Photosynthesis light reaction and dark reaction

1. Photosynthesis
By
Dr. Harinatha Reddy Aswartha
Christ University
Bangalore

2. • During photosynthesis, energy from sunlight is harvested and
used to drive the synthesis of glucose from C02 and H20 .
• In the light reactions, energy from sunlight drives the synthesis of
ATP and NADPH, coupled to the formation of 02 from H20.

3. • In eukaryotic cells, both the light and dark reactions of
photosynthesis occur within chloroplasts.
• The light reactions in the thylakoid membrane and the dark
reactions within the stroma.
6CO2 + 6H2O ------------> C6H12O6 + 6O2
light

4. • The photosynthetic pigments are organized into photocenters in
the thylakoid membrane.
• The photocenter act as antennae to absorb light and transfer the
energy to chlorophyll molecule that serves as a reaction center.
Electron Flow through Photosystems I and II:

5. • There are two distinct photosystems: photosystem I and
photosystem II.
• Photosystem I (Cyclic Photophosphorylation) absorbs
longer wavelength light (far-red light) and transfer energy to
reaction centre chlorophyll molecule called P700.
• Photosystem II (Non Cyclic Photophosphorylation) absorbs
light at shorter wavelengths (near red light) and transfer its
energy to the reaction centre chlorophyll molecules called
P680.

6. Plastaquinone.
ATP
ADP
Ferredoxin
b → f
plastocyanin
P700.
Photosystem I (Cyclic Photophosphorylation):

7. ADP
ATP
Photosystem II (Non Cyclic Photophosphorylation)

8. Photosystem I (Cyclic Photophosphorylation):
• This process uses only Photosystem I and the chlorophyll
P700.
• Electrons here travel in a cyclic manner and electrons travel
back to photosystem I and only ATP is produced.
• Another point to be noted is that photolysis or water splitting
is absent.
• Oxygen is not evolved and also this system is mostly
predominant in bacteria.

9. • When the photosystem I antenna chlorophylls transfer light energy
to the reaction centre chlorophyll P700, it gets excited.
• The excited or high-energy electron of P700 is transfer to primary
acceptor i.e Iron containing protein called Ferredoxin.
• From reduced Ferredoxin (FeS) the electrons is eventually
transferred to plastaquinone.

10. • From reduced plastaquinone the electrons transfer to
cytochrome b → cytochrome f → plastocyanin.
• During cyclic phosphorylation, ATP is generated in the region
of cytochrome b.
• From plastocyanin the electrons back to oxidized P700.

11. • Since the electrons travel in a cyclic pathway (i.e. they
originate from P700 and come back to the P700), the process is
called cyclic photophosphorylation in which only photosystem
I is involved.

12. Non-Cyclic Photophosphorylation.
• In this photophosphorylation both photosystem I and II are
involved.
• In this process, both NADPH and ATP are produced.
• The process of photolysis of water (splitting)is present and
oxygen is evolved as a byproduct.
• The system happens to be mostly predominant in green plants.

13. • The first step in photosystem I, the splitting of water
(photolysis) into oxygen atoms (1/2O2) and hydrogen ions
(2H+).
• Photolysis donates an electron to the oxidized P680 molecule
following the absorption of a quantum of light near 680 nm.

14. ADP
ATP

15. • The P680 molecule is now excited and reduces pheophytin.
• From reduced pheophytin the electrons subsequently travel
through, plastaquinone, cytochrome b6 (ATP is generated in
the region of cytochrome b6), cytochrome f and plastocyanin;
• The plastocyanin latter donates electrons to photosystem I.

16. • The electron is accepted by the oxidized reaction centre
chlorophyll ‘a’ of photosystem I (P700) and reduced.
• From Photosystem I the electrons transfer to Ferredoxin.
• From ferredoxin the electrons finally transfer to one molecule
of NADP+, It is reduce to NADPH in the presence of enzyme
NADPH redectase.