Membrane Function
Membrane Function
The incredibly thin, yet sturdy, plasma membrane that encloses every cell is vitally important in maintaining cellular integrity. Once believed to be a rather static entity that defined cell boundaries and kept cell contents from spilling out, the plasma membrane (also called the plasmalemma) is a dynamic structure having remarkable activity and selectivity. It is a permeability barrier that separates the interior from the external environment of the cell, regulates the vital flow of molecular traffic into and out of the cell, and provides many of the unique functional properties of specialized cells.
Membranes inside the cell surround a variety of organelles. Indeed, the cell is a system of membranes that divide it into numerous compartments. Someone has estimated that if all membranes present in one gram of liver tissue were spread out flat, they would cover 30 square meters! Internal membranes share many of the structural features of plasma membranes and are the site for many, perhaps most, of the cell’s enzymatic reactions.
A plasma membrane acts as a selective gatekeeper for the entrance and exit of the many substances involved in cell metabolism. Some substances can pass through with ease, others enter slowly and with difficulty, and still others cannot enter at all. Because conditions outside the cell are different from and more variable than conditions within the cell, it is necessary that the passage of substances across the membrane be rigorously controlled.
We recognize three principal ways that a substance may traverse the cell membrane: (1) by diffusion along a concentration gradient; (2) by a mediated transport system, in which the substance binds to a specific site that in some way assists it across the membrane; and (3) by endocytosis, in which the substance is enclosed within a vesicle that forms on and detaches from the membrane surface to enter the cell.
The incredibly thin, yet sturdy, plasma membrane that encloses every cell is vitally important in maintaining cellular integrity. Once believed to be a rather static entity that defined cell boundaries and kept cell contents from spilling out, the plasma membrane (also called the plasmalemma) is a dynamic structure having remarkable activity and selectivity. It is a permeability barrier that separates the interior from the external environment of the cell, regulates the vital flow of molecular traffic into and out of the cell, and provides many of the unique functional properties of specialized cells.
Membranes inside the cell surround a variety of organelles. Indeed, the cell is a system of membranes that divide it into numerous compartments. Someone has estimated that if all membranes present in one gram of liver tissue were spread out flat, they would cover 30 square meters! Internal membranes share many of the structural features of plasma membranes and are the site for many, perhaps most, of the cell’s enzymatic reactions.
A plasma membrane acts as a selective gatekeeper for the entrance and exit of the many substances involved in cell metabolism. Some substances can pass through with ease, others enter slowly and with difficulty, and still others cannot enter at all. Because conditions outside the cell are different from and more variable than conditions within the cell, it is necessary that the passage of substances across the membrane be rigorously controlled.
We recognize three principal ways that a substance may traverse the cell membrane: (1) by diffusion along a concentration gradient; (2) by a mediated transport system, in which the substance binds to a specific site that in some way assists it across the membrane; and (3) by endocytosis, in which the substance is enclosed within a vesicle that forms on and detaches from the membrane surface to enter the cell.
Support our developers
