2. In plants, algae and cyanobacteria:
Oxygenic and anoxygenic photosynthesis
2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2
carbon dioxide + water + light energy → carbohydrate + oxygen
Today, the average rate of
energy capture by
photosynthesis globally is
~130 terawatts which is about
six times larger than the current
power consumption of human
civilization.
3. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which
consumes carbon dioxide but does not release oxygen.
CO2 + (AsO3
3–) + photons → (AsO4
3–) + CO
carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other
compounds in subsequent reactions)
Examples of organisms belonging to anoxygenic photosynthesis are the filamentous green
bacteria, the green sulfur bacteria, the purple bacteria and the heliobacteria.
The basic difference between oxygenic and bacterial photosynthesis is that bacteria have only
one type of reaction center, whereas oxygenic species have two (Photosystem I and II).
These photosynthetic bacteria can use light energy to extract electrons from molecules other
than water such as sulfur compounds or simple organic molecules.
4. Green plants and algae have two different types of reaction centers that are part of larger
supercomplexes known as photosystem I (P700) and photosystem II (P680).
Transforming light energy into charge separation
5. The purple bacterial reaction center shares a striking similarity to Photosystem II of plants and
cyanobacteria.
Photosystem II
9. Photosystem II: harvesting light
Photosystems have large antennas of light-absorbing molecules that harvest light and transfer
their energy inwards to the reaction center.
10. The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino
acid, which then delivers it to the chlorophyll, making it ready to absorb another photon.
It contains cluster of manganese ions
(magenta), calcium (cyan), chloride ion
and oxygen atoms (red). It grips two water
molecules and removes four electrons,
forming oxygen gas and four hydrogen
ions.
It is surrounded by histidines, aspartates
and glutamates.
Photosystem II: oxygen-evolving center
11. Photosystem II: Synechococcus elongatus structure
The lumenal side reaction center proteins (D1 & D2): A through E. The peripheral antenna proteins
(CP43 & CP47): six membrane spanning α-helices. The PsbO protein: the water splitting enzyme of
photosynthesis. PsbV (cytochrome c-550): plays role in the oxygen-evolving complex of photosystem II.
12. Because of the oxidation of water and the release of dioxygen, the Photosystem II reaction
center does not operate in a cycle.
Bacterial RC vs Photosystem II
The released protons from water oxidation are deposited on one side of the thylakoid
membrane to help build the electrochemical gradient.
In the bacterial reaction centre, the electron is obtained from a reduced compound haem group
in a cytochrome subunit or from a water-soluble cytochrome-c protein.
In addition, Photosystem II differs from the bacterial reaction centre in that it has many
additional subunits that bind additional chlorophylls to increase efficiency.
13. Cytochrome b6f complex
It catalyzing the transfer of electrons from plastoquinol to plastocyanin.
Crystal structure from C. reinhardtii
Each monomer consists of
four large subunits: cytochrome f (32
kDa), a cytochrome b6 (25 kDa), a 19
kDa Rieske iron-sulfur protein (19 kDa),
subunit IV (17 kDa);
four small subunits (3-4 kDa): PetG,
PetL, PetM, and PetN.
The total molecular weight is ~217 kDa.
14. Reaction mechanism of cytochrome b6f complex
The cytochrome b6f complex is responsible for ‘non-cyclic’ and ‘cyclic’ electron transfer
between two mobile redox carriers, plastoquinone (QH2) and plastocyanin
H2O Photosystem II (1) QH2 Cyt b6f Pc Photosystem I NADPH
(2) QH2 Cyt b6f Pc Photosystem I Q
15. Plastocyanin
Plastocyanin (Cu2+Pc) is reduced by cytochrome f
according to the following reaction:
Cu2+Pc + e- → Cu+Pc
After dissociation, Cu+Pc diffuses through the
lumen until recognition/binding occurs with P700+
which oxidizes Cu+Pc according to:
Cu+Pc → Cu2+Pc + e-
