This document discusses diffusion mechanisms, including simple and facilitated diffusion, focusing on the transport of molecules across cell membranes. It explains the characteristics of protein channels, gating mechanisms, and the implications of these processes in conditions like cystic fibrosis, where impaired ion movement leads to thick mucus and digestion issues. The document also highlights the significance of various ion channels, their selectivity, and the physiological effects of their malfunction.

1. Diffusion

2. Diffusion
Diffusion refers to passive transport of molecules
from areas of higher concentration to areas of
lower concentration.
Two subtypes-
simple diffusion
facilitated diffusion

3. LIPID SOLUBLE
• O2
• Nitrogen
• CO2
• Steroid hormones
• Weak organic acids & bases
LIPID INSOLUBLE
• Water
• Urea
• Non hydrated sodium and
potassium ion
WATER SOLUBLE
• Glucose
• Most amino acids

4. Protein channels have following characteristics-
• Selective permeability
• Gating mechanism
Voltage gated
channel
Ligand gated
channel
or
Chemical gating
Mechanical gated
channel

5. Simple Diffusion- lipid soluble
substances
• All the molecules are at constant random movement ( thermal motion) except at
Absolute zero temperature.
• Physicist call it “heat”.
• There will be Net diffusion of the solute until the solute concentration between two
solutions become equal( Diffusional Equilibrium). This is called Flux or flow (J).
• Rate of diffusion α lipid solubility

6. Net movement of the molecules across a permeable
membrane where only simple diffusion is occurring is
expressed by Fick’s law of diffusion.
J= DA ( C1-C2)
T
J= net rate of diffusion
D= diffusion coefficient
A= cross-sectional area
T= thickness of the movement
C1-C2 = concentration gradient

7. Diffusion of Electrolyte
If there is a potential
difference across the
membrane.
That potential difference
will alter the net rate of
diffusion of a charged
solute.
When a charged solute
diffuse down a
concentration gradient
That diffusion itself generate a
potential difference across a
membrane called – Diffusion
potential.

8. Simple Diffusion- through pores or
channels
• Diffusion of water & other lipid-insoluble molecules through protein
channels.
• Protein channels or pores are transmembrane protein.
• Pores are called aquaporins. At least 13 different types of aquaporins
found in human body .
• They are highly selective.
• The rapidity of diffusion is very fast through this channels.
• More larger molecules < diffusion rate. ( Urea is 20% larger than water
molecule & diffusion rate 1000 times less than water).

9. • Some channels are always open and some channels
are gated channels.
• Aquaporins have narrow pore that permits water
molecules diffuse in a single file.
• The pore is too narrow to permit passage of any
hydrated ions.

10. • Selectivity results from characteristics of the channel i.e.
its diameter, its shape, nature of electrical charges &
chemical bonds along its inside surface.
• K+ ion larger than Na+ ion but diffusion of K+ is 1000
times more than Na+. So high degree of selectivity
cannot be explained by molecular diameter.

11. Lining of selectivity filter are carbonyl oxygen
Hydrated K+ ion enter the selectivity filter, interact with
carbonyl oxygen
Shed most of their bound water molecules
Permitting the dehydrated K+ ions to pass
through the channels.
Diffusion of K+:

12. Inner surface of Na+ channel are lined with amino acids
that are strongly negative charged
This strong negative charges can pull small
dehydrated sodium ions into these channels.
Actually pulling the Na+ ions away from their
hydrated water molecules
Diffusion of Na+:

13. Gating of protein channels
1. Voltage gating:- Na gate (outside) remain closed when
inside strong negative charge. Opens when inside
membrane loses its negative charge.
K gate (inside) opens when inside of
the cell membrane becomes positively charged.
2. Chemical/Ligand gating:- Gates open when some other chemical
molecules binds with the gate protein. Ligand may internal or
external.
i.e.- acetylcholine channels.
Extracellular ligand- first messenger
Intracellular ligand- second messenger( Ca2+,cAMP, G protein)
3.Mechanical gating:- opened by mechanical stretch.
Example- Pacinian corpuscles
-Hair cells in the organ
of corti

14. 1.Na + Channels
2.K+ Channels OUT
3.Ca 2+ Channels
4.Proton Channels (H+
)
5.Anion Channels ( Cl-
& HCO3-
) OUT

15. When the net driving force is negative, cations will enter the
cell and anions will exit.

17. Gap junction channels (also called hemichannels) are formed
between two adjacent cells and open to allow passage of ions
and small molecules between the cells. The hemichannels are
generally hexameric (i.e., consisting of six subunits, or
connexins).

18. Facilitated Diffusion-
• Downhill movement.
• Carrier protein used so called- Facilitated or Carrier mediated
diffusion.
• Requires no input of metabolic energy.
• Large water soluble molecules i.e. Glucose , Amino acids diffuse
through this process.
• Maintain saturation kinetics – rate of diffusion approaches a
maximum, called Vmax.
• Exhibits all the characteristics of carrier- mediated transport :
saturation , stereospecificity, competition .

19. • The rate of facilitated diffusion is greater than
that of passive diffusion at lower solute
concentrations. However, at higher solute
concentrations the rate of facilitated uptake
reaches its Vmax (i.e., the carrier is saturated),
whereas the rate of passive diffusion is not limited
by a carrier.
• Vmax can be increased by adding transport
proteins to the membrane, which is a key
regulatory aspect of the transport process.
• Glucose transport through membrane protein
(GLUT). At least 14 members of GLUT.
GLUT 4 activated by Insulin. Increase facilitated
diffusion 10-20 fold in insulin sensitive tissue.

20. Nernst Potential:-
• If an electrical potential is applied across the
membrane, the electrical charges of the ions
cause them to move through the membrane
even though no concentration difference exists
to cause movement.
• Therefore net diffusion occurs from left to
right.
• A concentration difference developed from rt
to left.
• At normal body temp (37 degree C), the
electrical difference that will balance a given
concentration difference can be determined by
Nernst Equation-
EMF (in millivolts) = + 61 log C1/C2
_

22. Over 100 years ago, midwives performed an
unusual test on the infants they delivered: The
midwife would lick the infant’s forehead. A salty
taste meant that the child was destined to die of a
mysterious disease.
Daniel’s (18mts) medical history tells a frightening story
of almost constant medical problems since birth:
recurring bouts of respiratory infections, digestive
ailments, and, for the past six months, a history of
weight loss.
APPLIED:-

23. Today, a similar “sweat test” will be performed in a major
hospital.
The name of the mysterious disease- Cystic fibrosis.
A culture taken from Daniel’s lungs raised a red flag for
cystic fibrosis: The mucus from his airways was unusually
thick and dehydrated. In cystic fibrosis, this thick mucus
causes life-threatening respiratory congestion and
provides a perfect breeding ground for infection causing
bacteria.

24. In people with cystic fibrosis, movement of sodium
chloride into the lumen of the airways is impaired.
Why would failure to move NaCl into the airways cause
the secreted mucus to be thick?
ANS- If NaCl is secreted into the lumen of the airways, the solute
concentration of the airway fluid increases. Water moves into
compartments with higher osmolarity.

25. ANS-Chemically gated channels open when a ligand binds to them.
CFTRs open when ATP binds to the channel protein.
ATP is a chemical ligand, which means CFTRs are chemically gated
channel proteins.

26. CFTR and Na+ channels move Cl - and Na+ out of the
sweat and back into the body. The duct cells are not
permeable to water, so that normal reabsorption of
NaCl creates sweat with a low salt content. However,
without functioning CFTR channels in the epithelium,
salt is not reabsorbed. In cystic fibrosis, salt
concentrations in the sweat can be four times the
normal amount.

27. Daniel is diagnosed with cystic fibrosis. Now, along with antibiotics to
prevent lung infections and therapy to loosen the mucus in his airways,
Daniel must begin a regimen of pancreatic enzymes to be taken
whenever he eats, for the rest of his life. In cystic fibrosis, thick mucus in
the pancreatic ducts blocks the secretion of digestive enzymes into the
intestine. Without artificial enzymes, he would starve.
Why will Daniel starve if he does not take artificial pancreatic
enzymes?
ANS-The pancreas secretes mucus and digestive enzymes into ducts that
empty into the small intestine. In cystic fibrosis, mucus in the ducts is thick
because of lack of Cl - and fluid secretion. This thick mucus blocks the ducts
and prevents digestive enzymes from reaching the small intestine.

29. Thank You…..