Comparison of photosynthesis in Cyanobacteria and purple sulphur bacteria

Differences between photosynthesis in Cyanobacteria and Purple sulphur bacteria:

SN	Features	Photosynthesis in Purple sulphur bacteria	Photosynthesis in Cyanobacteria
1	Type of light reaction	Anoxygenic	Oxygenic
2	Electron donor	Anoxygenic	Oxygenic
3	Photosynthetic pigments	Bacteriochlorophyll a and b as photosynthetic pigments in Purple bacteria	Chlorophyll a and phycobilins as photosynthetic pigments in Cyanobacteria.
4	site	Invaginated cell membrane (lamellae) in purple bacteria	Thylakoid membrane  in Cyanobacteria
5	Photosystems involved	P870	PSI (P700) and PSII (P680)
6	Electron first expelled from	Electrons are firstexpelled from the reaction center of P870.	Electrons are first expelled from the reaction center of PSII (P680).
7	Manner of electron movement	Electrons move in cyclic manner	Electrons follow Z scheme
8	Path of electron movement	In cyclic photophosphorylation, electrons go via the electron transport system (ETS) before returning to the P870.	NADP+ accepts the electrons and does not return them to the P680 reaction center.
9	Assimilatory powers	ATP produced. NADH can be produced by reverse electron flow where electron travels backward against thermodynamic gradient which requires ATP.	ATP produced. Proton Motive Force during electron transfer reduce NADP to NADPH.
10	Evolution of Oxygen	Oxygen is not evolved	Oxygen is evolved
11	Carbon source for assimilation reaction	Organic carbon and/or CO2	CO2

Similarities between Purple sulphur bacteria and Cyanobacteria:

• Both purple bacteria and cyanobacteria depend on light for their light-dependent processes. This is because H2O, S, and H2S are poor electron givers, and light is necessary to produce a good electron donor and an electron acceptor.
• Both can use H₂S as electron donor. This is especially important in case of Cyanobacteria which uses H₂O as electron donor. Cyanobacteria can utilize H₂S as electron donor during anoxygenic photosynthesis.
• Both can employ cyclic photophosphorylation when the reducing power is sufficient and ATP can be produced as instead of reducing NADP+, electrons from ferrodoxin are returned to travel the electron transport chain that connects PSII and PSI.
• Both processes involve photosystems which basically comprise light harvesting complex and reaction center embedded in photosynthetic membrane where the whole process of light dependent reaction take place.

Protons are pumped over a membrane to produce an electrochemical potential, and electrons pass through a number of membrane-bound carriers, such as cytochromes, quinones, and iron-sulfur proteins, to produce ATP in both processes.  ADP and Pi provide an electrochemical potential that drives the production of ATP, which is catalyzed by a membrane-bound ATP synthase complex.

• In both Purple sulphur bacteria and Cyanobacteria, carbon dioxide can be used as source of carbon in the carbon assimilation reaction (dark reaction).
• Both of them contain the enzyme RubisCO and phosphoribulokinase which are key ezymes in Calvin cycle which is operative in both organisms.
• Both processes are involved in the production of reducing power (NADH in purple bacteria and NADPH in Cyanobacteria) which can later be used in carbon assimilation in light independent reaction. In Purple bacteria (cyclic photophosphorylation), NADH is generated by reverse electron flow which requires ATP input. 

Comparison of cyclic and non-cyclic photophosphorylation:

Photophosphorylation is the process of utilizing light energy from photosynthesis in which high energy ATP molecules are produced by the electrons flow through a series of membrane-bound carriers including cytochromes, quinones, and iron-sulfur proteins, while protons are pumped across a membrane to create an electrochemical potential. A membrane-bound ATP synthase complex catalyzes the synthesis of ATP from ADP and Pi, which is driven by the electrochemical potential they generate.

The differences between cyclic and non-cyclic photophosphorylation:

SN	Features	Cyclic photophosphorylation	Non-cyclic photophosphorylation
1	Present	Purple and Green bacteria	Green plants, Cyanobacteria and Algae
2	Electron donor	H2, H2S, S	H20
3	Photosynthetic pigments	Bacteriochlorophyll a and b in purple bacteria Bacteriochlorophyll c, d, e and small amount of bacteriochlorophyll a in Green bacteria	Chlorophyll a in plants Chlorophyll a and phycobilins in Cyanobacteria.
4	site	Cell membrane in purple bacteria and Heliobacteria Chlorosomes in Green bacteria	Thylakoid in plants and  Cyanobacteria
5	Number of photosystem	One (PSI)	Two (PSI and PSII)
6	Electron first expelled from	Electrons are first expelled from the reaction center of PSI.	Electrons are first expelled from the reaction center of PSII .
7	Manner of electron movement	Electrons move in cyclic manner	Electrons follow Z scheme
8	Path of electron movement	Electrons return to the P700 after passing through electron transport system (ETS) in cyclic photophosphorylation.	Electrons are not returned to the reaction center of P680 and they are accepted by NADP+.
9	Assimilatory powers	Only ATP produced In case of purple bacteria, NADH can be produced by reverse electron flow which requires ATP.	Both ATP and NADHP produced
10	Evolution of Oxygen	Oxygen is not evolved	Oxygen is evolved

Similarities between Cyclic and non-cyclic photophosphorylation:

• Both the processes are light dependent processes because unlike NADH used as the major electron donor in oxidative phosphorylation, H₂O, H₂S, S and H₂S are poor donor of electrons and therefore light is required to create a good electron donor and a good electron acceptor.
• Both processes are predominant in phototrophic bacteria i.e cyclic photophosphorylation in green and purple bacteria and non-cyclic photophosphorylation in Cyanobacteria.
• Both processes involve photosystems which basically comprise light harvesting complex and reaction center embedded in photosynthetic membrane where the whole process of light dependent reaction take place.

In both processes ATP is generated whereby electrons flow through a series of membrane-bound carriers including cytochromes, quinones, and iron-sulfur proteins, while protons are pumped across a membrane to create an electrochemical potential. A membrane-bound ATP synthase complex catalyzes the synthesis of ATP from ADP and Pi, which is driven by the electrochemical potential they generate.

Both processes are involved in the production of reducing equivalents which can later be used in carbon assimilation in light independent reaction. In Purple bacteria (cyclic photophosphorylation), NADH is generated by reverse electron flow which requires ATP input.