PHOTOCHEMICAL AND BIOSYNTHETIC PHASES OF PHOTOSYNTHESIS

 PHOTOCHEMICAL AND BIOSYNTHETIC PHASES OF PHOTOSYNTHESIS:

Introduction:

The entire process of photosynthesis takes place inside the chloroplast. The structure of chloroplast is such that the light dependent (light reaction) and light independent (dark reaction) takes place at different sites in the same organelle. The thylakoids have the pigment and other necessary components to absorb light and transfer electrons to carry out the light reaction or Electron transport Chain (ETC). In ETC upon absorption of light, the electrons from PSII and PSI are excited to a higher energy level i.e. the electrons acquire excitation energy. As the electrons gain the energy, they are accepted by the electron acceptor which in turn reduced, leaving the reaction centre of PSII and PSI i.e. P680 and P700 molecule in an oxidized state. This represents the conversion of light energy into chemical energy. The electrons then travel downhill in energy in energy terms from one electron acceptor.

Photosynthesis comprises of two phases; first phase is the photochemical phase or light dependent process (light reaction) and second phase is the biosynthetic phase or dark reaction of photosynthesis. Since two phases, photochemical phase and Biosynthetic phase, photochemical phase or light reaction; this phase includes light absorption, splitting of water, oxygen release and formation of ATP and NADH. Biosynthetic phases or Dark reaction; it is light independent phase, synthesis of food material or sugar. Dark reaction occurs in the stroma of the chloroplast where they utilize the products of the light reaction and reduction of CO2 takes place result in the formation of glucose. Assimilatory powers (ATP and NADPH) produced during photochemical phase is used in the formation of CO2.

The whole process of photosynthesis takes place within chloroplast. Light reaction is a light dependent process which includes a series of events such as light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH. The light reaction of photosynthesis initiates only when it is supplied with light energy.  There are two Photosystem in plants; PSI and PSII, PSI absorbs light at a wavelength by 700nm whereas PSII absorbs light at a wave length of 680nm. 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 involves the conversion of light energy into chemical form. This phenomenon is known as Quantum conversion. When the light hits, chlorophyll a gets excited to higher energy state followed by a series of reaction, this energy is converted into energy molecule ATP and NADPH by using PSI and PSII.

 

PHOTOCHEMICAL PHASE:

It occurs inside the thylakoids especially in the Grana region. Light phases consist of photochemical reaction which is carried out by two different photosystems, PSI and PSII. The function of the phase is to produce assimilatory power consisting of reduced coenzyme NADPH and energy rich ATP molecule.  In the thylakoids, chlorophyll and other molecule are packed into units called photosystem. Each Photosystems unit contains from 250-400 molecule of pigment which serve as light trapping antennae. Some light energy is absorbed by one of the antennae pigments; it is bounced around among other pigment molecule of the PS until it reaches a special form of chlorophylla which is the reaction centre.

In PSI, the reactive chlorophyll a molecule is known as P700 because one of the peaks of its absorption spectrum is at 700nm. It is located in the stroma regions of the thylakoid. The reactive chlorophylla molecule of PSII is P680. They are located in the appressed regions of the grana in the thylakoid. It is believed that these chlorophyll molecules have unusual properties because of their association with special proteins in the membrane.

The main function of these two photosystems is to trap light energy and convert it into chemical energy (ATP) which is used by living cells.

The photochemical phase was explained by Arnon and his co- workers in 1958 whenever a chlorophyll molecule absorbs a photon of light it is said to move from the ground state to the excited state. The added energy lifts the electron from the chlorophyll molecule and is ultimately used to reduce NADPH2.

Chlorophyll a +hѵ -------------- chlorophylla*+ electron

(Ground state (photon          (Excited state   (excited

  Of chlorophyll) of light)               of chlorophyll) electron)

                                                  

The excited state is unstable and the chlorophyll molecule regains its lost electron from the water through an ETS and gets back to the ground state. P680 chlorophyll molecule having lost its electron is avidly seeking replacement. It finds it in the water molecule which dissociate into protons and O2 gas. The P700 chlorophyll molecule is oxidized and an electron is boosted to a primary electron acceptor from which it goes downhill to NADP. The electron removed from P700 molecule is replaced by its electron from PSII.

Photochemical phases involve the following reaction:

a)      Photolysis of water: The phenomenon of breaking up of water into H2 and O2 in the illuminated chloroplasts is called Photolysis or photo-catalytic splitting of water. Light energy, oxygen evolving complex (OEC) and electron carrier hydrogen is required. Oxygen evolving complex was formerly called Z- scheme. It is attached to the inner surface of the thylakoid membrane. The enzyme has four Mn+ ions. Light energized changes in Mn (Mn⁺2, Mn⁺3, Mn⁺⁴) removes electron from OH- component of hydrogen from oxygen. Liberation of oxygen also requires two other ions, Ca+2 and Cl- electron carrier Y2 transfers the released electron to P680. (It is a component of a protein connected with P680 centre.

4OH ------------------→ 2H2O +O2↑+ 4electron

b)      Production of Assimilatory Power (NADPH and ATP): The electrons released during photolysis of water are picked up by P680 photocentre of PSII. On receiving a photon of light energy the photocentre expels an electron with a gain of energy (23Kcal/mole). It is the primary reaction of photosynthesis which involves the conversion of light energy into chemical form. The phenomenon is also known as Quantum conversion. The electron extruded by the photocentre of PSII re-picked up by the quencher phaeophytin. From here the electron passes over a series of carriers in a downhill journey losing its energy at every step. The major carriers are Plastoquinone (PQ), Cytochrome b-f complex and Plastocyanin (PC) while passing over cytochrome complex, the electron loses sufficient energy for the creation of proton gradient and synthesis of ATP from ADP and Pi. The process is called Photophosphorylation. From Plastocyanin (PC) the electron is picked up by the trap centre P700 of PSI. On absorbing a photon of light energy P790 pushes out the electron with a gain of energy. The electrons passes over carriers X (a special chlorophyll molecule), FeS, ferrodoxin and NADP reductase. The latter gives electron to NADPH for combining with H+ ions to produce NDPH.

NADP⁺ + 2 electron +H⁺ -----------→ NADPH

                                       NADP reductase

  NADPH is a strong reducing agent. It constitutes the reducing power which also contains a large   amount of chemical energy.                               

BIOSYNTHETIC PHASE:

The phase in which synthesis of organic substances takes place is called Biosynthetic Phase. It is also known as Dark phase or Blackman’s reaction. It catalyzes assimilation of CO2 to carbohydrates. The reaction is called carbon reaction. The reaction does not require light instead assimilatory power (ATP, NADPH) produced during photochemical phase which is then used in the fixation and reduction of CO2. All the enzymes required for the process present in the matrix or stroma of the chloroplast.

·         Products of light reaction: ATP, NADPH, CO2 and water are the products of photosynthesis. The next phase after light reaction is the biosynthetic phase. In this phase ATP and NADPH are used for synthesizing the food.

·         CO2 is fixed to form CO2 fixation products. This stage is also called the dark phase as it is independent of light but deals with the products of light reaction. The reaction involved in the biosynthetic phase takes place in the stroma of chloroplast.

There are two main pathways for the biosynthesis or dark phases; Calvin cycle and C4 dicarboxylic acid cycle. A third pathway is CAM metabolism which is an intermediate between the two pathways. The plants exhibiting Calvin cycle and C4 dicarboxylic acid cycle are respectively called C3 and C4 plants. 

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