Transport Across the Membrane

 

                                Transport Across the Membrane

                                    

                                     Membrane Transport

                            

                  

 

                               Passive Transport    Active Transport

Passive transport mechanisms:

·         Diffusion

·         Facilitated Diffusion

·         Osmosis

·         Capillary filtration

·         Bulk Flow / Solvent drag

                                    DIFFUSION

 Diffusion is the net movement of molecules (or ions) from a region of their high concentration to a region of their lower concentration.

Characteristic features of diffusion

·         The molecules move down a concentration gradient.

·         Requires no energy

·         Molecules move about randomly by kinetic energy

·         As a result of diffusion, molecules reach equilibrium (no net movement of molecules from either side).

Factors Affecting  Diffusion

·         Cell membrane permeability

·         Concentration Gradient

·         Pressure gradient

·         Electrical potential gradient

     Cell membrane permeability depends upon:

·         Solubility of the substance in the lipid bilayer

·         Molecular size of the particle

·         Charge of the particle

·         Charge at the pore

·         Surface area

·         Thickness

·         Number of protein channels

·         Temperature

      Fick’s law of Diffusion:

                            J = DA (C1-C2)

                                  ---------------

                                       T

          J = Rate of Diffusion                    T = Thickness of membrane

          C1-C2 = concentration gradient   D = Diffusion coefficient

           A = Cross sectional area

Types of diffusion:

1.      Simple diffusion

2.      Facilitated diffusion

 

1. Simple Diffusion

                Diffusion without the help of a carrier protein

        Mode of Simple Diffusion

 

           Through Lipid Bilayer                      Through Channels

Molecules that pass through the lipid bilayer by diffusion

·         Gases (oxygen, carbon dioxide)

·         Water molecules (rate is slow due to polarity)

·         Lipids (steroid hormones)

·         Lipid soluble molecules (hydrocarbons, alcohols, some vitamins)

·         Small noncharged molecules (NH₃)

Molecules that pass through the Channels by diffusion

·         Ions (Na⁺, K⁺, Cl^-)

·         Small water soluble molecules

·         Water (faster rate)

 Gating of Protein channels

      Types of gated channels

         1. Voltage gated channels – open when there is a change in the resting membrane potential

                 e.g., Na+ channels along the nerve fiber

      Mechanical gated channels – Open when there is a mechanical stretch

                      e.g Channels in the smooth muscle fiber

      Drugs that block channels:

          Sodium channels – Tetrodotoxin (TTX) & Saxitoxin

          Potassium channels – TEA (Tetra Ethyl Ammonium)

          Calcium channels – Verapramil (to treat hypertension)

 

2. Facilitated diffusion

    Movement of solutes from high concentration to low concentration through the membrane

    with the help of a carrier protein

      Features of Facilitated Diffusion

·         Occurs along the concentration gradient

·         Does not require energy

·         Involves carrier protein

·         Carrier proteins are highly specific for molecules