photosynthetic pigments and their role | Chlorophyll and carotenoids |

When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light. The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer. The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed. The absorption spectra of three types of pigments in chloroplasts: chlorophyll a, the key light-capturing pigment that participates directly in the light reactions; the accessory pigment chlorophyll b; and a separate group of accessory pigments called carotenoids. The spectrum of chlorophyll suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis. Other accessory pigments include carotenoids, hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light. Carotenoids may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection. #LightSpectrumOfPhotosynthesis