Polymers are molecules. In fact, as repeatedly stated in the previous session, polymers are large molecules (macromolecules) made by linking together a series of small ones into chains. But what is a molecule? In today's session, you'll find that molecules are groups of two or more atoms held together by chemical bonds. But notice that I've introduced two additional terms: atoms and chemical bonds. Obviously in order to make sense of molecules, you're going to have to have some understanding of what these terms mean.
If you were to take a piece of any living thing -- a tomato slice, an apple, or a piece of your own skin -- and heat it very hot in the absence of air, you will end up with a solid blackish ash and a colorless gas. Upon analysis, a competent chemist will tell you that the solid consists almost entirely of three substances: carbon, phosphorous and sulfur. Likewise, the gas would be almost entirely made up of oxygen, nitrogen and hydrogen. If the tissue had some blood or muscle in it, the analysis would show some iron, and if there were bone present you would get some calcium, but apart from these and some other minor components, these six substances (carbon (C), hydrogen (H), oxygen (O), nitrogen(N), phosphorous (P) and sulfur (S)) are predominantly what we, and all living things, are composed of. These six substances are called elements, the result of what happens when you heat any substance to a high enough temperature.
There are just 92 different naturally occurring elements (Another 20 or so others have been created artificially by nuclear physicists). The six elements mentioned above are probably familiar to you, as they should be because they are you. Other elements that you probably have heard of are gold, silver, mercury, aluminum, neon and chlorine. And there are some that are not very well known like praseodymium or gadolinium.
Molecular biologists have it easy
Unlike chemists, who must learn about all the elements, the
molecular biologist generally has only to be concerned with the six
that are commonly found in the ashes and gases of living things.
We'll refer by them by their one letter designations: CHNOPS
(affectionately pronounced "shnops"): carbon (C), hydrogen (H),
nitrogen (N), oxygen (O), phosphorous (P) and sulfur (P). We need to
know about them for the same reason that an architect needs to know
something about the bricks and lumber a house is made of. It may seem
a relatively simple task to learn about six elements, and perhaps it
would be if it were not for carbon -- the most important member of
the sextet. Carbon is central to life as we know it because it has
the unique ability to combine with itself and with the five other
elements in a near endless variety of different combinations.
Atoms
What do we mean when we say that carbon, an element, can combine with itself and other elements? To answer this question we need to look at elements close up&emdash; at the individual entities that they consist of. These entities are called atoms.
An atom can be understood in terms of a simple conceptual
experiment first suggested by the ancient Greeks. The Greeks wondered
what would happen if they were to cut a cube of gold into smaller and
smaller pieces. Eventually, they expected, the pieces would get to a
size so small that they were too tiny to see. What would happen if
they were to continue cutting? One school of thought held that this
division would continue forever, yielding smaller and smaller pieces
of gold infinitely, without end. Others argued that the cutting would
eventually reach a point where only a single unit of gold remained.
Any further cuts, if they could be made at all, would destroy what
was being cut so that it no longer had the properties of gold. By
this logic, which we now believe to be correct, an atom is the
smallest unit of an element that retains the properties of that
element.
On the right are depicted atoms of CHNOPS magnified about 25 million times. They are drawn, as the ancient Greeks might have pictured them, as more or less featureless spheres, different only in size. To distinguish among them, I've assigned colors to each. In the picture at the right, I've even engraved initials on them. Even though these pictures make them appear very similar, except for their color, in the real world these atoms behave very differently from one another. To understand why, we must look more closely at their structure.
Electrons and atomic nuclei
At present there is very good evidence that an atom isn't simply
an indivisible sphere. Instead, it consists of a central, positively
charged nucleus surrounded by a swarm of one or more
negatively charged electrons. Each element is characterized by
a specific number of electrons. For example, hydrogen -- the simplest
element-- is surrounded by a solitary electron, while uranium -- the
most complicated naturally occurring element -- has 92. These
electrons fly around the nucleus, like bees buzzing around a flower,
in complicated paths. The spheres in the figure very roughly
represent the outer limits of where each atom's electrons are
located.
The nucleus is a cluster of two kinds of particles: protons, which
carry a positive charge, and neutrons that have no charge at all. The
number of protons is exactly the same as the number of electrons
surrounding the nucleus. This means that the positive and negative
charges precisely balance out, and that all atoms are electrically
neutral.
