POWER

Photosynthetic Pigments: The Light Receptors 

 

Light may be reflected, transmitted or absorbed when it contacts matter. Pigments are substances that absorb visible light. Different pigments absorb different wavelengths of light. Wavelengths that are absorbed disappear, so a pigment that absorbs all wavelengths appears black. When white light, which contains all the wavelengths of visible light, illuminates a pigment, the color you see is the color most reflected or transmitted by the pigment. For example, a leaf appears green because chlorophyll absorbs red and blue light but transmits and reflects green light. Each pigment has a characteristic absorption spectrum or pattern of wavelengths that it absorbs. It is expressed as a graph of absorption versus wavelength. The absorption spectrum for a pigment in solution can be determined by using a spectrophotometer, an instrument used to measure what proportion of a specific wavelength of light is absorbed or transmitted by the pigment. Since chlorophyll a is the light-absorbing pigment that participates directly in the light reactions, the absorption spectrum of chlorophyll a provides clues as to which wavelengths of visible light are most effective for photosynthesis. Chlorophyll a has absorption peaks at approximately 440 nm and 680 nm.

A graph of wavelength versus rate of photosynthesis is called an action spectrum and profiles the relative effectiveness of different wavelengths of visible light for driving photosynthesis.

The action spectrum of photosynthesis can be determined by illuminating chloroplasts with different wavelengths of light and measuring some indicator of photosynthetic rate, such as oxygen release or carbon dioxide consumption.

It is apparent from the action spectrum of photosynthesis that blue and red light are the most effective wavelengths for photosynthesis and green light is the least effective. Even though only special chlorophyll a molecules can participate directly in the light reactions, other pigments, called accessory pigments, can absorb light and transfer the energy to chlorophyll a.