Do you remember learning in school that the mass of a plant doesn't come from the soil, but from the air? Perhaps this tidbit is easy to forget, but it's true! Plants can suck carbon dioxide out the air and turn it into sugar. They use this sugar as energy to grow and reproduce. In order to stand up and reach for the sun, plants need to be able to build rigid structures can can hold weight. Trees are wizards at doing this.
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| Figure 1: The monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. In the background, an example of a lignin polymer. |
To build rigid structures, woody plants break down the sugars they obtained from capturing sunlight and reassemble the carbon pieces into monolignols - ring structures made of carbon, oxygen, and hydrogen. Some of the most common monolignols are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Fig. 1). Scientists suspect that the trees then produce enzymes (small molecular-scale machines) that can assemble these monolignols use what's called (I kid you not) 'radical' chemistry. Radicals are the technical name for unpaired electrons. In an unpaired state, electrons are highly reactive and it seems that it is this energy that the tree's molecular machines use to link the monolignols together.
After radical polymerization, very large polymers are formed - see the background in Fig. 1. This process consumes an incredible amount of carbon. In fact, the Carboniferous Period (350 - 300 million years ago) is in part defined by plants' evolution of the ability to sequester carbon from the atmosphere and store it in a way that allows them to stand up tall. It is these huge, cross-linked polymers that give wood its strength, and it is these same structures that are oxidized during the burning of wood, returning the carbon to the atmosphere after potentially hundreds of years in storage.
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