Mechanism of Photosynthesis

Deail Mechanism of Photosynthesis

Photosynthesis occurs in two phases - Photochemical and Biochemical, Photochemical phase is also called light Reaction or Hill reaction.  Biochemical phase is also termed as Dark reaction or Blackmann’s reaction.

Light Reaction 

This step occurs in presence of light. During this step ATP and NADPH₂ are produced.  ATP and NADPH₂ are together known as Assimilatory Power. This phase involves photolysis of water and production of assimilatory power.

(a) Photolysis of H₂O : It is breaking up of H₂O into hydrogen and oxygen in the illuminated chloroplasts.  A small complex of protein is attached to phytosystem II.  It is called oxygen evolving complex (OEC). OEC centre oxidises H2O into oxygen, H⁺ and electrons. Cl^–, Ca2+ and Mn²⁺ is required during this process.

(b) Production of Assimilatory power : The electrons released during photolysis of H2O are picked up by reaction centre (P680) of Ps II.  On receiving a photon of light energy P680 expels an electron.  It is primary reaction of photosynthesis which involves conversion of light energy into chemical energy (Quantum conversion). The electron extruded by P680 is picked up by pheophytin, from where electron passes over a series of carriers. While passing over cytochrome complex, the electron loses sufficient energy for creation of proton gradient and synthesis of ATP (Photophosphorylation). From plastocyanin the electron is picked up by trap centre (P700) of PS I. Electron is expelled by P700.  Which is picked up by FeS, ferredoxin and NADP - reductase. NADP reductase gives electrons to NADP+ for combining with H+ ions to produce NADPH.

Chemiosmotic Hypothesis of ATP formation

It was proposed by Mitchell (1961). Proton gradient develops in the outer chamber or intermembrane space of mitochondria and inside the thylakoid lumen in chloroplast during electron transport, both in respiration and photsynthesis. Three main events occur :

•  Due to photolytic splitting of water lumen of thylakoid becomes enriched with H+ ion.
• Primary acceptor of electron is located ont he outer side of thylakoid membrane. It transfers its electrons to an H-carrier. The carrier removes a proton from matrix while transporting electron to the innerside of the membrane. The proton is released into the lumen while the electron passes to the next carrier.
• NADP reductase is situated on the outerside of thylakoid membrane. It obtaines electron from PSI and protons from matrix to reduce NADP+ to NADP+ H+ state.

The consequences of the three events is that concentration of protons decreases in matrix or stroma region while their concentration in thylakoid lumen rises resulting in decrease in pH. A proton gradient develops across the thylakoid. The proton gradient is broken down due to movement of protons through transmembrane channels, CF0 of  ATPase (CF₀ – CF₁ particle). The rest of the membrane is impermeable to H⁺. CF₀ provides faciliated diffusion to H⁺ or protons. As protons move to the other side of ATP. They bring about conformational changes in CF1 particle of ATPase or coupling factor. The transient CF₁ particle of ATPase enzyme from ATP from ADP and inorganic phosphate

• Therefore, ATP synthesis through chemiosmosis requires a membrane, a proton pump, a proton gradient and CF0–CF1 particle or ATPase proton pump is energised by electron flow. It creates a proton ggradient or high concentration of H⁺ in the lumen. Protons diffuse across CF0 channels release energy that activates ATPase enzyme to catalyse ATP. One molecule of ATP is formed. When 2H+ pass through ATPase.

Dark reaction 

It does not require light.  During this reaction or biosynthetic pathway assimilatory power is used in fixation and reduction of CO₂. The enzymes required for this reaction are present in matrix of chloroplast.  There are two main pathways C₃ or Calvin cycle and C₄ or dicarboxylic acid cycle.