2. Passive Transport Mechanism
Diffusion
• Passive Transport Mechanism
• The passive transport mechanisms are diffusion and
osmosis.
• Diffusion: It is a passive process by which molecules
moves from area of higher concentration gradient to
that of lower concentration gradient and does not
require energy for the process.or spontaneous
admixture of the molecules of the two substances in
contact is called diffusion.
• The molecules are in constant motion. Motion is least
in solids, maximum in gases & intermediate in liquids.
• When a solid & liquid/two miscible liquids/ a liquid &
gas/ two gases, are in contact,molecules of two
substances will pass into each other until an uniform
admixture is obtained.
3. • The diffusion is of two types: Simple diffusion and
facilitated diffusion.
• .
• Simple diffusion : The movements of molecules
from area of higher concentration to that of
lower concentration due to kinetic motion of the
ions or molecules through the intermolecular
spaces or membrane openings without the aid of
any carrier protein is known as simple diffusion. It
does not require any energy (ATP) for transport
4. Examples: The lipidsoluble compounds (alcohol,
oxygen, nitrogen) diffuse through the lipid
bilayer; while water-soluble substances pass
through the aqueous channel ( urea, glucose
and ions).
Diffusion also occurs through leak channel and
gated channels. The leak channels are open
always, e.g. K+ channels.
Gated channels are voltage gated, ligand-
gated, mechanical-gated and temperature
gated as explained earlier.
5. • The main factors affecting diffusion are the concentration
gradients of the substances, number of channels and
opening in the membrane and kinetic motion of the
molecule.
• The factors that affect the diffusion rate across the cell
membrane are enlisted below:
• 1. Thickness: Greater the thickness of cell membrane
• slower will be the diffusion.
• 2. Temperature: The higher the body temperature
• faster is the diffusion.
• 3. Size of the molecule: Smaller the molecular size
• of substance rapid is the diffusion.
• 4. Concentration gradient difference: Larger the
• concentration gradient faster is the diffusion.
• 5. Cross-sectional area of membrane: Diffusion is
• directly proportional to the available cross sectional area
6. • 6. Diffusion coefficient (D) of the substance: D:
• Permeability X area of cross section available for
transport.
• Facilitated diffusion : Carrier protein mediated
• transport of substance across cell membrane
• is known as facilitated diffusion.
• Characteristic Features
• 1. The substance to be transported binds to the carrier
• protein. The binding causes confirmational change
• in carrier protein molecule and this aid in the
• transport and release of the substance to the other
• side of the membrane .
• 2. The energy for transport is provided by the
concentration gradient of the substance transported.
7. • No energy source (ATP) is required. This
carrier mediated transport of ions and organic
substrates into or out of the cell down their
concentration gradient passively is also called
passive carrier mediated transport.
• 3. The facilitated diffusion occurs faster than
simple diffusion.
• 4. When concentration of the transported
substance becomes high enough the
facilitated transporter gets saturated and this
limits the rate of transport
8. • 5. If more than one substance competes to bind with
• common transporter, it leads to competitive inhibition.
• Examples of Facilitated Diffusion
• 1. Insulin-mediated transport of glucose in muscle
cells. When more insulin is present, more of these
glucose transporters (GLUT) ( Glucose transporter
(GLUT) is a facilitative transport protein involved in
glucose translocation across the cell membrane). are
added to the membrane of cells of such muscle and
glucose moves into the muscle cells.
• 2. Transport of glucose along intestinal epithelium by
• glucose transporter.
9. Physiological importance of diffusion
• When two substances, come in contact, directly or
through a permeable membrane, diffusion will take
place, when the molecular concentration of a
substance in solution is higher in one part of a liquid
than in another & when any absolute barrier does
not intervene.
• E.g 1] Admixture of foodstuffs with digestive juices.
• 2] Absorption from the intestine
• 3] Exchange between plasma & red cells
10. • 4]Exchange in the capillary bed e.g food stuffs,
O2 etc. pass out from blood stream (higher
conc.) to the tissue fluid (lower conc)& then to
the tissue cells (lowest conc.) where they are
used up. The metabolites including Co2, on the
otherhand, come out of cells( higher conc) where
they are produced, to the tissue fluid (lower
conc) & then to the blood stream (lowest conc)
• 5] Exchange in the lung capillaries e.g O2 from air
(higher conc) enters the venous blood (lower
conc) while Co2 from venous blood (higher conc)
diffuses out into air (lower conc).
• 6] Admixture of gases in the lungs.
