全文获取类型
收费全文 | 15334篇 |
免费 | 1046篇 |
国内免费 | 1篇 |
专业分类
16381篇 |
出版年
2023年 | 114篇 |
2022年 | 226篇 |
2021年 | 447篇 |
2020年 | 294篇 |
2019年 | 360篇 |
2018年 | 450篇 |
2017年 | 413篇 |
2016年 | 637篇 |
2015年 | 961篇 |
2014年 | 1020篇 |
2013年 | 1269篇 |
2012年 | 1492篇 |
2011年 | 1349篇 |
2010年 | 835篇 |
2009年 | 717篇 |
2008年 | 880篇 |
2007年 | 864篇 |
2006年 | 811篇 |
2005年 | 667篇 |
2004年 | 631篇 |
2003年 | 556篇 |
2002年 | 446篇 |
2001年 | 100篇 |
2000年 | 77篇 |
1999年 | 101篇 |
1998年 | 84篇 |
1997年 | 75篇 |
1996年 | 68篇 |
1995年 | 51篇 |
1994年 | 55篇 |
1993年 | 40篇 |
1992年 | 40篇 |
1991年 | 30篇 |
1990年 | 28篇 |
1989年 | 25篇 |
1988年 | 12篇 |
1987年 | 20篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1975年 | 5篇 |
1972年 | 5篇 |
1961年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Summary Various tissues of common winkles,Littorina littorea (L.), experimentally exposed to cadmium (Cd) chloride were examined using light and electron microscopy and their elemental
composition determined by X-ray microanalysis and histochemistry. Membrane granules in gill epithelial cells with paddle cilia
contain carbonates, phosphates and sulphides associated with different cations in different types of granules. Traces of Cd
have been found only in those granules containing sulphur and iron. Nephrocytes also contain small amounts of this metal in
the cytoplasm of excretory cells. X-ray microanalysis reveals that concretions of basophilic cells are minor sites for Cd
sequestration while BTAN-ASSG stain for unbound Cd indicates that most of the Cd is located within the lysosomes of digestive
cells in association with proteins. Low amounts of the metal have been evidenced in the granules of epithelial mantle cells
rich in sulphur. The results also indicate that hemocytes contain granules of calcium phosphate and iron sulphide. Cd is also
associated to sulphur rather than to phosphate. These hemocytes may act as Cd carrier from gills to kidney and digestive gland.
A hypothetical pathway for Cd accumulation and detoxification is suggested. 相似文献
12.
Paulo E. Oliveira Peter E. Gibbs Ana Angélica Barbosa Salvador Talavera 《Plant Systematics and Evolution》1992,179(3-4):207-219
The pollination biology and breeding systems ofEriotheca pubescens andE. gracilipes have been studied. These two species occur as trees in cerrado vegetation, the neotropical savannas of Central Brazil, with partially sympatric distributions. They have similar phenology and floral structure, although the flowers ofE. pubescens are larger. Both species have nectar flowers pollinated by largeAnthophoridae bees but the main pollinators of each species differ in size. The species have markedly different breeding systems: late-acting self-incompatibility inE. gracilipes and apomixis stimulated by pollination inE. pubescens. 相似文献
13.
Fernando G. de Mello Jan N. Hokoç Ana L. M. Ventura Patrícia F. Gardino 《Cellular and molecular neurobiology》1991,11(5):485-496
1. Retina-cell aggregate cultures expressed glutamate decarboxylase activity (L-glutamate 1-carboxylase; EC 4.1.1.15) as a function of culture differentiation. 2. Glutamic acid decarboxylase (GAD) activity was low in the initial phases of culture and increased eight-fold until culture day 7, remaining high up to day 13 (last stage studied). 3. The addition of GABA to the culture medium 24 h after cell seeding almost totally prevented the expression of GAD activity. 4. In association with decreased enzyme activity, aggregates exposed to GABA did not display immunoreactivity for GAD, suggesting that GAD molecules were either lost from GABAergic neurons or significantly altered with GABA treatment. 5. Control, untreated aggregates showed intense GAD immunoreactivity in neurons. Positive cell bodies were characterized by a thin rim of labeled cytoplasm with thickest labeling at the emergence of the main neurite. 6. Heavily labeled patches were also observed throughout the aggregates, possibly reflecting regions enriched in neurites. 7. The GABA-mediated reduction of GAD immunoreactivity was a reversible phenomenon and could be prevented by picrotoxin. 相似文献
14.
Dr. María Cristina Maldonado Ana María Strasser de Saad Danley Callieri 《Current microbiology》1989,18(5):303-306
Several recent reports have described large numbers of monoclonal antibodies that cross-react with toxins A and B ofClostridium difficile; this suggests that the toxins share major epitopes. Our results show that monoclonal antibodies (MAb) against other antigens bind nonspecifically to both toxins. Therefore, we believe that the cross-reacting MAb bind by this manner and not by a true immune reaction. 相似文献
15.
Marcelo Hermes-Lima Ana Claudia Tessis Adalberto Vieyra 《Origins of life and evolution of the biosphere》1990,20(1):27-41
In this paper it is shown that the adsorption of 5-adenosine monophosphate (5-AMP) onto precipitated calcium phosphate exhibits a sigmoidal profile as revealed by isotherms at 45 °C. This result indicates a cooperative behavior in the adsorption of 5-AMP. The relationship between adsorption capacity and surface area of the sedimented matrix may be interpreted as an indication that there is a monolayer of the adsorbed nucleotide on the solid surface. The pH dependence of adsorption suggests that the negatively charged phosphoryl group of 5-AMP interacts with a positively charged site (possibly Ca2+) on the matrix surface. The adsorption of the nucleotide is markedly decreased at pH values above 8.0. The Dixon-like plot of the effect of pH suggests an inhibitory role of hydroxyl ions in the adsorption of 5-AMP. At pH 7.5, other anions such as pyrophosphate, tripolyphosphate and carbamyl phosphate also inhibit the adsorption of the nucleotide, probably by interacting with its adsorption site. We suggest that these phosphorylated molecules could have played a role in chemical evolution by modulating the amount of nucleotides adsorbed onto mineral surfaces. The significance of these phenomena in chemical evolution is discussed. 相似文献
16.
Ana M. Moura-da-Silva R. David G. Theakston Julian M. Cramptonl 《Journal of molecular evolution》1996,43(3):263-269
The evolution of the Metalloproteinase Disintegrin Cysteine-rich (MDC) gene family and that of the mammalian Matrix-degrading
Metalloproteinases (MMPs) are compared. The alignment of snake venom and mammalian MDC and MMP precursor sequences generated
a phylogenetic tree that grouped these proteins mainly according to their function. Based on this observation, a common ancestry
is suggested for mammalian and snake venom MDCs; it is also possible that gene duplication of the already-assembled domain
structure, followed by divergence of the copies, may have significantly contributed to the evolution of the functionally diverse
MDC proteins. The data also suggest that the structural resemblance of the zinc-binding motif of venom MDCs and MMPs may best
be explained by common ancestry and conservation of the proteolytic motifs during the divergence of the proteins rather than
through convergent evolution.
Correspondence to: J.M. Crampton 相似文献
17.
Ana María Planchuelo 《Brittonia》1996,48(2):263-265
Lupinus rubriflorus Planchuelo, from Rio Grande do Sul, Brazil, is described and illustrated. This species is related toL. bracteolaris s.l. differing in pubescence, stipules, and floral characters. 相似文献
18.
Sandra Irene Pitta-Alvarez Ana María Giulietti 《In vitro cellular & developmental biology. Plant》1995,31(4):215-220
Summary
Brugmansia candida hairy roots, obtained by infection withAgrobacterium rhizogenes LBA 9402, exhibit, after subculturing in liquid media, a tendency towards dedifferentiation. It has been found that the following
strategies can be applied to inhibit this dedifferentiation and preserve normal root morphology: (a) lowering both the mineral
and sucrose concentration in the media employed so as to diminish osmotic stress (a condition to which these roots appear
to be particularly susceptible); (b) employing antiauxins in appropriate concentrations; and (c) maintaining the hairy roots
on solid media prior to use in production processes in liquid media. The first strategy suggested does not favor alkaloid
productivity, but in this case a two-step method could be attempted: biomass with normal root morphology could be obtained
in a first stage using low sucrose concentrations, and in a second stage, sucrose could be increased in order to achieve higher
productivity. In all the clones ofB. candida obtained, alkaloid production was biased towards scopolamine. 相似文献
19.
20.
An Immunological Assay for Detection and Enumeration of Thermophilic Biomining Microorganisms 总被引:2,自引:1,他引:1 下载免费PDF全文
Ana M. Amaro Kevin B. Hallberg E. Brje Lindstrm Carlos A. Jerez 《Applied microbiology》1994,60(9):3470-3473
A specific, fast, and sensitive nonradioactive immunobinding assay for the detection and enumeration of the moderate thermophile Thiobacillus caldus and the thermophilic archaeon Sulfolobus acidocaldarius was developed. It employs enhanced chemiluminescence or peroxidase-conjugated immunoglobulins in a dot or slot blotting system and is very convenient for monitoring thermophilic bioleaching microorganisms in effluents from industrial bioleaching processes. 相似文献