Photosynthesis Light Reaction

In previous blog we have studied about photosynthetic pigments now in this blog we will study the arrangement of photosynthetic pigments and light dependent reaction of photosynthesis.

ARRANGEMENT OF PHOTOSYNTHETIC PIGMENTS:

For efficient absorption and utilization of light energy,these photosynthtic pigments(chlorophyll a and b,carotenoids) are embedded on the surface of thylakoid membranes where they arranged in the form of clusters.

These cluster of pigments molecules are called PHOTOSYSTEMS.

Photosystems mainly consist of two parts peripheral and central part.

The peripheral part of photosystem is called ANTENNA COMPLEX which contain acessory pigments like carotenoids and chlorphyll b .

The central part of photosystem is called reaction center which contain chlorophyll a + proteins.

Types of Photosystem:

There are two types of photosystem PS 1 (700nm) and PS2 (680nm) and they are categorized into different types because they absorb different wavelength of sunlight.

PS 1 photosystem absorb sunlight of wavelength 700 nm and it contain chlorophyll a molecule.

PS 2 photosystem absorb sunlight of wavelength 680 nm and it contain accessory pigments carotenoids and chlorophyll b which absorb high energy sunlight rays then PS 1.

Now we will discuss 2nd part of this blog Light dependent reaction of photosynthesis.

LIGHT DEPENDENT REACTION:

The reaction in which ATP,NADPH2 and oxygen produce as by product by using sunlight and photosynthetic pigments.

Light reaction in granum of chloroplast.

This reaction occurs only in the presence of light.

In light dependent reaction involves the absorption of light by photosystems,excitation and flow of excited electrons through electron transport chain and ATP ,NADPH2 and Oxygen production.

Excited electrons follows two type of electron transport chain:

1-NON CYCLIC ELECTRON TRANSPORT CHAIN:

In this reaction electron follows non cyclic path and pass throught different molecules and result in production of ATP,NADPH2 and Oxygen as by product.

Now we will discuss the non cyclic transport insteps:

1-ABSORPTION OF SUNLIGHT:

The Non cyclic transport chain starts when 2 photons strike the antenna complex of PS 2 (680nm),these two photons excited 2 electrons from chlorophyll b central magnesium atom which is instantly captured by primary electron acceptor of PS 2 called as PLASTOQUINONE and leaves 2 Electron holes in PS 2 and make PS 2 strong oxidizing agent.

These electrons are then transfer to PS 1 through Cytocrome complex (cyt b ,cyt b ) and plastocyanin.

ATP PRODUCTION:

When these excited electrons are transfer from cytochrome b to cytochrome f it change the ADP +Pi(inorganic phosphate) into ATP .

This process is called PHOTOPHOSPHORYLATION.

ADP + Pi = ATP

2-EXCITATION OF ELECTRONS FROM PS1:

When PS2 photosynthetic pigments capture sunlight they transfer this light to PS 1 which also result in excitation of 2 es from PS 1 and make PS 1 2 electron defecient and this defeciency of electrons filled by PS 2 excited electrons.

The excited electrons of PS 1 capture by primary electron transfer of PS 1 called as FERREDOXIN REDUCING SUSBTANCE(FRS) and then from FRS these excited electrons are given to oxidized NADP and result in REDUCED NADP.

NADP(OXIDIZED) + 2 es = NADP (REDUCED)

3-PHOTOLYSIS OF WATER:

When sunlight falls on photosystems at the same time this light falls on water present in chloroplast which result in breakdown of water molecules into H+ and OH- ions along with 2 es .

This two electrons are captured by PS2 and fulfill the defeciency of electrons in PS 2.

PS 2 (Electron defecient) + 2 es from water breakdown = PS 2

The hydrogen ions of water accepts by reduced NADP and result in production of NADPH2.

NADP + 2H- = NADPH2

Only oxygen remains which come out of chloroplast as a by product because it has ability to PS 2 so carotenoids protect PS 2 from oxidation and oxygen are come out of chloroplast as by product.

PRODUCTS OF NON CYCLIC TRANSPORT CHAIN:

1- ATP through photophosphorylation

2-NADPH2 (gain of es of PS 1 and 2 H - from water)

3-Oxygen

Now this ATP,NADPH2 will use in dark cycle of photosynthesis.

1 non cyclic transport chain produce 1 ATP.

2-CYCLIC ELECTRON TRANSPORT CHAIN:

In cyclic transport chain only PS 1 Involves and it occurs in rare conditions when activity of PS 2 is blocked.

This reaction starts when PS 1 (700nm) absorb sunlight and result in excitation of 2 es.

These excited electron captured by Ferredoxin reducing substance .From FRS these electrons falls back to PS 1 chlorophyll through series of electron carriers( Plastoquinone,cytb6,cytf and Plastocyanin).

Here ATP is produce when electrons transfer from cyt b to cyt f which is called photophosphorylation.

Products of cyclic photophosphorylation:

1 ATP ( PHOTOPHOSPHORYLATION).