Published: 14 July, 2005

Carbon's love of bonding makes it the key element in living things

Why is carbon the building block of matter?

First, carbon is actually not the building block of matter, atoms are. A look at the periodic table (which can be found at www.webelements.com) shows all the different kinds of atoms (elements) that are known. Carbon (element #6) is only one of the more than 100 different elements. I think what this questioner really wants to know is: why is carbon the building block of matter in living things? Carbon is a key element in living things, and this is a great question.

It turns out that life is surprisingly stingy with the elements it uses. Of all the elements known, only a few are found in living things. Life is mostly hydrogen (#1), carbon (#6), nitrogen (#7), oxygen (#8), and phosphorus (#15) along with smaller amounts of other atoms.

The story is a little more complicated, however. We are certainly not just a mixture of these elements. If all matter were just elements, there would only be about 100 different kinds of stuff in the world. Most of the matter that is around us (and inside us) is not pure elements but combinations of atoms of different elements bound together to make molecules. Some examples of simple molecules that you probably know are carbon dioxide, which is made from one carbon atom and two oxygen atoms; and water, which is made from two hydrogen atoms and one oxygen atom.

As you might imagine, the molecules in our bodies do some pretty amazing things. They detect light (so you can see), they move oxygen around your body from your lungs to your tissues, they can break down food molecules for energy and for use in building new muscle and tissue, they also contain all the information to make an organism from the first cell after fertilization. One thing chemists noticed early on in their studies on biological molecules such as DNA and proteins that perform these amazing jobs in living organisms is that they are really big. Now don't get me wrong, all molecules are very small. In fact, any molecule (even a big one) is about a million times smaller than anything you can see; but as molecules go, the ones in your body are whoppers. For example, if you compare the rather normal-sized water molecule and a molecule of hemoglobin (which makes your blood red and moves oxygen around your body) it would look like a ping-pong ball next to a car! Most big biological molecules are polymers, which are long molecules made from smaller "monomer" molecules bound together, just as a train is a long linkage of individual railroad cars. Biological monomers from various combinations of hydrogen, oxygen, carbon, nitrogen and phosphorous, are made into long polymers such as DNA and protein.

Carbon is a perfect element to build very, very big molecules with. If there is one thing in the world that carbon loves, it is bonding with other carbon, nitrogen and oxygen atoms. Some good examples of carbon's ability to form long polymers are the synthetic polymers that we make and use. Among these are nylon (seat belts), polystyrene (styrofoam), kevlar (bullet proof vests) and polycarbonate (CD cases). All these polymers are made from the same elements as biological polymers, showing that carbon (and its friends nitrogen, oxygen hydrogen and phosphorous) are ideal elements to build large molecules from.