Membrane Transport by 0A65RW0U

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									Membrane Transport




                     6-6
                Cell Membrane Structure
                  Fluid Mosaic Model
• Phospholipid bilayer
   – Two layers of phospholipids
   – Fluidic properties: lipids can
     move laterally within own
     bilayer
   – Proteins also move freely
• Integral proteins: span
  across entire membrane
• Peripheral proteins: only
  on surface of membrane
• Cholesterol: component of
  bilayer conferring rigidity
  to the membrane (ie:
  reduces flexibility)
          Membrane Proteins
• Structural
• Receptors: for hormones, neurotransmitters
• Carriers: transport specific molecules across
  membrane
• Channels: allow diffusion of materials through
  membrane
• Markers: identify cell as self
• Enzymes: provide digestion of materials
  in digestive tract
       Transport Across Plasma
             Membrane
• Plasma membrane is selectively permeable—
  ie: allows only certain kinds of molecules to
  pass
• Many important molecules have transporters or
  channels
  – Carrier-mediated transport involves specific
    membrane protein transporters
  – Non-carrier mediated transport occurs by diffusion
  – Through a protein channel
                                                     6-7
       Transport Across Plasma
           Membrane continued
• Passive transport moves compounds down
  concentration gradient via kinetic energy;
  requires no cell energy
• Active transport moves compounds against a
  concentration gradient; requires cell energy and
  membrane transporters



                                                6-8
                      Simple Diffusion
• Is the random motion of molecules due to kinetic energy
   – Net movement is from region of high concentration to region of low
     concentration (ie : concentration gradient)




                                                                          6-9
                    Diffusion continued
• Small, non-polar (lipid soluble) compounds readily
  diffuse thru cell membrane bilayer
  – Also some small molecules such as CO2 and O2
     • Gas exchange occurs this way




                                                       6-10
               Diffusion continued
• Cell membranes
  are impermeable
  to charged and
  most polar (water
  soluble)
  molecules
  – Charged
    molecules must
    have an ion
    channel or
    transporter to
    move across
    membrane
                                     6-11
                  Diffusion continued
• Rate of diffusion of a molecule/ion depends on:
  –   Magnitude of concentration gradient
  –   Permeability of membrane to compound
  –   Surface area of membrane
  –   Temperature: thermal energy
  –   Solubility of molecule in membrane




                                                    6-12
                            Osmosis
• Is net diffusion of H2O
  across a selectively
  permeable membrane
  – H2O diffuses down its own
    concentration gradient
  – H2O is less concentrated
    where there are more
    solutes
     • Solutes have to be
       osmotically active
        – ie: cannot freely move
          across membrane



                                      6-14
             Osmosis continued
• H2O diffuses down
  the concentration
  gradient until its
  concentration is
  equal on both
  sides of the
  membrane
• Water isn’t lipid
  soluble so some
  cells have water
  channels
  (aquaporins) to
  facilitate osmosis
                                 6-15
               Osmotic Pressure
• Is the force that would have to be exerted to stop
  osmosis
  – Indicates how strongly H2O wants to diffuse
• Is proportional to solute concentration gradient




                                                       6-16
       Molarity and Molality
• 1 molar solution (1.0M) = 1 mole of solute
  dissolved in solvent to produce 1L of solution
  (total)
  – Doesn't specify exact amount of H2O
• 1 molal solution (1.0m) = 1 mole of solute
  dissolved in exactly 1 kg of H2O




                                               6-17
Molarity
Molality
         Molarity and Molality continued
• Osmolality (Osm) is
  total molality of a
  solution
  – e.g: 1.0m of NaCl
     yields a 2 Osm
     solution
  – Because NaCl
     dissociates into Na+
     and Cl-
  – Ionic compounds
     dissociate in water
  - The individual particles
     each exert an osmotic
     force of 1

                                           6-18
                     Tonicity
• Is the effect of a solution on osmotic movement of H2O

• Isotonic solutions have same osmotic pressure

• Hypertonic solutions have higher osmotic pressure
  and are osmotically active

• Hypotonics have lower osmotic pressure



                                                      6-19
                    Tonicity

• Isosmotic solutions have same osmolality as plasma

• Hypo-osmotic solutions have lower osmotic pressure
  than plasma

• Hyperosmotics have higher pressure than plasma




                                                       6-19
     Carrier-Mediated Transport
• Molecules too
  large and polar
  to diffuse across
  membrane are
  transported by
  membrane
  protein carriers




                                  6-23
   Carrier-Mediated Transport continued
• Protein carriers
  exhibit:
  – Specificity for single
    molecules (substrate)
  – Competition among
    substrates for
    transport carrier
  – Saturation when all
    carriers are occupied
    • This is called Tm
      (transport maximum)


                                          6-24
               Facilitated Diffusion
• Is passive transport
  down a
  concentration
  gradient facilitated
  by carrier proteins
• The simple binding
  of ligand causes
  transport protein to
  change shape and
  move ligand across
  membrane




                                       6-25
            Active Transport
• Is transport of
  molecules
  against a
  concentration
  gradient
  – Cellular ATP
    energy is
    required to
    change shape
    of carrier
    protein

                               6-26
                 Na+/K+ Pump
• Uses ATP to
  move 3 Na+ out
  and 2 K+ in
  – Against their
    concentration
    gradients
  – Used in
    membrane
    potential, renal
    transport,
    action
    potentials, etc
               Bulk Transport
• Moves large
  molecules and
  particles across
  plasma
  membrane
• Requires cell
  ATP energy
• Occurs by
  endocytosis
  and exocytosis


                                6-34
               Membrane Potential
• Is difference in charge
  across membranes
• Results in part from
  presence of large
  anions being trapped
  inside cell
   – Diffusable cations
     such as K+ are
     attracted into cell by
     anions
• Na+ is not permeable
  and is actively
  transported out by
  Na/K pump

                                    6-36
               Equilibrium Potential
• Describes voltage difference across cell membrane if only 1 ion
  could diffuse
• If membrane permeable only
  to K+, it would diffuse until it
  reaches its equilibrium
  potential (Ek)
   – K+ is attracted inside by
     trapped anions but also driven
     out by its concentration
     gradient
   – At K+ equilibrium, electrical
     and diffusion forces are = and
     opposite
   – Inside of cell has a negative
     charge of about -90mV



                                                               6-37

								
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