5.1.3 Photosynthetic Pigments

Types of Photosynthetic Pigments

1. Chlorophyll a:

• The primary pigment in photosynthesis.
• Absorbs light mainly in the red and blue-violet regions of the spectrum.
• Found in the reaction centres of both Photosystem I (PSI) and Photosystem II (PSII).

2. Chlorophyll b:

• An accessory pigment that absorbs light at slightly different wavelengths than chlorophyll a.
• Transfers energy to chlorophyll a for the light-dependent reactions.

3. Carotenoids:

• Accessory pigments such as carotene and xanthophylls.
• Absorb light in the blue and green regions and appear yellow, orange, or red.
• Protect chlorophyll from photodamage by dissipating excess light energy.

Role of Photosynthetic Pigments

• Maximise light absorption by capturing light across a broader spectrum of wavelengths.
• Pigments work together in a light-harvesting complex to transfer absorbed energy to the reaction centre of photosystems.
• The energy excites electrons in chlorophyll a, which are used in the light-dependent reaction.

Organisation of Pigments in Photosystems

1. Photosystem II (PSII):

• Reaction centre absorbs light at 680 nm (called P680).
• Initiates the process of photolysis and the first electron transfer chain.

2. Photosystem I (PSI):

• Reaction centre absorbs light at 700 nm (called P700).
• Responsible for producing reduced NADP (NADPH) in the light-dependent reaction.

Key Concept

Pigments are arranged in a way that maximises light absorption, ensuring efficient energy transfer for photosynthesis.