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Action & Absorption Spectra in Photosynthesis

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

Learn about the action spectrum in plants; that is, what colors of light they use for photosynthesis. We will then discuss the absorption spectrum; that is, what colors of light they absorb, and whether they use them for photosynthesis or not.

Light and Color in Photosynthesis

Why are trees green? You may have heard that trees are green because their green pigments help them to trap light energy for photosynthesis. But why do pigments matter to photosynthesis? And couldn't trees have some other color of pigment to do the same thing?

When a pigment absorbs energy, one of three things can happen:

  1. it gives the energy off as heat
  2. it changes it into a longer wavelength (that is, it fluoresces), or
  3. it causes a chemical reaction (such as photosynthesis).

Plants use their green pigments to absorb light energy. When a photon of light hits a chlorophyll molecule, it excites an electron; that is, it puts the electron into a higher energy state. This excited electron sets off a chain of events called photosynthesis, that eventually converts the energy in that electron into stored energy in the form of sugar.

Plants are green because they reflect the most green light. Plants are more efficient at absorbing red and blue light. An absorption spectrum shows all the light typically absorbed by a leaf. An action spectrum, meanwhile, shows all the light that is actually used for photosynthesis.

Plant Pigments

There are three major types of pigments that are responsible for plant color:

  • chlorophylls - participate in photosynthesis directly
  • carotenoids - participate indirectly in photosynthesis
  • anthocyanins - do not participate in photosynthesis

Chlorophyll is the most well-known of the plant pigments, and it is what makes plants green. There are two kinds of chlorophyll, chlorophyll a and chlorophyll b. Chlorophyll a is a grassy green color and best absorbs light in the violet-blue and reddish-orange wavelengths. Chlorophyll b is yellow-green and absorbs blue-green and orange wavelengths. Chlorophyll is part of the absorption and action spectrum.

Chloroplast with chlorophyll.
chloroplast

Carotenoids are plant pigments that make plants orange or yellow. They're named after carrots, but can also be seen in sweet potatoes and yellow flowers. Carotenoids include lycopene, which makes tomatoes red; beta-carotene, which makes carrots orange; and lutein, which is particularly abundant in dark green leafy vegetables.

When you see yellow leaves on trees in the fall, you are seeing carotenoids. Carotenoids are called accessory pigments, which means that although plants can't photosynthesize with carotenoids, carotenoids can pass the light energy they receive along to chlorophyll. This allows plants to extend the range of wavelengths that they can use for photosynthesis. Carotenoids are also believed to help photosynthesis by protecting chlorophyll from sun damage. So though they contribute indirectly to photosynthesis, they are not part of the action spectrum.

Anthocyanins reflect red or blue and are often seen in flowers. One of the main purposes of anthocyanins is to attract pollinators. They can also help plants remain 'invisible' to herbivores that can't see red light. Anthocyanins do not directly participate in photosynthesis, however, like carotenoids, they are also believed to help protect plants from sun damage. Anthocyanins therefore contribute to a plant's absorption spectrum but not its action spectrum.

Why Aren't Trees Black?

So if pigments help plants trap light, wouldn't it make sense for plants to be black? Then they could absorb all the available light energy instead of reflecting back green light.

The visible spectrum of light seems to be better for photosynthesis. Waves such as ultraviolet might be too strong and destroy the chlorophyll molecule, while waves such as infrared might be too weak and not capable of exciting the electron enough.

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