Water is polar, so the phosphate heads are attracted to water. The cell membrane separates the cytoplasm, which contains water, and the interstitial fluid, which also contains water. Lipids are nonpolar, but the phosphate group is polar. And two fatty-acid tails make up the lipid part of a phospholipid. Phospholipids look kind of like brass brads, fasteners you have likely used to hold papers together or in a folder. This means it is made of two layers, or a bilayer, of phospholipids molecules. The cell membrane is composed primarily of a phospholipid bilayer. And like you in your home, if something cannot pass directly through the cell membrane, it may still be able to pass through a door. The chemical composition of the cell membrane determines what can and cannot pass. Instead, for cells, that control is done chemically. Cell membranes, however, unlike you in your home, have neither eyes nor brains to determine what should or should not come in or go out. Because of this the cell membrane is said to be semipermeable, or selectively permeable. But the membrane is impermeable, or not permeable, to other substances, so they cannot pass through it. The cell membrane is permeable to some substances, so they can pass directly through it. As fumes begin to build, you could open a window or a door to let them out and to bring fresh air in. And if someone else comes to the door, you decide whether or not to open the door. He can't just toss a pizza through the wall. For the majority of our cells, that outside world is fluid that fills the spaces between the cells, called the interstitial fluid. Cell membranes provide the same service, acting as a physical separation between the inside of the cell, called the intracellular compartment, and the world around the cell, the extracellular compartment. But you couldn't do this if you did not have walls around you providing a physical separation between the inside and the outside worlds.
Whether the temperature outside is sub-freezing or scorching hot, you can maintain a comfortable temperature inside. How can they maintain this constancy, called homeostasis, while allowing substances to both come and go? The answer is found in the cell membrane. Yet cells work best when the mini-world inside them stays relatively constant. They need to bring in building materials and nutrients for energy production so they can do their work, and they also generate waste products that must be cleared away. To do so, their needs must constantly be met.
Specifically, we will answer the following questions: what are some functions of the cell membrane, what is the structure of the cell membrane, and how are the cell membrane structure and function related? Our cells carry out the necessary functions to maintain not only their own lives, but ours as well. In this lesson we will examine the structure of the cell membrane.
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