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Autumn colours are a product of diminishing returns

As I was walking the dogs the other day, a stiff cold breeze was blowing. Fall is on its way. The leaves are starting to turn. Flowers are dying off. And a hint of frost lays on the ground first thing in the morning.

As I was walking the dogs the other day, a stiff cold breeze was blowing. Fall is on its way. The leaves are starting to turn. Flowers are dying off. And a hint of frost lays on the ground first thing in the morning.

Even though summer is not officially over until the autumnal equinox later this month, once school has started, most of us think of summer as done. It would seem that the natural world around us agrees. Trees have started to change colour and lose their leaves.

Despite the fact that I know this is just the first steps into another long, cold winter, I can't help but love the colours that are on display every fall. Yes, I know that it doesn't compare to the forests in eastern Canada, but I still think that our display is spectacular. Oranges, reds, browns, and even a few greens make for a wonderful symphony of colours at this time of year. But why are we treated to this visual display?

One of the first big words that most kids learn is "photosynthesis". Literally, it is the assembly or "synthesis" of molecules using light or "photons" for energy. It is the single most important biological process as it converts sunlight (energy), carbon dioxide, and water into simple sugars which are used to construct the cellulose of plants and provide food for almost every living organism on the planet.

The most important molecule in photosynthesis is "chlorophyll". In the grand scheme of biologically relevant molecules, it is relatively simple consisting of only a few hundred atoms. It is a large ring or "macrocycle" with a magnesium atom at its core and a long tail dangling from one side. Think of it as a disk with a string attached and, in the middle of the disk, is a marble. The marble is the magnesium; the disk is the macrocycle; and the string is the tail.

In leaves, chlorophyll is concentrated in "light gathering" assemblies within organelles called "chloroplasts". These assemblies capture passing photons and convert the energy to a useable form to drive the chemical transformations. The light that the chlorophyll captures is predominantly at the blue end of the visible spectrum, but there is also a significant amount of light absorbed at the red end. The consequence is that it is green light that is reflected and refracted as it bounces off or passes through a leaf.

This why leaves are green. They do not capture much of the green light but they do capture a bit. After all, why waste sunlight? 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 that absorb at different points in the visible spectrum. 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, blue colours.

It is the high energy "blue end" of the spectrum that is most useful for making sugars and oxygen - the products of photosynthesis. This means that chlorophyll, which is the best pigment at utilizing this region of the visible spectrum is the dominant pigment in leaves.

Chlorophyll is responsible for the colour that we see for plants and leaves during most of the summer.

However, the blue end of the visible spectrum - the "high energy" region of the spectrum - is where ionization occurs. It is where bonds get broken and molecules get destroyed. This is why ultraviolet light, which is at a slightly higher energy than even blue light, is so damaging as it breaks down the molecules in our skin.

This, alas, is also the fate of chlorophyll. It bravely sacrifices its existence to provide the energy that drives photosynthesis and feeds the plant. Okay. It is a molecule, composed of nothing more than bonded atoms, and doesn't know anything about "bravery". But the continued bombardment of chlorophyll by high energy photons means that it gets decomposed fairly rapidly and continually needs to be replaced.

As winter approaches, deciduous trees stop producing chlorophyll. As the days grow shorter and the light decreases, it takes more energy to synthesize chlorophyll than the plant can obtain from the captured sunlight. A negative return on investment means that plants shut down their manufacturing facilities, saving their energy and waiting for spring. Without chlorophyll around, the other pigments within a leaf increase in concentration to capture the last sun rays of the fall and their colours come to the fore.

It is the emergence of these reds, yellows, and oranges that have been hidden all summer long that leads to our spectacular fall displays. And it is these colours that make a walk in the woods with the dogs such as wonderful experience at this time of year.

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