Difference Between Photosystem 1 and 2

Main Difference – Photosystem 1 vs 2

Photosystem I (PS I) and photosystem II (PS II) are two multi-subunit membrane-protein complexes involved in oxygenic photosynthesis. Chlorophyll is the pigment involved in capturing light energy. PS 1 contains chlorophyll B, chlorophyll A-670, Chlorophyll A-680, chlorophyll A-695, chlorophyll A-700 and carotenoids. Chlorophyll A-700 is the active reaction center of PS 1. PS 2 contains chlorophyll B, chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, phycobilins and xanthophylls. Chlorophyll A-680 is the active reaction center of photosystem 2. The main difference between photosystem 1 and 2 is that PS I absorbs longer wavelengths of light (>680 nm) whereas PS II absorbs shorter wavelengths of light (<680 nm). 

This article examines, 

1. What is Photosystem 1
      – Definition, Characteristics, Function
2. What is Photosystem 2
      – Definition, Characteristics, Function
3. What is the difference between Photosystem 1 and 2

What is Photosystem 1

PS I is the collection of pigments of chlorophyll, absorbing mostly the wavelength of light at 700 nm. The final stage of the light reaction is catalyzed by PS I. The reaction center of PS I consists of chlorophyll A-700. The core of the PS I is made up of psaA and psaB subunits. Core subunits of the PS I are larger than the core subunits of PS II. PS I is made up of chlorophyll A-670, Chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B and carotenoids. Photons from light are absorbed by accessory pigments and passed into the reaction center. Reaction center itself is capable of absorbing photons. The energy of absorbed photons is released from the reaction center as high energy electrons. These electrons are transferred through series of electron carriers and finally taken up by NADP+ reductase. The enzyme, NADP+ reductase produces NADPH from these electrons. A schematic diagram of a photosystem is shown in figure 1.

Figure 1: A photosystem
1 – Sunlight, 2 – Pigments, 3 – Reaction center, 4 – high energy electron flow, 5 – photosystem

What is Photosystem 2

PS II is the collection of pigments of chlorophyll, absorbing mostly the wavelength of light at 680 nm. The first stage of the light reaction is catalyzed by PS II. The reaction center of PS II consists of chlorophyll A-680. PS II is an integral membrane protein, which consists of a core made up of D1 and D2 subunits. PS II consists of a lot of other proteins and pigments arranged in the photosystem. The pigments are chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B and phycobilins and xanthophylls. PS II achieves energy from absorbing photons or associated accessory pigments in the antenna complex. High energy electrons are generated from the energy of the absorbed photons. These electrons are passed through an electron transport chain. During the electron transport chain, PS II passes electrons to plastoquinone (PQ), which carries the electrons to cytochrome bf complex. In PS II, photolysis of water occurs in order to replace the released electrons from PS II. For each water molecule, that is hydrolyzed, two molecules of PQH2 are formed. The overall reaction in PS II is shown below.

2PQ (Plastoquinone) + 2H2O → O2 + 2PQH2 (Plastoquinol)

Figure 2: Photosystem 2

Difference Between Photosystem 1 and 2

Location

Photosystem 1: Photosystem 1 is located on the outer surface of the thylakoid membrane.

Photosystem 2: Photosystem 2 is located on the inner surface of the thylakoid membrane.

Photocenter

Photosystem 1: The photocenter of the photosystem 1 is P700.

Photosystem 2: The photocenter of the photosystem 2 is P680.

Absorbing Wavelength

Photosystem 1: Pigments absorb longer wavelengths of light (>680 nm).

Photosystem 2: Pigments absorb shorter wavelengths of light (<680 nm).

Photophosphorylation

Photosystem 1: Photosystem 1 is involved in both cyclic and non-cyclic photophosphorylation.

Photosystem 2: photosystem 2 is only involved in cyclic photophosphorylation.

Photolysis

Photosystem 1: No photolysis of water occurs in photosystem 1.

Photosystem 2: Photolysis of water occurs in photosystem 2.

Main Function

Photosystem 1: The main function of the photosystem 1 is NADPH synthesis.

Photosystem 2: The main function of the photosystem 2 is ATP synthesis and hydrolysis of water.

Electron Replacement

Photosystem 1: Released high energy electrons are replaced by the releasing energy of photolysis.

Photosystem 2: Released high energy electrons are replaced by the electrons released from photosystem II.

Pigments

Photosystem 1: PS 1 contains chlorophyll B, chlorophyll A-670, Chlorophyll A-680, chlorophyll A-695, chlorophyll A-700 and carotenoids.

Photosystem 2: PS 2 contains chlorophyll B, chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, phycobilins and xanthophylls.

Compositon of the Core

Photosystem 1: The core of the PS I is made up of psaA and psaB subunits.

Photosystem 2: The core of the PS II is made up of D1 and D2 subunits.

Conclusion

PS I and PS II are the two photosystems which drive the light reaction of photosynthesis. The first stage of the light reaction occurs in PS II whereas the final stage of the light reaction occurs in PS I. Each of the two photosystems are made up of a collection of proteins and pigments. Chlorophylls are the major pigments found in photosystems. The reaction center of PS I consists of chlorophyll A-700 and the reaction center of PS II consists of chlorophyll A-680. Other than chlorophylls, carotenoids are also present in photosystems. The core of the PS I is made up of large subunits of psaA and psaB proteins. The core of the PS II is made up of comparatively small subunits of D1 and D2. Water molecules are hydrolyzed at PS II in order to replace the releasing electrons of each of the two photosystems. Electrons released from PS I are used by NADP+ reductase, producing NADPH. However, the main difference between Photosystem 1 and 2 is the wavelengths of sunlight, which are absorbed by each of the reaction centers of photosystems.

Reference:
1. Caffarri, Stefano, Tania Tibiletti, Robert C. Jennings, and Stefano Santabarbara. “A Comparison Between Plant Photosystem I and Photosystem II Architecture and Functioning.” Current Protein & Peptide Science. Bentham Science Publishers, June 2014. Web. 17 Apr. 2017.

Image Courtesy:
1. “Schema-photosysteme” By –Pinpin 19:24, 24 May 2006 (UTC) – Own work, made with inkscape (CC BY-SA 3.0) via Commons Wikimedia
2. “Photosystem-II 2AXT” By Neveu,Curtis (C31004) (CC BY-SA 3.0) via Commons Wikimedia

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