Unit Membrane Hypothesis:          EM plasma cell membrane*
                   photomicrographs:          all membranes look alike*  

    Two Ways to Study Membranes & Transport of Solutes Across Membranes  
        a.  NATURE OF MEMBRANE ITSELF  - "its Molecular Makeup"
        b.  PERMEABILITY STUDIES               - "Physiological Properties of Membranes"  

    a. need a source of membranes for study (plasma membrane - RBC ghosts)
                     RBC-color*  and   RBC ghosts* 

 
 a. Current Molecular Model of Membrane Structure -

        Fluid Mosaic model...   has 2 major molecular components
  
        1.  lipids  =    Phospholipid structure (C11video tab)
                              phospholipids*   and their   their distribution* 

                  lipids spontaneously form bilayers via hydrophobic effect*
                            lipid fluidity - viscous vs. fluid (saturation vs. unsaturation)*
                            see fluidity video (C11)
                            exoplasmic vs. cytoplasmic lipid differences*

        2.  proteins 

              a)  Integral (intrinsic) proteins - part of... & denatured upon release from...
              b)  Peripheral (extrinsic)         - easily extractable from membrane
                            fig 7.3 (locale)         - How proteins are embed in membranes*            

 

                  Fluid Mosaic model*   -  extra-cellular matrix*   glycoproteins*       

                                                         -   roles of membrane proteins*^view@home & functions*
                                                         -   membranes result in compartmentation           
        
                                  a historical look at membrane structures
      

 
  b. Physiological Properties of Membranes

        Membrane are selective permeable* and membranes are mostly lipid, thus

               -  transport is based upon lipid solubility (more lipid soluble greater the transport)

               -  Partition Coefficient ( lipid solubility)   vs.   Permeability 
               -  an  ideal graph*  &   actual data  -->   molecular move thru bilayer*

 

 

         Movement of Water... is an anomaly

                                                    polar, not lipid soluble, yet readily permeable... How?

           1. Bulk flow...   hydrodynamics or mass flow of water is due to  

                                    potential energy of water (as in rivers & rapids via gravity).... 

                         Water potential is the chemical-energy potential of water 
                                   & is a measure of energy available for reactions or movement. 
                         Water potential measures the ability of water to "move" in a system. 
  
              "Water always moves from areas of High Water Potential (pure water)
                               to an area of Low Water Potential (diluted water)".
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. OSMOSIS - net movement of water from [High Water Potential] ---> [Low WP] 
           energy is not required:  passive transport has no energy requirement  osmosis*              

SOLUTIONS -  we can compare one solution to another solution
hypertonic solution  =  "more solute, less water"       (hyperosmotic)           hypotonic solution    =  "less solute, more water"       (hypoosmotic)                isotonic solution       =  "equal solute and water"         (isosmotic)         Cells & Solutions -  Plants & Animals balance water & solutes via:            tonicity*        state of water moving into or out of a cell by osmosis       osmolarity*    describes both solute and water concentrations       plasmolysis*  loss of cytoplasmic structure due to water loss        turgidity          swollen cells due to water gains
animation example of various osmotic situationsview@home [4.5 min]

 
            AQUAPORINS* -    Discovery of Water Channel proteins                  
                                                        2003 Nobel Prize in Chemistry

    

   
        How Things (solutes = molecules) Get Across a Membrane [4 Ways]

        1.     through a PORE*...  made by proteins embedded in bilayers*
                    allows ions & small hydrophilics to move through the pore.

 

        2.    by DISSOLVING in* membrane...   hydrophobic solutes via lipid bilayer

 

        3.    by CHANNEL and CARRIER PROTEINS* ...
                        proteins embedded in membrane w specificity for a solute
                        results in transport of solute through the protein by diffusion
 

        4.     by membranes ENGULFING particles* into membrane vesicles...
                    ENDOCYTOSIS - phagocytosis  (forms a phagosome - vesicle w solids)
                                             &  pinocytosis     (forms a liquid vesicle)
                    EXOCYTOSIS  -  exports vesicle contents via the endocytotic pathway
                                             -  constitutive vs. regulated exocytosis.

    Solute Transport across Membranes... 

            Animation of diffusion*    -       DIFFUSION*       
 
       A.  net thermal motion of solute down a concentration and/or electrical gradient
                  -  solutes move from area of higher to area of lower concentration
                  -  thus, diffusion requires no expenditure of cellular energy
 
       B.  Carrier Mediated Transport...      animation*  
                    Facilitated Diffusion* may be thru a channel or carrier protein...  
                                                           ex: Glucose Transport* 
                    defined as protein mediated passive transport facilitated
                    either by channel proteins or carrier proteins.

       C.  Diffusion is  PASSIVE* transport.

       D.  The rate of diffusion is directly proportional to the concentration gradient:
                    simple diffusion = (Δn/Δt) = [C]    &    faciltated diffusion =  graph*
    
    



















 
   
What about IONS with their  +/-  charges and an electrical gradient ?
     Ion transport is the movement of ions across a membrane by a tranporter, a
     transmembrane protein, either passively or actively through ion pumps,
    such as symporters and antiporters.
           ex: Potassium ions transport video (C12)  is through ion channels*       

      Will a charged ion [Na⁺or K⁺ ] behave differently when there is not only a
      Chemical Gradient, but also an Electrical Gradient* across a membrane ?
                                   
                         to answer this questions we can use the....

       Nernst Equation which defines PASSIVE Electrochemical Equilibrium by
            determining mathematically what the electrical potential (charge should be)
            across a cell based on the existing PASSIVE distribution of ions...

         E(mV)  =  (+/-) 62 log₁₀ [C_o/C_i]    Nernst equation for Na⁺*

 
 ACTIVE TRANSPORT -  cells expend energy (often via...  hydrolysis of ATP)
                                         to move a solute against a concentration gradient

                   animal cells -    NaK-ATPase animation* --->   Na-Pump*       
                               ex: sodium pump (C12)
                   lysosomes-       a  Proton Pump               --->   H⁺-ATPase*

                                ex: Bactriorhodopsin - H+-pump (C11)
                   pumps are ELECTROGENIC... move charge one way  =  voltage
  
 CO-TRANSPORT...  movement of 2 solutes together -
                                  often moves 1 solute passively  (down) & other actively (up gradient)
      ex:   1)  H⁺pump coupled to sucrose transport via a   H⁺symport* system
              2)  epithelial cotransport model [animation*]
                   ex: Na-glucose symport video (C12) - Na⁺/glucose* transport 

 CHEMICAL TRANSPORT...  of Cationic Peptide Drugs
       Researchers at U.Breman have developed chemical carriers (COSANs) with hydrophobic
       properties that when sandwiched with cationic peptides can facilitate the selective
       transport, while preventing the peptide degradation and achieving efficient delivery
       at desired locations. 

_______________________________________________________
  definitions:      uniport     -  single solute in one direction
                         symport   -  2 solutes simultaneously in same direction
                         antiport*  -  1 solute in & 1 solute out -opposite directions

           

                  Transport Carriers video (C12)
                        Examples of Carrier transport*
                       Summary of passive vs active transport*                           
______________________________________________________
       

 

 

 

  
   Exocytosis
       - releases (OUT) bulk material to outside...   fig 7.9 - secretory vesicles*    
                                                                                                        
   Endocytosis ...  takes IN solutes/particles by vesicles                               ^{view anim's@home}

       - phagocytosis* -  solid particle uptake into phagosome vesicle           - animation
 

       - pinocytosis*    -  liquid uptake into pinocytotic  vesicle                        - animation

       - receptor mediated*endocytosis -  uptake via membrane receptors    - animation

                            - coated pits with protein clathrin 
          both
                                                                    
 
  

    Key Concepts*            animation of inner Life of Cell - ^{view @ home  
              
                 next lecture}                
                 copyright c2023     Last update -  October 2024 
                   Charles Mallery,     Department of Biology,   U. of Miami,  Coral Gables, FL 33124
                    

Wiley Publishing animation of membrane transport

    *^view@home

                                   Mathematical Models for Simple Diffusion & Facilitated Diffusion.