Short Notes on the Absorption and Action Spectrum in the Process of Photosynthesis

Light must be absorbed in order to be effective in a photo process. Different wavelengths of light are effective in a photo process to different extent. An action spectrum is a graph that shows the effectiveness of light in inducing a particular process plotted as a function of wavelength.

To explain this in a very simpler way; an action spectrum is a graph that shows which wavelength of light is effective how much in inducing a photo process. Normally, the action spectrum for a light-dependent process closely resembles the absorption spectrum of the molecule or substance which acts as the mediator for the photo process.

With reference to photosynthesis, the graph showing the effectiveness of different wavelengths of light on the rate of photosynthesis is known is the action spectrum of photosynthesis The rate of photosynthesis is usually measured as *he amount of C0₂ taken up or as the amount of 0₂ evolved. The action spectrum of photosynthesis has pronounced peaks in the blue and red regions of the spectrum which correspond with the absorption peaks for chlorophyll.

The action spectrum of photosynthesis in Figure BI-i does not tell anything about the efficacy of the absorbed photons of different wavelengths of light on the rate of photosynthesis.

This can be determined by expressing the photosynthesis rate as the number of molecules 0₂ evolved or CO, taken up per photon of light absorbed. This is known as the quantum yield of photosynthesis. Quantum yield is a very important parameter of the light reactions of photosynthesis and is defined as follows:

Number molecules of 0₂ evolved or CO taken up Total number of photons absorbed photon absorbed by chlorophyll is more or less equally effective in driving photosynthesis. However, there is a sudden drop in the quantum yield of photosynthesis at wavelengths greater than 680 nine (far-red light) even though light is still absorbed phenomenon of red drop, in that range of the spectrum this drop in the quantum yield of photosynthesis is known as the red drop. The phenomenon of red drop was shown for the first time by R. Emerson and C.M. Lewis in 1943 in the green alga Chlorella.