IBBiologyHLRevision NotesInteraction & InterdependencePhotosynthesisPhotosystems (HL)

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First teaching 2023

First exams 2025

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Photosystems (HL) (HL IB Biology)

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Biology

Photosystems

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

photosystem-in-thylakoids-1

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

Exam Tip

Rather confusingly, the first photosystem to be activated in the light-dependent reaction is Photosystem II. Later in the reaction, Photosystem I is involved. This is because Photosystem I was the first to be discovered and therefore was named first.

Advantages of Photosystems

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

Pigment Role
Chlorophyll (a, b) Absorb wavelengths of light in the blue to violet and red regions of the spectrum
Carotenoid accessory pigments (xanthophyll, carotene) Absorb wavelengths of light in the blue to violet region of the spectrum
Light harvesting complex proteins: enzymes

To catalyse:

  • formation of ATP from ADP + Pi 
  • reduction of NADP+ to NADPH + H+
Light harvesting complex proteins: electron carrier molecules  Pass electrons down an electron transport chain
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Ruth

Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. She gained 10 years of teaching experience across the 3 key science disciplines and physical education. Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.