PHOTOSYNTHESIS

Presented by : - Patel Vanita L.

M.sc sem : - 3

Paper no : -( Botany ) 501

Department of Life sciences,

H. N. G. U., Patan

CONTENTS

What is Photosynthesis ?

Historical background

Photosynthetic Pigment system

Light harvesting complex ( Hill reaction)

Photo oxidation of water

Photophosphorylation and mechanism of electron transport

WHAT IS PHOTOSYNTHESIS

?

• Photosynthesis is one of the important biological functions. In this process green plant or green part of the plants trap the energy of sunlight transfer in into chemical energy.

HISTORICAL BACKGROUND

PHOTOSYNTETIC PIGMENTS

• The discovery of red drop and the Emerson's enhancement effects has led scientists to suggest that photosynthesis is driven by two photochemical processes.

• These processes are associated with two group of photosynthetic pigment system - 1 and pigment system - 2.

• Wave length of light shorter than 680nm affect both the pigment system while wavelength longer than 680nm affect only pigment system 1.

PIGMENT SYSTEM : — 1

• In green plant pigment system 1 contains chlorophyll a, b and

caroteins.

• The reaction centre of PS -1 consists of chlorophyll A 700.The core

of the PS -1 is made up PsaA and PsaB subunits. Core subunits of

the PS -1 are larger than the core subunits of PS-2.

• PS-1 is made up of chlorophyll A-670 ,Chlorophyll A-680,Chlorophyll

A-695, Chlorophyll A-700, Chlorophyll B and carotenoids.

• These electrons are transferred through series of electron and

finally taken up by NADP+ reductase.The enzyme NADP+ reductase

produces NADP form these electrons.

Pigment system :-2

• The pigment system 2 contains chlorophyll -b and some forms of chlorophyll -a (such as a chlorophyll 662, Chlorophyll a -670, chlorophyll a-677).

• a very small amount of special from of chlorophyll a called P680 the reaction center of pigments system 2.

• Carotenoids are present in both the pigment systems.

• In cash of the red and blue green algae the pigment systems contains phycobilins in place of chlorophyll -b.

Function of the cytochrome F:

• Cytochrome f play a role analogous to that of cytochrome C1, in spite of their different structures.

• Cytrochrome f has an internal network of water molecules that may function as a proton wire.

• Function of the plastoquione :• The role that plastoquione plays in photosynthesis

more specifically in the light dependent reactions of photosynthesis is that of a mobile electron carrier through the membrane of the thylakoid.

Function of the plastocyanine :

• In photosynthesis plastocyanin functions as an electron transfer agent between cytochrome F of the cytochorme b6 f complex from photosysystem 1.

• Function of the cytochrome b6 complex :• Electron transport via cytochrome b6 f is responsible

for creating the proton that drives the synthesis of ATP in chloroplast.

• In separate reaction the cytochrome b6 f complex plays a central role in not available to electrons from reduced ferrdoxin.

Different between pigment systems 1 and 2

LIGHT / DARK REACTION

1 ) Light reaction : —

• Light reaction required presence of light.

• Takes place into grana portion of the chloroplast.

• Light reaction is faster than dark reaction.

2 ) Dark reaction : —

• This reaction does not required presence of light.

• Takes place into stroma portion of the chloroplast.

• Dark reaction is slower than light reaction.

HILL REACTION

• Before 1930 it was though that the oxygen released during

photosynthesis comes from carbon dioxide.

• But for the first time Van Neil discover that the source of

oxygen evolution is not carbon dioxide but H2O.

• In this experiment Neil used green sulphur bacteria which do

not release oxygen during Photosynthesis.

• These bacteria require H2S in place of H2O.

• The idea of the Van Neil was supported by R. Hill.

• Hill observed that the chloroplasts extracted from leaves of

stalaria media and Lamium album when suspended in a test

tube containing suitable electron acceptors oxygen evolution

took place due to photochemical splitting of water.

• The splitting of water during Photosynthesis is called

photolysis of water.

• We now turn to another reaction sequence in which the flow of electrons is coupled to the synthesis of ATP light phosphorylation.

• The capture of solar energy by photosynthetic organisms and its conversion to chemical energy of reduced organic compound is the ultimate source of nearly all biological energy.

• Photosynthetic organisms solar energy and frome ATP and NADPH which they use as energy sources to make carbohydrates and other organic compounds from CO2 and H2O they release O2 into the atmosphere.

• Photosynthesis occurs in a variety of bacteria and in unicellur eukaryotes as well as in vascular plants.

• Mn, Ca and CI ions play prominent role in the photolysis of

water.

• This reaction is also known as Hill reaction. To release one

molecules of water are required.

4H2O Light 4H+ + 4OH -

4[OH ] - 4OH + 4e-

4OH 2H2O + O2↑

______________________________________

2H2O 4H+ + 4e- + O2 ↑

• When the Photosynthesis is allowed to proceed with H2O18 and

normal CO2, the evolved oxygen contains heavy isotope.

• If Photosynthesis is allowed to proceed in presence of CO218 and

normal water then heavy oxygen is not evolved.

6CO2. + 12 H2O Sunlight C6H12O6 + 6H2O + 6O2

Chlorophyll

Photo oxidation of water • A model for the oxidation of water in chloroplast is

suggested.

• This model is based on certain physico -chemical principles and it accomodates the experiment results associated with the O2 evolving system.

• The model consists of two pools of heterogeneously bound manganese ions are bound to an enzyme system containing a water -splitting site and four manganese ions are bound to another protein system which connects the reaction centre of photosystem 2 to the water -splitting enzyme.

PHOTOPHOSPHORYLATION

• Synthesis of ATP from ADP and inorganic phosphate ( Pi ) in

presence of light chloroplast is known as photophosphorylation.

• It was discovered by Arnon etal (1954) photophosphoryalation is

of two types : –

a) Cyclic Photophosphorylation

b) Non Cyclic photophosphorylation

a) Cyclic Photophosphorylation :—

• It is a process of photophosphorylation in which an electron

expelled by the excited photo center ( PSI) is returned to it

after passing through a series of electron carriers.

• It occurs under conditions of low light intensity wavelength

longer than 680nm and when CO2 fixation is inhibited.

• Absence of CO2 fixation results in non requirement of

electrons as NADPH2 is not being oxidized to NADP.

• Cyclic photophosphorylation is performed by photosystem 1 only.

• The expelled electron passes through a series of carriers

including X [ a special chlorophyll molecule ], Fes, ferredoxin,

plastoquinone, cytochrome b – f complex and plastoquinone

before returning to photo center.

• While passing between ferredoxin and plastoquinone and over

the cytochrome complex the electron loses sufficient energy to

from ATP from ADP and inorganic phosphate.

• As photosystem 2 is not used during cyclic photophosporylation, no oxygen is produced in the cyclic photophosphorylation.

• Generally cyclic phtophosphorylation occurs in photosynthesis bacteria such as green sulfur and non sulfur bacteria, purple bacteria, heliobacteria and acidobacteria.

• When the ATP supply drops and under high NADPH concentration chloroplasts also shift into cyclic phtophosphorylation.

b) Non cyclic photophosphorylation: —

• Non cyclic photophosphorylation is carried out in collaboration of

both photosystem 1 and 2.

• As electrons pass through the non cyclic pathway they do not

returns to the original photosystem. This does not create a cycle

hence the name non cyclic.

• Electron released during photoloysis of water is picked up by

reaction center of PS –2 called P680.

• Non cyclic photophosphorylation produces ATP using the energy

form excited electrons provided by photosystem 2.

• It passes through a series of electron carriers quinons, PQ,

cytochrome b -f complex and plastocyanin.

• The electron is handed over the reaction center P700 of PS - 1 by

plastocyanin.

• At P700 these electrons are taken up by NADP + producing NADPH.

• Generally non cyclic photophosphorylation occurs in plants algae and

cynobacteria.

• During non cyclic photophosphorylation both ATP and NADPH are

produced.

Similarities Between cyclic and non cyclic photophorylation

• Both cyclic and noncyclic photophosphorylation occur during the light reaction of photosynthesis.

• Cyclic and non cyclic photophosphorylation are two types ETS.

• Both cyclic and noncyclic photophosphorylation are light dependent.

• Both cyclic and non cyclic photophosphorylation generate ATP.

Different between cyclic and non cyclic photophosphorylation

REFERENCE

• Plant physiology by : — Taize & Zeigar

• Plant physiology by : — Verma P. S. and P. K. Agarwal

• Principle of biochemistry by : albert leningher

https://biologydictionary.net

https:// eagri.org

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