2. Transport through cell membrane
• Passive Transport
– Simple Diffusion
– Facilitated Diffusion
– Channel Diffusion
• Diffusion through
voltage gated channels
• Diffusion through ligand
gated channels
• Diffusion through leak
channels (ungated)
– Osmosis
– Filtration
• Active Transport
– Primary Active Transport
– Secondary Active Transport
• Counter transport
• Cotransport
– Exocytosis
– Endocytosis
5. Simple Diffusion
• Diffusion is the net movement of molecules (or ions)
from a region of their high concentration to a region of
their lower concentration.
The molecules move down a concentration gradient.
Molecules have kinetic energy, which makes them
move about randomly.
As a result of diffusion molecules reach an equilibrium
where they are evenly spread out.
In Equilibrium state there is no net movement of
molecules from either side.
6. Factors affecting rate of diffusion
1. The steepness of the concentration gradient. The
bigger the difference between the two sides of the
membrane the quicker the rate of diffusion.
( Electrical gradient, pressure difference etc)
2. Temperature. Higher temperatures give molecules or
ions more kinetic energy. Molecules move around
faster, so diffusion is faster.
3. The surface area. The greater the surface area the
faster the diffusion can take place. This is because the
more molecules or ions can cross the membrane at
any one moment.
4. The type of molecule or ion diffusing. Large molecules
need more energy to get them to move so they tend to
diffuse more slowly. Non-polar molecules diffuse more
easily than polar molecules because they are soluble in
the non polar phospholipid tails.
7. Examples of Simple Diffusion
1. Oxygen – Non-polar
so diffuses very
quickly.
1. Carbon dioxide –
Polar but very small
so diffuses quickly.
2. Water – Polar but
also very small so
diffuses quickly.
8. Facilitated Diffusion
• Large polar molecules such as glucose and amino
acids, cannot diffuse across the phospholipid
bilayer.
• These molecules pass through lipid bilayer with
the help of carrier proteins. Diffusion through
these carrier proteins is called FACILITATED
DIFFUSION
• Movement of molecules is still PASSIVE just like
ordinary diffusion, the only difference is, the
molecules go with the help of carrier protein
instead of passing between the phospholipids.
10. Concept of VMAX in Facilitated Diffusion
• Carrier proteins are
saturated by large
number of particles
which limits further
diffusion.
• The point where no
further movement of
paricles by carrier
protein is possible is
called VMax
13. Examples of Channel Mediated
Diffusion
• Sodium and Potassium – Voltage Gated
• Sodium – Ligand Gated (acetyl choline gated
channels)
• Potassium Leak channels
14. OSMOSIS
• Osmosis is the spontaneous net movement of solvent
molecules through a semi-permeable membrane into a region
of higher solute concentration, in the direction that tends to
equalize the solute concentrations on the two sides.
15. Filtration
• Filtration occurs through capillary walls, which allow
the passage of small ions and molecules but not of
plasma proteins
• Filtration occurs because of the pressure difference
• Filtration is regulated by
– Hydrostatic pressure
• capillary hydrostatic pressure
• tissue (interstitial) hydrostatic pressure
– Oncotic pressure
• capillary plasma oncotic pressure
• tissue (interstitial) oncotic pressure
ULTRAFILTRATION
17. Active Transport
• The ions or molecules move across the
membrane against an electrochemical gradient
by the utilization of energy
• The energy source is ATP
• Primary and secondary active transport
• In primary active energy is obtained directly from
ATP
• In secondary active transport energy is derived
secondarily from the active transport of Na ions
primarily
18. Primary Active Transport
• The substances transported mainly are Na, K,
H, Cl and few others
• The examples are Na-K pump, Ca–pump and
H-pump
19. Secondary Active Transport
• Primary active transport … Na pumped out … High Na
conc. outside cell and thus a conc. Gradient developed
• This gradient is a store house of energy because Na
tends to diffuse to the interior movement
• This energy is utilized to cause the movement of
substances either along with Na (Co-transport) or in
opposite direction (Counter-transport)
21. Examples of Secondary Active
Transport
• Sodium Glucose Co transport in Intestines
• Sodium Hydrogen Counter Transport in kidney
• Sodium calcium counter transport in majority
of cells
22. Endocytosis & Exocytosis
• Endocytosis is a form of active transport in
which a cell transports molecules (such as
proteins) into the cell by engulfing them in an
energy-using process.
• Two Types
– Phagocytosis
– Pinocytosis
