THE CELL’S PROTECTIVE WALL

To tour a prokaryotic cell, you would have to shrink to a size that is hundreds of times smaller than the period at the end of this sentence. Keeping you out of the cell is a tough, flexible membrane that acts like a brick and mortar wall surrounding a factory. It would take some 10,000 layers of this membrane to equal the thickness of a sheet of paper. But the membrane of a cell is much more sophisticated than the brick wall. In what ways?

Like the wall surrounding a factory, the membrane of a cell shields the contents from a potentially hostile environment. However, the membrane is not solid; it allows the cell to “breathe,” permitting small molecules, such as oxygen, to pass in or out. But the membrane blocks more complex, potentially damaging molecules from entering without the cell’s permission. The membrane also prevents useful molecules from leaving the cell. How does the membrane manage such feats?

Think again of a factory. It might have security guards who monitor the products that enter and leave through the doorways in the factory wall. Similarly, the cell membrane has special protein molecules embedded in it that act like the doors and the security guards.

Some of these proteins (1) have a hole through the middle of them that allows only specific types of molecules in and out of the cell. Other proteins are open on one side of the cell membrane (2) and closed on the other. They have a docking site (3) shaped to fit a specific substance. When that substance docks, the other end of the protein opens and releases the cargo through the membrane (4). All this activity is happening on the surface of even the simplest of cells.