Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 71

Just as the walls of your house separate the inside from the outside, so the cell membrane separates its inside (called the cytoplasm) from what’s outside the cell. The word cytoplasm (the inside of a cell) is derived from the Greek words cyto, meaning “cell,” and plasma, meaning “anything molded or formed.” The cell membrane is often called the plasma membrane.

All animal cell plasma membranes are made primarily of molecules called phospholipids. Plant cells also have phospholipid membranes, but plants also have cell walls made of more rigid molecules such as cellulose, which gives them their stiffness. These phospholipid membranes in cells are everywhere, so if you think they’re fundamental to life, you’re almost certainly right! Although we don’t know the details of how single cells evolved from the nonliving soup of organic molecules in ancient waters billions of years ago, the formation of stable phospholipid “bags” of organic molecules probably preceded the evolution of DNA that now controls the activity and reproduction of cells.

Why are phospholipid membranes so essential to life? Phospholipids are molecules that spontaneously form membranes in salt solutions such as seawater, which is where life probably evolved. They do this because they’re polarized, with different chemical properties at the two ends of the molecule (the next section explains this in more detail). The important thing about phospholipid membranes is that they’re very stable in salt solutions and almost totally impervious to the movement of water or ions through them. Phospholipid membranes keep the inside of the cell in and the outside of the cell out.