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We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction-separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy orWoS of the grain boundary, while the strength of the σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction-separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy-displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities. [ABSTRACT FROM AUTHOR]

Various stages of virus and mycoplasma budding indicated that both virus and, most probably some mycoplasma progeny developed by budding. Besides this alternative, binary fission was the mode of mycoplasma reproduction. Mycoplasma-virus and mycoplasma-mycoplasma connections by stems were observed. Circular scars, 40-80 nm in diameter, often in groups, were left in the membrane of mycoplasmas by the budding bodies. Cytoplasmic structures seen in cross-fracture are presented. A relatively small number of globular virus-like bodies, not identical with MV-Lg-L 172, were observed budding from mycoplasma cells in the non-infected host culture. [ABSTRACT FROM AUTHOR]

The main types of ultrastructures found in the freeze-fracture faces of Acholeplasma laidlawii S 2 and its virus MV-Lg-L 172 were (1) particles 7-19 nm in diameter, mostly located in the convex cytoplasmic fracture faces. (2) small bulges or aggregates, 13-25 nm in diameter. which occupied only limited areas of both inner and outer fracture faces of some mycoplasmas, (3) numerous tiny grains and/or spikes 2-6 nm in diameter, protruding from a finely structured background, especially in the outer concave mycoplasmal fracture faces, and (4) linear structures, most probably fibrils and thicker filaments, both in the fracture faces and around mycoplasmas and viruses and connected with them. There was a high degree of structural similarity between mycoplasmal and viral membranes; no obvious significant difference was found. [ABSTRACT FROM AUTHOR]

Fragments of Paramecium caudatum cells obtained by merotomy were fixed in 1% OsO within 5 seconds after cutting. The ultrastructure of the damaged area of the fragment was studied in oriented ultrathin sections and by scanning electron microscopy. The cytoplasm exposed by merotomy was covered during a few seconds with a new membrane. This was a typical trilaminar membrane continuous with the plasma membrane covering the undamaged surface of the cell. The surface over the wound was wrinkled into irregular grooves and ridges. The cytoplasm, mitochondria and trichocysts in the injured region were electron translucent. The cytoplasm under the new membrane contained an unusually high amount of small membrane vesicles, 20-90 nm in diameter. These were probably the remnants of subpellicular alveoli and the plasma membrane destroyed by microsectioning. The possibility that the exposed cytoplasm would be covered by mere shifting of the existing plasma membrane can be excluded. The complex structure of the cortex with its subpellicular alveoli and regularly distributed cilia provide a strong argument against this notion. It seems probable that the new membrane was built up from the available molecular material, e.g., phospholipids and proteins present in the cytoplasm. Fragments of the membrane and alveolar membranes in the form of small vesicles may have also been included into the new membrane. [ABSTRACT FROM AUTHOR]

An abstract of the article "From Micro to Macro: Determining Hydrodynamic Properties of Stalk Forming Diatoms," by J.G. Stepanek, R. Janisch and M.L. Julius is presented.