Essay on the Light Raction 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 science photosynthesis is an oxidation reduction process in which water is oxidised to release O2 and CO2 is reduced to form carbohydrates (sugar).

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

(a) Light reaction or primary photochemical reaction or hill’s reaction:

The phase of reactions in photosynthesis which invloves 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 xxygen and photolysis of water. These assimilatory powers help in the fixation of carbon dioxide during dark phaseofreaction.

Light reaction involves following steps.

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

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

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

iv. Chlorphyll pigment systems are the trapping centers of light. There are several forms of chlorophyll a and chlorophyll -b constitutes the photo system along with other accessory pigment.

v. Emerson and Arnold (1932) showed that 2500 chlorophyll molecules required to fix one molecule of co2 is photosynthesis.

vi. Steinman (1952) observed that granular structures in chlorplast lamella may be the morphological expression at the physiological photosynthetic units called them as quantosomes.

(b) Transfer of light energy from accessory pigment to chlorophyll.

Light energy absorbed by the pigment other that pholotophyll a transferred to chlorophyll a by reasons of photons. I n the chlorophyll a primary photochemical reaction takes place.

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

(c) 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 with release of electrons. The normal state of molecules is called as ground state or singlet states, when light is absorbed it raised to higher energy level called excited second singlet state, then it comes to meta level called excited second sinlet 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.

(d) Photolysis of Water:

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

When pigment system II is active it receives light the water molecule split into OH and H* ions in presence of Mn** and Ci ions. The OH ions unite to form some water molecules again and O2 and electrons are released.

(e) Electron transport and the production of assimilatory power.

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

There are two distinct paths of electron transport invlove inlight reaction.

(i) Noncylic photophosohorylation :

According to Arnon, the electron which ejected from pigment system 1 after photo excitation causes an electron hole in the molecule. This ejected electron is trapped by FRS which is an unknown oxidation reduction system with a Redon potential. The electron then transferred to a not heme iron protein called feredoxin with. From ferredoxin electron transferred to NADP to that NADP is reduced to NADPH.

When a photon of light absorbed by 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 I through a series of electron carries or cytochrome systems and plastocyanin. The electron which passes from phosphoorylation of ADP takes place as shown in above diagram.

Cytochromeb has electropotential to 40 005 V and cytochrome -f has +0.36 volt.

Cyclic photophophorylation:

Besides non-cyclic electron transport there may be transport of electrons takes place in light reaction in cylic way. It invloves only pigment system -I.

There 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 in no need of photolysis of water.

When p700 molecule is excited in pigmentsystem 1 to absorbing a photon oflight the ejected eldctron is captured by ferredoxin.

The electron then instead of reducing NANP falls back to p700 molecule involving elctorn carries like there is phosphory lation takes place in previous manneir so that ATP molecule is generated.

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