How Things Get In/Out of Cells       
  The Cell Membrane*...

    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)
*   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 
   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)

& SolutionsPlants & Animals balance water & solutes via:

*        state of water moving into or out of a cell by osmosis
    describes both solute and water concentrations
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]

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






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

        1.     through a
*...  made by proteins embedded in bilayers*
                    allows ions & small hydrophilics to move through the
        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)
-  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...
Facilitated Diffusion
* may be thru a channel or carrier protein...  
Glucose Transport

                    defined as protein mediated passive transport
                    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 =



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 ?
                                    is ion transport
                         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 [Co/Ci]    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               ---> 
                                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  
* 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
*  -  1 solute in & 1 solute out -opposite directions
                  Transport Carriers video (C12)
             Examples of Carrier transport*
Summary of passive vs active transport*                           








       - 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 


    a paradigmKey Concepts*            animation of inner Life of Cell - cellview @ 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


                                   Mathematical Models for Simple Diffusion & Facilitated Diffusion.