AP Syllabus focus:
‘In light reactions, photosystems I and II capture light energy and transfer excited electrons through an electron transport chain.’
Light reactions convert light energy into high-energy electrons by coordinating pigments, membrane protein complexes, and redox carriers. Photosystems II and I work in series to move electrons along an electron transport chain embedded in the thylakoid membrane.
Big picture: what photosystems do
Photosystems are pigment–protein complexes that absorb photons and convert that energy into electron excitation. Each photosystem has two functional parts:
An antenna (light-harvesting) complex that absorbs light across multiple pigments
A reaction center that performs the primary photochemical event: transferring an excited electron to an electron acceptor
This satisfies the syllabus focus because absorbed light energy is converted into excited electrons, which are then passed to an electron transport chain (ETC).
Practice Questions
FAQ
They are named for the wavelength (in nm) at which the reaction-centre chlorophyll a absorbs light most strongly: about 680 nm for PSII and 700 nm for PSI.
This reflects differences in the local protein environment around chlorophyll, which slightly shifts absorption properties.
Cyclic flow occurs when electrons leaving PSI return to the electron transport chain instead of reducing NADP$^+$.
It tends to happen when:
NADP$^+$ is limited
The chloroplast needs relatively more ATP than NADPH
It helps rebalance energy output without producing additional NADPH.
Carotenoids can absorb excess energy and dissipate it as heat, reducing formation of reactive oxygen species.
They also help prevent damage by quenching excited chlorophyll states that would otherwise generate harmful reactions.
Photoinhibition is light-induced damage that reduces photosynthetic efficiency, often affecting PSII reaction-centre proteins.
Repair commonly involves:
Removal of damaged D1 protein
Synthesis and insertion of a replacement D1 protein
Reassembly of functional PSII complexes
Common approaches include chlorophyll fluorescence measurements (tracking how much absorbed light is re-emitted) and monitoring oxygen evolution as a proxy for PSII water-splitting activity.
Spectroscopic methods can also detect redox changes in PSI/PSII electron carriers under different light conditions.
