Selective permeability is the ability of a cell membrane to control which molecules can enter or leave the cell. This can occur through passive transport, active transport, or vesicles. The size, charge, and solubility of molecules affect permeability, which is essential for sustaining life.
In cell biology, selective permeability is the property of a living cell membrane that allows the cell to control which molecules can pass through the membrane, entering or leaving the cell. To understand this property, it is helpful to be aware that there are three different methods by which molecules can enter or leave cells: passive transport, active transport and transport by the use of vesicles.
In passive transport across a selectively permeable membrane, molecules move across the membrane without the cell having to expend extra energy. When water molecules passively enter or leave a cell, for example, this is called osmosis. Other small molecules can move across the membrane by the diffusion process. This means that they move across the cell membrane from an area of high concentration to an area of low concentration. Oxygen molecules can diffuse from the lung cavity by passive transport into the blood cells of the lung.
Active transport is a vital mechanism used by living cells for selective permeability. This method is required for small molecules to move across the cell membrane in situations where the molecules must move against a concentration gradient. Unlike passive transport, active transport gives the cell the ability to move molecules from an area of low concentration to an area of high concentration. It works through special channels called pumps, which are present in the cell’s plasma membrane and which expend energy when they move molecules across the membrane. Active transport is often used by the cells lining the stomach to absorb glucose, amino acids and other nutrients.
Vesicles are tiny pockets that can form in the cell membrane to help transport larger molecules. Vesicles allow the cell to take in or excrete these molecules across the cell membrane. This process is called endocytosis when molecules are moved into the cell and exocytosis when molecules are moved out of the cell.
The selective permeability of membranes generally depends on the size of the molecules, the positive or negative charge of the molecules and their solubility in water or oil. In plasma cell membranes, it also depends on many biological functions and biochemical reactions both inside and outside each cell. It is one of the most vital biochemical attributes of a living cell and is a key part of most vital processes required to sustain life.
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