Grana lamellae

FIGURE 1. Thin section of part of an isolated chloroplast showing the internal thylakoid membrane system which consists of appressed grana lamellae (g) and non-appressed stroma lamellae (s) embedded in the stroma protein matrix and surrounded by a double membrane envelope (e). 

Our results reveal two classes of PSII proteins those that are present only in grana lamellae, and those that display lateral heterogeneity in the thylakoids. The LHCII and the three PSII reaction center proteins, 32 kDa, D2 and cytochrome b559 are present both in grana and stroma lamellae. Such distribution is in agreement with previous reports on the dual location of PSII centers in the two membrane regions (1-3). In contrast, we find that the 51 kDa and 43 kDa PSII proteins are located exclusively in grana lamellae. 

Previous studies of the wild type ultrastructure of Cblamydomonas (see, for example, Ohad, et al., 1967)have demonstrated a clear differentiation between appressed, grana lamellae and unstacked, stroma lamellae, similar to that seen in higher plants. 

The study presented In this paper demonstrates that the path taken by pLHCP is different than what was previously suggested (11), showing that insertion of pLHCP into thylakoids occurs mainly into the stroma-lamellae rather than being restricted to the grana. The path taken by pLHCP to the grana-lamellae is similar to that found for the 32 kDa protein of the reaction center of photosystem II. The precursor form of this protein is also inserted into stroma thylakoids where it is processed and palmitoylated before migrating to its location of activity in the grana-lamellae (18). 

Key CM, chloroplast double membrane (each 35—50 A thick) S, stroma G, granum (made up of a cylindrical pile of discs) SL, stroma lamella GL, grana lamella D, disc OD, osmiophilic droplet. 

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. 

Chloroplasts fill most of the cytoplasm around the junction of three cells of Arabidopsis thaliana seen in this micrograph. Both grana stacks and stroma lamellae (pictured in more detail in Fig. 23-19) can be seen. Also present are several small mitochondria. Portions of the large vacuoles, characteristic of plant cells, are seen at top, right, and bottom. Micrograph courtesy of Kenneth Moore. 

Chloroplasts of higher plants are saucer-shaped, and from 4 to 10 ym in diameter and 1 to 3 ym thick. The chlorophyll is concentrated in bodies within the chloroplasts called grana, which are about 0.4 ym in diameter. Under the electron microscope, the grana appear as highly organized, precisely stacked lamellae, to which the chlorophyll is bound, imbedded in a stroma matrix. The light and associated electron transport reactions take place in the lamellae, whereas enzymes involved in carbon dioxide fixation are located in the stroma. 

Because Photosystem II tends to occur in the grana and Photosystem I in the stromal lamellae, the intervening components of the electron transport chain need to diffuse in the lamellar membranes to link the two photosystems. We can examine such diffusion using the time-distance relationship derived in Chapter 1 (Eq. 1.6 x je = 4Djtife). In particular, the diffusion coefficient for plastocyanin in a membrane can be about 3 x 10 12 m2 s-1 and about the same in the lumen of the thylakoids, unless diffusion of plastocyanin is physically restricted in the lumen by the appres-sion of the membranes (Haehnel, 1984). For such a D , in 3 x 10-4 s (the time for electron transfer from the Cyt b(f complex to P ), plastocyanin could diffuse about [(4)(3 x 10-12 m2 s-1) (3 x 10-4 s)]1/2 or 60 nm, indicating that this complex in the lamellae probably occurs in relatively close proximity to its electron acceptor, Photosystem I. Plastoquinone is smaller and hence would diffuse more readily than plastocyanin, and a longer time (2 x 10-3 s) is apparently necessary to move electrons from Photosystem II to the Cyt b(f complex hence, these two components can be separated by greater distances than are the Cyt b f complex and Photosystem I. 

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. 

Precursor Processing. Hie precursor protein is synthesized on stromal lamellae and is processed there to the 32kDa mature form (21). Processing of the precursor is a posttranslational event (12) and takes place at the carboxy terminus (22). Following processing, the mature protein translocates to spatially-distinct chloroplast membranes, the grana, where functional photosystem II reaction centers are mainly located (21, 23). 

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