Types of Photosystems

What is a Photosystem?

The light-absorbing pigments of thylakoid or bacterial membranes arranged in functional arrays are called photosystems. All the pigments of the photosystem can absorb photons that are being used in photosynthetic reactions but only a few molecules of the pigment are directly involved in the transduction of photon’s light energy to electrochemical energy.

In this article, we will discuss the types of photosystem- Photosystem I and Photosystem II. Students can also refer to this article to know the difference between photosystem 1 and 2.


Constituents of Photosystem

Photosystem contains mainly two types of pigment that are, chlorophyll and accessory pigments. The main function of chlorophyll is to absorb light energy in the form of photons to perform photosynthesis whereas accessory pigments as the name suggest act as secondary light-absorbing pigments which enhances the overall performance of chlorophyll. An example of accessory pigment is carotenoids. 

The carotenoid pigments absorb light at a wavelength not absorbed by the chlorophyll, they may be yellow, red, or purple. The most important is beta carotene, which is a red-orange isoprenoid, and yellow carotene lutein.

The main constituents of the photosystem are photochemical reaction centers and antenna molecules, also known as the light-harvesting complex. Photochemical reaction system constitutes chlorophyll and is the site of photosynthetic reaction, specialized chlorophyll molecule converts light energy into electrochemical energy Light-harvesting complex constitutes of the accessory pigments which absorbs a photon and transfers it to the reaction system. This is known as exciton transfer. Together these systems form a complete functioning photosystem.


Types of Photosystem

The photosynthetic apparatus of modern cyanobacteria, algae, and vascular plants are highly evolved and complex as compared to their primitive counterparts found in bacteria. These evolved organisms constitute 2 photosystems namely Photosystem I and photosystem II, both of these works in a coordinated fashion to carry out photosynthesis. The coordinated pathway of PSI and PSII is known as Z-scheme.

  1. Photosystem I

  2. Photosystem II


Photosystem I 

PSI absorbs photons at 700 nm, conversely known as P700. Apart from its absorption, it receives electrons from the plastocyanin of PSI. Major components include chlorophyll, phylloquinone, iron-sulfur protein, ferredoxin: NADP oxidoreductase, and NADP+. Given below is the description of the individual component of PSII.


1. Chlorophyll

Designated as A0, chlorophyll molecules of PSI act as electron acceptors. Reduction of it causes chlorophyll to move in an excited state where it transfers electrons to the next carrier. It causes A0 to move back to its ground state.


2. Phylloquinone 

It is designated as A1, it is the second electron carrier in photosystem II. It transfers electrons to the iron-sulfur protein cluster.


3. Iron-Sulfur Protein 

It is a cluster of proteins located in the thylakoid lumen. There are about 3 Fe-S protein functioning in PSI 


4. Ferredoxin 

It is a flavin-containing protein that receives electrons from the Fe-S cluster, thus getting reduced.


5. Ferredoxin: NADP oxidoreductase

It is the fourth electron carrier protein, it oxidizes Fd and simultaneously transfers electrons to NADP generating NADPH.

2Fdreduced + 2H+ + NADP+  2Fdoxidized + NADPH + H+


Photosystem II

 It absorbs light at 680nm (wavelength), the photosystem is shortly written as PS II. The main constituents of PSII are chlorophyll molecules at the reaction center, pheophytin (pheo), plastoquinone (PQ), cytochrome- b6f complex (cyt b6f), and plastocyanin. PSII is associated with the oxygen-evolving complex. Oxygen evolving complex is responsible for the production of oxygen mediated by oxidation of H2O yielding O2. 


Steps Included in Photosystem II

Given below is the description of the photosystem II steps:

1. Chlorophyll 

It is a five-membered ring coordinated with Mg2+, there is a presence of phytol side chain esterified with ring IV, its ring structure is similar to protoporphyrin except with only difference that Fe2+ is substituted with Mg2+. There are two types of chlorophyll molecule namely chlorophyll a and chlorophyll b.  The main role of the chlorophyll molecule present in a reaction center is to absorb a photon, directly as well as supplied by accessory pigments, this absorption leads to excitation in the chlorophyll molecule, excited chlorophyll molecule then transfers the electron to subsequent compounds of the photosystem. This electron transfer causes chlorophyll molecules to return to their ground state. Excitation is coupled with a reduction reaction that is gaining electrons and the subsequent transfer of the electron is known as an oxidation reaction. The components of the photosystem undergo a series of oxidation-reduction reactions to yield the final product.


2. Pheophytin

It is a transfer protein. Pheo receives electrons from chlorophyll thus getting reduced and then it transfers the electron to plastoquinone. 


3. Plastoquinone

It is a major transfer protein of the photosystem, there are 2 molecules of plastocyanin involved namely PQA and PQB. PQA receives electrons from plastocyanin, thus getting reduced, it then transfers it to PQB. The reduced state of plastoquinone is denoted by PQBH2, H2 here shows that there is a net gain of 2 electrons during reduction reaction. The electrons are not transferred in a single reduction reaction rather 2 separate reduction reactions take place transferring one electron at a time. PQBH2 then transfers electrons to the next complex. This overall process of electron transfer is known as the Q cycle.


4. Cytochrome-b6f Complex

It receives electrons from plastoquinone, it is a major complex photosystem responsible for creating proton gradient, that is, for each pair of electrons four protons are pumped into the thylakoid lumen from the stroma of the chloroplast. The proton gradient is responsible for ATP synthesis. The structural component includes b type cytochrome, f type cytochrome, and Rieske iron-sulfur protein.


5. Plastocyanin

 It is a copper-containing protein, it is responsible for the transfer of electrons from photosystem II to photosystem I.


6. Oxygen Evolving Complex

It is also known as a water-splitting complex, responsible for the generation of oxygen by the process of photolysis. Four protons are responsible for the photolysis of water. Photolysis is mediated by photons (light energy).

2H2O 4H+ + 4electron + O2

These four protons are pumped into the thylakoid lumen.


Function of Photosystem I

The main role is the generation of NADPH molecules. It is also involved in cyclic electron flow.


Cyclic Electron Flow

Also known as electron cycling, is the process by virtue of which electrons are transferred from ferredoxin of PSI to cyt b6f complex of PSII, this recycling of electron is responsible for proton gradient which in turn is responsible for the ATP synthesis.


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Photosystem Structure


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Z-scheme


Photosystem 2 Function

The main role of PSII (photosystem 2) is to absorb light energy in the form of photons and transfer it to PSI via intermediate proteins, it also creates the proton gradient required for ATP generation.


Differences Between Photosystem 1 and 2

Photosystem I

Photosystem II

Cyclic flow of electron only occur in PSI

Cyclic flow of electron does not occur

Generates NADPH

Involved in ATP synthesis

P700 is the reaction center

P680 is the reaction center

No photolysis of water

Photolysis of water in the oxygen-evolving complex

No oxygen production

Oxygen is produced.

FAQs (Frequently Asked Questions)

1.  Define Photosystem.

Ans. It is a highly organized functional array of light-absorbing pigments. The photosystem is responsible for performing photosynthetic reactions. The electron carriers of photosystems undergo a series of oxidation-reduction reactions to yield the final product of photosynthesis.

2. State the Location of Photosystem 1 and 2.

Ans. PSI is located at the outer surface of the thylakoid membrane and PSII is located at the inner surface of the thylakoid membrane.

3. What is Photosystem II Photosynthesis?

Ans. PSII reactions are the first step of photosynthesis, they are also known as the light reactions that are photons mediated photolysis of water and electron transfer to produce NADPH and ATP.