Left Hands and Right Hands

Consider your left and right hands. They are very similar, but there is no way that one can be placed on top of the other without at least one aspect of them being reversed. Another example could be left and right-handed threads on a bolt. It is impossible to get a lefthanded nut to work on a right-handed bolt. These types of comparisons can be carried right down to the molecular level, and are critical to living systems.

You can read and "digest" a newspaper, but you can't literally eat and digest it even though the paper is almost pure sugar. Starch is a name given to large digestible compounds made up of sugar molecules, such as potatoes and flour. Cellulose (paper, cotton, etc.) is essentially the same large molecule, but with one right- or left-hand link between the sugar molecules reversed. Humans don't have the correct tool (enzyme) to fit between the sugar molecules or cellulose and break it down into usable sugar. A termite does, which permits it to survive on wood.

Hold a piece of plain white bread in your mouth for about a minute. You will note that it becomes sweeter as your enzymes start to break down the large starch molecules into the smaller sugar molecules from which they are built. Only the smaller sugar molecules, not the larger starch molecules, can pass through your cell membranes to provide you with nutrition. The key difference between the toast (starch) you ate for breakfast and the cotton shirt (cellulose) you wear is just one right- or left-hand link between the sugar molecules out of which both are constructed.

Spearmint and caraway have two noticeably different aromas. Yet the molecule that causes those aromas is exactly the same in all respects except for the left- or right-handed arrangement of one carbon atom in the compound.

Larger molecules of living systems have so many variable combinations of left- or right-handed ways of being put together that the diameter of the Milky Way measured in inches would be a smaller number.

Modern medicines would have no effect if just one of the numerous left or right-handed centers in them were reversed. This is only one reason why the potential of genetic engineering is so promising.