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The most important structure of the cell is its membrane, for it is the cell membrane that controls and regulates what enters and leaves the cell. The cell membrane is made of two layers of lipids, called a lipid bilayer. The lipid bilayer is made of lipids (fats) and phosphates, normally called phospholipids Phospholipids have a polar end, and a non-polar end. In making membranes, the non-polar ends of both layers face each other, while the polar end faces the outside and inside of the cell. Thick of a soap bubble. Soap is also made of phospholipids. When you wash your hands with soap, the soap surrounds oils, pointing their non-polar ends at the oils, which are themselves non-polar. Soap's polar ends face the water, allowing water to remove oils from your hands. It is this very nature that makes a cell's membrane so impervious to things moving in and out of a cell. Water and other polar molecules can approach the membrane, running along its outer surface. But if anything polar tries to move through the membrane, its non-polar middle pushes any polar molecule out, like positive ends of magnets pushing each other away. Of course, any non-polar molecule will be pushed away from the cell membrane at the membrane's surface.
So how can anything move into or out of a cell? Some things, like ions, are very small. So even if they are slightly polar or are non-polar, they can slip into and out of the cell. Such small molecules just simple diffuse through the membrane in a process called osmosis. By the way, I lied a little about water. It is actually small enough to escape the effects of the non-polar middle of the lipid bilayer. Larger molecules can not slip through so easily. So how do large molecules enter and leave the cell?
Cell membranes have passages, or channels, made from proteins. These channel proteins allow water, other proteins, lipids, and sugars to enter and leave the cell. The channel proteins help the passage of larger molecules, facilitating their passage in a process called facilitated transport. Sometimes the channel proteins need a little energy to push a molecule through. In this case, we would call the process active transport.