Difference Between Photosystem I and Photosystem II

Introduction image

The difference between Photosystem I and Photosystem II is primarily due to the following factors:
Active reaction centre: P700 is the active reaction centre of PS-I, while P680 is the active reaction centre of PS-II.
Photon absorption: PS-I absorbs light of longer wavelengths (ranging between 725-1035 nm), while PS-II absorbs light of shorter wavelengths (<680 nm).

Photosystem I (PS-I) and photosystem II (PS-II) are two multi-subunit complexes that laid inside the thylakoid membrane of chloroplast and involved in the process of photosynthesis. A photosystem possesses an antenna complex (contains around 200-300 light-harvesting pigment molecules) and a reaction centre. The auxiliary accessory pigments trap photon and hand over it to a specialized pigment of the reaction centre.

Content: Photosystem I Vs Photosystem II

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Similarities
  5. Conclusion

Comparison Chart

PropertiesPhotosystem IPhotosystem II
MeaningPS-I is the photo centre that absorbs photons by the association of light harvesting pigments, reaction centre (P700) and other accessory pigments to produce NADPHPS-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
LocationFound on the outer surface of the thylakoid membraneFound in the inner surface of the thylakoid membrane
Reaction centreP700 is an active reaction centreP680 is an active reaction centre
Photon absorptionThe 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 photophosphorylationIt is involved cyclic as well as non-cyclic photophosphorylationIt is only involved in non-cyclic photophosphorylation
Photolysis of waterPS-I do not carry out photolysis of waterPS-II results in the photolysis of water
SubunitsIt comprises psaA and psaB subunitsIt comprises D1 and D2 subunits
Protein pigmentsPhotosystem I includes chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, and carotenoids as the photo pigmentsPhotosystem 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 ratio20-30 :13-7 :1
Oxygen productionDoes not occurOxygen production occurs
NADPH productionNADPH production occursDoes not occur
Type of reaction centrePS-I possesses iron sulphur or type-I RC PS-II possesses Q (Quinone) type or type-II RC
FunctionIts primary produces NADPH Its primary produces ATP and causes water hydrolysis

Definition

Photosystem I or PS I can define as 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 the cyclic phosphorylation and produces NADPH. Photosystem I has a reaction centre composed of a chlorophyll a molecule. It is capable of absorbing the light at wavelength 700 nm.

The light-harvesting or antenna complex of PS I absorbs photon and hands over to its reaction centre that contains chlorophyll a molecule. The reaction centre excites and releases high energy electrons. These high energy molecules pass through electron carriers and release energy (NADPH) via travelling across the electron transport chain.

Photosystem II or PS II can define as the 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 molecule that having an absorption peak of 680 nm (P680). Furthermore, PS II contains pigment molecules that absorb photons and funnel it to the reaction centre composed of a chlorophyll a molecule.

The reaction centre of PS II excites and releases high energy molecules. Consequently, the primary electron acceptor molecules pick high energy electron and hand over to PS I via the number of carrier molecules. When electrons are transferred through electron carriers of low energy levels, some of the energy released is used in the synthesis of ATP from ADP through a process called photophosphorylation.

Key Differences Between Photosystem I and Photosystem II

  1. The light-harvesting pigments of photosystem I and II absorb photons having wavelengths of 700 nm (P700) and wavelengths of 680 nm (P680), respectively.
  2. Photosystem I is involved cyclic as well as non-cyclic photophosphorylation, photosystem II is only engaged in non-cyclic photophosphorylation.
  3. PS-I comprises of 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 the light-dependent photosystems that participate in the light reactions of photosynthesis.
  • The light-harvesting pigments (collectively called antenna complex) and the reaction centre are two common elements common in both the photosystems.
  • Furthermore, Photosystem I and II comprises light-harvesting pigments for photon absorption and active reaction centre for the electron excitation.
  • The location of PS-I and PS-II is somewhat common that both are found in the thylakoid membrane.

Conclusion

Therefore, we can conclude that the photosystem I and photosystem II plays a fundamental role in trapping photons of selective wavelength and channelizing it to the active reaction centre. The light energy harnessed by the cells produces chemical potential energy like ATP that is used by the plant cells to drive chemical energy or to synthesize glucose in the dark reaction of photosynthesis.

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