Photosynthetic pigments and function

Photosynthetic Pigments

Photosynthetic pigments are light-absorbing molecules present in plants, algae, and photosynthetic bacteria that capture solar energy and convert it into chemical energy during photosynthesis. These pigments are mainly located in the thylakoid membranes of chloroplasts and play a crucial role in absorbing light of different wavelengths.

They absorb light energy and transfer it to the reaction center where it is used for the synthesis of ATP and NADPH, ultimately leading to carbohydrate formation.

Functions of Photosynthetic Pigments in Photosynthesis

Photosynthetic pigments contribute to photosynthesis in the following ways:

1. Absorption of Light Energy

Pigments absorb light from the visible spectrum (400–700 nm), known as photosynthetically active radiation (PAR).

• Different pigments absorb different wavelengths.
• This broadens the range of light that plants can use.

2. Energy Transfer

Accessory pigments capture light energy and transfer it to chlorophyll a, the primary pigment in the reaction centre of photosystems.
This process occurs through resonance energy transfer.

3. Initiation of Photochemical Reactions

Chlorophyll a in the reaction centre converts light energy into chemical energy by exciting electrons.
These high-energy electrons enter the electron transport chain, leading to formation of ATP and NADPH.

4. Protection Against Photooxidation

Carotenoids protect chlorophyll from damage caused by excess light and reactive oxygen species by dissipating excess energy.

Classification of Photosynthetic Pigments

Photosynthetic pigments are mainly classified into three major groups:

1. Chlorophylls

These are the most important photosynthetic pigments.

Types:

• Chlorophyll a – Primary pigment present in all photosynthetic organisms; directly involved in light reactions.
• Chlorophyll b – Accessory pigment in higher plants and green algae; transfers energy to chlorophyll a.
• Chlorophyll c, d, e – Found in certain algae.

Functions:

• Absorb mainly blue and red light
• Participate directly in photochemical reactions

2. Carotenoids

These are accessory pigments and include:

(a) Carotenes

• Orange pigments (e.g., β-carotene)

(b) Xanthophylls

• Yellow pigments (e.g., lutein, zeaxanthin)

Functions:

• Absorb blue and violet light
• Transfer energy to chlorophyll a
• Protect chlorophyll from photooxidation

3. Phycobilins

Water-soluble pigments found in cyanobacteria and red algae.

Types:

• Phycocyanin (blue)
• Phycoerythrin (red)

Functions:

• Absorb light in wavelengths not absorbed by chlorophyll
• Transfer energy to chlorophyll a

          Photosynthetic pigments are essential for capturing and utilizing solar energy in photosynthesis. Chlorophyll a acts as the primary pigment, while accessory pigments like chlorophyll b, carotenoids, and phycobilins expand the absorption spectrum and protect the photosynthetic apparatus. Together, they ensure efficient conversion of light energy into chemical energy, sustaining life on Earth.