Cubic membranes, continued

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... 

The lattice-like membrane arrangement of the cristae of mitochondria in the giant amoeba was interpreted by Pappas and Brandt [22] to be composed of membranous tubules. If instead of discrete tubules, a continuous surface is assigned in such a way that it links the sections, a cubic membrane can be... 

Figure 7.4 A cubic membrane in leucoplasts of root tip cells that are actively involved in "protein storage", (a) Shows two protein crystals which are encompassed by a membrane which is continuous with the cubic membrane, (b) Shows the direct continuities (arrows) between one space of the two defined by the cubic membrane and the amorphous protein-sac. From [53], reproduced with permission.

Among the Arthropoda, we have identified cubic membrane structures in mitochondria [63-66], in spermatids [67, 68] and in mitochondria of spermatids [63]. The latter is an interesting observation, since the occurrence of cubic membranes in the mitochondria of spermatids and of oocytes, as well as in association with the ER and the NE, suggests a close relationship between these organelles during cell differentiation, and perhaps in organelle development. Indeed, mitochondria have occasionally been observed in close relationship with, and continuously linked to, cubic membranes (see, e.g. [61]). Further, as pointed out above, mitochondria in which the cristae form cubic membranes have been seen in continuous association with the NE [22, 63,69]. 

Several details, described by Bassot and coworkers [73, 76], such as the presence of a continuous membrane, and especially its bicontinuity, are consistent with our cubic membrane model. Indeed, similarities between the structure of cubic phases in lipid-water systems (c/. Chapters 4 and 5) and that of the photosome membrane system have been discussed by Bassot and coworkers [77]. These particularly well developed cubic membranes have been exhaustively studied surpassed only by studies of the PLB cubic membrane. 

Many of the cells listed in Table 7.1 and 7.2 are involved in active membrane flow and other mass-cooperative transport phenomena. Since cubic membranes offer a high surface to-volume ratio, they may also be actively involved in these processes, perhaps as membrane storage bodies, or as transport guides. It is of interest to note that aggregates of "s3maptic vesicles" often resemble cubic membranes (see Chapter 5 and [136]). This can be taken as an indication of a possible on-off mechanism of membrane continuity, which might accovmt for a regulative capacity of the release of transmitter substance. 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

The enzyme membrane reactor (EMR) is an established mode for running continuous biocatalytic processes, ranging from laboratory units of 3 mL volume via pilot-scale units (0.5-500 L) to full-scale industrial units of several cubic meters volume and production capacities of hundreds of tons per year (Woltinger, 2001 Bommarius, 1996). The analogous chemzyme membrane reactor (CMR) concept, discussed in Chapter 18, Section 18.4.5, is based on the same principles as the EMR but is far less developed yet. 

Other Reported Studies Brady [105] has reported on UF systems (start-up 2000) for mill water recovery and reuse. The Koch tubular HFP-276 (PVDF) membrane used removes 50% of COD, TDS, and total organic carbon (TOC). The UF operating cost is 1.6 per cubic meter. The plant operates continuously atarecovery rate of 98.7%. The feed stream and the null are not reported. 

Fig. 24. a) Schematic illustration of the "stretching" of water channel junctions during the continuous transformation between the D and G cubic phases, which occur with no disruption of the bilayer topology. A junction of four water channels in the Qu° phase is converted into two three-way junctions in the Qu° phase, b) Possible mechanism of membrane fusion the monolayers of two apposed lipid bilayers mix to form a stalk intermediate that expands radially to a trans monolayer contact (TMC), leading to rupture as a result of curvature and interstitial stresses and finally to the formation of a fusion pore. 

Figure 6.—Continued. C, Predicted electrical drift contribution to molecular transport across one of the two cubic cell membranes. (Weaver, J. C. Barnett, A. Wang, M. W. B/iss, J. G., unpublished). A hypothetical series of molecules, a// with unit charge (zs = l) was used to test the relative importance of different size pores in the pore population. More realistic predictions would use estimates of the size (radius rs), shape (a form factor), and the Bom energy repulsion (zs>eff — zm, where m is a number in the range 1 < m < 2).

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