Dear Science

Dear Science,

What makes the leaves turn such pretty colors before they fall off? Is it a last-ditch suicide mission to cheer us up before a season of depressing, bare sky-sticks?

Leaf Lover

The energy in light brings the fall colors. Every wave of light is intrinsically loaded with a pulse of energy from the fusion inferno of the sun; the greater the energy loaded, the higher the frequency of the light wave. Of visible waves, blue light contains the most energy, followed by green, and then, lastly, red.

When a light wave finally reaches the earth and strikes a leaf, all that energy is transferred as the light is absorbed into the cells of the leaf. Inside these cells, during the summer and spring, are vast amounts of chlorophyll. Chlorophyll absorbs both red and blue light. Through the lovely ring shape of the molecule, the energy in the absorbed light is used to move around electrons in a structured way, ultimately capturing the energy in the form of bonds between hydrogen and carbon. You would be well advised to take a moment to marvel at the beauty of this: Energy formed by the fusion of atoms millions of kilometers away—after a brief few minutes of travel across the void of space—is transformed into energy in chains of carbon and hydrogen. Few things humans have designed come close to this crisply efficient and enduring system.

The problem arises from the green light that chlorophyll cannot absorb. (This inability to absorb green light is why leaves are green during the summer.) Without a pigment beyond chlorophyll, those green light waves would be absorbed into another component of the living leaf, with their energy transferred to some critical structure. Electrons would slosh around in an uncontrolled manner, and critical molecules would be ruined when random, unwanted new chemical bonds were formed with this energy. To defend against this, plants have evolved the ability to make other pigments that absorb the green wavelengths of light that chlorophyll cannot. These pigments—akin to the melanin in your skin—safely disperse the green light’s energy to protect the delicate molecules within the leaf.

As winter approaches and the chlorophyll molecules are broken down into pieces to be stored for the next season, these pigments remain behind. Unmasked, they show their yellow, orange, and red glory. The fall colors are essentially from plant sunblock.

Thanks to the many layers of the atmosphere, including the ozone layer, most of the waves of light with higher energy than blue light cannot make their way to the surface. In places where high-energy radiation is common—say in the abandoned ruins of the Chernobyl reactor—pigments have evolved to capture the energy of gamma and X-rays. Radiotrophic fungus blissfully generates energy from gamma rays in the ruins of the greatest nuclear disaster.

Absorbingly Yours,

Science

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