interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis

Photosynthesis as an Energy Transfer Process

Photosynthesis converts light energy into chemical energy stored in organic molecules. Understanding how light energy is captured by chloroplast pigments and how this energy drives the light‑dependent reactions requires interpreting two fundamental graphs:

• Absorption spectra of individual pigments (chlorophyll a, chlorophyll b, carotenoids, etc.).
• Action spectra for the overall photosynthetic rate of a plant.

1. Absorption Spectra of Chloroplast Pigments

An absorption spectrum plots the percentage of light absorbed (y‑axis) against wavelength (nm, x‑axis). Each pigment has characteristic peaks (λ_max) where absorption is highest.

When pigments are embedded in the thylakoid membrane, the combined absorption spectrum of the chloroplast is the sum of the individual spectra, producing a broad band covering most of the photosynthetically active radiation (PAR, 400‑700 nm).

2. Action Spectrum for Photosynthesis

The action spectrum measures the rate of photosynthetic oxygen evolution (or CO₂ fixation) at different monochromatic wavelengths, keeping photon flux constant. It reflects the efficiency with which absorbed photons are converted into chemical energy.

Key observations:

• Peaks in the action spectrum align closely with the λ_max of chlorophyll a and chlorophyll b.
• The green region (≈550 nm) shows a trough because chlorophylls absorb poorly there, even though photons are abundant.
• Carotenoid absorption contributes modestly to the action spectrum, mainly protecting the photosystems and extending usable light into the blue region.

3. Interpreting the Relationship Between the Two Spectra

1. Identify pigment peaks. Locate the λ_max values on the absorption spectra.
2. Overlay on the action spectrum. If the action spectrum is plotted on the same wavelength axis, peaks that coincide with pigment λ_max indicate that those pigments are the primary contributors to photosynthetic efficiency at those wavelengths.
3. Assess mismatches. A discrepancy (e.g., a high absorption but low action) suggests energy loss pathways such as fluorescence, heat dissipation, or non‑photochemical quenching.
4. Quantify contribution. The relative height of the action spectrum at a given wavelength can be approximated by the weighted sum of pigment absorbances, where the weighting factor reflects quantum efficiency (ϕ) of each pigment.

Mathematically, the expected photosynthetic rate \$R(\lambda)\$ at wavelength \$\lambda\$ can be expressed as:

\$R(\lambda) = \sum{i} \phii \, A_i(\lambda) \, I(\lambda)\$

where \$Ai(\lambda)\$ is the absorbance of pigment \$i\$, \$\phii\$ is its quantum yield, and \$I(\lambda)\$ is the incident photon flux (kept constant in an action spectrum experiment).

4. Practical Interpretation of Experimental Data

When presented with real spectra, follow this checklist:

• Mark the λ_max of each pigment on the absorption graph.
• Locate corresponding peaks on the action spectrum.
• Note any broad shoulders – these often arise from pigment–protein interactions that shift absorption slightly.
• Consider the role of accessory pigments (carotenoids) in extending the usable range and protecting against photodamage.
• Discuss how environmental factors (e.g., light intensity, leaf age) might alter the shape of the action spectrum without changing pigment absorption.

5. Summary of Key Points

• Chlorophyll a and b are the principal light‑harvesting pigments; their absorption maxima define the most efficient wavelengths for photosynthesis.
• The action spectrum mirrors the combined absorption spectrum but is modulated by quantum efficiency and energy‑loss processes.
• Interpretation requires linking spectral peaks to pigment identity and evaluating any deviations as evidence of physiological regulation.
• Understanding these spectra underpins experimental design (e.g., selecting growth‑light LEDs) and explains why plants appear green.

6. Sample Examination Question

Question: A monochromatic light source at 660 nm produces a photosynthetic rate that is 80 % of the maximum observed at 430 nm. Explain, using absorption and action spectra, why the rate is lower despite the high absorbance of chlorophyll a at 660 nm.

Answer outline:

1. Both 430 nm (blue) and 660 nm (red) correspond to absorption peaks of chlorophyll a.
2. Quantum yield (\$\phi\$) is generally slightly lower in the red region because energy per photon is lower (E = hc/λ).
3. Photochemical efficiency can be reduced by increased non‑photochemical quenching in the red, leading to a modest drop in the action spectrum.
4. Thus, even with similar absorbance, the lower photon energy and possible regulatory mechanisms result in a reduced photosynthetic rate.