|MadSci Network: Cell Biology|
Selectively permeable means that only specific molecules can cross the plasma membrane. This is a very important characteristic of cellular membranes. Inside each cell is a specific combination of water, salt, sugars, nucleic acids, fats, and proteins. If that combination is disrupted, the cell won't work properly. Sometimes it can even die. For example, if too much water crosses the plasma membrane, the cell will lyse, or burst apart from the pressure. Alternatively, if too much salt crosses the membrane, there won't be enough liquid inside the cell for it to perform its functions. For some proteins, specific movement of materials across the membrane is required for function. For example, when a nerve cell wants to send information to another cell, the rapid transport of Sodium and Potassium across it's membrane results in an electrical current that passes through the axon of the cell and causes the release of chemical information that is picked up by another cell. Without selective permeability, this electrical current could not be achieved, and our nerves wouldn't work.
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