Plants use only some colors in the solar spectrum

Now that biologists at the University of North Carolina Chapel Hill have discovered that plants can detect shadows, a fair follow-up question might ask: When sunlight is shining on the plant, just how much of the solar light spectrum do plants use in photosynthesis? 

Remember, white light is made up of all of the colors of the spectrum. Plants use the portion of the spectrum that falls in the visible range (400-700nm wave length). That’s all of the different colors of light that we see. And plants use all of those colors to generate biological energy through photosynthesis. 

Ironically, visible light makes up about 45 percent of the total solar spectrum. That means plants can only use about half of the sun’s energy. 

Additionally, the color of light used for photosynthesis depends on the pigment of the plant. In other words, whatever color the plant is, that is the color in the spectrum that the plant is reflecting instead of absorbing and using for photosynthesis. Chlorophyll absorbs all sunlight colors besides green, which is why the plant appears green to the human eye. The green light is what’s being reflected. 

Chlorophyll uses sunlight to make sugar. Chlorophyll is the key substance within chloroplasts, which are the food production centers of a plant cell. Sunlight hitting chloroplasts in plants is absorbed by chlorophyll and then combined with carbon dioxide and water to create glucose or sugar. Mitochondria then use the sugar produced by the chlorophyll to create energy that is usable for the plant. The process also creates oxygen, which is a good thing, because the plants release that for all of us to use. The stroma, which are tiny openings on the underside of leaves take in carbon dioxide, which is critical to the whole process. 

Leaves slowly lose their chlorophyll during the fall months because trees shut down their photosynthesis processes. The days are growing shorter, which means there isn’t enough sunlight to complete the process and they need to conserve energy. As the green from chlorophyll fades away, the yellows and oranges that were in the leaves all along are then seen because of food stored in the leaves. 

It’s amazing what plants accomplish using only half of the full color spectrum!

—Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog! 

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