Photosynthesis has two photosystems, Photosystem I and Photosystem II, that work sequentially to harness light energy to produce chemical energy. Photosystem II uses light energy to split water, releasing electrons that are transferred through an electron transport chain, pumping protons across the membrane and producing oxygen. The energized electrons are then passed to Photosystem I, which uses them to reduce NADP+ to NADPH to be used in the Calvin cycle for carbon fixation. Together, the two photosystems convert light energy to chemical energy in the form of ATP and NADPH.

1. PS II and PS I complex

2. Most important physio-biochemical process of the world on which existence of life
on earth depends
It’s the ability of green plants to utilize the energy of light to produce carbon
containing organic matter from stable inorganic matter by photosynthetic process
The oxidation of organic compound release store energy which is utilized by
organism to drive essential metabolic process
PHOTOSYNTHESIS

3. In simple terms photosynthesis can be defined as the formation of carbon containing
compounds from carbon dioxide and water by illuminated green cells, water and oxygen
being the by-products
Plants use sunlight, carbon dioxide, and water to produce carbohydrate with oxygen as a
byproduct.
The overall chemical reaction summarizes the process as:
6 CO2 + 12 H2O + light energy  C6H12O6 + 6 H2O + 6 O2

4. Light
energy
Light-dependent
reactions
H2O O2
Chemical
energy
Calvin cycle
ATP, NADPH CO2
Chemical
energy
Sunlight
Thylakoid Reactions Stroma Reactions
Light reactions Dark reactions
(CH2O)n
Mechanism of photosynthesis can be divided into two phases
Light reaction phase of photosynthesis is a considerably complicated process & can
be briefly discussed with the help of following subheadings
1) Red drop, emersion effect & two pigment systems
2) Production of assimilatory powers
3) Energy relationships & efficiency of photosynthesis
4) Interrelationships between light and dark reactions

5. RED DROP AND EMERSON EFFECT
Photosynthesis is considered as two quanta process, i.e. it takes two light quanta energy to
drive an electron
•Number of oxygen molecules released can be used to determine the quantum yield of the
process.
•Quantum yield is defined as the no. of O2 molecules released per light quanta absorbed
•Emerson & Lewis worked on quantum yield of photosynthesis in monochromatic light of
different wavelength. They observed the quantum yield declined sharply at wavelength
greater than 680nm in the red zone . This decline is called red drop
•Later Emerson found that the sharp decline in the quantum yield of photosynthesis beyond
680nm can be brought to full efficiency by simultaneously providing short wavelength of
light. This photosynthetic enhancement is called Emerson effect
CO2 +4 H+ CH2 O + H2 O
4H2 O 4 ( H+ + e+ ) + 2H2 O + O2

6. TWO PIGMENT SYSTEMS
•Discovery of the red drop & Emerson effect concluded that at least two pigment
systems are involved in photosynthesis
•These two pigment system has been referred as pigment system I & pigment
system II
•The presence of two such systems has been supported by studies based on
chloroplast fractionation process which showed two type of particles within the
chloroplast membrane, Smaller & lighter particles of PS I & larger & heavier particle
of PS II
•Each photosystem is a network of chlorophyll a molecules, accessory pigments,
and associated proteins held within a protein matrix on the surface of the
photosynthetic membrane

7. There are two processes in photosynthesis that capture light and produce energy
rich compounds that are used in carbon fixation. These are termed
Photosystem I, and
Photosystem II.
Photosystem:
Reaction center surrounded by several light-harvesting complexes
Light-harvesting complex:
light-harvesting complexes consist of pigment molecules bound to particular
protein. They funnel the energy from photons of light to the reaction center
Photosystems
Reaction center :
Protein complex that includes two special chlorophyll a molecules & a primary e-
acceptor molecule
When a reaction-center chlorophyll a molecule absorbs energy, one of its
electrons gets jumped up to a primary electron acceptor

8. Light reactions occur in
the thylakoids (PSII) and
stroma lamella (PSI).
Dark reactions in
occur in the stroma

9. Architecture of a Photosystem
Each photosystem is a network of chlorophyll a molecules, accessory pigments,
and associated proteins held within a protein matrix on the surface of the
photosynthetic membrane.
A photosystem channels the excitation energy gathered by any one of its pigment
molecules to a specific molecule, the reaction center chlorophyll.
This molecule then passes the energy out of the photosystem so it can be put to
work driving the synthesis of ATP and organic molecules.

10. A photosystem thus consists of two closely linked components:
(1) an antenna complex of hundreds of pigment molecules that gather photons and
feed the captured light energy to the reaction center; and
(2) a reaction center, consisting of one or more chlorophyll a molecules in a matrix of
protein, that passes the energy out of the photosystem.
Basic concept of
energy transfer
during
photosynthesis

11. How the antenna complex works.
When light of the proper wavelength strikes
any pigment molecule within a photosystem,
the light is absorbed by that pigment molecule.
The excitation energy is then transferred from
one molecule to another within the cluster of
pigment molecules until it encounters the
reaction center chlorophyll a. When excitation
energy reaches the reaction center chlorophyll,
electron transfer is initiated.

12. Reaction
center
Fluorescence
Heat
Photon
Photon
e–
Electron
acceptor
Chlorophyll molecules in antenna complex Reaction centerChlorophyll moleculeLower
Higher
e–
The excited-state energy of pigments increases with distance from the
reaction centre. Pigments closer to the reaction centre are lower in
energy than those farther from it. This energy gradient ensures that
excitation transfer toward the reaction centre is energetically favourable
and that transfer back out to the peripheral portions of the antenna is
energetically unfavourable.

13. Chlorophyll donates a light-
energized electron to the primary
electron acceptor, reducing it. The
oxidized chlorophyll then fills
its electron “hole” by oxidizing a
donor molecule.
Converting light to chemical energy. The reaction center

14. Photosystem I
PS I complex consist of ˜200 chlorophylls, ˜ 50 carotenoids, a mol of P700, one cyt
f, one plastocyanin, two cyt. B 563, FRS (ferredoxin reducing substance), one or
two membrane bound ferredoxin molecules etc. It is rich in chl a, iron & copper.PS I
controls the process of producing a strong reductant to reduce NADP into NADPH+
H+
Photosystem II
PS II complex consist of ˜ 200 chlorophylls, ˜ 50 carotenoids, a mol of P680, a primary
e acceptor Q, a plastoquinone, 4 plastoquinone equivalents, 4 Mn molecules bound
to one or more proteins, two cyt. b 559, one cyt. b and chloride. PS II is concerned
with the generation of strong reductant and weak reductant coupled with the
release of oxygen

15. Some basic difference between photosystem I & II

16. Two photosystems work sequentially. First, a photon of light ejects a high-energy
electron from photosystem II; that electron is used to pump a proton across the
membrane, contributing chemiosmotically to the production of a molecule of ATP.
The ejected electron then passes along a chain of cytochromes to photosystem I.
When photosystem I absorbs a photon of light, it ejects a high-energy electron used
to drive the formation of NADPH.
Z diagram of photosystems I and II

17. 4e–
4 Photons
2 H+
2 NADP+
2 NADPH
Lower
Higher
Photosystem I
Ferredoxin
+
4e–
4 Photons
4e–
Photosystem II
4 H+
PQ
PC
P700
ATP
produced via
proton-motive force
Cytochrome
complex
Pheophytin
P680
+ O22 H2O
The Z scheme linking Photosystem II and Photosystem I
When electrons reach the end of the Photosystem II electron chain they are passed to a protein
plastocyanin that can diffuse through the lumen of the thylakoid and donate electrons to
Photosystem I. Shuttle rate of 1000 electrons per second between photosystems.

18. ChlorophyllLower
Photon
Pheophytin
Cytochrome
complex
Higher
PQ
1. When an electron in the reaction center chlorophyll is
excited energetically the electron binds to pheophytin
and the reaction center chlorophyll is oxidized
2. Electrons that reach pheophytin are transferred to
plastoquinone (PQ), which is lipid soluble,
passed to an electron transport chain
(quinones and cytochromes)
In photosystem II, excited electrons feed an electron
transport chain.
2H2O O2+ 4H+ + 4e-
Pheophytin has the structure of chlorophyll
but without the Mg in the porphyrin-like ring
and acts as an electron acceptor.

19. Photosystem II Feeds an ETC that Pumps Protons
Cytochrome
complex
PQ
PQ
e–
e–
e–
Pheophytin
Antenna
complex
Reaction
center
Photosystem IIStroma Photon H+
H+
(low pH) H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Stroma
Thylakoid Lumen
3. Passage of electrons along the chain involves a
series of reduction-oxidation reactions that results in
protons being pumped from stroma to thylakoid
lumen
Plastoquinone carries protons to the
inside of thylakoids, creating a proton-
motive force.
An essential component of the
reaction is the physical transfer of the
electron from the excited chlorophyll.
The transfer takes ~200 picoseconds
The ph of the lumen reaches 5 while
that of the stroma is around 8 - the
concentration of H+ is 1000 times
higher in the lumen than the stroma.
+
The oxidized reaction center of the chlorophyll that donated an electron is re-reduced by a secondary
donor and the ultimate donor is water and oxygen is produced.
H2O
O2

20. Thank you