Photosystems (HL) (HL IB Biology)

What are photosystems?

• Chloroplasts contains the pigment chlorophyll, plus other accessory pigments
• These are grouped together as structures called photosystems which are located in the thylakoid membranes in cyanobacteria and photosynthetic eukaryotes
• Photosystems contain many chlorophyll molecules and accessory pigments (carotene and xanthophylls) as well as a reaction centre
• Two types of photosystems exist:
  • Photosystem I - contains the reaction centre P700 (as it is activated by a wavelength of light of 700nm)
  • Photosystem II - contains the reaction centre P680 (as it is activated by a wavelength of light of 680nm)
• Chlorophyll molecules and accessory pigments within Photosystem II absorb light energy, in the form of photons, and pass it to a chlorophyll molecule in reaction centre P680
• Electrons within the reaction centre of Photosystem II are then excited to a higher energy level by the photons of light
• The chlorophylls within the reaction centre are said to be photoactivated
• Excited electrons are able to be donated to an electron acceptor in a reduction reaction

Diagram to show excitation of electrons in a photosystem

A photosystem used in the light-dependent reaction to excite electrons

Why are there multiple pigments in a photosystem?

• In each photosystem, the presence of many different types of pigment, each with a specific role, allows the photosystem to efficiently absorb light of different wavelengths
  • The structured arrangement of these pigments and accessory pigments allows for electrons to be excited in a controlled manner
  • These can then be directed along the electron transport chain
• All the pigments in photosystem I and II are required in order for photosynthesis to occur
  • A single pigment molecule would not be able to perform any part of photosynthesis

Table to show the pigments involved in light harvesting in the light dependent stage of photosynthesis