Photosynthetic apparatus in bacteria

1. Photosynthetic apparatus in
bacteria
Lecture by:
Dr. Aswartha Harinath Reddy M.Sc, Ph.D
Department of Life Sciences
Srikrishnadevaraya University
Anantapur –A.P. India

2. • Certain bacteria have the ability to perform photosynthesis.
Example: Green sulphur bacteria, Purple sulphur bacteria.
• Photosynthetic bacteria cannot use water as the Electron and
hydrogen donor and are incapable of evolving oxygen.
2H2O → 4H+ + 4e- + O2
• Bacteria that contain bacteriochlorophyll do not use water as an
electron donor and therefore do not produce oxygen.
• They are therefore it is called anoxygenic photosynthetic
bacteria.

3. 6CO2 + 6H2O ------> C6H12O6 + 6O2 (Plants)
Oxygenic photosynthetic
6CO2+ 12 H2S------------------ C6H12O6 + 12S (Bacteria)
Anoxygenic photosynthetic

4. Green sulfur bacteria:
• These cannot grow aerobically, and H2S (hydrogen sulfide) is
an inorganic sulphur is a source of electrons and CO2 is the
source of carbon.
• These bacteria are called photoautotrophic anaerobes.
sunlight
6CO2+ 12 H2S------------------ C6H12O6 + 12S

5. Photosynthetic apparatus in Bacteria
• Photosynthetic bacteria do not have specialized organelles
such as the chloroplasts of green plants.
• Chloroplasts are absent in prokaryotes and the photosynthetic
pigments are integrated into internal membrane systems.
There are two types of systems present i.e
1. Lamellae or Sacs
2. Chlorosomes

6. Lamellae or Sacs
• Which are the invaginations of the plasma membrane, appear
like a flat sheets (parallel layers) in which their chlorophyll
pigments are inserted.
• In some bacteria invaginations of the plasma membrane appear
like a spherical-shaped vesicles.

7. Chlorosomes:
• Chlorosome or chlorobium vesicles is an rod shape vesicle attached to the
plasma membrane contain chlorophylls.
• Chlorosome bacteriochlorophylls (bacteriochlorophyll C, D and E) absorb
light and transfer energy to reaction centre through light harvesting
bacteriochlorophyll A molecules a located in the plasma membrane.

9. Reaction centers.
• A photosynthetic reaction centre is a complex of several
proteins, pigments and other co-factors that together
execute the primary energy conversion reactions of
photosynthesis.

10. Photophosphorylation
• In phototrophic bacteria 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.

11. 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.

12. • 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.

13. • From reduced plastaquinone the electrons transfer to
cytochrome b → cytochrome f → plastocyanin.
• During cyclic phosphorylation, ATP is generated in the region
of cytochrome b6.
• From plastocyanin the electrons back to oxidized P700.

14. Plastaquinone.
ATP
ADP

15. • 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.

16. Non-Cyclic Photophosphorylation.
• In this photophosphorylation both photosystem I and II are
involved.
• In this process, both NADHPH 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.

17. • The first step in oxygenic electron flow, 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.

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

19. • 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.

21. THANK YOU