Cue Column | Note-Taking Area
Q: What is the primary pigment and why is it central? Chlorophyll a is the primary pigment. It is the ONLY pigment that directly converts light to chemical energy at the reaction centre. It exists as P680 (in PS II) and P700 (in PS I). All other pigments are accessory — they transfer energy TO Chl a.
Q: Absorption peaks of each pigment?
| Pigment | Colour | Absorption Peaks | Unique Feature |
|---|---|---|---|
| Chl a | Blue-green | ~430 nm (B), ~662 nm (R) | Reaction centre (P680, P700) |
| Chl b | Yellow-green | ~453 nm (B), ~642 nm (R) | Energy transfer to Chl a |
| Carotenoids | Yellow-orange | ~400–500 nm (blue-violet) | Photoprotection |
| Xanthophylls | Yellow | ~400–500 nm (blue-violet) | Photoprotection |
| Phycoerythrin | Red | ~490–570 nm (green) | In red algae; deep water adaptation |
Q: What is the difference between absorption and action spectrum?
- Absorption spectrum = wavelengths absorbed by pigment extract (spectrophotometer)
- Action spectrum = rate of photosynthesis vs. wavelength of light
- Key fact: Action spectrum closely matches Chl a absorption spectrum → proves Chl a drives photosynthesis
Q: Why are leaves green? Chl a and Chl b absorb blue and red light → they REFLECT green light (~550 nm) → leaves appear green. Green light has the lowest photosynthesis rate in the action spectrum.
Summary Box
- Primary: Chl a (reaction centre, P680 in PS II, P700 in PS I)
- Accessory: Chl b, carotenoids, xanthophylls, phycoerythrin
- Action spectrum = functional proof of which wavelengths drive photosynthesis
- Green light = least effective; Blue and Red = most effective