ml> Polarity II The Polarity of MoleculesWe have seen that atoms differ in their capacity to "hold onto" theirelectrons; some gain electrons, some lose electrons. Certain atoms,oxygen and nitrogen, for example, do not have sufficientelectron-attractive power to become fully charged negative ions.However, the attractions of electrons is sufficiently great so that,when covalently bonded to hydrogen, the electrons are not equally sharedbetween the two nuclei. The electrons tend to spend more time around theoxygen nucleus and consequently less time around the hydrogen nucleus.This means that one portion of a molecule is slightly positive or slightlynegative in relation to another portion of the same molecule. When suchan uneven distribution of charge occurs, the molecule is said to exhibitpolarity. The molecule has a positive and a negative end,separatedfromeachother like the poles of a bar magnet. Because this is not a full -1or +1 charge but a smaller charge, it is represented as delta positive ordelta negative.Water is a good example to illustrate this point. Although, as awhole, the water molecule is electrically neutral, it does have a positiveand a negative end. The geometric configuration of the molecule placesboth hydrogen atoms at one end. The nucleus of the oxygen atom attractselectrons more than the nuclei of the hydrogen atoms. This results in twoslightly positively charged regions on one end of the molecule and asingle slightly negatively charged region on the other. The molecule thushas a positive and a negative end, or two poles. The water molecule is apolar molecule.The significance of molecular polarity to the biological sciences comefrom two main areas: First, polar molecules tend to become oriented withrespect to other molecules. Because of this, polar molecules areimportant in helping to establish the three-dimensional structure ororientation of other larger molecules. For example, molecules of fattyacids (Chapter 8), found in all living matter are composed of a nonpolarcarbon chain with a polar carbon-oxygen group (COOH) at one end. Whenplaced in water, the polar ends of the fatty acid molecules are attractedto water molecules, which are also polar. The nonpolar carbon chains areat the same time repelled by the water. As a result, fatty acid moleculesare oriented on the water"s surface.
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Of particular importance to living things is the orientation ofphospholipid molecules, which are a combination of a fat moleculewith a phosphate group. Phospholipids are among the most important partsof cell membranes. They tend to become oriented on surface or boundaryregions in a manner similar to the fatty acids on water. It is partly inthis way that cell membranes are given a distinct structure.Second, polarity is important in understanding both the geometry andthe chemical characteristics of large molecules, such as proteins.Proteins are so large that they may possess a number of polargroups on one molecule. Polar groups, like radicals (Section 3-4),are simply groups of atoms which bear as a unit a partial positive or apartial negative charge. The specific geometry of proteins exists in partbecause polar groups on one part of the molecule attract polar groups onanother part of the same molecule. This stabilizes the specific twistingand folding of the molecule which is all-important to the chemicalcharacteristics it displays.Polarity thus tends to bring small molecules, or specific regions oflarge molecules, into definite geometric relation. In this way, thechemical bonding between individual molecules or between specific portionsof large molecules is brought ab. Hydrogen bondsare produced by the electrostatic attraction between positively(partially)out more easily.In living organisms, one of the most common types of chemical bondsproduced by polar attraction is the hydrogen bond charged hydrogen atoms (protons) on one part of the molecule,and negatively charged atoms of oxygen or nitrogen on the same or anothermolecule. The oxygen and nitrogen atoms are partially negatively chargedbecause their nuclei attract large numbers of electrons around them.Because the hydrogen bond occurs between polar regions of a molecule, itis, like all polar attractions, relatively weak. A simple example of hydrogen bonding can be seen between watermolecules. The hydrogen atoms of one water molecule form a hydrogen bondwith the oxygen atom of the adjacent molecule. Polar molecules, such as fatty acids, tend to orient themselvesin respect to other polar molecules. Here the molecules of a fatty acidline up in a specific fashion on the surface of water.


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The COOH groupsare in the water (also polar) and the carbon chains stick out into theair.