The difference between photosystem I and photosystem II is primarily due to the difference in active reaction centre and photon absorption. P700 is the active reaction centre of PS-I, while P680 is the active reaction centre of PS-II. PS-I absorbs light of a longer wavelength (between 725-1035 nm), while PS-II absorbs light of a shorter wavelength (<680 nm).
Photosystem I (PS-I) and photosystem II (PS-II) are two multi-subunit complexes that play a pivotal role in the light-dependent reaction of photosynthesis. Chlorophylls are the light-harvesting pigments that constitute the photosystem and pigment system.
A photosystem possesses an antenna complex (200-300 light-harvesting pigment molecules) and a reaction centre. Based on chlorophyll’s different absorption wavelength, two photosystems (PS I and II) generally exist. The auxiliary accessory pigments trap photons and hand them over to the specialized protein pigments of the reaction centre.
Content: Photosystem I Vs Photosystem II
Comparison Chart
| Properties | Photosystem I | Photosystem II |
|---|---|---|
| Meaning | PS-I is the photo centre that absorbs photons by the association of light harvesting pigments, reaction centre (P700) and other accessory pigments to produce NADPH | PS-II is the photo centre that absorbs photons by the association of light harvesting pigments, reaction centre (P680) and other accessory pigments to produce ATP and oxygen via photolysis of water |
| Location | Found in the outer surface of the thylakoid membrane | Found in the inner surface of the thylakoid membrane |
| Reaction centre | P700 is an active reaction centre | P680 is an active reaction centre |
| Photon absorption | The light harvesting pigments of photosystem 1 absorb photons having wavelengths of 700 nm (P700) | The light harvesting pigments of photosystem2 absorb photons having wavelengths of 680 nm (P680) |
| Involvement in photophosphorylation | It is involved in cyclic as well as non-cyclic photophosphorylation | It is only involved in non-cyclic photophosphorylation |
| Photolysis of water | PS-I do not carry out photolysis of water | PS-II results in the photolysis of water |
| Subunits | It comprises psaA and psaB subunits | It comprises D1 and D2 subunits |
| Protein pigments | Photosystem I includes chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, and carotenoids as the photo pigments | Photosystem II includes chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, xanthophylls and phycobilins as the photo pigments |
| Chlorophyll to carotenoid ratio | 20-30 :1 | 3-7 :1 |
| Oxygen production | Does not occur | Oxygen production occurs |
| NADPH production | NADPH production occurs | Does not occur |
| Type of reaction centre | PS-I possesses iron sulphur or type-I RC | PS-II possesses Q (Quinone) type or type-II RC |
| Function | It primarily produces NADPH | It primarily produces ATP and causes water hydrolysis |
Definition of Photosystem I
PS I refers to a photosystem that participates in a light reaction of photosynthesis. In contrast to PS II, PS I carries more chlorophyll-a content compared to chlorophyll b. Furthermore, PS I participates in cyclic phosphorylation and produces NADPH. Photosystem I has a reaction centre that possesses chlorophyll-a molecules. It is capable of absorbing light at a wavelength of 700 nm.
The light-harvesting or antenna complex of PS I absorbs photons and hand them over to its reaction centre containing chlorophyll-a. The reaction centre excites and releases high energy electrons. The high energy molecules enter the electron transport chain through electron carriers and release NADPH.
Definition of Photosystem II
PS II refers to a light-dependent photosystem that participates in the photosynthetic light reactions. Opposite to PS I, it contains more chlorophyll b pigments compared with chlorophyll a. The PS II reaction centre contains chlorophyll-a that has an absorption peak of 680 nm (P680). Furthermore, the pigment molecules of PS II absorb photons and funnel them to the reaction centre or antenna complex.
The PS II reaction centre excites and releases high energy molecules. Consequently, the primary electron acceptor molecules pick high energy electrons and hand them over to the PS I through many carrier molecules. The excitation of electrons through electron carriers of low energy levels releases free energy that the cells harness to synthesize ATP from ADP via photophosphorylation.
Key Differences Between Photosystem I and Photosystem II
- The light-harvesting pigments of photosystems I and II absorb photons with wavelengths of 700 nm (P700) and 680 nm (P680), respectively.
- PS-I resides in the thylakoid’s outer membrane of the chloroplast, i.e. non-appressed region. Conversely, PS-II is present inwards the thylakoid membrane, i.e. appressed region.
- A photosystem possesses two typical classes of reaction centres (type I and II RCs) responsible for photochemical reactions. PS-I has type-I RC or P-700, whereas PS-II has type-II RC or P-680.
- Photosystem I shows involvement in cyclic and non-cyclic photophosphorylation, while photosystem II only participates in non-cyclic photophosphorylation. The difference between cyclic and noncyclic photophosphorylation is due to the different mechanisms of ATP synthesis.
- PS-I comprises psaA and psaB subunits and possesses iron sulphur or type-I RC, whereas PS-II includes D1 and D2 subunits and possesses Q (Quinone) or type-II reaction RC.
Similarities
- Both PS I and PS II are light-dependent photosystems that participate in the light reactions of photosynthesis.
- The light-harvesting pigments and the reaction centre (collectively called antenna complex) are two common elements in both the photosystems.
- Furthermore, photosystem I and II comprises light-harvesting pigments for photon absorption and an active reaction centre to excite electrons.
- The location of PS-I and PS-II is somewhat common in that they are found in the thylakoid membrane.
Conclusion
Therefore, we can conclude that photosystems I and II play a fundamental role in trapping photons of selective wavelength and channelizing them to the active reaction centre. Plant cells harness the light energy harnessed to generate chemical potential energy like ATP and NADPH that is again used by the plant cells to synthesize glucose during the dark reaction of photosynthesis.