White light is actually composed of a spectrum of colors, each with a different wavelength and energy. Photopigments absorb light of specific wavelengths. The color visible to the eye depends on the wavelengths that are not absorbed. For example, leaves are green because the major light-absorbing pigment in plants, chlorophyll, absorbs red and blue light and reflects green light. We see the reflected green light.

When photopigments in plants absorb light, they absorb light energy. This is the first step in photosynthesis—the process that converts light energy into chemical energy. The absorbed light energy initiates a series of electron transfer reactions. The energy from the flow of electrons is captured in a proton gradient and in the reduced coenzyme NADPH. When the energy stored in the proton gradient is released, ATP is made.

The chemical energy stored in ATP and NADPH is used to drive the reduction of carbon dioxide to sugar. The Calvin-Benson Cycle describes these reactions.

Not all wavelengths of light are absorbed equally by water. Water molecules absorb wavelengths in the red end of the visible spectrum. Underwater scenes appear blue because primarily the blue wavelengths of light penetrate the water. Some species of marine bacteria take advantage of how light penetrates water. They have evolved photopigments tuned for specific wavelengths of light. Life on Earth depends on the absorption of light energy from the sun.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education