Mechanism of Photosynthesis

Photosynthesis takes place in two phases:-

a) Photochemical phase or Light Reaction ( Hill reaction)

b) Biosynthetic phase ( Dark reaction)

a) Photochemical Phase:- This phase is dependent on light. The ATP and NADPH2 are synthesized. The water molecule undergo splitting ( photolysis of water ) resulting in the release of molecular O.2.  The electron released during photolysis of water are picked by photocentre of photosystem II. On receiving a photon of light energy the photocentre expels an electron with a gain of energy.It is the primary reaction of photosynthesis which involve the conversion of light energy into chemical form. This phenomenon is also known as quantum conversion.
b) Dark reaction :- In this phase reduction of CO2 takes place result in the formation glucose. This is also known as the Blackman's reaction. The reaction do not require light. All the enzymes required for this process are present in the matrix or stroma of the chloroplast. Assimilatory powers ( ATP and NADPH ) produced during photochemical phase is used in the fixation of CO2. 

Chemiosmotic Hypothesis of ATP Formation

This hypothesis was proposed by P. Mitchell in 1961. Photosynthesis and respiration both processes are involved in the synthesis of ATP. Electron transport in both photosynthesis and respiratiosn creates proton gradient inside the thylakoid or in the outer chamber of the mitochondria respectively. The movement of hydrogen ions from high proton concentration to lower proton concentration and an electrochemical concentration gradient of proton synthesize the ATP with the help of enzyme ATPase. Three important events during photosynthesis are associated to create proton gradient inside the thylakoid. These are :-
i) Lumen of thylakoid become enriched with hydrogen ions (H+) due to photolysis of water.
ii) Primary acceptor of electron is present on the outer side of the thylakoid and it transfer electron to the H-carrier. This H-carrier removes proton from the matrix while transporting electron to the inner side of the membrane. The proton is released into the lumen and electron passes to the next carrier.
iii) NADP reductase present on the outerside of thylakoid membrane obtains electron from the PS I and H+ ions from the matrix to reduce NADP+ to NADPH.
These three events constantly helps in increasing of proton concentration inside the lumen of thylakoid and decreases proton concentration in the matrix of chloroplast. A proton gradient develops across the thylakoid and proton movements from higher concentration to lower concentration through special channels ( CF0 - CF1 ) activates the enzyme ATPase will make the ATP from ADP. Because movement of proton is from higher to lower concentration like water moement in osmosis this process is named as chemiosmosis synthesis of ATP.

                               Mechanism of chemiosmotic synthesis of ATP in chloroplast

Photophosphorylation

In photosynthesis photophosphorylation is the process in which there is synthesis of ATP from ADP by using the light energy. photophosphorylation was discovered by Arnon et al in 1954. It is of two main types :-

i) Cyclic Photophosphorylation
ii) Non-Cyclic Photophosphorylation

i) Cyclic photophosphorylation:- It is a type of photophosphorylation in which electron expelled from the photocentre returned back to photocentre after passing the downhill journey through series of       electron carriers. Cyclic photophosphorylation is performed by photosystem I or P700. Its photocentre extrudes the electron after absorption of light energy ( 23kcal/mole). P700 will become oxidised after losing the electron. The expelled electron will pass through the series of carriers including X then A. FeS proteins ( FeSx, FeSA, FeSB), plastoquinone (PQ), cytochrome b-f complex and to plastocyanin before returning to the photocentre. While passing from carriers electron energises passage of proton to create a proton gradient which actually involved in the synthesis of ATP.

                                                   Cyclic Photophosphorylation

ii) Non Cyclic Photophosphorylation:- It is a type of photophosphorylation in which electron expelled from photocentre does not return to photocentre. Both PS I and PS II participated in non-cyclic photophosphorylation. Photolysis of water will provide the electron to PS II or P680 when PS II is deficienion of electrons. PS II extrudes the electron after absorption of light energy. The extruded electron has an energy equivalent to 23kcal/mole. These electrons passes a series of carriers like Phaeophytin, PQ, cytochrome b-f complex and plastocyanin. While passing from cytochrome complex electron loses energy for the synthesis of ATP. After this electron is handed over to PS I by plastocyanin. Electron again expelled by the PS I after absorption of light energy. The extruded electron passes the journey of electron carries- Chlorophyll X, Fs-S, ferrodoxin to finally reach NADP+. This NADP+ will combine with the H+ ( released from photolysis of water) with the help of NADP reductase to form NADPH. 

                                                        

This is called Z scheme due to zig-zag journey of electrons through the series of electron carriers. The release of molecular oxygen into atmosphere is the result of non-cyclic photophosphorylation.

Electron Transport Chain

The process of photosynthesis is completed in two main steps - light reaction and dark reaction. Light reaction occurs in the thylakoids of the chloroplast. This phase requires light for chemical reactions therefor, called photochemical phase. Dark reaction occurs in the stroma region of the chloroplast and does not require any light for the reaction to complete. Therefor, it is also called thermochemical phase. Light reaction begins as soon as the light falls on green leaves. It results in the excitation of electrons and facilitates the downhill journey of these electron with the synthesis of ATP molecules. This process is called photophosphorylation. Two photosystems are involved in the light reaction or in the synthesis of ATP molecules. P700 or photosystem I participated in the cyclic photophosphorylation and both photosystem I and photosystem II are associated in a non-cyclic photophosphorylation. The two photosystems are connected with each other by electron transport chain.
Electron transport chain:- It was first described by Hill in 1939. Electron transport chain is a series of electron carriers over which electron pass downhill journey and releasing energy at each step which helps in the synthesis of ATP. Photosynthesis electron transport chain has two main components connected to each other photosystem I and photosystem II. P680 or Photosystem II absorbs light energy and get excited. Excited photosystem transfer electron to the next carrier phaeophytin. after losing the electron photosystem II will become strong oxidant ( who has the ability of taking electron from others is called oxidant). Meanwhile the photolysis of water or splitting of water will provide the electron to the deficient photosystem II ( Photlysis is the splitting of water in the light which results in the release of molecular oxygen during the process). Phaeophytin after accepting electron becomes strong reducing agent ( who is ready to gives extra electrons to others). Then phaeophytin donates its electron to downhill carriers of ETC. At the end plastocyanin will transfer electrons to P700 or photosystem I.  P700 will hand over electrons to special chlorophyll molecule X from where electron transfer first to the iron sulphur protein (FeS) and then to the protein ferrodoxin (Fd). The latter can pass electron to the reductase coplex which helps in reducing NADP+ to NADPH. In electron transport chain two main events are involved first the production of ATP and second the release of molecular oxygen into atmosphere.

                            Electron transport pathway in photosynthesis involving two phosystems

Photosystems or Pigment Systems

    Photosynthesis involve the participation of two distinct groups called photosystem I and      photosystem II. Each photosystem has a pigment protein complex composed of chlorophyll a, chl b, carotenoids and other components required for electron transport during light reaction. These pigment systems are as fllows:-

Photosystem I (PS I) :- It is a photosynthetic pigment system with some electron carriers is present on both nonappressed part of the grana thylakoids as well as on the stroma thylakoids. This system has more chlorophyll a molecules and chlorophyll b and carotenoids are less.  Photosystem I consists of a photocentre, light harvesting complex and some electron carriers. Photocentre contains molecules of chlorophyll a700 and P700 ( a special chlorophyll molecules that absorb visible light near 700 nm wavelength). PS I has a reducing agent A0, A1, Fe Sx, Fe SA and Fe SB ( iron sulphur protein), Fd ( ferrodoxin), cytochrome b6 - f complex and plastocyanin. PS I takes part in both cyclic and non cyclic photophosphorylation ( it is the synthesis if ATP in the presence of sunlight). PS I can carry cyclic photophosphorylation independently.photosys

Photosystem II :- It is a photosynthetic pigment system present in appressed part of the thylakoid. PS II contains a chl a, chl b and carotenoids. Chl a and b are in equal content. PS II complex contains a molecule of chl a680 or P680  ( a special chlorophyll a molecule that absorb visible light near 680 nm wavelength).  Carotenoids are higher as compared to PS I. PS II consists of a photocentre, oxygen evolving complex, light harvesting comples and some electron carriers. Oxygen evolving complex contains a Mn2+, Ca2+ and Cl ions. PS II is only participate in the non cyclic photophosphorylation. 

Photosynthetic Units

A photosynthetic unit is a small group of pigment molecules which takes part in photochemical reaction and changing light energy into chemical energy. In one unit there will be one photocentre or reaction centre which will be feed by about 200 harvesting pigment molecules. The photocentre consists of special chlorophyll a molecules, P700 and P680. Reaction centre absorbs light energy at longer wavelength. Harvesting molecules are also called light harvesting complex ( LHC). There are different LHCS for two photosystems I and II. Two types of molecules are presesnt in LHC these are called antenna molecules and core molecules. The antenna molecules absorbs variou wavelength of light shorter than photocentre. After absorbtion antenna molecules get excited and in excited form these molecules hand over energy to core molecules by resonance and come to ground state. The energy from core molecules will supply to the photocentre. On absorbtion of light energy photocentre get excited and by changing the state of electrons from excited to again grond state helps the photocentre to change the light energy to chemical energy. The frequency of excitation of photocentre is very high so it cannot meet by direct absorption of energy.

                                                   Harvesting of light by photosynthetic unit