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Passive transport:-


Facillated Diffusion


Diffusion – The movement of ions or molecules from regions of higher concentration to regions of lower concentration.

(Down a concentration  gradient)

Factors influencing the rate of diffusion:-

a)Concentration gradient – Increased rate .

b)Membrane permeability – Increased rate.

c)Increased surface area – Increased rate.

d)Increased molecular weight – Decreased rate.

e)Increased thickness of membrane – Decreased rate.

Osmosis - Osmosis is the movement of water from a region of high water concentration to
                    a region of lower water concentration thought a semi permeable membrane.

Facillated diffusion – Movement of specific molecules across cell membranes through protein channels.

– Similar to simple diffusion but the rate of diffusion is greatly accelerated by the action membrane proteins that act as

carrier molecules and aid in diffusion.


Active transport :-

Active transport

a)Transport of a substance (protein) across a cell membrane against the concentration gradient.

b)Transport across a cell membrane which requires energy generated by the hydrolysis of some energy carrier (usually ATP).



Structure of Plasma Membrane


1. Most of the plasma membrane contains phospholipids in a lipid bilayer
         a. The polar ends are hydrophilic (dissolve in water easily) while the
              fatty acid tails are hydrophobic (dissolve in water with difficulty). 
     2. Plasma membrane is more structurally complex than a lipid bilayer.
          a. Numerous proteins are imbedded in the lipid bilayer that have 
              many different functions.  
               1)  act as ion channels, as in nerve function
               2)  act as a receptor for a hormone/neurotransmitter
               3)  are carrier molecules for specific substances
               4)  docking proteins for vesicle identification
               5)  glycoproteins are markers for cell-cell identification.
     3. Cholesterol embedded in the membrane provides membrane fluidity.

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