Biology · Photosynthesis in Higher Plants · NEET
The absorption spectrum shows how much light a pigment absorbs at each wavelength. The action spectrum shows the rate of photosynthesis at each wavelength. For chlorophyll the two graphs overlap closely: both show high peaks in blue and red light. This overlap is proof that chlorophyll a is the main pigment doing photosynthesis. For NEET, remember absorption = light taken in by the pigment; action = actual photosynthesis rate (O2 or glucose).
Chlorophyll absorbs blue and red light strongly but absorbs green light very poorly. The green light that is not absorbed is reflected back to your eyes. So the colour you see is the colour the leaf does NOT use. This is a common NEET trap: green light is the least useful colour for photosynthesis, not the most.
Blue and red light give the highest rate of photosynthesis because chlorophyll absorbs them best. Engelmann's experiment with the alga Cladophora and aerobic bacteria showed the bacteria gathered mainly in the blue and red regions, where oxygen release (photosynthesis) was maximum. Green light gives the lowest rate.
Chlorophyll a has two main absorption peaks: one in the blue region near 430 nm and one in the red region near 662-680 nm. The reaction-centre chlorophyll a of PS I absorbs maximally at 700 nm (P700) and that of PS II at 680 nm (P680). These exact numbers are directly asked in NEET.
Yes. Chlorophyll b, xanthophylls and carotenoids absorb light at wavelengths that chlorophyll a absorbs weakly. They widen the range of light that can be used and pass the energy to chlorophyll a. This is why the action spectrum is broader than the absorption spectrum of chlorophyll a alone.
One scientist cultured Cladophora in a suspension of aerobic bacteria and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
The reaction centre in PS II has an absorption maxima at:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a graph showing chlorophyll absorbs light strongly in the blue and red regions and weakly in the green region of visible light.
Chlorophyll a peaks near 430 nm (blue) and 662-680 nm (red). Chlorophyll b peaks near 453 nm and 642 nm, slightly shifted from chlorophyll a.
Because chlorophyll reflects and transmits green light instead of absorbing it, so very little green light energy is captured for photosynthesis.
NEET asks direct facts: blue and red are absorbed most, green is reflected, PS I absorbs at 700 nm, PS II at 680 nm, and the absorption spectrum overlaps the action spectrum proving chlorophyll a is the chief pigment.
