PASSIVE TRANSPORT

PASSIVE TRANSPORT

• In Passive processes, substances cross the membrane from an area of high concentration to an area of low concentration (move with the concentration gradient, or difference), without any expenditure of energy (ATP) by the cell.
• Passive process or Passive transport includes (i) Simple Diffusion, (ii) Facilitated Diffusion and (iii) Osmosis.

(i) SIMPLE DIFFUSION

• Simple diffusion is the net (overall) movement of molecules or ions from an area of high concentration to an area of low concentration.
• For Example - After a dye pellet is put into a beaker of water, the molecules of dye in the pellet diffuse into the water from an area of high dye concentration to areas of low dye concentration.
• The movement continues until the molecules or ions are evenly distributed. The point of even distribution is called Equilibrium.
• Cells rely on simple diffusion to transport certain small molecules, such as Water, Oxygen and Carbon-di-oxide across their cell membranes.

(ii) FACILITATED DIFFUSION

• In Facilitated diffusion, an integral membrane proteins function as channels or carriers that facilitate the movement of ions or large molecules across the plasma membrane. Such Integral proteins are called Transporters or Permeases.
• Facilitated diffusion is similar to Simple Diffusion in that the cell does not expend energy, because the substance moves from a high to a low concentration. The process differs from Simple Diffusion in its use of Transporters.

Facilitated Diffusion in Prokaryotes

• Some transporters (Non-specific Transporters) permit the passage of mostly small, inorganic ions that are too hydrophilic to penetrate the non-polar interior of the lipid bilayer. These transporters, which are common in prokaryotes, are nonspecific and allow the passage of a wide variety of ions (or even small molecules).
• Glycerol is transported by Facilitated Diffusion in many Bacteria, Facilitated Diffusion does not seem to be the major uptake mechanism.

Facilitated diffusion through Non – specific transporter

Facilitated Diffusion in Eukaryotes

• Other transporters (Specific Transporters), which are common in Eukaryotes, are specific and transport only specific, usually larger, molecules, such as simple sugars (Glucose, Fructose and Galactose) and Vitamins. In this process, the transported substance binds to a specific transporter on the outer surface of the plasma membrane, which undergoes a change of shape; then the transporter releases the substance on the other side of the membrane.
• Facilitated diffusion is much more prominent in eukaryotic cells where it is used to transport a variety of Sugars and Amino acids.

Facilitated diffusion through Specific transporter

(iii) OSMOSIS

• Osmosis is the net movement of solvent molecules across a selectively permeable membrane from an area with a high concentration of solvent molecules (low concentration of solute molecules) to an area of low concentration of solvent molecules (high concentration of solute molecules).
• In living systems, the chief solvent is water. Water molecules may pass through plasma membranes by moving through the lipid bilayer by Simple Diffusion or through Integral membrane proteins, called “Aquaporins” that function as water channels.

• A bacterial cell may be subjected to any of three kinds of Osmotic solutions: (a) Isotonic solutions, (b) Hypotonic solutions or (c) Hypertonic solutions.

a)    Isotonic solutions

ü  Water concentration is equal inside and outside the cell, thus rates of diffusion are equal in both directions.

ü  No net moment of water.

b)   Hypotonic solutions

ü  Water moves into the cell.

ü  If the Cell wall is strong, the cell swells and if the cell is weak or damage, the cell bursts (Osmotic lysis). 

c)    Hypertonic solutions

ü  Water moves out of the cell and the cell becomes shrink (Plasmolysis).