What Is a Membrane?
A membrane is the outer layer of a cell that wraps around it to protect the inner material from the external environment. It is composed of protein and fat, and is semipermeable, which means selected molecules can pass in and out of it. The membrane is structured by the cell's cytoskeleton, the scaffolding inside the cell that gives it shape and boundaries. In many ways, the membrane of the cell functions in the same way our skin functions: it provides a barrier between our innards and the outside world, blocking harmful material from entering while also allowing needed nutrients in and certain waste to come out.
Function of the Membrane
The main function of the membrane is to moderate what materials pass in and out of the cell. It works by allowing material to pass either passively or actively through it to move items in and out of the cell. Passive movement, also known as osmosis, is effectively the effort-free way material can be passed: neither the cell nor the surrounding cells need to exert any effort to move material. Active movement implies that the cell, membrane and surrounding cells must use energy to move items through it. The semipermeable nature of the membrane typically allows only items the size of a protein molecule to pass in or out of the cell. Movement is spurred by a molecule's electric charge or pH.
Another, secondary function of the membrane is to aid the cytoskeleton in providing structure to the cell. While the membrane relies on the cytoskeleton to give it shape and a framework, the membrane in turn also helps the cytoskeleton stay attached to the innards of the cell and connect it to other cells. This is called the extracellular matrix, and it is how cells stick together to create tissue, muscle, skin and other larger biological entities.
Cell Membrane and Disease
Membranes play a crucial role in the protection of cells, and the larger entities they form, from disease and destruction. Likewise, they play a crucial role in our health. The membrane is a cell's first line of defense, and certain diseases, like HIV, target the membranes of immune system cells and make them more malleable an easy to permeate. Other serious illnesses, like cancer and polio, are also linked to a weakening of the permeability of the membrane, allowing harmful material to pass into the cell and destroy it. Heart disease, one of the top reasons for death in the United States, is caused by cell membranes repairing themselves incorrectly.
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