I love the fall. It is such a wonderful, colourful time of the year. It's the time of the year for long walks down forest paths amongst a symphony of oranges, browns, reds, and even a few greens. The fall colours are truly spectacular, one of nature's better displays. But why are we treated to this symphony?
The process by which plants collect and use sunlight is called photosynthesis - probably one of the first big words kids learn to say. As a kid, it was explained to me that there were little men inside each leaf that capture a sun beam and turn it into sugar and oxygen. (There were always little men inside of everything making them run!) As an adult, I know that photosynthesis involves the interaction of light with atoms and molecules. Indeed, this is the sort of thing that I study as a chemist.
The most important molecule in photosynthesis is chlorophyll. It is a relatively simple biological molecule, containing only a few hundred atoms. It is a large ring with a magnesium ion at its centre and a tail dangling from one side.
In leaves, chlorophyll is concentrated in "light gathering" assemblies where it captures passing photons and converts their energy to a useable form. The light that chlorophyll captures is predominantly in the blue part of the spectrum but some is in the red region. This leaves behind the greens and yellows, hence, the colour of leaves. With the blue and red absorbed, it is the green that is reflected for our eyes to see.
Why waste sunlight, though? Well, plants don't. They have evolved over the course of time to gather as much energy as possible from light by using a whole range of pigments. For example, the carotenoids and open chain tetrapyrroles gather light from the green portion of the spectrum, leaving behind the red and, to a lesser extent, the blue colours.
But it is the high energy "blue end" of the visible spectrum that is the most useful for making sugars and oxygen which are the products of photosynthesis. This means that chlorophyll, which is the best pigment at utilizing this region of the spectrum, is the dominant pigment in leaves - the most concentrated compound.
It is the colour of this compound that we see most of the summer. Or, more accurately, it is the colour of light that is not absorbed by chlorophyll that gives plant life its unmistakable hue.
However, the blue end or high energy end of the spectrum is also where bonds get broken and molecules get destroyed. And this, too, alas is the fate of chlorophyll. It bravely sacrifices its existence to provide the energy that drives photosynthesis... well, OK, it is a molecule and probably doesn't know anything about bravery.
Indeed, the whole photosynthetic process happens with no conscious thought or action and little in the way of outside influence. But the continued bombardment of chlorophyll with high energy photons does mean that it gets decomposed fairly rapidly and that it continually needs to be replaced during the long growing season.
In the fall when deciduous trees prepare for winter, they stop producing chlorophyll. As the days grow shorter and the light decreases, at some point it takes more energy to produce chlorophyll than the plant can obtain from the sunlight. That is, even though chlorophyll is a fairly small natural molecule, its synthesis is still an energy expense for the plant and requires a fair amount of work and energy.
At some critical point, the law of diminishing returns takes over and the investment in new chlorophyll is not offset by the energy and chemicals being generated during photosynthesis.
As a consequence, deciduous trees shut down their manufacturing facilities, saving energy and effort. The arrival of fall triggers this temporary cessation of chlorophyll production. And without the dominant colour of chlorophyll to mask their colours, all the rest of the pigments come to the fore and get a chance to show off their hues.
It is this emergence of colours that have been hidden all summer long that leads to our spectacular fall displays.
It is these colours that make a walk in the woods at this time of year such a wonderful experience because - like it or not - it would appear that fall is upon us.
