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 the solar energy efficiently in the thylakoid membranes. Each photosystem is composed of two parts.
Antenna Complex: It is 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 reaction center.
Reaction center: It converts the 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 form the reaction center. It then passes this electron to the series of electron carriers.
Types of Photosystem
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.
- Photosystem I: It has chlorophyll a molecules. It absorbs maximum light of 700nm. So it is called P700.
- Photosystem lI: lt has also chlorophyll a molecules in its reaction center. This chlorophyll absorbs best the light of 680nm. So this chlorophyll is called P600.
Types of Electron Transport or Electron Flow in Photosystem
There are two types of electron transport:
- Non-cyclic electron flow: It is most common type of electron flow. In this case, the electron passes through two photosystems. The formation of ATP during non-cyclic flow is called non-cyclic phosphorylation.
- Cyclic electron flow: It is a less common type of electron flow. In this case, only photosystem I is involved. The formation of ATP during cyclic electron flow is called cyclic phosphorylation.
Non- cyclic phosphorylation
The path of electron through the two photosystems during non-cyclic photophosphorylation is called Z- scheme. It forms Z-shape path.
- Photosystem II absorbs light. An electron is exited to a higher energy level in the reaction center of the chlorophyll P680. This electron is captured by the primary electron acceptor of PS II. The oxidized chlorophyll is now a very strong oxidizing agent. Its electron hole must be filled.
- An electron is extracted from the water by an enzyme. This electron fills the hole of the chlorophylls P680. This reaction splits the water molecules into two hydrogen ions and an oxygen atom. This oxygen atom combines with another oxygen atom to form O2. This oxygen is the main source of replenishment of the atmospheric oxygen. The splitting of water and release of oxygen during photosynthesis is called photolysis.
- Each photoexcited electron passes form the primary electron acceptor of the photosystem II to photosystem I through an electron transport chain. This electron transport chain has following electron carriers:
- Plastoquinone (Pq).
- A complex of two Cytochromes.
- Plastocyanin (PC): It is a copper containing protein.
- As the electrons move down the chain, their energy goes on decreasing. This energy is used by the thylakoid membranes to synthesize ATP. “The synthesis of ATP due to light energy is called photophosphorylation”. The ATP synthesis during non-cyclic electron flow is called non-cyclic photophosphorylation. This ATP produced during light dependent reaction will be used during synthesis of sugar in the Calvin cycle (dark reaction).
- The P700 chlorophyll of Photosystem absorbs light energy and drive electrons to the primary acceptor of the photosystem I. It creates hole in the molecule of P700.
The electrons of the photosystem II reaches the bottom of the electron transport chain and fill the electron hole in Chlorophyll P700 molecule of photosystem I.
- The primary electron acceptor of the photosystem I transfers the photo excited electrons to a second electron transport chain. This second transport chain transfers these electrons to the ferredoxin (Fd). The Fd is an iron containing protein. An enzyme NADP reductase transfers the electron from Fd to NADP. This is the redox reaction. It stores the high-energy electrons in NADPH. The NADPH molecule will provide reducing power for the synthesis of sugar in the Calvin cycle.
Sometime, the photoexcited electrons take an alternative path. This path is called 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 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 ATP supply fulfills the demand. So the cyclic flow is a short circuit. Following steps take place during cyclic phosphorylation:
- P700 of the photosystem I absorbs light. This light energy drive electrons form P7 of the photosystem I to primary electron acceptor. It produces electron hole in the chlorophyll.
- The primary electron acceptor of photosystem I transfers the photoexcited electrons to ferredoxin (Fd).
- The electrons are transferred from ferredoxin (Fd) to Cytochromes complex (ETC).
- Finally the Cytochromes complex returns these electrons to exited 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 the ATP synthesis is chemiosmosis in cyclic and non- cyclic phosphorylation. It is a process that uses membranes during 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 electron through the ETC.
This energy is transferred into potential energy. This potential energy is stored in the form of H+ gradient across the membrane. Then these hydrogen ions move down form the gradient through ATP synthase complex. The ATP synthase complexes are present with in the thylakoid membranes. The energy of the electrons is used for the synthesis of ATP during the passing of electron through ATP synthase enzyme.