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