MEDIATED TRANSPORT MECHANISMS By Dr. Ishtiaq Ali 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. 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. 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. 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. 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 receptor- mediated 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. 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 receptor- mediated 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. 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. 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. 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. 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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