Photosynthesis

1. PHOTOSYNTHESIS
Presented by : - Patel Vanita L.
M.sc sem : - 3
Paper no : -( Botany ) 501
Department of Life sciences,
H. N. G. U., Patan

2. CONTENTS
 What is Photosynthesis ?
 Historical background
 Photosynthetic Pigment system
 Light harvesting complex ( Hill reaction)
 Photo oxidation of water
 Photophosphorylation and mechanism of electron transport

3. 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.

5. HISTORICAL BACKGROUND

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. Different between pigment systems
1 and 2

14. 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.

15. 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.

16. • 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.

17. • 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.

19. • 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 ↑

20. • 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

21. 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.

22. 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

23. 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.

24. • 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.

25. • 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.

27. 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.

28. • 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.

30. 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.

31. Different between cyclic and non cyclic
photophosphorylation

32. 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

33. THANK YOU