Granal membranes

Photosystem II reaction centres have been shown to be located mainly in the appressed granal membranes by a number of independent but indirect methods, including thylakoid fractionation, de-stacking and re-stacking experiments and chemical modification. Further evidence has come from freeze-fracture investigations of mutants specifically deficient in photosystem II in barley (Simpson et al., 1977), tobacco (Miller and Cushman, 1979) and Chlamydomonas (Olive etal. 1979). In all cases, there was a partial or almost... 

Lane 1 molecular weight standards, lanes 2, 11 dark adapted stromal and granal membranes kept at 15 C lanes 16, 18 granal and stromal membranes after 7 hr in the light at 25 C lanes 19, 22 stromal and granal membranes after 7 hr in the light at 3 C. 

Fig. 3. Negative image of chloroplast granal membranes of Phaseolus (osmium vapor fixation, air dried). Note membrane "particles," arrows. 85,000x...

When electron micrographs showed particles in the chloroplast granal membranes which were about the right size to contain 2500 chlorophyll molecules and the necessary associated compounds it seemed that the photosynthetic unit had been structurally identified. However, further work has indicated that these quantasomes, as they are sometimes termed, probably do not contain chlorophyll and may not, therefore, represent the intact photosynthetic unit. Nevertheless, the concept of the photosynthetic unit remains and it may yet be shown that the numbers discussed above represent structural rather than statistical relationships. 

If the model proposed by Andersson and Anderson [109] of total separation of PS I and PS II in the granal chloroplasts were to be accepted, electron transport from the PS II acceptors to P-700 would require a mobile electron carrier(s) which should diffuse laterally in the membrane fast enough to account for the observed electron transport rate. Plastoquinone [112] and plastocyanin are the candidates of choice for this role. The former has been shown to be present at approximately the same activity in the partitions and in the stroma-exposed membranes [43], while PC is known to be located in the intrathylakoid space [113],... 

Early fractionation studies with thylakoids that had been fragmented by detergents [54] or mechanical means [55] followed by collection of the thylakoid submembrane fractions by centrifugation provided the first evidence for lateral heterogeneity. Submembrane fractions derived from granal stacks were enriched in PS II but they also contained PS 1, whereas the stroma thylakoids had mainly PS I. Sane et al. [55] proposed that appressed membranes were the site of non-cyclic electron transport, while the non-appressed membranes carried out cyclic photo-... 

The membranes of chloroplast are rather complex. The lamellae may occur singly, and these are called stromal or intergranal lamellae (intergrana) or they may be stacked like coins, when they are termed granal lamellae (grana), and the individual "coins" are called thylakoids. In cross section, the lamellae look like pairs of membranes separating a narrow internal space (the lumen) from the external stroma. 

The four fracture faces in Pig. 18 (C) show imbedded particles of various sizes. In particular, the EFg face shows a profusion of 16 nm-diameter particles. The complementary fracture face PFs contains relatively tightly-packed, but rather indistinct and smaller particles. It is known that fractionation of a thylakoid membrane disrupted by detergent or by mechanical means yields separate granal and stromal fractions. The granal fraction has been found to be enriched in photosystem-II components and activity. 

Freeze-fracture electron microscopy of thylakoid membranes has clearly revealed an asymmetric lateral distribution of the various photosynthetic complexes in the granal and stromal membranes, i.e., the distribution of the protein complexes in the membrane is nonrandom. This lateral asymmetry was further substantiated by the results of electron microscopy of the inside-out vesicles discussed in Section Vll. These findings by electron microscopy are summarized by the model shown in Fig. 21 (A). It is a transverse cross section of the thylakoids shown earlier in Fig. 13 (D) and (D ), with the various photosynthetic protein complexes appropriately placed in the granal and stromal regions. 

THE GRANAL MARGINS OF PLANT THYLAKOID MEMBRANES NONAPPRESSED DOMAIN... 

---