Dominant Hypothesis

The biological nitrogen fixation process is Introduced. Discussion focusses on the Dominant Hypothesis of nitrogenase composition and functioning. The enzyme system catalyzes the six-electron reduction of N2 to 2 NH3 concomitant with the evolution of H2. ATP hydrolysis drives the process. The two protein components of the enzyme,... 

Clearly, the spectroscopic properties of the P clusters in the proteins do not reveal their structural nature. However, extrusion of these clusters from the protein leads to the clear identification of 3-4 Fe S clusters(13.291. Despite the uncertainties inherent in the extrusion procedure (due to possible cluster rearrangement) the extrusion result supports the Dominant Hypothesis, which designates the P centers as Fe S units, albeit highly unusual ones. The P clusters are thought to be involved in electron transfer and storage presumably providing a reservoir of low potential electrons to be used by the M center (FeMo-co) in substrate reduction. 

It should be pointed out that while the protease-antiprotease theory of emphysema is currently the dominant hypothesis used to explain the etiology of emphysema, other factors must also be considered as contributing to the development of this disease. For example, oxidants, present in smoke, probably contribute in other fashions besides inactivating a -Pl. Such substances are capable of oxidizing a variety of cellular components (lipids, enzymes, and nucleic acids) and in this way may destroy parts of the alveolar epithelium, thus facilitating the exposure of elastin fragments present in the interstitial space to elastase. 

A schematic model of these activation events is shown in the following figure. Two possible models are envisaged expansion or fusion. The fusion model is the dominant hypothesis in the field. In assessing the evidence, the finding that the ATPase is present in tubules, not vesicles, suggests that fusion is restricted to the tips of the tubules and does not really resemble exocytosis in the strict sense. After fusion, there must be eversion of the tubules to form the microvilli. Presumably, the KCl pathway is at the base of the microvilli to allow effective reabsorption of K. 

Vacuum Flow When gas flows under high vacuum conditions or through very small openings, the continuum hypothesis is no longer appropriate if the channel dimension is not very large compared to the mean free path of the gas. When the mean free path is comparable to the channel dimension, flow is dominated by collisions of molecules with the wall, rather than by colhsions between molecules. An approximate expression based on Brown, et al. J. Appl. Phys., 17, 802-813 [1946]) for the mean free path is... 

DMS has been observed in the marine atmosphere since the early 1970s, but it was not until the mid-1980s that there was interest in this gas as being a natural source for sulfate CCN. Sulfate aerosols are, in number terms, the dominant source of CCN. The major role clouds play in the climate system leads to possible climatic implications if changes to DMS production occurred. Furthermore, the dependence of this production on environment conditions means that scope for a feedback process arises this feedback is called the Charlson hypothesis. ... 

Support for the hypothesis that the relative position of the transition state is responsible for the dominant (kinetic) formation of either the (Z)- or the (ii)-isomer comes from a comparison of Hammett /7-values for rates of addition of nucleophiles with p-values for the corresponding equilibria. 

The sum of all results is consistent with the formation of both the aryl cation and the aryl radical in the aqueous acid system without copper, and with the dominance of the aryl radical in the presence of copper. The product ratios are also qualitatively consistent with the hypothesis that the reactivity of aryl cations with nucleophiles is close to that of a diffusion-controlled process (see Sec. 8.3), and that aryl radicals have arylation rate constants that are about two orders of magnitude smaller than that for diffusion control (0.4-1.7 X 107 m-1s-1 Kryger et al., 1977 Scaiano and Stewart, 1983). Due to the relatively low yields of these dediazoniations in the pentyl nitrite/benzene systems, no conclusions should be drawn from the results. 

Much of the variation in these time series for the past 700 kyr can be described by a combination of a 100 kyr cycle plus additional cycles with periods of 20 and 40 kyr. This result immediately suggests that the ice-age cycles are caused by variations in the amount and seasonality of solar radiation reaching the Earth (insolation), because the 20, 40, and 100 kyr periods of climate history match the periods of cyclic variations in Earth s orbit and axial tilt, line hypothesis that these factors control climate was proposed by Milutin Milankovitch in the early part of the 20th century and is widely known as "Milankovitch Theory." It is now generally accepted that the Milankovitch variations are the root cause of the important 20 and 40 kyr climate cycles. The 100 kyr cycle, however, proves to be a puzzle. The magnitude of the insolation variation at this periodicity is relatively trivial, but the 100 kyr cycle dominates the climate history of the last 700 kyr. Further,... 

A current working hypothesis is that F. and chemoinvestigative behaviour are closely linked, but that it is not restricted to urinalysis by the male. Other signalling functions are indicated by the behaviour of captive and feral groups. Flehmen s role in female female interactions is clearly of equal value since it is highly correlated with rank in the Sable antelope (Hippotragus niger). Dominant females not only perform... 

Detailed analysis of the DDIF spectra showed a constant microporosity for almost all samples. This lends strong support to the hypothesis that the original rocks after burial, compaction and the initial diagenesis had a common pore geometry. Such a base rock appears to be dominated by the micropores with approximately 2/3 micropores, 1 /3 mesopores and very few macropores. The effect... 

There are three subcases to consider. First suppose that e is not a branch node. Then e is directly connected to exactly one node n. Clearly e chain dominates n and G = (e) U S(n,d). The subscheme corresponding to e is well-structured and e is not in S(n,d) which is line-like by Lemma 4.12. Thus P is the block composition of the subschemes corresponding to e and to S(n,d). By the induction hypothesis S(n,d) corresponds to a well-structured subgraph, and so P is well-structured. 

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