Passage of substances across biomembranes occur by three methods -
1) Passive transport
2) Active transport
3) Bulk transport
1) Passive transport:- It is the mode of membrane transport in which cell does not spend any energy. it is of two types:-
a) Passive diffusion by tunnel protein molecules:- The tunnel protein molecules have the channels. these channels permit water and water soluble substances to pass through them. The channels only permit the molecules to pas from higher concentration to lower concentration gradient.
b) passive transport by carrier proteins:- The proteins are known as permeases. They transport the material down the concentration gradient. A carrier protein combines with a specific substances to be transported and moves it from one side of the membrane to another through a channel in it.
Facilitated diffusion moves glucose molecules in liver and it occurs without expenditure of energy.
2) Active transport:- Active transport moves substances against the concentration by utilizing energy which is privided by ATP ( Adenosine triphosphate ). The carries protein has a binding site for ATP in addition to the binding site for the substrate. ATP molecules binds to the carrier protein. The energy set free brings the substrate binding site of the protein to the surface of the membrane. The substrate present in the medium joins the binding site of protein, forming carrier-substrate complex. It undergoes conformational changes and carries the substrate through a channel in it to the cytoplasmic site of the membrane. Here the substrate is released. This type of transport by using energy is supported by various evidences:- i) Absorption is reduced with the decrease in oxygen content of the surrounding environment ( cells are using oxygen in the respiration involved in the liberation of energy-ATP).
ii) Metabolic inhibitors like cyanides inhibit absorption.
iii) Absorption of different substances is selective.
iv) Decrease in temperature decreases absorption. ( again respiration process is regulated by temperature)
v) Active transport is more rapid then diffusion.
3) Bulk transport:- It occurs by two methods:- pinocytosis and phagocytosis. They involve the enclosure of the material under transport in the vesicles of the membrane. The inward transport by means of carrier vesicles is called endocytosis. The outward transport of substance by means of carrier vesicles is known as exocytosis.
Pinocytosis:- It is the bulk trasport of fluid matter and substances dissolved in it. pinocytosis is also called cell drinking.
Phagocytosis:- It is also called cell eating. Phagocytosis is the transport of solid matter like food, foreign particles, pathogens etc. across the membrane by forming vesicles. These vesicles are called phagosomes. phagosomes fuses with lysosomes to produce a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole with indigestible substance is called residual vacuole. The undigested part are usually thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood cells is an important defence mechanism of the animal body.
1) Passive transport
2) Active transport
3) Bulk transport
1) Passive transport:- It is the mode of membrane transport in which cell does not spend any energy. it is of two types:-
a) Passive diffusion by tunnel protein molecules:- The tunnel protein molecules have the channels. these channels permit water and water soluble substances to pass through them. The channels only permit the molecules to pas from higher concentration to lower concentration gradient.
b) passive transport by carrier proteins:- The proteins are known as permeases. They transport the material down the concentration gradient. A carrier protein combines with a specific substances to be transported and moves it from one side of the membrane to another through a channel in it.
Facilitated diffusion moves glucose molecules in liver and it occurs without expenditure of energy.
2) Active transport:- Active transport moves substances against the concentration by utilizing energy which is privided by ATP ( Adenosine triphosphate ). The carries protein has a binding site for ATP in addition to the binding site for the substrate. ATP molecules binds to the carrier protein. The energy set free brings the substrate binding site of the protein to the surface of the membrane. The substrate present in the medium joins the binding site of protein, forming carrier-substrate complex. It undergoes conformational changes and carries the substrate through a channel in it to the cytoplasmic site of the membrane. Here the substrate is released. This type of transport by using energy is supported by various evidences:- i) Absorption is reduced with the decrease in oxygen content of the surrounding environment ( cells are using oxygen in the respiration involved in the liberation of energy-ATP).
ii) Metabolic inhibitors like cyanides inhibit absorption.
iii) Absorption of different substances is selective.
iv) Decrease in temperature decreases absorption. ( again respiration process is regulated by temperature)
v) Active transport is more rapid then diffusion.
3) Bulk transport:- It occurs by two methods:- pinocytosis and phagocytosis. They involve the enclosure of the material under transport in the vesicles of the membrane. The inward transport by means of carrier vesicles is called endocytosis. The outward transport of substance by means of carrier vesicles is known as exocytosis.
Pinocytosis:- It is the bulk trasport of fluid matter and substances dissolved in it. pinocytosis is also called cell drinking.
Phagocytosis:- It is also called cell eating. Phagocytosis is the transport of solid matter like food, foreign particles, pathogens etc. across the membrane by forming vesicles. These vesicles are called phagosomes. phagosomes fuses with lysosomes to produce a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole with indigestible substance is called residual vacuole. The undigested part are usually thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood cells is an important defence mechanism of the animal body.
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