Thylakoid Membrane of Cyanobacteria

In addition to the presence of PBSs, cyanobacteria are also distinct from eukaryotic chloroplasts with respect to respiratory and photosynthetic electron transport. Both bioenergetic processes function within the thylakoid membrane of cyanobacteria, and thus share a common PQ pool and a common Cyt bi/f complex (Scherer, 1990 ... 

The water-plastoquinone photo-oxidoreductase, also known as photosystem II (PSII), embedded in the thylakoid membrane of plants, algae and cyanobacteria, uses solar energy to power the oxidation of water to dioxygen by a special centre containing four Mn ions. The overall reaction catalysed by PSII is outlined below ... 

The photosynthetic apparatus of modem cyanobacteria, algae, and vascular plants is more complex than the one-center bacterial systems, and it appears to have evolved through the combination of two simpler bacterial photocenters. The thylakoid membranes of chloroplasts... 

Photosystem II is the multi-enzymatic chlorophyll-protein complex (water-plastoquinone oxido-reductase) located in the thylakoid membrane of algae, cyanobacteria and higher plants. It is an integral part of the electron transport chain that catalyses primary charge separation. This protein complex consists of over 25 polypeptides, which make up a light-harvesting chlorophyll protein... 

Figure 23-18 Schematic view of photosynthetic reaction centers and the cytochrome //complex embedded in a thylakoid membrane. Plastocyanin (or cytochrome c6 in some algae and cyanobacteria) carries electrons to the PSI core.

An elegant example of this is the monitoring of herbicide residues via the photosynthetic electron transport (PET) pathway by utilising cyanobacteria or thylakoid membranes (5). For many herbicides the mode of action is as inhibitors of PET, often acting between the 2 photosystems as indicated in figure 3, and the result is a decrease in the photocurrent. 

The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. 

The inner membrane is folded inwards to form sacs called lamellae. The membranes these lamellae are made of are called thylakoid membranes, which look just like those of cyanobacteria (blue-green algae) (Fig 13.5). 

Fig. 2. Electron micrographs of phycobilisomes in the red alga Rhodella violacea (A) and phycobilisomes isolated from Porphyridium cruentum (B). (C) shows a membrane model consisting of the electron-transfer complexes of PS I, PS II, the cytochrome bet complex, the ATP synthase, CFq CFi, and the phycobilisomes. (A) and (C) from MOrschel and Rhiel (1987) Phycobilisomes and thylakoids The light-harvesting system of cyanobacteria and red algae. In JR Harris and RW Horne (eds) Membranous Structure, pp 216, 248. Acad Press (A) kindly furnished by Dr. Erhard Mbrschei and (B) kindly furnished by Dr. Alexander Glazer.

As seen earlier in Fig. 2, a phycobilisome is a fan-shaped set ofsupramolecular assemblies, consisting of a triangular core with six short rods radiating outward, with each phycobilisome being attached to the thylakoid membrane through the core complex. In crude extracts of cyanobacteria, these short rods are seen in electron micrographs to have a diameter of 12 nm, as expected for stacks of hexamers, with clearly marked divisions 6 nm apart along the rod and faint divisions midway between. 

The phycobilisome core is the site of attachment to the thylakoid membrane. Hemidiscoidal phycobilisomes, shown earlier in Fig. 2, are present in both cyanobacteria and some red algae, while hemiellipsoidal phycobilisomes are found only in certain other red algae. The phycobilisome in the thylakoid-less cyanobacterium Gloeobacter violaceus is present as a bundle of six rods in the phycobilisome of Synechococcus 6301, the core (attached to the thylakoid) has only two short rods attached. 

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