Chlorophyll Absorbs Mainly Red and Blue Light and Reflects GreenDefinitionPhotosynthetic pigmentsPhotosynthetic pigments are specialized molecules that absorb light energy and convert it into chemical energy during photosynthesis.The main photosynthetic pigments are chlorophyll a, chlorophyll b, and carotenoids.Each pigment absorbs light at its own specific wavelengths, which is the light energy that drives photosynthesis.The wavelengths a pigment does not absorb are reflected, and this reflected light is the colour we see.TipChlorophyll appears green because it reflects green light rather than absorbing it.Pigments Absorb Only Wavelengths That Match Their ElectronsLight is made of photons, each with a particular wavelength and energy.Shorter wavelengths such as blue light carry more energy, while longer wavelengths such as red light carry less.A pigment can only absorb a photon whose energy matches the energy levels of its electrons.This is why each pigment absorbs only certain wavelengths and not others.NoteWhen a pigment absorbs a photon, an electron gains that energy and jumps to a higher energy level.This is the first step in turning light energy into chemical energy.The Absorption Spectrum Shows Which Wavelengths Each Pigment AbsorbsAn absorption spectrum is a graph of how much light a pigment absorbs at each wavelength.The x-axis shows the wavelength of light in nanometres.The y-axis shows how much light is absorbed.Light absorption by chlorophyll and chlorophyll bWhere Each Pigment AbsorbsChlorophyll a: absorbs strongly in the red (around 660 to 680 nm) and blue-violet (around 430 to 450 nm) regions.Chlorophyll b: absorbs in the blue (around 450 to 470 nm) and red (around 640 to 660 nm) regions, at slightly different peaks from chlorophyll a.Carotenoids: absorb mainly in the blue and blue-green region (around 400 to 500 nm) and reflect yellow and orange light.None of these pigments absorb much green light, so green is reflected and the plant looks green.NoteHaving several pigments lets a plant absorb a wider range of wavelengths than chlorophyll a could alone.Carotenoids also help protect the plant from damage caused by too much light.Reflecting Green Light Is Why Plants Are Not BlackIf plants absorbed every wavelength, they would appear black, because no light would be reflected.Instead, chlorophyll reflects green light rather than absorbing it.The wavelengths absorbed best, red and blue, are also the ones that drive photosynthesis fastest, a link explored in the action spectrum.Common MistakeDo not confuse the absorption spectrum with the action spectrum.The absorption spectrum shows how much light a pigment absorbs at each wavelength.The action spectrum shows the rate of photosynthesis at each wavelength.Different pigments absorb different wavelengths, and together they capture a wide range of light.Chlorophyll absorbs mainly red and blue light and reflects green.Active recallWhich two regions of the spectrum does chlorophyll a absorb most strongly?Why do leaves appear green to us?What does the y-axis of an absorption spectrum represent?How does having carotenoids as well as chlorophyll benefit a plant?Why can a pigment absorb only certain wavelengths of light?
