Our Biosphere contains all biotic and abiotic factors, and talking about abiotic. They are completely dependent on the biotic (or living) factors for their survival. So, are plants which is the most important part of the biotic factors in our biosphere. Plants prepare food with the help of the process known as photosynthesis. On a short note, it seems very easy to understand and a simple process, but if we dig up a little inside, it is a very complicated process with many limiting factors. Photosystems I and II are just a little part of the cycle that is to be completed to form plant food.
Photosystem I vs Photosystem II
The main difference between Photosystem I and Photosystem II is that Photosystem I absorb sunlight at a wavelength of about 700nm while Photosystem II absorbs sunlight at a wavelength of 680nm in the red region. Also, Photosystem I is present in both granum and stroma thylakoid, while Photosystem II is present only in the granum thylakoid.
Photosystem I is also written as P700. Its main function is to form a molecule of NADPH. The primary electron acceptor of Photosystem I is Plastocyanin. It contains six electron carriers, namely – Cytochrome b6, Cytochrome f553, Plastocyanin, Ferredoxin reductase NADP+, X-Ferredoxin reducing substance. Photosystem I gain an electron from Photosystem II and takes part in both cyclic and non-cyclic photophosphorylation. The end product of the non-cyclic photophosphorylation is used in Calvin Cycle.
Photosystem II s also written as P680nm. The main function of the Photosystem is to perform hydrolysis of water along with ATP synthesis. The primary electron acceptor of the same is Plastoquinone, and the three major electron acceptors of Photosystem II are – Unknown Q, Plastoquinone, Cytochrome b559.
Comparison Table Between Photosystem I and Photosystem II
| Parameters of Comparison | Photosystem I | Photosystem II |
| Present In | Photosystem is present in the granum and stroma thylakoid. | Photosystem II is present in the granum thylakoid only. |
| Wavelength Absorb | It absorbs a wavelength of about 700nm. | It absorbs a wavelength of about 680nm. |
| Number of Electron Carriers | It has six in total electron carriers. | It has three electron carriers in total. |
| Electron Acceptor | Plastocyanin | Plastoquinone |
| Formation of NADPH | The end product is NADPH. | There is no formation of NADPH. |
| Reaction Centre | P700 nm | P680 nm |
| Photolysis of Water | Photosystem I is not used in the photolysis of water. | Photosystem II is used in photolysis. |
| Content of Chlorophyll | The content of chlorophyll a is more compared to the chlorophyll b content. | The content of chlorophyll b is more than chlorophyll a. |
What is Photosystem I?
Photosystem I is present in both granum thylakoid as well as stroma thylakoid of green plants and algae. Photosystem I contain two components – Photosynthetic Unit and Electron Carrier. The Photosynthetic Unit further consists of Reaction Centre and Light-Harvesting Complex while there are six major electron carriers of the Photosystem I, which we have already mentioned above.
Photosystem I is made up of two protein-rich subunits, which are – psaA and psaB. It absorbs a wavelength of about 700nm. Along with the presence of chlorophyll a and b, many other pigments such as – carotenoids, chlorophyll A–670, chlorophyll A-680, and chlorophyll A-695. It is also said that the content of the chlorophyll a then the chlorophyll b present.
The function played by the Photosystem is that it helps in the formation of NADPH and ATP in the light reaction.
What is Photosystem II?
Photosystem II is present in granum thylakoid only in the green plants and algae. It also contains two components like Photosystem I, which are – Photosynthetic Unit and Electron Carrier. These are further sub-divided in Reaction Centre and Light-Harvesting Complex, while Electron Carrier is three in number, which is named above.
The reaction center consists of chlorophyll, a molecule that absorbs the wavelength of 680nm, while the light-harvesting complex has 200 molecules of chlorophyll a and b, which absorbs light less than 680nm along with 50 molecules of carotenoids.
The core composition of the photosystem is said to be made up of two subunits named D1 and D2. It is known as a membrane-embedded protein complex that has 20 subunits and more than 50 cofactors.
The major role performed by Photosystem II is that it helps in the hydrolysis of water and ATP synthesis in the mitochondria.
Main Differences Between Photosystem I and Photosystem II
- The location of the presence of Photosystem I is that it is present in the stroma and granum thylakoid, while Photosystem II is present only in the granum thylakoid.
- The wavelength at which Photosystem I is 700nm while the wavelength at which Photosystem II is 680nm.
- The six different electron carriers of Photosystem I are – Plastocyanin, Cytochrome f553, Cytochrome b6, Ferredoxin reductase NADP+, Ferredoxin, X-Ferredoxin reducing Substances while there are only three electron carriers of Photosystem II which are – Plastoquinone, Cytochrome b559, and an unknown name carrier which is assumed as Q.
- Photosystem I is used to producing NADPH, while Photosystem II does not involve any reaction to produce NADPH.
- The reaction center of Photosystem I is said to be P700, and on the other hand, the reaction center of Photosystem II is said P680.
- Biologists have found Photosystem I having more amount of chlorophyll content than b, while that is the opposite with Photosystem II as it has more content of chlorophyll b as compared to chlorophyll a.
- Also, while light and dark reactions, the electron acceptor for Photosystem I is Plastocyanin, while for Photosystem II, it is none other than Plastoquinone.
Conclusion
Plants produce energy via making food with the help of two types of reactions named Light Reactions and Dark Reactions. The light reactions involve both cyclic and non-cyclic photophosphorylation, while dark reactions involve all the carbon assimilation reactions.
And the Photosystems are a major part of Light Reactions. Both the photosystems have different absorbing wavelengths as the first absorbs at a higher wavelength of about 700nm while the second absorbs the sunlight at a lower wavelength of 680nm.
The presence of chlorophyll contents also differs. The core component of both the photosystems are also different, but one same thing is that both are made up of in two subunits which is psaA and psaB for Photosystem I and D1 and D2 is said to be the two subunits for Photosystem II.
Photosystem I is a major part of non-cyclic photophosphorylation, generally known as Hills Reaction, while Photosystem II plays a major role in non-cyclic photophosphorylation.
References
- https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-3054.1992.tb01328.x
- https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201303671
- https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1751-1097.1987.tb08413.x
- https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-3054.1991.tb05101.x