How the light reaction of Photosynthesis occurs

1. PRESENTED BY-
ALAKESH DAS
PG 3RD SEMESTER
ENROLLMENT- BOT1662008
DEPARTMENT OF BOTANY
COTTON UNIVERSITY
PHOTOSYNTHESIS: THE
LIGHT REACTION

2. INTRODUCTION
 Photosynthesis is a physiochemical process by which photosynthetic
organisms (either eukaryotes or prokaryotes) convert light energy into
chemical energy in the form of reducing power (as NADPH) and ATP
by using some inorganic compounds & utilize this energy to drive CO2
fixation.
SU
N
LIGHT
REACTION
NADP
H
+
ATP
DARK
REACTION
CO2
GLUCO
SE

3. TYPES
 Based on the generation of Oxygen (O2) in the process, photosynthesis
can be of 2 principle types-
(a) Oxygenic: generation of O2 occurs via the photolysis/photo-oxidation
of H2O, who act as the ultimate e- donor; e.g. -
Eukaryotes- Green plants
Prokaryotes- Cyanobacteria
CO2 + 2H2O CH2O + H2O + O2
(b) Anoxygenic: light energy is harvested without evolution of O2 , where
some other inorganic molecule (e.g.- H2S) act as a electron donor;
seen only in prokaryotes. E.g.- Purple Photosynthetic Bacteria and
Green Photosynthetic Bacteria.
CO2 + 2H2S CH2O + H2O + 2S

4. SITE OF PHOTOSYNTHESIS : Structure of
Photosynthetic Apparatus
 Most active photosynthetic tissue in higher plants is the “Mesophyll”
tissue having specialized organelle called “Chloroplast”, which contains
light absorbing pigments the “Chlorophylls” (for Bacteria-
Bacteriochlorophyll).
 Chloroplast has a extensive system of internal membranes known as
“Thylakoids” & is the site of ‘light reaction’.
Fig: A eukaryotic
Chloroplast showing
internal machinery of
membrane system.

5. PHOTOSYNTHETIC PIGMENTS: Light absorbing
molecules.
 Major Photosynthetic Pigments
(a) Chlorophyll (a, b)
(b) Bacteriochlorophyll
Fig: Structure of Chlorophylls & Bacteriochlorophyll.

6.  Accessory Photosynthetic Pigments
(a) Carotenoids : are long chain conjugated hydrocarbons, further
divided into two groups-
(1) Carotenes (pure hydrocarbons)
(2) Xanthophylls (contain oxygen)
(b) Phycobilins : possess a linear non-cyclic tetrapyrrole ring structure,
similar to bile pigment ‘bilirubin’, and are 3 principle types-
(1) Phycoerythrobilin
(2) Phycocyanobilin
(3) Allophycocyanobilin

7. ABSORPTION SPECTRA : Concept of “Action Spectra”
 Absorption Spectra : the light of wavelength at which absorption is
maximum by the pigment and is specific for every molecule.
 Action Spectra : the light of specific wavelength necessary for the
generation of oxygen (O2).
Fig : T.W Engelmann’s experiment with
Spirogyra filament showing action spec-
tra for the chloroplast which is blue-violet
& far-red region.

8. ORGANIZATION OF LIGHT ABSORBING PIGMENT
MOLECULES : Concept of “photosynthetic unit”
 Majority of pigments serve as an antenna (mostly the accessory
pigment), collecting and transferring the light energy to the adjacent
molecules & finally releasing into the reaction centre.
Photosynthetic unit
 Only one specialized Chlorophyll molecule known as “Photochemical
Reaction Centre” traps the energies from antenna and drives a series
of oxidation-reduction reactions via e- transport.

9. FATE OF THE LIGHT ENERGY ABSORBED BY
PHOTOSYNTHETIC PIGMENTS
a) Conversion of energy into Heat : through a process called
“Bioluminescence”; which may be either-
(1) Fluorescence : emits light of a longer wavelength.
(2) Phosphorescence : radioactive decaying of light.
Fig : Schematic presentation of
the process “Bioluminescence”.

10. b) Excitation Transfer : transfer of energy to the neighboring
molecules but not the e- directly, through a process called FRET
(Forster Resonance Energy Transfer) . Shown by pigment
molecules of ‘Antenna Complex’.
c) Electron Transfer : transfer of energy in the form of an e- to a
nearby molecule (e- acceptor) and in turn re-reduced by taking an e-
from an another molecule (e- donor). Shown by ‘Photochemical
reaction centre’.

11. CONCEPT OF PIGMENT SYSTEM : “Red Drop” & “Emerson
Enhancement effect”
 Robert Emerson & Charlton Lewis (1943); experiments with Chlorella
pyrenoidosa.
a) Experiment 1 : alga was illuminated with visible light & O2 evolution per
photon absorbed (quantum yield) fairly constant up to 680 nm, beyond which
it sharply declined in the far red region, known as “Red Drop” effect.
b) Experiment 2 : alga was illuminated with two separate beams of light, one ≤
680 nm (red region) & one > 680 nm (far red region); resulting rate of
photosynthesis was 3-4 times greater when lights were applied
simultaneously rather than separately referred to as “Emerson Enhancement

12. PIGMENT SYSTEM / PHOTOSYSTEM
 The photosynthetic unit, i.e. “antenna complex” & “photochemical reaction
centre” together with some proteins form a “pigment-protein” complex in
thylakoid membrane called “Pigment System/ Photosystem (PS)”.
o Photosynthetic organisms may carry two types of photosystem -
(a) Photosystem(PS)-I : consist of ‘Fe-S type’ of photochemical reaction
centre that mediates e- transfer from ‘Plastocyanin’ to ‘Ferredoxin’, also
known as “P700“ because of the action spectra of 700 nm; together with
the antenna complex forms ‘LHC-I(Light Harvesting Complex-I)’.
(b) Photosystem(PS)-II : consist of ‘Pheophytin-Quinone’ type reaction
centre constitutes ‘LHC-II’ together with antenna complex & activated by
the light of wavelength 680 nm, thus called “P680”.
o Various type of proteins are remain associated with these photosystems.

13. LOCATION OF PHOTOSYSTEMS IN “THYLAKOID
MEMBRANE”
 PS-II : located predominantly in the “Appressed Membrane” of grana
thylakoid whose surface is in contact with the other membranes of
thylakoid.
 PS-I : found almost exclusively in the “Non-Appressed Membrane”
(i.e. the exposed membranes which are not in contact with other
membranes) of stroma thylakoid & margins of grana thylakoid.

14. LIGHT REACTION : The transport of e- & protons (H+)
 Carried out by four major protein complexes – PS-II, Cytochromeb6f, PS-I and
ATP Synthase; resulting in O2 evolution in the luminal side and reduces NADP+
to NADPH on the stromal side of the membrane. Two major types in oxygenic
eukaryotes-
(a) Non-cyclic pathway : mediated from H2O through a series of protein to
NADP+, which leads to the generation of PMF( Proton Motive Force) results in
ATP synthesis called “Non-cyclic photophosphorylation”.
Fig: The “Z-scheme”
mechanism of non-cyclic
e- transfer.

15.  It all starts when PS-II transfers the e- by absorbing photon to the Pheophytin(a modified
Chl molecule). The process of Non-cyclic e- flow can be sub-divided into three steps-
(1) Photolysis of water (H2O) : mediated by PS-II and catalyzed by protein complex
OEC (Oxygenic Evolving Complex) which extracts e- from H2O and transfers to the PS-
II one at a time to re-reduce it. The reaction involves –
2H2O 4e- + 4H+ + O2
OEC cycles through five different states (S0 , S1 , S2 , S3 , S4) of a photon driven redox-
reaction which initiated a gear wheel that collects 4e- from 2 mol of H2O to release 1
mol of H2O. This is known as “S-State Mechanism” often called “Kok-Clock” (Kok et. al).

16. (2) The Q – Cycle : involve two major component – the “Plastoquinone” and the
“Cytochrome b6f complex”. The process include –
 Pheophytin transfers the e- to Plastoquinone & reduces it into Plastohydroquinone(by
pumping 2 protons from outside) through it’s two forms QA & QB and finally releases the e-
into the Cyt b6f complex.
 Cytochrome b6f complex contains several prosthetic groups – two Cyt-b , one Rieske Fe-
S protein and one Cyt-C(Cyt f), through which e- is passed following linear and cyclic
pathway as follows-

17. (3) Reduction of NADP+ : in this process PS-I is involved, which by absorbing photon
transfers one of it’s e- to ‘A0’ (a modified Chl molecule) and re-reduced by taking e- from
Plastocyanin (blue colored Co containing protein). Additional steps includes-
 ‘A0’ reduces ‘A1’ (a member of Quinone known as Phylloquinone/Vitamin K1 ).
 The e- is then transferred through a series of Fe-S protein – FeSx , FeSA ,
FeSB to a water soluble protein ‘Ferredoxin’ (Fd).
 At the last stage Ferredoxin reduces NADP+ to NADPH and the reaction is
catalyzed by a flavoprotein ‘Ferredoxin-NADP Reductase’ (FNR).

18. (b) Cyclic pathway : only the PS-I is involved in this process without any evolution of
oxygen(i.e. no photolysis of water) and reduction of NADP+ ; instead protons are
pumped across the membrane to generate PMF for the synthesis of ATP called ‘Cyclic
Photophosphorylation’.
Fig- Cyclic electron transport chain

19. SYNTHESIS OF ATP : The Chemiosmotic Mechanism
 When there is a difference in ion concentration across membranes, ions tends to move
from higher concentration to lower concentration, creating a force PMF which is the
energy available for the synthesis of ATP (Peter Mitchell, 1979).
 Catalyzed by a large enzyme complex ‘ATP Synthetase’ often known by several names
which can be divided into two parts – CF0 (hydrophobic) and CF1 (hydrophilic).
Fig- the coupling factor with it’s two
Parts CF0 and CF1 showing the synthesis
of ATP.

20. LIGHT REACTION IN OXYGENIC PROKARYOTES
 Cyanobacteria are the only group of organisms that can carry out oxygenic
photosynthesis through non-cyclic and cyclic e- transport chain. Instead of
chloroplast they bear a striking resemblance to chloroplast themselves.
 Almost similar to the eukaryotes, they carry PS-I , PS-II and Cyt b6f complex
and also extract e- from water.
 Major differences includes-
 They contain Phycobilins as their accessory pigment molecules.
 Plastocyanin is replaced by an another protein complex called Cytochrome C6
 Except these differences overall electron transport chain is similar to that of
eukaryotes mentioned above.

21. LIGHT REACTION IN ANOXYGENIC PROKARYOTES
 Anoxygenic photosynthetic bacteria, which do no generate oxygen in the process; i.e. e-
donor is some inorganic compounds like H2S, S etc. and carry only one type of reaction
centre. Members include –
(a) Purple Photosynthetic Bacteria- ‘Pheophytin-Quinone type reaction centre’
(b) Green Photosynthetic Bacteria- ‘Phe-Q’ (if non-sulphur) & ‘Fe-S’ (if sulphur) reaction
centre.
Fig- Cyclic electron transport chain in
Purple Photosynthetic bacteria.

22. CONCLUSION
At last it can be concluded that “Light Reaction” of
photosynthesis is the powerhouse for the production of energy
necessary for the fixation of CO2.
Therefore, it simply means that without light reaction there will
be no food available for the consumers, which itself defines the
importance of light reaction in nature.

23. THANK YOU