Envelope Thickness Research Articles

Heterocyst envelope thickness, heterocyst frequency and nitrogenase activity in Anabaena flos-aquae: influence of exogenous oxygen tension

SUMMARY: The heterocyst envelope of the N2-fixing cyanobacterium Anabaena flos-aquae thickened as exogenous O2 partial pressure (pO2) was increased from 5 to 40 kPa. The majority of the thickening occurred in the glycolipid layer area of the envelope. Such thickening appears to be an O2-induced mechanism for providing a greater O2 diffusion barrier against O2 inhibition of nitrogenase. Nitrogenase activity decreased at pO2 levels above ambient (20 kPa), indicating that a thicker envelope is not completely effective as a barrier to O2 diffusion. However, when cultures grown at 10 kPa and 40 kPa pO2 were transferred to ambient pO2, the 40 kPa pO2 cells showed higher nitrogenase activity 24 h after transfer compared to those grown at 10 kPa or ambient pO2, indicating O2-protection of nitrogenase by thicker heterocyst envelopes. Heterocyst frequency was lowest at 20 kPa O2.

Risk of burn-through-a quantitative assessment of the capability of gas-insulated equipment to withstand internal arcs

The author introduces the notion of the risk of burn-through, borrowed from the risk of failure widely used in the statistical approach to insulation coordination, to evaluate the capability of gas-insulated equipment and substations (GIS) to withstand burn-through due to internal arcs. A sensitivity analysis performed to identify the main parameters affecting the risk of burn-through showed that the risk increases with the mean fault current. Both the fault-clearing characteristics of the switching apparatus and the GIS design, namely the envelope thickness and the GIS section length between two consecutive insulators, were found to have a significant influence on the risk of burn-through. To keep the risk of burn-through reasonably low, appropriate coordination must be made among the fault-clearing characteristics of the switching apparatus, the GIS design, and the distribution of the local fault currents.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Growth, nodule morphology, and nitrogenase activity of Myrica gale with roots grown at various oxygen levels

Myrica gale L. plants were inoculated with Frankia strain HFP M+g15 and grown for 28 days with root systems exposed to 2, 5, 10, 21, and 40 kPa O2. Plant growth was similar under all treatments, except for a small decrease in final size of plants at 2 kPa O2. At the end of the experiment nitrogenase activity of representative plants was analyzed over a variety of Po2 levels using an open-flow cuvette. Optimum nitrogenase activity was found at Po2 levels close to the growth Po2 in all cases and plants showed no short-term adaptation to oxygen. Specific activity of maximum nitrogenase was similar for all treatments, being within a factor of two. Nitrogenase activity showed rapid transient responses to step shifts in Po2 during assay and irreversible decline in activity at Po2 levels above optimum. Morphological responses to changing Po2 include a dramatic increase in nodule-root growth, inversely proportional to ambient Po2, and a variety of internal structural changes reducing nodule ventilation with Po2 increase. No modification in vesicle envelope thickness was observed over the range of Po2 studied. We conclude that oxygen protection in M. gale nodules operates through a complex suite of morphological controls, but that modification of nodule-root surface area is probably the prime mechanism.

The structure of Phaethon and detonation of its icy envelope

It is suggested that Phaethon — a hypothetical planet whose breakup gave rise to the asteroid belt — has a structure similar to that of Callisto, and thus consisted of a rocky core (40%; in mass) and an ice envelope (60%). Total breakup of the planet becomes possible in an explosition of the electrolysis products accumulated in the ice in the form of a solid solution if the planet mass M ≲ 0.5 M. Assuming M = 0.5 M we obtain 1750 km for the planet's radius with the envelope's thickness of 750km. Application of the hydrodynamic theory of detonation to the (2H2 + O2) solution in ice reveals that depending on the actual critical temperature which for conventional explosives lies in the range 700–900 K the minimum (2H2 + O2) concentration in ice required for its explosion is 13–18%;.

Continuation of Solutions and the Retarded Electromagnetic Fields in the Mathematical Formulation of Huygens' Principle

Chroococcidiopsis sp. cells present hypolithically in quartz flints collected in the Negev Desert (Israel) and in wet and desiccated laboratory cultures were examined by light and transmission electron microscopy. The composition of Chroococcidiopsis cell envelopes was characterized by cytochemical tests for detection of sporopollenin-like compounds, and acid, sulphated and beta-linked polysaccharides, positively charged glycoproteins, lipids and proteins. These compounds were present as very elaborate structures in the envelope of cells from stones and in desiccated cultures. An increased envelope thickness is probably useful in the prevention of water loss, although it could cause cell death by reducing exchange with the environment. Isolated living cells that occurred in desiccated cultures might be regarded as resting forms allowing the survival of the cyanobacterium.

Automated Optimum Design of Refrigerated Warehouses

The main consideration in the design of refrigerated commercial warehouses, apart from energy required for operation, is the total cost (initial plus operating costs). There is no simple explicit relation between the total cost and the energy required to maintain the desired inside conditions. Qualitatively, one feels that a design involving less initial cost requires larger energy input and hence a higher operating cost. With limited energy resources, it is necessary to conserve energy or use it optimally. A unified approach to find the optimal combination of initial cost and operating cost (energy) is presented in this paper. Since the thickness of insulation is one of the important factors to be considered in reducing the external load and hence the energy requirements, the optimum building envelope and insulation thicknesses are found for a specified volume and location by using the interior penalty function method of optimization. The procedure outlined in this paper can be used for new as well as existing building to fulfill the functional requirements optimally and thus conserve the energy to the greatest possible extent. For the computation of heat gain, the design day, based on the average maximum solar air temperature computed from the hourly meteorological data, is chosen. The resulting computer program is used to find the effect of some of the parameters like wall thickness, type of insulation, orientation of building and economics model on the optimum design.