What is photosystem 1 vs 2?
Photosystem I (PSI) vs. Photosystem II (PSII)
Both Photosystem I (PSI) and Photosystem II (PSII) are crucial components of the light-dependent reactions of photosynthesis, but they have distinct roles and characteristics:
Photosystem II (PSII):
- Location: Primarily found in the grana lamellae (stacked thylakoids) of the chloroplast.
- Function: PSII is the first protein complex in the light-dependent reactions. Its primary function is to split water molecules (photolysis) to obtain electrons, protons (H+), and oxygen (O2). This is the source of nearly all the oxygen in Earth's atmosphere.
- Reaction Center: The reaction center of PSII contains a special pair of chlorophyll a molecules known as P680. P680 absorbs light most strongly at a wavelength of 680 nm.
- Electron Flow: Electrons energized by light absorption are passed from P680 to the primary electron acceptor, pheophytin. These electrons are then passed down an electron%20transport%20chain to PS I.
- Water Splitting Complex (Oxygen Evolving Complex): PSII is associated with the water%20splitting%20complex, which catalyzes the oxidation of water.
- Role in ATP Production: PSII contributes to the generation of a proton gradient across the thylakoid membrane, which is then used by ATP%20synthase to produce ATP (photophosphorylation).
Photosystem I (PSI):
- Location: Found primarily in the stroma lamellae (unstacked thylakoids) and at the edges of the grana.
- Function: PSI's main function is to re-energize electrons received from PSII and pass them to NADP+, reducing it to NADPH.
- Reaction Center: The reaction center of PSI contains a special pair of chlorophyll a molecules known as P700. P700 absorbs light most strongly at a wavelength of 700 nm.
- Electron Flow: Electrons energized by light absorption are passed from P700 to a series of electron acceptors, ultimately leading to the reduction of NADP+ to NADPH.
- Role in NADPH Production: PSI is crucial for generating NADPH, which is a reducing agent used in the Calvin cycle (light-independent reactions) to fix carbon dioxide.
Key Differences Summarized:
| Feature | Photosystem II (PSII) | Photosystem I (PSI) |
|---|
| Primary Function | Water splitting, ATP production | NADPH production |
| Reaction Center | P680 | P700 |
| Location | Grana lamellae | Stroma lamellae, grana edges |
| Electron Source | Water | From PSII (via electron transport chain) |
| Final Electron Acceptor | Plastoquinone (PQ) | NADP+ |
| Primary Products | Oxygen, Proton Gradient (for ATP synthesis) | NADPH |