Vesicles, mineral surfaces

The great importance of minerals in prebiotic chemical reactions is undisputed. Interactions between mineral surfaces and organic molecules, and their influence on self-organisation processes, have been the subject of much study. New results from Szostak and co-workers show that the formation of vesicles is not limited to one type of mineral, but can involve various types of surfaces. Different minerals were studied in order to find out how particle size, particle shape, composition and charge can influence vesicle formation. Thus, for example, montmorillonite (Na and K10), kaolinite, talc, aluminium silicates, quartz, perlite, pyrite, hydrotalcite and Teflon particles were studied. Vesicle formation was catalysed best by aluminium solicate, followed by hydrotalcite, kaolinite and talcum (Hanczyc et al., 2007). 

Earlier reports [50] showed that vesicles composed of oleic acid can grow and reproduce as oleoyl anhydride spontaneously hydrolyzed in the reaction mixture, thereby adding additional amphiphilic components (oleic acid) to the vesicle membranes. This approach has recently been extended by Hanczyc et al. [51], who prepared myristoleic acid membranes under defined conditions of pH, temperature, and ionic strength. The process by which the vesicles formed from micellar solutions required several hours, apparently with a rate-limiting step related to the assembly of nuclei of bilayer structures. However, if a mineral surface in the form of clay particles was present, the surface in some way catalyzed vesicle formation, reducing the time required from hours to a few minutes. The clay particles were spontaneously encapsulated in the vesicles. The authors further found that RNA bound to the clay was encapsulated as well. 

The diffusion barrier. Much attention has been directed toward primitive amphiphile vesicles, inasmuch as they self-assemble from simple components and have an obvious ancestral connection with the more complex membranes that enclose modem cells. A review has been provided by Monnard and Deamer.55 The papers by Segre et al. and Hanczyc et al. contain additional discussion.56 57 Other prominent alternatives that would limit loss by diffusion have been electrostatic forces at mineral surfaces,58 iron sulfide membranes,59 and aerosols at the ocean-atmosphere interface.60 Section 2.7.1 discusses the function of compartmentalization in Earth life today. 

In the case of mineral particles, by making them smaller and smaller, you gain a larger and larger total surface. Is this also true for the total surface of micelles and vesicles when they divide What about the total volume ... 

In these tissues the distribution of vesicles correspond closely to the patterns of matrix mineralization. Furthermore, it has been suggested that crystals of HA are deposited within the vesicles. Subsequently apatite is deposited within the vesicles and upon their surfaces to produce typical modular clusters of mineral. Such morphological observations strongly implicate the matrix vesicles in the formation of apatite crystals (Fig. 9). Once the first crystals are formed, they mineralize further by epitactic crystal growth458. ... 

In other coccoliths species the intracellular vesicles may be mineralized and completed after the scale has been extruded onto the cell surface but still covered by an organic membrane. In the genus Chyrsotila, nonmotile cells produce a mucilaginous sheath in which spherulitic aragonite crystallites form, which eventually encapsulate the cell (Green and Course, 1983). There are... 

Skeletal tissue mineralization (bone formation) is initiated by osteoblasts, which secrete the osteoid matrix (Fig. 9.4). They express type I procollagen in secretory vesicles together with matrix vesicles that pinch off from the membrane. The matrix vesicles are pushed away from the cell surface, possibly by the flow of fluid containing calcium and phosphate ions that are also transported through the cell from the extracellular fluid on the outer surface. Collagen fibers develop further away from the cell surface than from fibroblasts. 

Nucleation of calcium phosphate precipitation within the matrix vesicles is mediated by phosphatidylserine, which comprises about 8% of the phospholipids of the inner cytosolic membrane surface (Fig. 9.5a). Calbindin in the vesicle (Fig. 9.5b) may also contribute. Rapid mineral growth within the vesicle keeps the concentration of dissolved calcium and inorganic phosphate ions so low that additional Ca2+ and Pi ions spontaneously enter from the extracellular fluid via their respective transporters. Attached type II and type X collagens from cartilage in the growth plate enhance calcium ion transport and calcification during endochondral ossification (Fig. 9.5b). 

A physiologic phosphate concentration is required for bone mineralization. Lowering the concentration prevents mineralization, but raising it does not ensure precipitation because pyrophosphate is present to inhibit precipitation. The concentration of PPi in cartilage and bone is controlled by three enzymes, two on the outer surface of matrix vesicles (Fig. 9.5b). One is tissue-nonspecific alkaline phosphatase (TNAP), which decreases stromal pyrophosphate and the other is NTP-PPi hydrolase (also called plasma cell membrane glycoprotein-1), which increases it. The progressive ankylosis gene product (ANK protein) is expressed by osteoblasts to add to the pyrophosphate of the osteoid matrix from osteoblast cytosol. 

M Mg + and of about 10" tration. In seawater Ca concentration exceeds 10 M while that of Mg approaches 10 M. However while cells do not usually accumulate Mg in vesicles the content of Ca there can be at least as high as 10 M allowing precipitation. Thus there are different mechanisms for handling the two ions in the cells before crystallization. Given these complications, as well as those stated above, it is then simplest to consider extreme cases where we know that mineralization occurs only on the surface of the organisms (as is the case of coral) or only in vesicles (as is tme of... 

Fig. 4.14 a, b. Epitaxial deposition of minerals within vesicles is limited by the spherical geometry of the compartments a aqueous ions can be transported across the vesicle membrane and precipitated on an atomic net located at the inner membrane surface b epitaxial growth from the membrane sites results in incoherent development of the mineral nuclei and the formation of a polycrystalline material... 

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