Why Are Plant Leaves Usually Green?

Photoautotrophs use pigments to absorb certain wavelengths of sunlight and convert that energy into food. The type of light that they 'eat' results in their green color.

What Is a Photoautotroph?

Although it's big, daunting word, as is the case with most scientific terms, you just need to pick it apart to figure out its meaning. Photo refers to 'light' and troph to 'feeder.' Photoautotrphs are plants, and other photosynthetic organisms, that can create their own food using sunlight.

The raw materials that they use to make food are:

• carbon dioxide (the stuff we breathe out)
• water
• sunlight
• and inorganic elements obtained from the environment

Photosynthetic organisms also need the right equipment to turn sunlight into food energy; a variety of chemicals called pigments. Chlorophyll is the main pigment of photosynthesis, and this pigment is able to absorb certain wavelengths of light.

The Visible Spectrum of Light

The sunlight is actually a mixture of all the visible colors of light. You can see all the colors when sunlight is refracted through a prism, or shone through raindrops, creating a rainbow. Together, all of the colors of the rainbow are referred to as the visible spectrum, and include red, orange, yellow, green, blue, indigo and violet.

Light energy travels in wavelengths, with the red end of the visible spectrum representing the longest, lowest energy wavelengths and the violet end of the spectrum, the shortest and highest energy wavelengths. A slick trick to help you recall the colors of visible light in order from longest to shortest wavelength is to remember the fictitious name Roy G. Biv.

Light Absorption and Photosynthetic Pigments

Photosynthetic pigment, such as chlorophyll, will only absorb certain wavelengths of light, and it's only the wavelengths that are absorbed which can influence the photosynthetic reaction. Wavelengths that are reflected or transmitted (pass through) are not captured and therefore do not influence photosynthesis.

When we look at a green leaf, green is the wavelength of light that is being reflected and not absorbed. This is true of any color we see. If you have a yellow shirt on, the reason that it appears yellow is because the fabric is not absorbing yellow light, but reflecting it.

Blue and red light are the wavelengths absorbed by chlorophyll and used to drive photosynthesis. Green light is not absorbed, it is reflected, and that is why leaves appear green.Sources

Campbell, N. & Reece, J. (2004) Biology Seventh Edition. Pearson Benjamin Cummings.
Thorpe, P. Ed. (2007) Biology 120 General Biology I Laboratory Experiments and Exercises. Grand Valley State University.