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).

Photosystem structure and key terms

Most photosystems contain:

• Chlorophyll a in the reaction center (the chlorophyll that directly donates excited electrons)

• Accessory pigments (e.g., chlorophyll b, carotenoids) that broaden the range of absorbed wavelengths

• Protein scaffolds that position pigments and electron carriers for rapid energy/electron transfer

Antenna complexes: capturing and funnelling energy

A photon absorbed by an accessory pigment raises an electron to an excited state.

Side-by-side schematic of Photosystem II and Photosystem I embedded in the thylakoid membrane, highlighting the antenna (light-harvesting) complex feeding excitation energy into the reaction-center chlorophyll pair. The diagram emphasizes the key photochemical step: an excited reaction-center electron is transferred to a primary electron acceptor, initiating directed electron flow. Source

That energy is passed (not the electron itself) by resonance energy transfer among pigments until it reaches the reaction-center chlorophyll pair.

Key idea:

• Antenna pigments increase capture efficiency under variable light conditions by acting as a “solar panel” that feeds excitation energy to one reaction center.

Reaction centers: converting energy into electron flow

The reaction center contains a “special pair” of chlorophyll a molecules:

• P680 in Photosystem II (PSII)

• P700 in Photosystem I (PSI)

These names reflect their peak absorption wavelengths (in nm).

Photosystem II (PSII): starting electron flow with water

PSII acts first in the standard light-reaction sequence. When P680 absorbs energy:

• P680 becomes excited (P680*)

• P680* transfers an electron to a primary electron acceptor, beginning electron flow into the ETC

• The reaction-center chlorophyll becomes oxidised (P680⁺) and must be reduced again to continue functioning

PSII replaces lost electrons by oxidising water via the oxygen-evolving complex (a catalytic system associated with PSII). This is the source of most atmospheric oxygen produced by photosynthesis.

The electrons produced replenish PSII, allowing repeated cycles of photon capture and electron donation into the electron transport chain.

Electron transport chain: moving excited electrons through carriers

After leaving PSII, electrons move through carriers in the thylakoid membrane.

Thylakoid-membrane overview of the light reactions showing linear electron flow from PSII through electron carriers to PSI and onward to NADP+ reduction. The figure also indicates proton translocation into the lumen and ATP synthase use of the resulting electrochemical gradient, linking electron transport to energy conservation. Source

Core principles students should know:

• Each transfer step is a redox reaction (one carrier is reduced, the previous is oxidised).

• Electron transfers are energetically “downhill” overall, which allows the cell to conserve some of that energy in useful forms.

• The ETC provides the connection between PSII and PSI, ensuring that light-driven excitation results in directed electron flow rather than random loss as heat.

Photosystem I (PSI): re-energising electrons for high-energy reduction

Electrons arriving from the ETC reach PSI. When P700 absorbs light energy:

• P700 is excited (P700*)

• P700* transfers an electron to a primary acceptor associated with PSI

• P700 becomes oxidised (P700⁺) and is then reduced by electrons delivered from the ETC

A central outcome of PSI activity is production of high-energy electrons capable of reducing downstream carriers (commonly leading to NADPH formation), linking light capture to cellular reducing power. For AP Biology, the critical mechanism is the same as PSII:

• Light absorption → excited reaction-center electron → electron transfer to an acceptor → electron movement through an ETC component set

Coordinated function of PSII and PSI

PSII and PSI operate in series to keep electrons flowing:

• PSII extracts electrons from water and sends excited electrons into the ETC

• The ETC transfers electrons to PSI

• PSI absorbs light to excite those electrons again, enabling transfer to high-energy acceptors

Important takeaways for regulation and efficiency:

• Two photosystems are needed because one excitation event does not typically raise electrons to a high enough energy level to support the full set of reductions required downstream.

Z-scheme energy diagram tracing electron transfer from water oxidation at PSII (P680) through intermediate carriers to PSI (P700), where absorption of a second photon re-energizes the electrons. The “zig-zag” profile summarizes how light capture at two reaction centers enables high-energy reduction steps (commonly culminating in NADPH formation). Source

• The use of antenna pigments and closely packed reaction components increases the probability that absorbed light becomes productive electron transfer rather than being lost as heat or fluorescence.