The major difference between photosystems 1 and 2 is that photosystem 1 lies on the outer surface of the thylakoids and it receives electrons from photosystem 2 while photosystem 2 lies on the inner surface of the thylakoids and it receives electrons from photolytic dissociation of water.
|Photosystem I||Photosystem II|
|The analysis of water does not occur.||It is related to the photolysis of water.|
|The reaction center is P700.||Its reaction center is P680.|
|It is rich in chlorophyll A then Chlorophyll B||It is rich in chlorophyll B then Chlorophyll A|
|Molecular oxygen is not evolved.||Photosystem II, as a result of the photolysis of water molecular oxygen, is evolved.|
|Receive electrons from photosystem II.||Receive electrons from photolytic dissociation of water.|
|Pigments absorb longer (>680nm) wavelengths of light||Pigments absorb shorter (<680nm) wavelengths of light|
|In this reaction, NADPH is formed.||While in this reaction, NADPH is not formed.|
|It can participate in both cyclic and non-cyclic photophosphorylation.||Just participates in non-cyclic photophosphorylation.|
|The core complex is composed of a smaller number of proteins.||The core complex is composed of a multi-subunit of about 25-30 sub-units.|
|Lies on the outer surface of the thylakoid membrane||Lies on the inner surface of the thylakoids.|
|PS I have an iron-sulfur type reaction center.||PS II is a Quinone-type reaction center|
|The major function is NADPH synthesis.||Its main function is the hydrolysis of water and ATP synthesis.|
The photosynthetic pigments absorb the sunlight. This sunlight drives the process of photosynthesis. Photosynthetic pigments are organized into clusters called photosystems. These photosystems absorb and utilize solar energy efficiently in the thylakoid membranes. Each photosystem is composed of two parts.
Antenna Complex: It is a light-gathering part. It is composed of many molecules of chlorophyll a, chlorophyll b, and carotenoids. Light energy absorbed by the antenna complex is transferred to the reaction center.
Reaction center: It converts light energy into chemical energy. It has one or more molecules of chlorophyll a. Chlorophyll a molecule of reaction center and other associated proteins are closely linked to nearby primary electron acceptor and electron transport system. These associated parts are:
(i) Primary Electron Acceptor: It is associated with the reaction center. It traps the high-energy electron from the reaction center. It then passes this electron to the series of electron carriers.
(ii) Electron Transport Chain: It is associated with chlorophyll a molecule. The electron transport chain plays an important role in the synthesis of ATP by chemiosmosis.
There are two types of photosystems photosystem I (PS I) and photosystem II (PS Il). They are named so due to their order of discovery.
There are two types of electron transport:
The path of an electron through the two photosystems during non-cyclic photophosphorylation is called Z- scheme. It forms the Z-shape path.
The electrons of photosystem II reach the bottom of the electron transport chain and fill the electron hole in the Chlorophyll P700 molecule of photosystem I.
Sometimes, the photoexcited electrons take an alternative path. This path is called a cyclic electron flow. This path uses only photosystem I. It does not use photosystem II. This cycle may take place when there is less amount of ATP for the Calvin cycle. It slows down the cycle.
So, the NADPH accumulates in the chloroplast. This rise in NADPH may simulate the temporary shifting from non-cyclic to cyclic electron flow. The cyclic electron flow continues until the ATP supply fulfills the demand. So the cyclic flow is a short circuit. The following steps take place during cyclic phosphorylation:
4. Finally, the Cytochromes complex returns these electrons to excited chlorophylls of the P700. A molecule of ATP is produced during this transfer of electrons through ETC by chemiosmosis. The NADPH is not produced and oxygen is also not released. As the same excited electrons are returned back to the excited chlorophyll by producing a molecule of ATP, so it is called cyclic phosphorylation.
The mechanism for ATP synthesis is chemiosmosis in cyclic and non-cyclic phosphorylation. It is a process that uses membranes during a redox reaction for ATP production. The electron transport chain (ETC) pumps the protons (H+) across the thylakoids. The energy used for this pumping is provided by the movement of an electron through the ETC.
This energy is transferred into potential energy. This potential energy is stored in the form of an H+ gradient across the membrane. Then these hydrogen ions move down to form the gradient through the ATP synthase complex. The ATP synthase complexes are present within the thylakoid membranes. The energy of the electrons is used for the synthesis of ATP during the passing of electrons through the ATP synthase enzyme.
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