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Pigments

Passing white light through a prism separates the light into different wavelengths, appearing as a rainbow of colors. The order of colors is determined by the wavelength of light. For visible light, red has the longest wavelength and violet has the shortest wavelength. However, visible light is just a small portion of the electromagnetic spectrum. Energy is inversely porportional to the wavelength – longer wavelengths have less energy than shorter wavelengths. For example, UV light has a shorter wavelength and more energy than visible light.

A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light that are reflected, or in other words, those wavelengths not absorbed. Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects. This is why plants appear green to us. Black pigments absorb all wavelengths of visible light that strike them. White pigments reflect most of the wavelengths striking them.

Each pigment has a characteristic absorption spectrum describing how it absorbs or reflects different wavelengths of light. Wavelengths absorbed by chlorophyll and other photosynthetic pigments generate electrons to power photosynthesis. All photosynthetic organisms have chlorophyll a which absorbs violet-blue and reddish orange-red wavelengths. Chlorophyll a reflects green and yellow-green wavelengths.

Accessory photosynthetic pigments, including chlorophyll b and beta-carotene, absorb energy that chlorophyll a does not absorb. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane, such as in thylakoid membranes of the chloroplast or membrane infoldings found in photosynthetic prokaryotes.

Overview | Food Web | Leaf Structure | The Chloroplast | Pigments | Oxygen

Light Dependent Process | Light Independent Process

Problem Set

The Biology Project > Biochemistry > Introduction to Photosynthesis >