IB Biology Topic C1.3.10: Advantages of the Structured Array of Different Types of Pigment Molecules Notes

Many Pigment Types Around One Reaction Centre Capture Light Far Better

1. A photosystem arranges many pigment molecules around a single reaction centre.

2. This structured array captures light far more efficiently than one pigment could alone.

Advantage 1: More Photons Are Captured

1. A single pigment molecule would absorb only a few photons each second.

2. Hundreds of pigments working together greatly increase the total light captured per reaction centre.

Advantage 2: A Broader Range of Wavelengths Is Absorbed

1. Different pigment types absorb different wavelengths of light.

   1. Chlorophyll a absorbs mostly red and blue light.
   2. Carotenoids absorb blue and blue-green light that chlorophyll a misses.

2. Using several pigment types lets a photosystem use a wider part of the spectrum than any one pigment could.

Advantage 3: Energy Is Funnelled Efficiently to the Reaction Centre

1. Pigments are packed close together in a precise arrangement.

2. Energy absorbed by any pigment is passed along to the reaction centre (P680 in PSII, P700 in PSI).

3. Without this funnelling, much of the energy would be lost as fluorescence or heat before reaching the reaction centre.

Advantage 4: Accessory Pigments Also Protect the Photosystem

1. Accessory pigments such as chlorophyll b and carotenoids do more than widen the spectrum.

2. Carotenoids give photoprotection by safely releasing excess absorbed energy as heat.

3. This stops surplus energy from damaging chlorophyll in bright light.

Advantage 5: Different Pigment Mixes Suit Different Habitats

1. Species with different pigment combinations can match the light available where they live.

2. For example, algae and cyanobacteria use phycobilins to absorb the wavelengths that reach deeper water.