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All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed
of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited
by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures
arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc.
Natl. Acad. Sci. 95, 7215–7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the
direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal
wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations.
The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether
the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain,
(ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions
do change, we find that in all cases the overall character of the helicoidal solutions is preserved. 相似文献
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T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献
29.
To maximize fitness, many animals must trade off their need to forage efficiently against their need to avoid predators. We studied such a trade-off in four species of tits (Paridae) in a forest near Oxford, UK. During winter, tits form flocks which increase feeding efficiency and reduce predation risk. These flocks feed extensively on beech (Fagus sylvatica) seeds, the abundance of which may be critical for winter survival. Because these seeds drop to the ground, where birds are exposed to sparrowhawk (Accipiter nisus) attack, tits need to trade off their need to find seeds against the proximity to protective cover, provided by dense clusters of hawthorn (Crataegus spp.). The quality of the beech crop differs markedly between trees and years. During a year of abundant beechmast, most tits searched for seeds close to protective cover. This 'safety-first' strategy precluded visits to superabundant food patches if they were too far from protective cover. Among beech trees near to cover, tits tended to prefer those with high seed density. Tits benefited from foraging under trees with high seed density because this correlated significantly with seed mass per square metre and because mean search times decreased with increasing seed density. Finally, we show experimentally that great tits, Parus major, can discriminate between edible (viable) and inedible (empty) seeds. 相似文献
30.
(Z)- and (E)-4-amino-2-(trifluoromethyl)-2-butenoic acid (4 and 5, respectively) were synthesized and investigated as potential mechanism-based inactivators of gamma-aminobutyric acid aminotransferase (GABA-AT) in a continuing effort to map the active site of this enzyme. The core alpha-trifluoromethyl-alpha,beta-unsaturated ester moiety was prepared via a Reformatsky/reductive elimination coupling of the key intermediates tert-butyl 2,2-dichloro-3,3,3-trifluoropropionate and N,N-bis(tert-butoxy-carbonyl)glycinal. Both 4 and 5 inhibited GABA-AT in a time-dependent manner, but displayed non-pseudo-first-order inactivation kinetics; initially, the inactivation rate increased with time. Further investigation demonstrated that the actual inactivator is generated enzymatically from 4 or 5. This inactivating species is released from the active site prior to inactivation, and as a result, 4 and 5 cannot be defined as mechanism-based inactivators. Furthermore, 4 and 5 are alternate substrates for GABA-AT, transaminated by the enzyme with Km values of 0.74 and 20.5 mM, respectively. Transamination occurs approximately 276 and 305 times per inactivation event for 4 and 5, respectively. The enzyme also catalyzes the elimination of the fluoride ion from 4 and 5. A mechanism to account for these observations is proposed. 相似文献