Brief notes on the Mechanism Light Reaction of Photosynthesis

Photosynthesis is the synthesis of carbohydrates by green organs of a plant in the presence of sunlight from CO2 and h2O taken up from the air and soil respectively with release of oxygen.

In modern sense photosynthesis is an oxidation reduction process in which water is oxidises to release O2 and CO2 is reduced to from carbohydrates (sugars).

Mechanism of photosynthesis involves two distinct phases of reactions, viz. Primary photochemical reaction or light reaction and dark reaction or blackman’s reaction or Calvin cycle.

Light reaction or primary photochemical reaction or hill’s reaction:

The phase of reactions in photosynthesis which involves the direct sunlight is called light reaction. This reaction takes place inside the grana of chloroplast. In this reaction assimilatory power ATP and NADPH2 are generated with evolution of oxygen and photolysis of water. These assimilatory powers help in the fixation of carbon dioxide during dark phase of reaction.

Light reaction involves following steps:

(1) Absorption of light energy by chlorophyll pigment:

a. Chief source of light energy for this reaction is the sun.

b. All the incident light energy falling on green parts of plant is not absorbed only 1% a fraction of it is absorbed by pigments.

c. Photosynthesis pigments absorb light energy only in the visible part of spectrum i.e. within 390 to 760 am.

d. Chlorophyll pigment systems are the trapping centers of light. There are several form of chlorphyll a, and light. There are several form of chlorolyll and chlorophyll – b constitute the photosystem along with other accessory pigment.

e. Emerson and Arnold (1932) showed that 2500 chlorphyll molecules required to fix one molecule of CO2 in photosynthesis.

f. Steinman (1952) observed that granular structure in chloroplast lamella may be the morphological expression at the physiological photosynthetic units called them as quantosomes.

(2) Transfer of light energy from accessory pigment to chlorophyll:

Light energy absorbed by the pigment other than chlorophyll a transferred to chlorophyll -a by resonance of photons. In the chlorophyll-a primary photochemical reaction takes place.

There are two pigment systems in the chlorophyll with p-690 and p-700.

(3) Activation of chlorophyll-a molecule and photo excitation of the pigment:

When pigment molecule in a photo system receives a photon of light, become excited and expels the extra energy level called excited second singlet state; then it comes to Meta stable state is called triplet state. From which it receive electron from outer source and return back to ground state.

(4) Photolysis of water:

The electron being released by photo excitation makes the pigment unstable, for which supply of electron in bet by photolysis of water.

When pigment system II is active it receive light, the water molecule split into OH and H+ ions in presence of Mn++ and CL- ions. The OH ions unite form some water molecules again and O2 and electron are released.

(5) Electron transport and the production of assimilatory:

Excited electrons from the pigment system travels through a number of electron carriers reducing the NAND to NADPH2. The extra energy which carried along with electrons utilized in phoshorylation of ADP to form ATP. This above process is called electron transport or photophosphorylation.

There are two distinct paths of electron transport involves in light reaction.

i. Noncyclic Photosphosphorylation:

According to Arnon, the electron which ejected from pigment system -1 after photo excitation causes an electron hole in the p700 molecule. This ejected electron is trapped by FRS (Ferredoxin reducing substance which is an unknown oxidation reduction system with a redox notential E of 0.6 volts. The electron then transferred to a non heme iron protein called ferrodoxin (FD) with E, 0 of 0.432 volt. From gerredoxin electron transferred to NADP, so that NAND is reduced to NANDPH.

When a photon of light absorbed by P690 form of chlorophyll a molecule in pigment system II, it gets excited and release electron. This ejected electron trapped by plastoquinone from where the electron follow down hill direction towards the pigment system 1, through a series of electron carriers or cytochrome systems and plastocyanin (PQ). The election while pass from Cyt-b to Cyt-f phosphorylation of ADP takes place as shown in above diagram.

Cytochromeb has electro potential to 40.055 v and cytochrome -f has +0.36 volt.

ii. Cyclic photophosporylation:

Besides non-cycles electron transport there may be transport of electrons takes place in light reaction in cyclic way. It involves only pigment system 1.

This is rarely happen when the pigment system-II is totally impaired or blocked by inhibitors like i-dimethy urea, or 3-4 dichloro phynyl etc.

There is no need of photolysis of water.

When p700 molecule is excited in pigment system I by absorbing a photon of light the ejected electron is captured by ferredoxin.

The electron then instead of reducing NADP fals back to p700 molecule involving electron pass from F.R.S to P.C. there is phosphpry lation takes place in previous manner; so that ATP molecule is generated.

Thus light reaction is fully dependant on the radiant energy in terms of photon by which radian-energy from sun transformed into assimilatory powers NADPH2 and ATP which are utilised in the dark phase of reaction in order to fix CO2 forming sugar.