Membrane Transport and the Membrane Potential by uExqTD45


									Membrane Transport and the
   Membrane Potential
    Cell membrane

Composed primarily of phospholipids
 Separates intracellular fluid from
           and proteins
         extracellular fluid
         Cell membrane

Proteins may serve as: carriers, channels
            and receptors
Mechanisms of Transport Across
       Cell Membrane
• According to the means of transport there are
  two categories:

          1- Non-carrier-mediated transport
                - Simple diffusion       Passive

          2- Carrier-mediated transport
                 - Facilitated diffusion   Passive
                 - Active transport

Random motion of molecules due to their
      Drops of ink
    thermal energy is called diffusion
 Molecules in a solution tend to reach a
 uniform state, for example a drop of ink
  in a water container spreads uniformly
   Higher                Lower
       Equal Concentrations
Concentration       Concentration

          Net diffusion
         No net diffusion
       Diffusion Through the Cell
•    Two major groups of molecules can pass the
        cell membrane by simple diffusion:

    1. Molecules that can dissolve in the lipid bilayer
       membrane, non-polar molecules such as:
       •   O2, Hormones

    2. Small polar molecules which are uncharged such
       •   CO2, alcohol, and urea
  = CO2          Extracellular   Tissue
  = O2           Environment      cell

Gas exchange
 between the
   CO2 out
     O2 in
  occurs by
Diffusion Through Protein Channels
Inorganic ions such as Na+ and K+ are able to penetrate the
        Small through pores within integral proteins
    • membraneions can use channels in the that
      span the thickness of the double phospholipid layers

      Integral                         Ions     bilayer
            Rate of Diffusion

•   The number of diffusing molecules passing
       through the membrane per unit time
                  Rate of Diffusion
  •    Rate of diffusion depends on:

       1.   Concentration difference across the membrane
       2.   Permeability of the membrane to the diffusing molecule
       3.   Surface area of the membrane
       4.   Molecular weight of the diffusion molecule
       5.   Distance
       6.   Temperature

                      Concentration gradient x Surface area x Temperature
Rate of diffusion a
                                MW x distance

• The net diffusion of water across the
  membrane is called osmosis
• Osmosis can occur only if the
  membrane is semipermeable
• Semipermeable means that the
  membrane must be more permeable to
  water than the solute dissolved in water
                              = Solute
                              = Water

               More concentrated
             Osmotic Pressure

•   What is osmotic pressure?

    The force needed to prevent osmotic
      movement of water from one area to
      another across a semipermeable
        Volume = X      Volume = X


      Pure water      180-g/L glucose

Force preventing volume change
            Osmotic Pressure
•   If a selectively permeable membrane
    separates pure water from a 180-g/L
    glucose solution, water will tend to move
    by osmosis into the glucose solution, thus
    creating a hydrostatic pressure that will
    push the membrane to the left and expand
    the volume of the glucose solution
•   The amount of pressure that must be
    applied to just counteract this volume
    change is equal to the osmosis pressure of
    the glucose solution

Equivalent of one molecular weight
   (g) of a substance dissolved in
   water to make a total one liter
     solution is called a Molar
             solution (1M)
180 g              1 mole of glucose
                          (180 g)


        1.0 mole per liter
                                                 1.0 liter
            solution- one
                molar            1.0 M glucose

When equivalent of one molecular
   weight (g) of a substance is
 added to one liter (Kg) of water,
   this solution is called Molal
           solution (1m)
180 g           1 mole of glucose    1.0 Kg of H2O        1.0 Kg
                       (180 g)            (1 liter)

        1.0 mole per
                                              1.0 liter
           water- one         1.0 m glucose
       Molarity and Molality

• Molal solution is a better indication of
  solute to solvent ratio, therefore it is a
  better indicator of osmosis
• However, in the body since the
  difference between Molal and Molar
  concentration of solutes is very small,
  Molarity is often used
•   Total molality of substances in a solution is
    called osmolality (Osm).

•   e.g. A solution containing 1m glucose and 1 m
    fructose has osmolality of 2 osmol/L (2 Osm).

•   Electrolytes such as NaCl are ionized when in
    solution, therefore one molecule of NaCl in
    solution yields tow ions. So 1m of NaCl has
    osmolality of 2 Osm.
• Solutions that have the same total
  concentration of osmotically active
  solutes and the same osmotic pressure
  as plasma* are said to be isotonic

* In the body plasma has osmolarity of 0.28
   Osm (280 mOsm)
                       Effect of isotonic
                          solution on
                          cell volume


No change

            280 mOsm/L solution

• Solutions that have a lower total
  concentration of osmotically active
  solutes and a lower osmotic pressure
  than plasma are said to be hypotonic
                         Effect of hypotonic
                             solution on cell


 Cell swells

               200 mOsm/L solution

• Solutions that have a higher total
  concentration of osmotically active
  solutes and a higher osmotic pressure
  than plasma are said to be hypertonic
                        Effect of hypertonic
                            solution on cell


 Cell shrinks

                360 mOsm/L solution
           Regulation of Blood Osmolarity

• Blood osmolarity is maintained within a narrow range and when
  this osmolarity changes several regulatory mechanisms come
  into action.

                                              Negative feedback
             Carrier-Mediated transport

• Unlike the simple diffusion, carrier-mediated transport

      1- Specificity

      2- Competition

      3- Saturation

           Simple diffusion
                Carrier-Mediated transport

• There are two major types of carrier-mediated transport:

        a) Facilitated diffusion:
           like simple diffusion facilitated diffusion is powered by
Passive     thermal energy of the diffusing molecules. But the transport
            of molecules across the membrane is helped by a carrier
            protein. For example glucose is transported to the cells of
            the body by facilitated diffusion. the net transport is along
            the concentration gradient.

       b) Active transport:
          Movement of molecules against their concentration gradient
Active     which requires energy (ATP). For example movement of
           calcium from inside to outside of the cell.
Facilitated Diffusion
   Outside of cell     = Glucose
Higher concentration
                       = carrier

   Inside of cell
Lower concentration
              Active Transport
a) Primary active transport:
   ATP is directly needed for the carrier
   protein in the following sequences:

 1- Binding of molecule to the carrier protein

 2- ATP is hydrolysed to provide energy for transport.

 3- Carrier changes its shape and moves the
 molecule across the membrane.
a) Primary active transport:

e.g Transport of Ca++   from
  inside to outside of the
Low Ca2+                 High Ca2+

                       Carrier protein

       ADP + Pi ~

Ca2+                     Ca2+

Cytoplasm           Extracellular Fluid
a) Primary active transport:
   e.g Na/K pump.
               Active Transport
b) Secondary active transport (Co-transport):

  The energy required is obtained from downhill
  transport of Na+ into cell:

     ECF        ICF     Na

      Na         Na               Na       Na
        K        K           Glucose       Glucose
    b) Secondary active transport (Co-transport):
       e.g Transport of glucose in kidney.

 Active Transport

Primary                                             Facilitated
Active Transport                                    Diffusion
b) Secondary active transport (Co-transport):
   e.g Co-transport of Na+ and glucose.
                   Membrane Potential

The difference in ionic distribution between inside and outside of the cell
result in electrical potential difference across the cell membrane which is
called membrane potential.

Membrane potential is produced by:

      1- The action of Na/K pump at the cell membrane is
         essential for the production of membrane potential.

      2- Proteins, ATP and other organic molecules in the
         cell are negatively charged, and can not cross the
         cell membrane therefore this makes inside of the
         cell negative.
Membrane Potential

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