The two main multi-subunit membrane protein complexes differ in their absorbing wavelength, where the photosystem I or PS 1 absorbs the longer wavelength of light which is 700 nm while photosystem II or PS 2 absorbs the shorter wavelength of light 680 nm.
Secondly, each photosystem is replenished by the electrons, after the loss of an electron, but the sources are different where PS II gets it electrons from water while PS I gains electrons from the PS II through an electron transport chain.
The photosystems are involved in photosynthesis and are found in thylakoid membranes of algae, cyanobacteria and mainly in plants. We all know that plants and other photosynthetic organisms collect solar energy which is supported by the light-absorbing pigment molecules present in the leaves.
The absorbed solar energy or light energy in leaves is converted to chemical energy at the first stage of photosynthesis. This process undergoes a series of chemical reaction known as light-dependent reactions.
The photosynthetic pigments like chlorophyll a, chlorophyll b and carotenoids are present in the thylakoid membranes of the chloroplast. The photosystem constitutes the light-harvesting complexes, that comprises of 300-400 chlorophylls, proteins, and other pigments. These pigments get excited after absorbing the photon, and then one of the electrons is switched to higher-energy orbital.
The excited pigment passes their energy to neighbouring pigment by the resonance energy transfer, and this is the direct electromagnetic interactions. Further, in turn, the neighbouring pigment transfer energy to pigment and the process is repeated multiple times. Together these pigment molecules collect their energy and pass towards the central part of the photosystem known as reaction center.
Though the two photosystems in the light-dependent reactions got their name in the series, they were discovered, but the photosystem II (PS II) comes first in the path in the electron flow and then the photosystem I (PSI). In this content, we will explore the difference between the two types of pf photosystem and a brief description of them.
Content: Photosystem I and Photosystem II
Comparison Chart
| Basis for Comparison | Photosystem I (PS I) | Photosystem II (PS II) |
|---|---|---|
| Meaning | Photosystem I or PS I uses light energy to convert NADP+ to NADPH2. It involves the P700, chlorophyll and other pigments. | Photosystem II or PS II is the protein complex that absorbs light energy, involving P680, chlorophyll and accessory pigments and transfer electrons from water to plastoquinone and thus works in dissociation of water molecules and produces protons (H+) and O2. |
| Location | It is located on the outer surface of the thylakoid membrane. | It is located on the inner surface of the thylakoid membrane. |
| Photocenter or reaction centre | P700 is the photo center. | P680 is the photo center. |
| Absorbing wavelength | The pigments in the photosystem 1 absorb longer wavelengths of light which is 700 nm (P700). | The pigments in the photosystem2 absorb shorter wavelengths of light which is 680 nm (P680). |
| Photophosphorylation | This system is involved in both cyclic as well as non-cyclic photophosphorylation. | This system is involved in both cyclic photophosphorylation. |
| Photolysis | No photolysis occur. | Photolysis occurs in this system. |
| Pigments | Photosystem I or PS 1 contains chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, and carotenoids. | Photosystem II or PS 2 contains chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, xanthophylls and phycobilins. |
| The ratio of the chlorophyll carotenoid pigments | 20-30 :1. | 3-7 :1. |
| Function | The primary function of the photosystem I is in NADPH synthesis, where it receives the electrons from PS II. | The primary function of the photosystem II is in the hydrolysis of water and ATP synthesis. |
| Core Composition | The PSI is made up of two subunits which are psaA and psaB. | The PS II is made up of two subunits made up of D1 and D2. |
Definition of Photosystem I
Photosystem I or PSI is located in the thylakoid membrane and is a multisubunit protein complex found in green plants and algae. The first initial step of trapping solar energy and the then conversion by light-driven electron transport. PS I is the system where the chlorophyll and other pigments get collected and absorb the wavelength of light at 700nm. It is the series of reaction, and the reaction center is made up of chlorophyll a-700, with the two subunits namely psaA and psaB.
The subunits of PSI is larger than the subunits PS II. This system also consists of the chlorophyll a-670, chlorophyll a-680, chlorophyll a-695, chlorophyll b, and carotenoids. The absorbed photons are carried into the reaction center with the help of the accessory pigments. The photons are further released by the reaction center as high energy electrons, that undergoes a series of electron carriers and finally used by NADP+ reductase. The NADPH is produced through NADP+ reductase enzyme from such high energy electrons. NADPH is used in the Calvin cycle.
Therefore, the main aim of the integral membrane protein complex that uses light energy to produce ATP and NADPH. Photosystem I is also known as plastocyanin-ferredoxin oxidoreductase.
Definition of Photosystem II
Photosystem II or PS II is the membrane-embedded-protein-complex, consisting of more than 20 subunits and around 100 cofactors. The light is absorbed by the pigments such as carotenoids, chlorophyll, and phycobilin in the region known as antennae and further this excited energy is transferred to the reaction center. The main component is peripheral antennae which are engaged in the absorbing light along with the chlorophyll and other pigments. This reaction is done at the core complex which is the site for the initial electron transfer chain reactions.
As discussed earlier that, PS II absorbs light at 680 nm, and enters at high-energy state. The P680 donates an electron and transfer to the pheophytin, which is the primary electron acceptor. As soon as the P680 loses an electron and gains positive charge, it needs an electron for replenishment which is fulfilled by splitting of water molecules.
The oxidation of water occurs at manganese center or Mn4OxCa cluster. The manganese center oxidizes two molecules at once, extracting four electrons and thus producing a molecule of O2 and releasing four H+ ions.
There is the various contradicting mechanism of the above process in PS II, though protons and electrons extracted from water are used to reduce NADP+ and in ATP production. Photosystem II is also known as water-plastoquinone oxidoreductase and is said as the first protein complex in the light reaction.
Key Differences Between Photosystem I and Photosystem II
Given points will exhibit the variation between the photosystem I and photosystem II:
Conclusion
So we can say that in plants photosynthesis encompasses two processes; the light-dependent reactions, and the carbon-assimilation reaction which is misleadingly also known as dark reactions. In the light reactions, the photosynthetic pigments and chlorophyll absorb light and convert into ATP and NADPH (energy).
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