Photosystems (DP IB Biology)
Revision Note
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
A photosystem used in the light-dependent reaction to excite electrons
Examiner Tips and Tricks
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:
|
Light harvesting complex proteins: electron carrier molecules | Pass electrons down an electron transport chain |
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