How Things Get In/Out of Cells          

The Cell Membrane...
    1. Unit Membrane Hypothesis - "all membranes look alike*"
                        photomicrographs:          EM*  &  EM-neurons*  &  SEM* 

    2. Source for study (plasma membrane - RBC ghosts)
                                   RBC-bw*RBC-color  and   RBC ghosts*  
  
                                                                                              
 
  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. Current Molecular Model of Membrane Structure -
    Fluid Mosaic model...   has 2 major molecular components
           1.  lipids  =  mostly phospholipids
*    &   cholesterol*
                                           lipid fluidity - viscous vs. fluid (unsaturation)*
          2.  proteins  =    
                  a)  Integral (intrinsic) proteins  -  part of & denatured upon release
                  b)  Peripheral (extrinsic)            -  easily extractable from membrane

                  Structural Models... (a historical look at membrane structures)     
                       Langmuir-troughs              Davson-Danielli model*
               Singer-Nicolson model*    glycoproteins secreted by cell = a cell "wall"
               Fluid Mosaic model*    &    Extra-cellular Matrix*-common to animal cells
 
                Functions of Membrane Proteins*
                                                                             Concept Activity - chapter 7.1 - Membrane Structure         end
    

 

 

 

 

 

 



 B. Physiological Properties of Membranes

     Solute Movement... since membrane is mostly lipid, movement of molecules across
        a membrane is based upon lipid solubility (more lipid soluble greater transport)
                               -graph of Partition Coefficient   vs.   Permeability
                                            actual data    and an    ideal graph*
 
     Water Movement... an anomaly
                    not lipid soluble, yet readily permeable

           1. Bulk flow...   hydrodynamics or mass flow of water 
                                   due to potential energy of water (as in rivers & rapids)....
                    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, 
             
"water always moves from areas of high water potential to area of low potential".
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2. Osmosis - net movement of water from [high] ---> [low] 
           energy required:  passive transport of water, no energy required    osmosis*

      SOLUTIONS -  comparing one solution to another solution

            hypertonic solution  =  "more solute, less water"       (hyperosmotic cell)
  
           
hypotonic solution    =  "less solute, more water"       (hypoosmotic cell)
       
            isotonic solution       =  "equal solute and water"         (isosmotic cell)

        Cells and Solutionsplant vs. animal and water balance  tonicity  &  osmolality*
                plasmolysis* -  loss of cytoplasmic structure due to water loss 
                turgidity
       -  swollen cells due to water gain
                                        Investigation - Process of Science - Chapter 7.3
- How do salts Affect Cells?
                       
             Concept Activity - chapter 7.3 - Osmosis and Water Balance
  
AQUAPORINS* -  Water Channel proteins (?)    2003 Nobel Prize in Chemistry
 
 
 
 

 

 

 

   Membrane Solute Transport
                   How Things (molecules - solutes) Get Across a Membrane [4 Ways]

        1.     through a PORE
*...  made by proteins embedded in bilayers*
                                               allows ions & small hydrophilics to move through [ a pore ]
 
        2.    by DISSOLVING IN* membrane...   hydrophobic solutes via lipid bilayer
 
        3.    by 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 - solid)
                                   &              &  pinocytosis (makes a liquid vesicle)
                        EXOCYTOSIS    -  export vesicles   [ figure
*]

   

 

 

 

  

 

 

 

 

Membrane Transport 
 
                                                           Concept Activity - chapter 7.2 - Selective Permeability of Membranes,
                 
                                           Concept Activity - chapter 7.3 - Diffusion   &   Osmosis
  DIFFUSION* 
                   -  net thermal motion of solute down a concentration and/or electrical gradient

                                                  ...solutes move from area of higher to area of lower concentration
                   -  is PASSIVE...  i.e., requires no expenditure of cellular energy
 

            CARRIER MEDIATED TRANSPORT... a special type of diffusion
                    also called Facilitated Diffusion
* ...   ex: Glucose Transport* 
                    defined as protein mediated passive transport
                                                                               Concept Activity -
chapter 7.3 - Facilitated Diffusion
 

     IONS?  Nernst Equation:   defines PASSIVE electrochemical quilibrium...
                       determines mathematically what the electrical potential (charge) should
                       be  across a cell based on existing PASSIVE distribution of ions
                       
      E(mV)  =  (+/-)  62   log10 [Co/Ci]        flux equation
*    K
  
   
               
 
  

 

 

 


 
 ACTIVE TRANSPORT - cells expends energy (often... splitting of ATP)
              to move a solute against a concentration gradient
*
                         ex:     animals-     Na-Pump*        :    NaK-ATPase
                                     plants-     Proton Pump    :    H+-ATPase
*
                                                 both are ELECTROGENIC... move charge one way  =  voltage
                                                                                               Concept Activity - Chapter 7.4 -
Active Transport
 CO-TRANSPORT... movement of 2 solutes together -
                                                 often moves 1 solute passively & other actively
      ex:   1)  H+pump coupled with sucrose transport ( H+symport
* )
              2)  epithelial transport* Na+glucose model ( glucose absorption
* )
_______________________________________________________
  definitions:     uniport     -  single solute in one direction
                         symport   -  2 solutes simultaneously in same direction
                         antiport
*  -  1 solute in & 1 solute out -opposite directions
   
                                        Summary of passive vs active transport*                           
______________________________________________________
       
 
 
 

 

 

 
   Exocytosis
            - releases (out) bulk material to outside...   fig 7.16
*    
                                                                                                        

   Endocytosis ...  takes in solutes/particles by vesicles
            - phagocytosis
* -  solid particle uptake into phagosome vesicle
            - pinocytosis
*    -  liquid uptake into pinocytotic  vesicle
            - receptor mediated
* endocytosis...   uptake @ membrane sites w receptors
                            - coated pits with protein clathrin 
                              Sumanas, Inc. animation - Receptor Mediated Endocytosis of cholesterol               

                                                                  Concept Activity - chapter 7.5 - Exocytosis and Endocytosis
                                    
both  

                        a paradigm
Key Concepts*                 cell animation of inner Life of Cell

 
     go to next lecture on Cell Communication*

 

 

 

 

   model organisms   

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Last Update - 02 June, 2008

 

 

Wiley Publishing animation of membrane transport