(c) (i) the importance of photosynthetic pigments in photosynthesis (ii) practical investigations using thin layer chromatography (TLC) to separate photosynthetic pigments

To include reference to light harvesting systems and photosystems.

Photosynthetic pigments

• Are within the thylakoid membranes, which absorb different wavelengths of light

• two types of pigments: chlorophylls and carotenoids

  • Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum

    • They reflect green light, causing plants to appear green

  • Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum

Pigment group	Name of pigment	Colour of pigment
Chlorophylls
Carotenoids

Pigment group	Name of pigment	Colour of pigment
Chlorophylls	Chlorophyll a Chlorophyll b	Blue-green Yellow-green
Carotenoids	β carotene Xanthophyll	Orange Yellow

Photosystems 

• are pigment molecules that are attached to proteins that are arranged in light-harvesting clusters

• the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane (each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre)

• There are two different photosystems, each with a specific form of chlorophyll a

  • Photosystem 1 (PSI), often referred to as P700

    • The chlorophyll a in this system has a maximum absorption of light at 700nm

  • Photosystem 2 (PSII), often referred to as P680

    • The chlorophyll a in this system has a maximum absorption of light at 680nm

Photosynthetic pigments

• a photosystem contains two types of photosynthetic pigments

  • primary pigments : are reaction centres where electrons are exited during the light-dependent reaction - in most chloroplasts the primary pigment is chlorophyll a

  • Accessory pigments: make up light harvesting systems. These surround reaction centres and transfer light energy to them to boost the energy available for electron excitement to take place