LIGHT REACTION OR PHOTOCHEMICAL PHASE (photophosphorylation) OR HILL’S REACTION

The process in which NADPH, and ATP are formed in the presences o light is called light reaction or photochemical phase of photosynthesis. I was studied by Robert Hill. So it is also called Hill’s reaction. There are two types of electron transformation during light reactions.
1. Non —cyclic electron flow (non-cyclic phosphorylation)2. Cyclic electron flow (cyclic phosphorylation)
Non-cyclic phosphorylation
Non -cyclic electron flow (Z-Scheme)
The formation of ATP during non-cyclic flow is called non-cyclic phosphorylation. In this case. the electron passes through two photosystems. It is most common type or electron flow. The path of electron through the two photosystems during non-cyclic photophosphorylation is called /- scheme. It forms Z-shape path.
I. Excitation of electron at PS II: Photosystem II absorbs light. An electron is exited to a higher energy level in the reaction centre of the  chlorophyll P6S(1. This electron is captured by the pheophytin (primary electron acceptor of PS II). The P680 becomes positively charged. It attracts electron from the Mil-protein. Mn-protein becomes oxidized and it acts as strong reducing agent.
2. Photolysis of water: The splitting of water and release of oxygen
during photosynthesis is called photolysis. This reaction splits the water molecules into two hydrogen ions and an oxygen atom. Mn-protein attracts electrons from water. The -oxygen atom combines with another oxygen atom to form 02. it is by product of the photosynthesis.
3. Electron transport chain (ETC): Each photoexcited electron passes form the pheophytin (primary electron acceptor) of the photosystem Ito photosystem I through an electron transport chain. This electron transport chain has following electron carriers:
• Plastoquinone (PQ): Electrons are accepted by PQ and it
reduces to form PQI-12. This reduction requires two electron and two protons 0-1). These proions are absorbed from stroma side of the membrane. The PQH, donates electrons to cytochromes and again become PQ. The two protons remain in the thylakoids. They reduce the pH there. Low pH helps in the photophosphorylation.
• A complex of .two Cytochromes: It is composed of two cytochromes b, (Fe-S protein) and cytochrome f.
• Plastocyanin (PC): It is a copper containing protein It finally donates electron to PSI.
Photophosphorylation: The synthesis of ATP due to light energy is called photophosphorylation. As the electrons move down the chain, their energy goes on decreasing. This energy is used by the

thylakoid membranes to synthesize ATP. The thylakoid have ATP synthase complex. ATP is synthesized during chemiosmosis. This ATP will be used during synthesis of sugar in the Calvin cycle (dark reaction .

  1. Photosy.tem I: The No chlorophyll of Photosystem I absorb light energy. It ;yes its electrons to the Fe- S protein (primary acceptor of the phoo.. •9m I). It creates a hole in the molecule of P700

The electrons of ,Thotosystem II reaches the bottom of the electron transport chain t id till the electron hole in Chlorophyll P7m, molecule of photosystem

  1. Synthesis of N4DPH2: Fe — S protein transfer the electrons to ferredoxin (Fd). The Fd is an iron containing protein. An enzyme NADP reductase transfers the electrons from Fd to NADP. This is the redo,: 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.

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Cyclic phosphorylation (Cyclic electron flow)

The returning back of the same excited electrons to the excited chlorophyll by producing a molecule of ATP it is called cyclic phosphorylation. It is a ,ess common tnpe of electron flow. In this case. only photosvstent involved. The photoescited electrons take an alternative path. It does not use photosystem


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This cycle may • t;,e place when there is less amount of ATP for Calvin Cycle. It slow ,down the cycle.. Therefore, the NADPII 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 demand of ATP fulfilled. Therefore, the cyclic flow is a short circuit. Following steps take place during cyclic phosphorylation: I. P700 of the photosystem I absorbs light. This light energy drive

electrons form P700 of the photosystem Ito primary electron acceptor.

It produces electron hole in the chlorophyll.

  1. The primary electron acceptor of photosystem I transfers the photoexcited electrons to Fe —S protein.
  2. Fe — S protein transfer the electron to ferredoxin (Fd).
  3. The electrons are transferred from ferredoxin (Fd) to Cytochromes complex (ETC).
  4. Finally, the Cytochromds complex returns these electrons to exited chlorophylls of the P700. A molecule of ATP is produced during this transfer of electrons through ETC by chem iosmosis. The NADPH is not produced and oxygen is also not released.

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CHEMIOSMOSIS

Mechanism of photophosphorylation

The coupling reaction in which synthesis of ATP molecule takes

during movement of H+ across an 111+ gradient is called chemiosmosis. Peter Mitchell proposed chemiosmosis hypothesis in 1961. 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.

I. The membranes are impermeable to passive flow of protons.

  1. The electron donors and acceptors are arranged in the membrane. They provide passage for the movement of protons.
  2. The electron transport chain (ETC) pumps the protons (H’) across

the thylakoids. The movement of electron provides energy for the pumping of electron through the ETC. This energy is transferred into potential energy. The potential energy is stored in the form of proton motive force.

  1. The hydrogen ions move down form the gradient through ATP synthase complex. The ATP synthase complexes are present with in the thylakoid membranes. The ATE’ synthase enzyme ‘uses the proton motive force for the synthesis of ATP
  2. .05-28b_Non-CyclicPhoto_L

 

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