全文获取类型
收费全文 | 730306篇 |
免费 | 77541篇 |
国内免费 | 370篇 |
专业分类
808217篇 |
出版年
2018年 | 10903篇 |
2017年 | 10546篇 |
2016年 | 11481篇 |
2015年 | 11269篇 |
2014年 | 13145篇 |
2013年 | 18531篇 |
2012年 | 22602篇 |
2011年 | 26995篇 |
2010年 | 19125篇 |
2009年 | 17930篇 |
2008年 | 23858篇 |
2007年 | 25788篇 |
2006年 | 18738篇 |
2005年 | 18813篇 |
2004年 | 18617篇 |
2003年 | 17991篇 |
2002年 | 17607篇 |
2001年 | 28517篇 |
2000年 | 28623篇 |
1999年 | 22865篇 |
1998年 | 8368篇 |
1997年 | 8629篇 |
1996年 | 8092篇 |
1995年 | 7753篇 |
1994年 | 7612篇 |
1993年 | 7588篇 |
1992年 | 19435篇 |
1991年 | 19014篇 |
1990年 | 18608篇 |
1989年 | 18045篇 |
1988年 | 17158篇 |
1987年 | 16473篇 |
1986年 | 15102篇 |
1985年 | 15148篇 |
1984年 | 12664篇 |
1983年 | 11047篇 |
1982年 | 8618篇 |
1981年 | 7723篇 |
1980年 | 7396篇 |
1979年 | 12445篇 |
1978年 | 9600篇 |
1977年 | 8966篇 |
1976年 | 8533篇 |
1975年 | 9203篇 |
1974年 | 10201篇 |
1973年 | 10162篇 |
1972年 | 9875篇 |
1971年 | 9036篇 |
1970年 | 7554篇 |
1969年 | 7376篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
G. E. Tanyi 《Bulletin of mathematical biology》1982,44(4):501-535
The Thom gradient model of morphogenesis poses the followinga posteriori problem: “From the observed morphology of a given natural process (effect) determine the dynamics of the process (cause)”. In this paper we consider the classicala priori problem: “Given the cause (dynamics) determine the effect (resultant morphology)”.
We find that in biochemical processes the mechanisms for energy activation, energy-matter interaction and energy dissipation
determine the dynamics. Furthermore there exists basic energy mechanisms which drive the equilibrium states through the elementary
catastrophes of Thom. A comparison with current theories shows that our models describe open ecological food chains and their
dynamical systems generalize the equations of organisation posed by M. Eigen.
Work supported by a Research Associateship of the International Centre for Theoretical Physics, P.O.B. 586, Miramare, 34100
Trieste, Italy. 相似文献
92.
T B Hargreave 《BMJ (Clinical research ed.)》1982,284(6333):1948-1949
93.
O Klinge E Alexandrakis 《Virchows Archiv. B, Cell pathology including molecular pathology》1981,37(3):369-379
The main hepatic change in erythropoietic protoporphyria is the deposition of protoporphyrin. Brown deposits of this pigment occur in bile canaliculi and ductules, discretely in hepatocytes, and secondarily in macrophages and Kupffer cells. The pigment is deposited in a crystalline form. Under the fluorescence microscope with a mercury maximum pressure burner (HO 50) at a wave length of 380--500 nm, it shows a typical red fluorescence even after paraffin embedding. Its crystalline structure results in a characteristic double refraction under the polarising microscope. Light-microscopically, hepatocellular reactions are characterised mainly by discrete alterations in the ergastoplasm. However, cell damage is indicated by diffusely distributed, hyaline single cell necrosis and by cytolytic piecemeal necrosis at the peripheries of hepatic lobules. Numerous, often disturbed mitoses produce binuclear and multinuclear hepatocytes. The obligatory secretion of protoporphyrin into the bile ducts leads to an alteration in the canalicular and ductular excretion apparatus which involves distinct ductular proliferation and accompanying fibrosis. Piecemeal necrosis is a further consequence of this process. The resulting histological picture is similar to sclerosing cholangitis with which it also has in common the slowly progressive development of hepatic cirrhosis. 相似文献
94.
Ryanodine was found to initially inhibit calcium uptake by cardiac sarcoplasmic reticulum. This initial depression was followed by a later marked stimulation of calcium uptake. These effects were noted when calcium uptake was measured in the presence or absence of oxalate. The requirement for preincubation with ryanodine was highly dependent on ryanodine concentration and temperature. The mechanism of action of ryanodine clearly was not an effect on oxalate entry or calcium oxalate precipitation because the effects were also observed in the absence of oxalate. Ryanodine also had no effect on passive calcium efflux from actively loaded vesicles. Because ryanodine had no effect on Ca2+-ATPase activity under defined conditions of an ATP-regenerating system and no calcium gradient, we suggest ryanodine does not change the stoichiometry of the pump. Our results are consistent with the hypothesis that ryanodine closes a calcium channel in a subpopulation of the vesicles. 相似文献
95.
96.
A. Knob C. R. F. Terrasan E. C. Carmona 《World journal of microbiology & biotechnology》2010,26(3):389-407
β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented. 相似文献
97.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms. 相似文献
98.
EEG activity was recorded in rats submitted to osmotic opening of the BBB by intracarotid mannitol infusion.This procedure produced an immediate short-lasting depression of the EEG and a tardive paroxysmal EEG activity. Both these phenomena were more relevant on the ipsilateral hemisphere. In some instances a tonico-clonic seizure was recorded.Pre-treatment with diazepam abolished the occurrence of the tardive EEG and behavioral modifications.In accord with previous findings, focal seizure activity is likely to be responsible for the metabolic abnormalities associated with osmotic opening of the BBB. This preparation therefore produces in the brain unphysiological states in respect to local metabolism and electrical function. 相似文献
99.
N. V. Ozolina E. V. Kolesnikova V. N. Nurminsky I. S. Nesterkina L. V. Dudareva L. I. Donskaya R. K. Salyaev 《Biochemistry (Moscow) Supplemental Series A: Membrane and Cell Biology》2010,4(3):297-301
The changes in transport activity of tonoplast proton pumps under the influence of exogenous NO donator and modulation of
Ca2+ concentration jointly and separately were investigated at different stages of ontogenesis and under hyperosmotic stress.
The results suggest that both exogenous NO donator and Ca2+ ions can influence the activity of transport processes related to tonoplast and this influence is especially evident in the
period of growth and accumulation of metabolites. Under hyperosmotic stress, H+-pyrophosphatase plays a more important role than H+-ATPase: the activity of the former increases 2.3-fold compared to the control osmotic conditions, whereas the activity of
H+-ATPase is practically unchanged. H+-pyrophosphatase was more responsive to the presence of exogenous NO donator and to variations in Ca2+ concentration. The effects of exogenous NO donator on tonoplast proton pumps depended on the concentration of Ca2+, which apparently can mediate NO action. 相似文献
100.
Summary A stream-breeding race of small-mouthed salamanders (Ambystoma texanum) in central Kentucky produces ova that are twice as large as those of a pond-breeding race found nearby. Embryos of stream-breeders also hatch at a more advanced developmental stage than those of pond-breeders. Morphological evidence indicates that stream-breeders were derived from pond-breeding stock. Assuming that differences between stream and pond-breeders reflect evolutionary change, and that the ancestral pond stock that invaded streams was similar to extant pond-breeders, we examined three hypotheses that might explain changes in ovum size and stage at hatching following the invasion of streams. (1) Larger ovum size evolved indirectly as a consequence of selection for rapid development which minimizes mortality risk from stream drying. (2) Increased ovum (hatchling) size and stage at hatching of stream-breeders are adaptations to resist stream current. (3) Increased ovum (hatchling) size and stage at hatching are adaptations to reduce predation on hatchlings from stream invertebrates. The results of field and laboratory studies only support hypotheses (2) and (3). Hatchlings that were relatively large or at a more advanced developmental stage had slower drift rates and were less vulnerable to predation by Phagocata gracilis, a flatworm abundant in streams in central Kentucky. Developmental and growth parameters were not correlated significantly with ovum size in populations of either geographic race. Differences in degree of parental care among races also cannot explain variation in ovum size since both races abandon their eggs immediately after oviposition. 相似文献