Photosynthetic membranes

The reaction centres of purple and green sulphur bacteria are localised in membranes, often called chromatophore membranes, which lie close to or include the outer cell membrane. In purple photosynthetic bacteria. the LH proteins are also intrinsic to the chromatophore membrane. However, in green bacteria the very large LH chlorosome, packed with many thousands of molecules of bacteriochlorophyll, is stacked into rodlike structures attached to the cytoplasmic side of the photosynthetic membrane, which does not invaginate as it does in purple bacteria (Fig.l.8)

In oxygenic photosynthetic organisms, the photosynthetic apparatus involved in light reactions is embedded in the specialised thylakoid membrane (see Fig.l.3). In cyanobacteria, the thylakoid membranes tend to form concentric rings within the cytoplasm and are characterised by the presence of the large LH phycobilisomes attached to their surfaces, which induces a considerable spacing between them. In the green oxyphotobacteria (prochlorophytes), the same concentric rings are present but the membranes lie more closely together because of the absence of bulky phycobilisomes. The presence of phycobilisomes in the chloroplast of red algae leads to a thylakoid membrane organisation reminiscent of cyanobacteria. In striking contrast, the thylakoid membranes of higher plant chloroplasts, and to a lesser extent those of green algae, are arranged in stacked (grana) and unstacked regions (see Fig.l.3). The granal thylakoids are highly enriched in PSII, while PSI is found in the unstacked regions. However, this extreme lateral separation does not seem to occur in the thylakoid membranes of cyanobacteria and many forms of algae and therefore cannot be an absolute requirement for oxygenic photosynthesis to occur.