Action & Absorption Spectra

Action Spectra

In 1881, the German plant physiologist T. W. Engelmann placed a filamentous green alga under the microscope and illuminated it with a tiny spectrum of visible light.

In the medium surrounding the strands were motile, aerobic bacteria.

After a few minutes, the bacteria had congregated around the portions of the filament illuminated by red and blue light.

Assuming that the bacteria were congregating in regions where oxygen was being evolved in photosynthesis, Engelmann concluded that red and blue light are the most effective colors for photosynthesis.

With modern instruments, a plot of the rate of photosynthesis as a function of wavelength of light produces a graph like this. More precise than Engelmann's but telling the same story.

Absorption Spectra

An absorption spectrum is a spectrum of radiant energy whose intensity at each wavelength is a measure of the amount of energy at that wavelength that has passed through a selectively absorbing substance.

• produces a beam of monochromatic ("single-color") radiation that can be shifted progressively across the spectrum;
• passes the beam through a solution of the substance, and
• measures the radiation that gets through.

Note that both chlorophylls absorb light most strongly in the red and violet portions of the spectrum. Green light is poorly absorbed so when white light (which contains the entire visible spectrum) shines on leaves, green rays are transmitted and reflected giving leaves their green color.

The similarity of the action spectrum of photosynthesis and the absorption spectrum of chlorophyll tells us that chlorophylls are the most important pigments in the process. The spectra are not identical, though, because carotenoids, which absorb strongly in the blue, play a role as well.

Many substances absorb ultraviolet and/or infrared rays. Spectrophotometers are available for measuring absorption spectra in these regions as well.