ions through the plasma membrane

  
• Lipid bilayers


  Impermeable to ions and most uncharged polar molecules

  
• Molecules rate of diffusion

  Depends on size and solubility properties
• Molecules that can diffuse across lipid bilayer

  • Small, non-polar molecules: dissolve easily in lipid bilayers and diffuse faster
  • Uncharged, polar molecules: will diffuse across lipid bilayer if smaller; moderate rate of diffusion
  • Large, uncharged, polar molecules: hardly cross

  
• Transfer of the water-soluble molecules depends on membrane transport proteins

  
• Membrane transport proteins


  Each transfers a particular type of molecule, allowing an uneven concentration of that molecule to build up on either side of the membrane

  
• Each type of membrane has its own characteristic set of transport proteins
• Cells maintain internal ion concentrations that differ from extracellular environment

  
• Na+ is most abundant outside the cell; K+ most abundant inside cell
• Number of positively charged ions inside and outside of the cell must be balanced to avoid comprising the cell
• Membrane potential


  Arises from differences in the concentration of inorganic ions across a cell membrane

  
• The positive external charge (Na+) is largely balanced by anions (Cl-)
• The positive internal charge (K+) is balanced mainly by nucleic acids and proteins
• Membrane potential


  The inside of the cell membrane is negative and the outside is positive

  
• Ion gradients


  Result of specific transporters that move specific ions

  
• Net driving force to move charged molecule through membrane

  Electrochemical gradient = sum of force from [molecule] + membrane potential

  
• Membrane potential and concentration gradient can work together or in opposite directions

  
• Membrane transport proteins


  • Inorganic ions and small (polar) organic molecules cross cell membrane via channels or protein transporters
  • Protein channels form pore-like structures spanning the lipid bilayer through which molecules diffuse
  • Protein transporters undergo conformational changes to transfer small solutes across bilayer

  
• Passive transport (diffusion)


  Does not require external energy, only a concentration gradient where the molecule travels down its concentration gradient (from high concentration to low)
• Active transport


  Moves proteins against their concentration gradient, requires a membrane transport protein that is coupled to an energy-consuming reaction (hydrolysis of ATP)

  
• Classes of transport proteins (transporters)

  • ATP-powered pumps, 1-1000 molecules/sec
  • Channel proteins (ions), 107-108 molecules/sec
  • Carrier proteins (transporters), 102-104 molecules/sec
• Passive transporters


  Move a solute along its electrochemical gradient
• Pumps


  Actively transport a solute against its electrochemical gradient
• Na+ - K+ Pump

  1. Moves Na+ ions out of the cell and K+ ions inside the cell with the help of ATP hydrolysis

  2. Undergoes a series of conformational changes to allow for the transport of each ion

  
• Uniports


  Work in passive diffusion and are selective for one type of molecule, move molecules down their concentration gradients
• Symports


  Move a pair of solutes in the same direction
• Antiports


  Move a pair of solutes in opposite directions
• Differences between uniport and simple diffusion: 1) Rate of substance movement is higher for uniporters 2) Partition co-efficient is irrelevant for uniporters 3) Uniport transport is limited by the number of uniporters in the membrane 4) Transport with a uniporter is specific
• Glucose-Na+ symport


  1. Uses Na+ electrochemical gradient to actively move glucose and Na+ through membrane

  2. Transporter oscillates between alternate states: one that faces extracellular space ("outward-open") and one that faces cytosol ("inward-open")

  3. Since [Na+] is higher outside of the cell, ions readily bind in the "outward-open" state, but must wait for a glucose molecule before changing conformation again to inward-open state

  
• Epithelial cells have two (2) types of glucose transporters located at opposite end of the cell to ensure glucose is released back for use by other cells in the body

  
• Ion channels


  • Highly selective for the ions they transport
  • Contain an inner selectivity filter which allow specific ions to interact with inner channel wall to ensure ions with appropriate charge and size are passing through
  • Selectivity depends on diameter and shape of ion channel
  • Channels are often gated; fluctuate between open and closed states

  
• Membrane potential


  • Changes in membrane potential are the basis of cell signalling
  • Changes in potential are mediated by changes in ion permeability through the cell membrane

  
• When a cell is in a "rested" state, negative charges inside the cell are balanced by K+ ions
• Cell membranes also contain K+ leak channels