The Biology of Belief - Bruce H. Lipton [32]
Polar molecules include water and things that dissolve in water. Nonpolar molecules include oil and substances that dissolve in oil; there are no positive or negative charges among their atoms. Remember the adage “water and oil don’t mix”? Neither do oily nonpolar and watery polar molecules. To visualize the lack of interaction between polar and nonpolar molecules, think of your bottle of Italian salad dressing. You do your best to get vinegar and oil to bond by shaking the bottle, but when you set the bottle down, they separate. That’s because molecules, like people, prefer environments that offer them stability. For their stability, polar (vinegar) molecules seek out watery polar environments and nonpolar (olive oil) molecules seek out nonpolar environments. Phospholipid molecules, comprised of both polar and nonpolar lipid regions, have a difficult time in seeking stability. The phosphate portion of the molecule is motivated to seek water, while its lipid portion abhors water and seeks stability by dissolving in oil.
Electron micrograph showing the cell membrane at the surface of a human cell. The dark-light-dark layering of the cell membrane is due to the ordering of the barrier’s phospholipid molecules (inset). The lighter center of the membrane, the equivalent of the butter in our sandwich, represents the hydrophobic zone formed by the legs of the phospholipids. The dark layers above and below the central lipid zone, the equivalent of the bread slices, represent the molecule’s water-loving phosphate heads.
Getting back to our sandwich, the membrane’s phospholipids are shaped like lollipops with an extra stick (see illustration above). The round part of the lollipop has polar charges among its atoms; it corresponds to the bread of our sandwich. The molecule’s two stick-like portions are nonpolar; they correspond to the butter part of our sandwich. Because the “butter” portion of the membrane is nonpolar, it does not let positively or negatively charged atoms or molecules pass through it. In effect, this lipid core is an electrical insulator, a terrific trait for a membrane designed to keep the cell from being overwhelmed by every molecule in its environment.
But the cell could not survive if the membrane were the equivalent of a simple bread and butter sandwich. Most of the cell’s nutrients consist of charged polar molecules that would not be able to get past the formidable nonpolar lipid barrier. Neither could the cell excrete its polarized waste products.
Integral Membrane Proteins
The olives in our sandwich are the truly ingenious part of the membrane. These proteins allow nutrients, waste materials, as well as other forms of “information” to be transported across the membrane. The protein “olives” allow not just any old molecules to get into the cell but only those molecules necessary for the smooth functioning of the cytoplasm. In my sandwich, the olives represent Integral Membrane Proteins (IMPs). These proteins embed themselves into the “butter” layer of the membrane, just as I have embedded olives in the illustration.
How do IMPs embed themselves into the butter? Remember that proteins are composed of a linear backbone assembled from linked amino acids. Of the twenty different amino acids, some are water-loving, polar molecules and some are hydrophobic, nonpolar molecules. When a region of the protein’s backbone is made up of linked, hydrophobic amino acids, this segment of the protein seeks stability by finding an oil-loving environment like the membrane’s lipid core (see arrow below). That’s how hydrophobic parts of the protein integrate themselves into the middle layer of the membrane. Because some regions of a protein’s backbone are made up of polar amino acids and other regions are nonpolar, the protein strand will weave itself in and out of the bread-and-butter sandwich.
There are lots of IMPs with lots of different names, but they can be subdivided into two functional classes: receptor proteins and