Inner envelope

PSTIC55 T1C55, chloroplast inner envelope import complex P. sativum 004127, 004422, S74825 24... 

The inner envelope of these surfaces defines the equilibrium shape. 

Chloroplasts are a typical type of plastid that performs various metabolic reactions as well as photosynthesis. Their envelope consists of two membranes the outer envelope membrane and the inner membrane (Fig. 7). The space between these two membranes is called the intermembrane space, and the space enclosed by the inner envelope membrane is called the stroma. In addition, chloroplasts have another membrane system within the stroma the thylakoid membrane forms the lumen. Therefore, there are six different localization sites and, of course, multiple pathways to each site. Naturally, their sorting mechanisms are very complicated. 

Like the Tom and Tim systems on mitochondrial outer and inner membranes, chloroplasts use the Toe and Tic systems on their outer and inner envelope membranes. Although there may not be a direct correspondence between both subunits, their functions for protein translocation appear quite similar. Thus, most of the sorting mechanisms within the envelope membranes are recognized as variations of the general sorting pathway to the stroma. 

The inner envelope membrane proteins have a cleavable N-terminal transit peptide, as well as some hydrophobic domain (s) in their mature portion. There are two possibilities on the role of this hydrophobic domain it may work as an N-terminal signal peptide after the translocation into the stroma and the subsequent cleavage of the transit peptide. Alternatively, it may work as a stop-transfer signal. One more important question is how the distinction is made between the outer membrane proteins, the inner membrane proteins, and the thylakoid membrane proteins. It is still an enigma. 

Note that when the interfacial energy is isotropic and the 7-plot is a sphere, the Wulff shape will also be a sphere. However, if the 7-plot possesses deep depressions or cusps at certain inclinations such as in Fig. C.4a, the planes normal to the radii of the plot at these inclinations will tend to dominate the inner envelope, and the Wulff shape will be faceted. In such cases, the system is able to minimize its total interfacial energy by selecting patches of interface of particularly low energy even though the total interfacial area increases. 

The means to determine the minimum-energy shape for a crystal of fixed volume was developed by Wulff (38), who showed that the equilibrium shape can be determined if the surface tension, y, at all crystallographic orientations is known. As illustrated in Fig. 2, on a polar y plot of the surface tension as a function of orientation, the inner envelope of the planes drawn perpendicular to and at the ends of the radius vectors gives the equilibrium shape of a crystal of constant volume. Faceting in the equilibrium crystal shape is due to cusps in the polar y plot. 

Fig. 2. A schematic Wulff construction for an equilibrium crystal shape using the polar y plot of the surface tension, (a) The equilibrium shape is that found from the inner envelope of tangents to the y plot, (b) An ECS with (001) facets produced by cusps in the y plot (39). (Reprinted from Prog. Surf. Sci., Volume 39, E. H. Conrad, Page 65, Copyright (1992), with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 IGB, UK.)...

II. The outer envelope a complex layer filling the space between the capsule and the inner envelope (Fig. 7.11). 

III. The inner envelope a syncytial layer showing much variation. Some workers divide this layer into two zones - zone I, a cytoplasmic layer and zone II, a gelatinous layer (Fig. 7.14). Part of this embryonic layer gives rise to the embryophore (Fig. 7.4) and also to the oncospheral membrane (Figs. 7.4, 7.11 and 7.14) (a very thin layer surrounding the oncosphere), which is often counted as a fourth layer. Additional layers, which may be further derived from the above basic envelopes have been reported in some species (e.g. H. nana 204), but it is beyond the scope of this text to discuss all the various modifications which can occur. Only those features which have a special physiological significance are discussed below. 

The structure, ultrastructure and formation of the hymenolepidid egg has been reviewed in detail by Ubelaker (888). Its general morphology is shown in Fig. 7.14). Although there are only the usual three basic embryonic membranes (p. 179) in the developing egg - shell/capsule, outer envelope, and inner envelope - the fully formed egg often appears to be more complex due to further differentiation of these layers. The following structures can be recognised (Fig. 7.11). 

This envelope is a syncytial layer involving two cells Ailing the space between the embryo and the egg shell it is responsible for the formation of the embryophore and oncospheral membrane (204). After the latter is formed, what is left of the inner envelope becomes the cytoplasmic and gelatinous layers-zones I and II, respectively, in H. diminuta (Fig. 7.14). The latter has the ability to swell readily in dilute salt solutions, a property which may facilitate the escape of the oncosphere during hatching (p. 191) (442). 

The proteinaceous embryophore is synthesised by the inner envelope and in H. nana this process is highlighted by the presence of granular endoplasmic... 

Test Indicates presence of Variety" Shell Cytoplasmic layer Inner envelope Gelatinous layer Embryophore Penetration gland Hooks... 

This structure (Fig. 7.14) has received much attention due to the role it plays in the hatching process (see below). It is not a typical unit membrane but resembles a membranous lamina and consists of a layer of regularly arranged granules bounded on both sides by a number of lamina (442). Its chemical composition has not been determined but, in taeniid cestodes, there is some histochemical evidence that it may be a lipoprotein (442). It is apparently formed by the delamination of the inner part of the inner envelope, detaching from it as a thin, separate layer. 

One of the structures which shows the greatest degree of variation is the embryophore, especially in cyclophyllideans, where it has been most widely studied. Even in species of the same genus, the form of the embryophore can vary greatly. In H. nana, after the embryophore is formed within the inner envelope (Fig. 7.11), it does not grow inwards but remains a thin, discontinuous, peripheral layer of the inner envelope. In H. diminuta, in contrast, it increases in thickness and moves towards the inner region of the inner envelope (Fig. 7.11) (204). 

Chloroplasts in higher plants have three membranes the outer and inner envelope membranes and the thylakoid membrane. Very little is known about membrane protein assembly into the two envelope membranes (Soil and Tien, 1998). The thylakoid has been better studied and in fact appears to use mechanisms very similar to those found in E. coli for membrane protein insertion (Dalbey and Robinson, 1999). Thus, SRP, SecA, SecYEG, YidC, and Tat homologues are all present in the thylakoid membrane or in the stroma (the Tat system was first identified in thylakoids, in fact). In contrast to E. coli, however, there are thylakoid proteins that appear to insert spontaneously into the membrane, insofar as no requirement for any of the known translocation machineries has been detected (Mant et al, 2001). 

PEND protein DNA-binding protein in the inner envelope membrane of the developing chloroplast... 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

In the plastids, acyltransferases provide a direct route for entrance of acyl groups from ACP to membrane lipids. Since this is the standard pathway for phosphatidic acid synthesis in E. coli and cyanobacteria, both the enzymes of phosphatidic acid synthesis in plastids and the glycerolipid backbones they produce are termed prokaryotic . In both chloroplasts and non-green plastids, the glycerol-3-phosphate acyltransferase is a soluble enzyme that, unlike the E. coli enzyme, shows preference for 18 1-ACP over 16 0-ACP. The lysophosphatidic acid acyltransferase, which is a component of the inner envelope of plastids, is extremely selective for 16 0-ACP. The presence of a 16-carbon fatty acid at the... 

Thermal Envelope Houses - An architectural design (also known as the double envelope house), sometimes called a "house-within-a-house," that employs a double envelope with a continuous airspace of at least 6 to 12 inches on the north wall, south wall, roof, and floor, achieved by building inner and outer walls, a crawl space or subbasement below the floor, and a shallow attic space below the weather roof. The east and west walls are single, conventional walls. A buffer zone of solar-heated, circulating air warms the inner envelope of the house. The south-facing airspace may double as a sunspace or greenhouse. 

The inner envelope in Figure 8.12 is the line of incipient mechanical instability the line separating states that are only diffusionally unstable from states that are both diffusionally and mechanically unstable. The line of incipient mechanical instability is the locus of points at which (8.1.31) is first violated that is, the points at which... 

Figure 8.20 Txx diagram for liquid-liquid or solid-solid equilibria in binary mixtures that obey the Porter equation (8.4.32) with parameter A given by (8.4.38). Filled square is the critical point filled circles lie on the isotherm at 30°C. The inner envelope, with labels C and D, is the spinodal and satisfies (8.4.37). The outer envelope is the equilibrium curve, which satisfies the equilibrium conditions (8.4.35).

A specific transport of malate and oxaloacetate across the inner envelope membrane enables a transfer of redox equivalents from the chloroplast stroma to the cytosol (8). The translocator involved is half saturated at very low concentrations of oxaloacetate (Km 9 pM). OAA transport is insensitive to even high concentrations of malate (Ki 1.4 mM) as determined in spinach chloroplasts. The large redox gradient between the NADPH/NADP in the stroma and the NADH/NAD in the cytosol. 

Predicted folding pattern of the chloroplast phosphate translocator in the inner envelope... 

Flg. 13. The chloroplast envelope plays a predominant role in the assembly of the three parts of the galactolipid molecule (galactose, glycerol, fatty acids). Saturated and monounsat-urated fatty acids are synthesized in the stroma by a multienzyme complex (fatty acid synthetase). Then, the different steps occur on the envelope, probably at the level of the inner membrane. Under these conditions, massive transport of galactolipids should occur very rapidly between the inner layer of the inner envelope membrane and the thylakoids (reproduced from Douce and Joyaid, 1979a, by permission). 

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