Photosynthetic lamellae

In this group there are quite a few reports dealing with PLB-like membrane assemblies of photosynthetic lamellae, however few can be unambiguously identified as cubic membranes. One example in which we can make that identification has been observed in an aged blue-green alga [19] whose photosynthetic thylakoids were observed to continuously fold into a... 

Dubacq JP, and Tremolieres A. Occurence and function of phosphatidylglycerol containing delta3-trans-hexadecenoic acid in photosynthetic lamellae. Physiol Veg. 1983 21 293-312. 

Dubacq, J.-P., and A. Tr molibres, Physiol. Veg. 21 293-312 (1983). Occurrence and function of phosphatidyIglycerol containing 3-trans hexadecenoic acid in photosynthetic lamellae. 

In photosynthetic eukaryotic cells, both the light-de-pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19-38), membrane-bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the internal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) contains most of the enzymes required for the carbon-assimilation reactions. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Despite the physiological diversity of the purple bacteria the photosynthetic apparatus is much the same in all species. All purple bacteria contain only one type of chlorophyll, either BChl a or b. The light-harvesting and RC chromophores are all located in the cytoplasmic membrane or elaborate invaginations of it in the form of vesicles, tubules or lamellae [40]. 

The membranes of the lamellae contain photosynthetic pigments and an electron transport system. The lumen of the lamellae contains the oxygen generating system. The region around the lamellae (stroma) contains the CO2 fixing system (dark reactions). 

The green photosynthetic organelles of plants resemble mitochondria. Chloroplasts are about 0.2 fJLm in diameter and surrounded by a double-layered lipoprotein membrane about 100 A thick. The two major components of chloroplasts are the membrane system and the stroma. The membrane system contains grana and interconnecting lamellae which carry the chlorophyll molecules, whereas the stroma contains ribosomes (70S) and both RNA and DNA. In spite of the presence of DNA, chloroplasts (like mitochondria) are not autonomous but depend for many of their proteins on nuclear DNA and cytoplasmic ribosomes. Liposoluble spheres containing carotinoids are also present (Mercer, 1960). See Section 4.6 for the many herbicides whose selectivity is exerted on the chloroplasts. 

Hattersley P W, Browning A J 1981 Occurrence of the suberized lamella in leaves of grasses of different photosynthetic types. I. In parenchymatous bundle sheaths and PCR ( Kranz ) sheaths. Protoplasma 109 371-401... 

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