LIGHT-THE DRIVING ENERGY

Light is a form of energy called electromagnetic energy or radiations. Light behaves as waves as well as sort of particles called photons. The radiation most important to life are the visible light ranges from about 380 to 750 nm in wave length.

      It is the sunlight that is absorbed by chlorophyll, converted into chemical energy, and drives the photosynthesis process. Not all the light falling on the leaves is absorbed. Only about one percent of the light falling on the leaf surface is absorbed, the rest is reflected or transmitted.

     

      Absorption spectrum for chlorophylls indicates that absorption is maximum in blue and red parts of the spectrum, two absorption peaks being at around 430 nm and 670 nm respectively. Absorption peaks of carotenoids are different from those of chlorophylls.

     Different wavelength are not only differently absorbed by photosynthetic pigments but are also differently effective in photosynthesis. Relative effectiveness of different wavelength (colours) of light is driving photosynthesis is called action spectrum of photosynthesis.

      Action spectrum can be obtained by illuminating plant with light of different wavelength (or colours) and then estimating relative carbon dioxide consumption or oxygen release during photosynthesis.

      Action spectrum of photosynthesis corresponds to absorption spectrum of chlorophyll. The same two peaks and the valley are obtained for absorption of light as well as for carbon dioxide consumption. This also shows that chlorophyll is photosynthetic pigment.

      However, the action spectrum of photosynthesis does not parallel the absorption spectrum of chlorophyll exactly. Compared to peaks in absorption spectrum, the peaks in action spectrum are broader, and the valley is narrower and not as deep. (Photosynthesis is the most absorbed range is more than the absorption itself. Likewise, photosynthesis in 500-600 nm (including green light) is more than the absorption of green light by the chlorophyll). This difference occurs because the accessory pigments, the carotenoids, absorb light in this zone and pass on some of the absorbed light to chlorophylls which then convert light energy to chemical energy. When equal intensities of light are given, there is more photosynthesis in red than in blue part of spectrum.