Question
Differentiate between absorption spectrum and action spectrum of photosynthesis. Why do plants appear green? Which spectrum more accurately represents the effectiveness of different wavelengths in photosynthesis, and why?
(NCERT Class 11, high-frequency NEET question)
Solution — Step by Step
The absorption spectrum shows how much light of each wavelength is absorbed by a particular pigment. When we pass white light through a solution of chlorophyll and measure which wavelengths are absorbed most, we get the absorption spectrum.
For chlorophyll a: maximum absorption at 430 nm (blue-violet) and 662 nm (red). There is very low absorption in the 500-560 nm range (green region).
For chlorophyll b: peaks shift slightly — 453 nm and 642 nm.
The action spectrum measures the actual rate of photosynthesis at each wavelength of light. We illuminate the plant with monochromatic light of different wavelengths and measure evolution or fixation at each.
The action spectrum shows peaks in both blue and red regions, closely matching the combined absorption of all photosynthetic pigments — not just chlorophyll a alone.
Chlorophyll pigments absorb blue and red wavelengths strongly but reflect and transmit green light. Since our eyes receive mostly the reflected green wavelengths, leaves appear green.
This is not “waste” — it’s simply that the energy gap in chlorophyll molecules matches red and blue photons, not green ones. Green photons don’t have the right energy to excite chlorophyll’s electrons to the required excited states.
The action spectrum does not perfectly match the absorption spectrum of chlorophyll a alone. There’s a notable difference: the action spectrum shows higher photosynthesis in the green-yellow region than the absorption spectrum of chlorophyll a would predict.
Why? Because accessory pigments (chlorophyll b, carotenoids, xanthophylls) absorb wavelengths that chlorophyll a misses and transfer that energy to chlorophyll a via the antenna complex. The action spectrum reflects the combined contribution of ALL pigments, while the absorption spectrum is measured for individual pigments.
Why This Works
The reason the action spectrum is a better indicator of photosynthetic effectiveness is straightforward: photosynthesis uses multiple pigments working together, not just chlorophyll a in isolation.
Engelmann’s experiment (1882) demonstrated this beautifully. He used a prism to split white light across a filament of Spirogyra and observed where aerobic bacteria clustered (indicating release). The bacteria accumulated most in the red and blue regions — directly giving us the action spectrum.
The slight mismatch between absorption and action spectra was the first evidence that accessory pigments play a real role in photosynthesis, not just photoprotection.
NEET has asked “Which spectrum represents the actual effectiveness of light in photosynthesis?” multiple times. The answer is always action spectrum. The reasoning: it accounts for energy transfer from accessory pigments to the reaction centre, while the absorption spectrum only tells you what one pigment absorbs.
Alternative Method — Tabular Comparison
| Feature | Absorption Spectrum | Action Spectrum |
|---|---|---|
| What it measures | Light absorbed by a pigment | Rate of photosynthesis at each wavelength |
| Method | Spectrophotometry of pigment extract | evolution / fixation measurement |
| Specific to | Individual pigment | Whole photosynthetic apparatus |
| Peaks for Chl a | 430 nm, 662 nm | ~430 nm, ~660 nm (broader peaks) |
| Green region | Very low absorption | Low but non-zero (accessory pigments contribute) |
For CBSE board exams, always mention Engelmann’s experiment when discussing action spectrum. Examiners expect it. Draw the setup: prism, Spirogyra filament, aerobic bacteria clustering at red and blue zones.
Common Mistake
The most common error: writing that “green light is useless for photosynthesis.” This is incorrect. Green light IS absorbed to some extent by accessory pigments and even by chlorophyll (weakly). The action spectrum shows a non-zero rate in the green region. Green light is least effective, not completely ineffective.
Another NEET trap: confusing absorption spectrum with emission spectrum. The absorption spectrum shows which wavelengths are absorbed (dark bands on a continuous spectrum). The emission spectrum shows wavelengths emitted when excited atoms return to ground state. These are completely different concepts — absorption spectrum is relevant to photosynthesis, emission spectrum is not.