The figure shows the absorption spectrum for chlorophyll a and the action spectrum for photosynthesis. Why are they different?

• A. Green and yellow wavelengths of light inhibit the absorption of red and blue wavelengths.
• B. Oxygen given off during photosynthesis interferes with the absorption of light.
• C. Accessory pigments are absorbing light in addition to chlorophyll a which can be used in photosynthesis.
• D. Aerobic bacteria take up oxygen, which changes the measurement of the rate of photosynthesis.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the difference between the absorption spectrum and the action spectrum. The absorption spectrum shows the wavelengths of light absorbed by chlorophyll a, while the action spectrum shows the overall rate of photosynthesis at different wavelengths.
2. Step 2: Observe the graph carefully. Notice that chlorophyll a absorbs mainly in the blue (~430 nm) and red (~680 nm) regions, as indicated by the peaks in the absorption spectrum (solid line).
3. Step 3: Compare this to the action spectrum (dashed line), which shows photosynthesis occurring over a broader range of wavelengths, including some green and yellow light where chlorophyll a absorption is low.
4. Step 4: Recognize that the difference arises because other pigments, called accessory pigments (such as chlorophyll b and carotenoids), absorb light in wavelengths where chlorophyll a does not absorb strongly. These accessory pigments transfer the absorbed energy to chlorophyll a, contributing to photosynthesis.
5. Step 5: Conclude that the action spectrum is broader than the absorption spectrum of chlorophyll a alone because it reflects the combined light absorption of chlorophyll a and accessory pigments, which together drive photosynthesis.