Transcript MEDIATED TRANSPORT MECHANISMS

MEDIATED
TRANSPORT
MECHANISMS
By
Dr. Ishtiaq Ali
MEDIATED TRANSPORT MECHANISMS
Many nutrient molecules, such as amino
acids and glucose, can not enter the cell by
the process of diffusion , and many
substances, such as proteins, produced in
cells cannot leave the cell by diffusion.
Carrier molecules within the cell membrane
are involved in carrier-mediated transport
mechanisms, which function to move large,
water-soluble molecules or electrically
charged ions across the cell membrane.
After a molecule to be transported binds
to a specific carrier molecule on one side of
the membrane, the three-dimensional shape
of the carrier molecule changes, and the
transported molecule is moved to the opposite
side of the cell membrane. The transported
molecule is then released by the carrier
molecule, which resumes its original shape
and is available to transport another molecule.
Carrier-mediated transport mechanisms
exhibit specificity, that is, only specific
molecules are transported by the carriers.
Kinds of carrier-mediated transport
There are three kinds of carrier-mediated
transport
a. Facilitated diffusion
b. Active transport
c. Secondary active transport.
1. FACILITATED DIFFUSION
Facilitated diffusion is a carrier-mediated
transport process that moves substances
into or out of cells from a higher to a lower
concentration of that substance. Because
movement is with the concentration
gradient, metabolic energy in the form of
ATP is not required.
2. ACTIVE TRANSPORT
Active transport is a carrier-mediated
process that moves substances across the
cell membrane from regions of lower
concentration to those of higher
concentration against a concentration
gradient. Consequently, active transport
processes accumulate substances on one
side of the cell membrane at
concentrations many times greater than
those on the other side.
Active transport requires energy in the form of
ATP, and if ATP is not available, active
transport stops. Examples of active transport
include the movement of various amino acids
from the small intestine into the blood.
In some cases, the active transport
mechanism can exchange one substance for
another. For example, the sodium-potassium
exchange pump moves Na+ and K+,
established by the sodium-potassium
exchange pump, are essential in maintaining
the resting membrane potential.
Cystic fibrosis:
Cystic fibrosis is a genetic disorder that affects
the active transport of chlorine ions into cells. It
occurs at a rate of approximately one per two
thousand births (2000) and currently affects at
33,000 people in the united states. It is the most
common lethal genetic disorder among whites.
The diagnosis is based on the existence of
recurrent respiratory disease, increased Na in
the sweat, and high levels of unabsorbed fats in
the stool. Approximately 98% of all cases of
cystic fibrosis are diagnosis before the patient is
18 years old.
3. SECONDARY ACTIVE
TRANSPORT-CO TRANSPOT
AND COUNTER TRANSPORT
When sodium ions are transported out of cells by
primary active transport, a large concentration gradient
of sodium ions across the cell membrane usually
develops high concentration outside the cell and low
concentration inside. The gradient represents a
storehouse of energy because the excess sodium the
cell membrane is always attempting to diffuse to the
interior. Under approximate conditions, the diffusion
energy of sodium can pull other substances along with
the sodium through the cell membrane. This
phenomenon is called co-transport; it is one form of
secondary active transport
For sodium to pull another substance along it, a
coupling mechanism is required. This is
achieved by means of still another carrier protein
in the cell membrane. The carrier in this instance
serves as a attachment point for both the sodium
ion and the substance to be co-transported.
Once they both are attached the energy gradient
of the sodium ion causes both the sodium ion
and the other substance to be transported
together to the interior of the cell.
ENDOCYTOSIS AND
EXOCYTOSIS
Endocytosis (endon, within+ kytos, cell+
osis, condition) is the uptake of material
through the cell membrane by the
formation of a membrane-bound sac
called a vesicle. The cell membrane
invaginates to form a vesicle containing
the material to be taken into the cell. The
vesicle is then taken into the cell.
Endocytosis usually exhibits specificity. The
cell membrane contains specific receptor
molecules, which bind to specific
substances. When a specific substance
binds to the receptor molecule, endocytosis
is triggered, and the substance is transported
into the cell. This process is called receptormediated endocytosis. Cholesterol and
growth factors are examples of molecules
that can be taken into a cell by receptor
mediated endocytosis. Bacterial
phagocytosis is also receptor mediated.
Phagocytosis:
Phagoctosis (cell eating) is a term often
used for endocytosis when solid particles
are ingested. A part of the cell membrane
extends around a particle and fuses so
that the particle is surrounded by the
membrane. That part of the membrane
then “pinches off” to form a vesicle
containing the particle.
The vesicle is within the cytoplasm of the
cell, and the cell membrane is left intact.
White blood cells and some other cell
types phagocytize bacteria, cell debris,
and foreign particles. Phagocytosis is an
important means by which white blood
cells take up and destroy harmful
substances that have entered the body.
Pinocytosis “cell drinking” is distinguished
from phagocytosis in that much smaller
vesicles are formed that contain liquid
rather than particles.
In some cells, membrane-bound sacs
called secretory vesicles accumulate
materials for release from the cell. The
secretory vesicles move to the cell
membrane, where the vesicle membrane
fuses with the cell membrane, and the
material in the vesicle is eliminated from
the cell. This process is called
exocytosis.
Secretion of digestive enzymes by the
pancreas, of mucus by the salivary glands,
and of milk from the mammary glands are
examples of exocytosis. In many respects
the process is similar to that of
endocytosis, but it occurs in an opposite
direction. Endocytosis results in the uptake
of materials by cells, and exocytosis in the
release of materials from cells. Both
endocytosis and exocytosis and
exocytosis require energy in the form of
ATP to form vesicles.
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