Page 10 - 11-Bio-11 Kingdom plantae
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11. Bioenergetics eLearn.Punjab
Action spectrum can be obtained by illuminating plant with light of different wavelengths (or
colours) and then estimating relative C02 consumption or oxygen release during photosynthesis.
Fig. 11.4 (a) Absorption spectrum of chlorophyll and carotenoids.
(b) Action spectrum for photosynthesis.
As is evident from above figure 11.4, 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 CO2 consumption. This also shows that chlorophyll is the photosynthetic pigment.
However, the action spectrum of photosynthesis does not parallel the absorption specturm of
chlorophyll exactly. Compared to the peaks in absorption spectrum, the peaks in action spectrum
are broader, and the valley is narrower and not as deep.
(Photosynthesis in 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.
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