Being Green

Green. Why has green become the byword for things sustainable? And what is green - origin, function, symbol?

Green is on all of the color charts; it is the complement of red - when you focus on a green object and then close your eyes, the residual image glows red.

Green is at the center of the human visual spectrum - we detect and differentiate tens of thousands (even millions) of greens. In physics, that means we are most aware of impacts on light waves around 570 nanometers, at the center of our visual span, which runs from violet, at about 380 nm, to far red, at 750 nm.

This is not just the visible spectrum of light, it is also the biologically active spectrum. Wavelengths of light that are shorter than 380 nm border on the ultraviolet; they are more energetic than longer wavelengths and can be harmful to organic molecules, denaturing and destroying them. Wavelengths longer than 750 nm are increasingly infrared, less energetic than most visible light, playing out as heat. Light in the visual spectrum has the useful ability to affect organic reaction centers, to cause temporary, non-destructive activity that can drive chemical change.

Thus photosynthesis. The world we perceive from our green-centric viewpoint is powered through the blue and red light that drives photosynthesis. When blue and red light strikes photosynthetic reaction centers, electrons are energized, then lost into a chain of events, transported through a series of reactions that harvest the extra energy, storing it in chemical bonds. This newly bound-up chemical energy can be used at a later point to bond carbon atoms into chains that yield sugars and eventually the myriad of other organic compounds that make a plant.

But in passing along those electrons, the photosynthetic reaction center suffers an ongoing need for other electrons to reset the photochemical mechanism. Those replacement electrons, in effect, come from a powerful capacity to disassemble molecules of water. The photosynthetic apparatus is practically unique in its capacity to split water into oxygen, protons (hydrogen), and electrons. The oxygen we breathe was freed from water during ancient and on-going acts of photosynthesis.

The direct and curious relationship, then, between water and green is the capture of light during photosynthesis, from which there are by-products. Oxygen is left over from the water that was split. Green light is left over when the red and blue light are taken from the spectrum (though it is not this simple; there are other light-absorbing compounds involved.)

So plants are green because they do not use green light. Our world is green because green, photosynthesizing plants are the basis for life on Earth. How convenient! Green describes the range of colors we perceive best. And that all makes sense because our vision depends on non-destructive light-driven reactions. So it is no surprise that our vision and the photosynthetic process depend on the same special spectrum of radiation - visible light.

To flash red is to send an alert, to call a stop, to wave a flag. A green light signals the coast is clear, an open field. Green is our comfort color. Green is fresh, it is crisp lettuce. Green is living, like lawn or trees. So green has become the color associated with ecology, and ecology has become synonymous with environmental protection, which we associate with stewardship and sustainability. Thus green has become the color of sustainability, about which we have much to learn. And as Kermit tells us, it isn't easy being green.