Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It absorbs light energy, strips electrons from water (splitting it into oxygen, protons, and electrons), and feeds those electrons into the photosynthetic electron transport chain.
Photosystem II (PSII) is where oxygenic photosynthesis actually begins. When a photon hits the pigments in PSII, the energy gets passed along until it boots an electron up to a high-energy state. That excited electron leaves PSII and heads into the electron transport chain. PSII then needs to replace the electron it just lost, so it pulls electrons off water. Splitting water (called photolysis) is the step that releases the O₂ you breathe, plus protons (H⁺) that help build the gradient for ATP synthesis.
Here's the part that trips people up: even though it's named "II," Photosystem II comes first. The numbering reflects the order they were discovered, not the order electrons flow through them. Electrons go PSII → electron transport chain → Photosystem I → NADP⁺ (making NADPH). PSII is the only one of the two that can split water, which is why it's the source of all the oxygen in oxygenic photosynthesis.
Photosystem II lives in Unit 3: Cellular Energetics. It's part of how cells move energy through oxidation-reduction reactions and build electrochemical gradients across membranes, which is the same logic the CED uses for the mitochondrial electron transport chain in EK 3.5.A.3. The big-picture theme is Energetics: living things capture energy from the environment and convert it into usable forms. PSII is the entry point for that capture in photosynthesis. Understanding it means you understand where the O₂ comes from, why the light reactions need water, and how the proton gradient that drives ATP synthase gets started.
Keep studying AP Biology Unit 3
Photosystem I (Unit 3)
PSII and PSI work as a relay. PSII grabs electrons from water and sends them down the chain; PSI re-energizes those electrons with another photon and hands them to NADP⁺ to make NADPH. PSII is the water-splitter and oxygen-maker; PSI is the NADPH-maker.
Electron Transport Chain (Unit 3)
The electrons PSII excites flow through an ETC that pumps protons across the thylakoid membrane. It's the same oxidation-reduction logic as the mitochondrial ETC in cellular respiration, just powered by light instead of NADH.
ATP synthase (Unit 3)
The proton gradient PSII helps build (both from splitting water and from the ETC pumping H⁺) drives ATP synthase, which makes ATP. Same machine, same chemiosmosis idea you see in mitochondria.
Oxygenic Photosynthesis (Unit 3)
PSII is literally what makes photosynthesis 'oxygenic.' No PSII splitting water means no O₂ released, which is why this complex is central to how early life changed Earth's atmosphere.
Expect Photosystem II in light-reaction questions and experimental setups. One classic stem describes isolated chloroplasts producing oxygen in light but no glucose, and asks you to explain it; the answer is that the light reactions (including PSII splitting water, which releases O₂) can run even when carbon fixation can't. Another common stem describes a herbicide that binds the D1 protein in PSII and asks for the most direct consequence; blocking PSII stops electron flow out of PSII, which shuts down water-splitting and oxygen production. On the 2023 set-based FRQ, Photosystem II showed up in a question contrasting noncyclic and cyclic electron flow, where electrons pass through Photosystem II first in the noncyclic pathway. You should be able to trace electrons from water through PSII to PSI and explain what stops if PSII is inhibited.
Despite the names, Photosystem II acts first and Photosystem I acts second. PSII splits water and releases oxygen; PSI does not split water and instead reduces NADP⁺ to NADPH. If a question mentions oxygen production or water, that's PSII. If it mentions NADPH, that's PSI.
Photosystem II is the first complex in the light-dependent reactions, even though it's labeled 'II' (the numbers reflect discovery order, not flow order).
PSII splits water (photolysis), which releases the O₂ produced in oxygenic photosynthesis and supplies replacement electrons.
Electrons flow from PSII through the electron transport chain to Photosystem I, building a proton gradient that powers ATP synthase.
Only PSII splits water, so blocking PSII (like a herbicide binding the D1 protein) halts oxygen production.
The light reactions, including PSII, can run and make O₂ even when carbon fixation isn't happening.
It absorbs light energy, excites electrons, and splits water to replace those electrons, releasing oxygen and protons in the process. Those excited electrons then enter the electron transport chain on their way to Photosystem I.
The numbering is based on the order scientists discovered them, not the order electrons travel. Electrons actually flow PSII first, then PSI.
Yes. PSII is the only complex that splits water (photolysis), and that water-splitting is the source of the O₂ released in oxygenic photosynthesis.
PSII acts first, splits water, and releases oxygen. PSI acts second, doesn't split water, and reduces NADP⁺ to NADPH. If a question mentions oxygen or water, think PSII; if it mentions NADPH, think PSI.
Electrons can't be pulled from water, so water-splitting and oxygen production stop, which is exactly what a herbicide targeting the D1 protein in PSII does.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.