What is Photosystem 1 (PS1)?

• Photosystem I (PSI), scientifically termed plastocyanin–ferredoxin oxidoreductase, is a pivotal component in the photosynthetic light reactions of algae, plants, and cyanobacteria. This integral membrane protein complex harnesses light energy to catalyze the transfer of electrons across the thylakoid membrane, moving from plastocyanin to ferredoxin. Consequently, these electrons contribute to the production of the moderate-energy hydrogen carrier NADPH. Moreover, the photon energy absorbed by PSI facilitates the creation of a proton-motive force, instrumental in generating ATP. Notably, PSI’s structure is intricate, comprising over 110 cofactors, surpassing the complexity of Photosystem II.
• Historically, PSI’s identification preceded that of Photosystem II, although subsequent research revealed that Photosystem II actually initiates the photosynthetic electron transport chain. The foundational aspects of PSI emerged in the 1950s, but their significance was not immediately understood. In 1960, Louis Duysens first conceptualized Photosystems I and II. In that same year, Fay Bendall and Robert Hill proposed a theory integrating these discoveries into a cohesive explanation of serial photosynthetic reactions. Their hypothesis gained empirical support through experiments conducted in 1961 by the Duysens and Witt groups.
• Central to PSI are two primary subunits, PsaA and PsaB. These closely related proteins are integral to binding essential electron transfer cofactors, including P700, Acc, A0, A1, and Fx. Each subunit, comprising 730 to 750 amino acids, features 11 transmembrane segments. A notable feature is the [4Fe-4S] iron-sulfur cluster, Fx, coordinated by four cysteines, with each subunit contributing two cysteines. These cysteines are proximally located in a loop between the ninth and tenth transmembrane segments. Additionally, a leucine zipper motif, possibly facilitating PsaA/PsaB dimerization, is situated downstream of these cysteines. The terminal electron acceptors FA and FB, also [4Fe-4S] iron-sulfur clusters, reside in a 9-kDa protein, PsaC, which binds near FX to the PsaA/PsaB core.
• Therefore, Photosystem I plays an indispensable role in photosynthesis, effectively converting light energy into chemical energy, crucial for the survival of many organisms. Its complex structure and sophisticated electron transfer mechanisms highlight the intricate nature of photosynthetic processes.

What is Photosystem 2 (PS2)?

• Photosystem II (PSII), also known as water-plastoquinone oxidoreductase, serves as the initial protein complex in the light-dependent reactions of oxygenic photosynthesis. Located in the thylakoid membranes of plants, algae, and cyanobacteria, PSII plays a crucial role in the absorption of light photons, facilitating the energization of electrons. These electrons are subsequently transferred through a series of coenzymes and cofactors, reducing plastoquinone to plastoquinol. In this process, the electrons lost by PSII are replenished through the oxidation of water, yielding hydrogen ions and molecular oxygen.
• The significance of PSII lies in its ability to provide electrons for the entire photosynthetic process by splitting water. The generated hydrogen ions contribute to establishing a proton gradient, which ATP synthase utilizes to produce ATP. The electrons passed to plastoquinone participate in reducing NADP+ to NADPH or engage in non-cyclic electron flow. DCMU, a chemical inhibitor, is used in laboratory settings to obstruct the electron flow from PSII to plastoquinone, demonstrating PSII’s role in electron transport.
• The structural core of PSII is characterized by a pseudo-symmetric heterodimer, composed of two homologous proteins, D1 and D2. A distinct feature of PSII, compared to other photosystems, is the localization of the positive charge on the chlorophyll dimer involved in initial photoinduced charge separation; it is predominantly concentrated on one chlorophyll center. This characteristic makes P680+ highly oxidizing, capable of participating in water splitting.
• Furthermore, PSII’s composition varies depending on the organism, typically encompassing around 20 subunits and additional accessory, light-harvesting proteins. Each PSII contains a minimum of 99 cofactors, including chlorophyll a, beta-carotene, pheophytin, plastoquinone, heme, bicarbonate, lipids, the Mn4CaO5 cluster (incorporating two chloride ions), non-heme Fe2+, and putative Ca2+ ions per monomer. PSII’s structural complexity is highlighted by several crystal structures, with PDB accession codes such as 3WU2, 3BZ1, 3BZ2, 2AXT, 1S5L, 1W5C, 1ILX, 1FE1, and 1IZL.
• Therefore, PSII’s role in photosynthesis is fundamental, initiating the electron transport chain and facilitating the conversion of light energy into chemical energy. Its complex structure and function underscore the intricate mechanisms underlying photosynthetic processes.

Difference between Photosystem 1 (PS1) and Photosystem 2 (PS2)

1. Location:
   • PSI is situated on the outer surface of the thylakoid membrane.
   • PSII is found on the inner surface of the thylakoid membrane.
2. Photocenter:
   • The photocenter of PSI is P700.
   • In contrast, PSII has a photocenter of P680.
3. Absorbing Wavelength:
   • PSI pigments absorb longer wavelengths of light (>680 nm).
   • PSII pigments absorb shorter wavelengths of light (<680 nm).
4. Photophosphorylation:
   • PSI is involved in both cyclic and non-cyclic photophosphorylation.
   • Conversely, PSII is only involved in non-cyclic photophosphorylation.
5. Photolysis:
   • No photolysis of water occurs in PSI.
   • PSII engages in the photolysis of water.
6. Main Function:
   • The primary function of PSI is NADPH synthesis.
   • PSII mainly functions in ATP synthesis and the hydrolysis of water.
7. Electron Replacement:
   • In PSI, released high-energy electrons are replaced by those from photolysis.
   • PSII replenishes its high-energy electrons with electrons from PSII itself.
8. Pigments:
   • PSI contains chlorophyll B, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, and carotenoids.
   • PSII includes chlorophyll B, chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, phycobilins, and xanthophylls.
9. Composition of the Core:
   • The core of PSI comprises psaA and psaB subunits.
   • The core of PSII consists of D1 and D2 subunits.
10. Electron Donors and Acceptors:
    • PSI: Receives electrons from PSII, using them to reduce NADP+ to NADPH.
    • PSII: Donates electrons to PSI and is responsible for the initial step of splitting water molecules, releasing oxygen.
11. Position in the Electron Transport Chain:
    • PSI is downstream in the electron transport chain, receiving electrons from PSII.
    • PSII is the initial step in the electron transport chain, supplying electrons to PSI.
12. Reaction Center Chlorophyll:
    • PSI: Contains chlorophyll a molecules with peak absorption at 700 nm (P700).
    • PSII: Contains chlorophyll a with peak absorption at 680 nm (P680).
13. Chlorophyll Carotenoid Pigment Ratio:
    • PSI: Exhibits a chlorophyll-carotenoid pigment ratio of 20-30 : 1.
    • PSII: Shows a chlorophyll-carotenoid pigment ratio of 3-7 : 1.
14. Location within the Thylakoid Membrane:
    • PSI: Primarily located in the appressed regions of grana.
    • PSII: Found in the stroma and non-appressed regions of grana.
15. Role in Oxygen Evolution:
    • PSI: Does not play a direct role in the evolution of molecular oxygen.
    • PSII: Directly involved in the photolysis of water, leading to the evolution of molecular oxygen.
16. Subunit Composition and Complexity:
    • PSI: Has a simpler core complex, with about 15 subunits.
    • PSII: More complex, consisting of approximately 25-30 subunits.
17. Type of Reaction Center:
    • PSI: Features an iron-sulfur type reaction center.
    • PSII: Has a quinone type reaction center.
18. Pigment Composition:
    • PSI: Rich in chlorophyll a compared to chlorophyll b.
    • PSII: Contains more chlorophyll b relative to chlorophyll a.