Biology · Photosynthesis in Higher Plants · NEET
Absorption spectrum plots how much light a pigment absorbs at each wavelength. Action spectrum plots the rate of photosynthesis at each wavelength. One measures pigment absorption, the other measures the actual work (photosynthesis) done. For NEET, remember: absorption is about the pigment, action is about the process.
They overlap because the light that chlorophyll a absorbs most (blue and red) is also the light that drives the highest rate of photosynthesis. NCERT concludes from this near-overlap that chlorophyll a is the chief pigment of photosynthesis. The overlap is not a perfect one-to-one match because accessory pigments also absorb some light and pass energy to chlorophyll a.
The action spectrum shows the rate of photosynthesis plotted against wavelength of light. The absorption spectrum only shows how much light a pigment absorbs, not the rate. This is a common NEET trap, so keep them separate.
Photosynthesis is maximum in the blue region (around 430 to 450 nm) and the red region (around 650 to 680 nm) of the visible spectrum. Some photosynthesis still happens at other wavelengths because of accessory pigments. Green light is absorbed least, which is why leaves look green.
T. W. Engelmann described the first action spectrum. He split light with a prism, shone it on the green alga Cladophora placed in aerobic bacteria, and saw bacteria gather in the blue and red light zones (where oxygen was released). This showed those wavelengths drove the most photosynthesis.
One scientist cultured Cladophora in a suspension of Azotobacter and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. They are similar and roughly overlap, both peaking in blue and red, but they are not a perfect one-to-one match. Accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb extra wavelengths and pass the energy to chlorophyll a, so some photosynthesis occurs at wavelengths where chlorophyll a absorbs little.
Chlorophyll reflects and transmits green light instead of absorbing it, so little green light drives photosynthesis. That is why both the absorption and action spectra dip in the green region and why leaves appear green.
The wavelengths where chlorophyll a absorbs most (blue and red) are also where the rate of photosynthesis is highest. Because the action spectrum closely matches chlorophyll a's absorption spectrum, NCERT concludes chlorophyll a is the chief pigment of photosynthesis.
Accessory pigments absorb wavelengths that chlorophyll a misses and transfer that energy to chlorophyll a. This widens the range of usable light and is why the action spectrum is broader than chlorophyll a's absorption spectrum alone. They also protect chlorophyll a from photo-oxidation.
