全文获取类型
收费全文 | 64868篇 |
免费 | 4664篇 |
国内免费 | 139篇 |
出版年
2021年 | 470篇 |
2020年 | 416篇 |
2019年 | 485篇 |
2018年 | 991篇 |
2017年 | 953篇 |
2016年 | 1446篇 |
2015年 | 2223篇 |
2014年 | 2402篇 |
2013年 | 3146篇 |
2012年 | 3864篇 |
2011年 | 3589篇 |
2010年 | 2305篇 |
2009年 | 1799篇 |
2008年 | 3046篇 |
2007年 | 3044篇 |
2006年 | 3005篇 |
2005年 | 2876篇 |
2004年 | 2823篇 |
2003年 | 2641篇 |
2002年 | 2621篇 |
2001年 | 1682篇 |
2000年 | 1752篇 |
1999年 | 1436篇 |
1998年 | 685篇 |
1997年 | 560篇 |
1996年 | 532篇 |
1995年 | 533篇 |
1994年 | 522篇 |
1993年 | 499篇 |
1992年 | 911篇 |
1991年 | 910篇 |
1990年 | 855篇 |
1989年 | 802篇 |
1988年 | 774篇 |
1987年 | 693篇 |
1986年 | 665篇 |
1985年 | 639篇 |
1984年 | 633篇 |
1983年 | 566篇 |
1982年 | 515篇 |
1981年 | 516篇 |
1980年 | 406篇 |
1979年 | 591篇 |
1978年 | 476篇 |
1977年 | 401篇 |
1975年 | 456篇 |
1974年 | 429篇 |
1973年 | 405篇 |
1972年 | 408篇 |
1971年 | 364篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
61.
62.
63.
Steven D. Johnson Marco G. Balducci Alison Bijl Miguel Castañeda-Zárate Ruth J. Cozien Carmen R. Ortmann Timotheüs van der Niet 《Ecological Entomology》2020,45(3):751-755
1. Diel rhythms of foraging activity by animal flower visitors can reflect niche partitioning and are considered an important component of selection on floral traits. However, it has been notoriously difficult to obtain objective information on the patterns of flower visitation by crepuscular and nocturnal insects. 2. Motion-activated cameras were used for field-based studies of hawkmoth foraging behaviour on six African plant species. 3. The results showed that short-tongued hawkmoth species forage mainly around dusk and then sporadically throughout the night, whereas long-tongued hawkmoth species feed consistently throughout the night, with a peak shortly before midnight. 4. These results provide the first quantitative estimates of diel patterns of interactions between multiple hawkmoth and plant species and, when combined with qualitative reports from other studies, suggest that differences in diel activity between the two main hawkmoth functional groups (short- and long-tongued) are consistent across the Old and New Worlds. 相似文献
64.
65.
Hans-Günther Döbereiner Olaf Selchow Reinhard Lipowsky 《European biophysics journal : EBJ》1999,28(2):174-178
We present measurements of the effective spontaneous curvature of fluid lipid bilayers as a function of trans-bilayer asymmetry.
Experiments are performed on micrometer-scale vesicles in sugar solutions with varying species across the membrane. There
are two effects leading to a preferred curvature of such a vesicle. The spontaneous curvatures of the two monolayers as well
as their area difference combine into an effective spontaneous curvature of the membrane. Our technique for measuring this
parameter allows us to use vesicle morphology as a probe for general membrane-solute interactions affecting elasticity.
Received: 3 June 1998 / Revised version: 18 August 1998 / Accepted: 21 August 1998 相似文献
66.
Andreas K. Nussler Gianna Vergani Susanne M. Gollin Kenneth Dorko Susanne Gansauge Sidney M. Morris Jr. Antony J. Demetris Minoru Nomoto Hans G. Beger Stephen C. Strom 《In vitro cellular & developmental biology. Animal》1999,35(4):190-197
Summary The isolation and characterization of human liver cell lines are rather difficult due to limited material and poor growth
in cell culture. In this report, we present the isolation, culture and characterization of a new epithelial-like liver cell
line (AKN-1) with a heterogeneous cell population and many characteristics of the biliary epithelium. The AKN-1 cell line
stained positively with antibodies to epithelial cytokeratin polypetides CK 8, 18, and 19. In addition, the cell line expressed
the anti-human epithelial-related antigen (MOC-31), the human epithelial antigen (HEA), and the gamma-glutamyl transpeptidase,
the hematopoietic growth factor, stem cell factor, and also its receptor, c-kit. The cell line failed to express albumin and factor 8 by immunohistochemistry. It did show, however, a twofold increase in
7-ethoxyresorufin-O-deethylase activity. Cytogenetic characterization revealed rare breakpoints in chromosome 2, which to our knowledge, have
not yet been reported in liver cells. 相似文献
67.
We have established a series of 20 colorectal cancer cell lines and performed cytogenetic and RFLP analyses to show that the
recurrent genetic abnormalities of chromosomes 1, 5, 17 and 18 associated with multistep tumorigenesis in colorectal cancer,
and frequently detected as recurrent abnormalities in primary tumours, are also retained in long-term established cell lines.
Earlier studies by us and other investigators showed that allelic losses of chromosomes 1 and 17 in primary colorectal cancers
predicted poorer survival for the patients (P = 0.03). We utilized the cell lines to identify specific chromosomal sites or gene(s) on chromosomes 1 and 17 which confer
more aggressive phenotype. Cytogenetic deletions of chromosome 1p were detected in 14 out of the 20 (70%) cell lines, whereas
allelic deletions for 1p using polymorphic markers were detected in 13 out of 18 (72%) informative cell lines for at least
one polymorphic marker. We have performed Northern blotting, immunohistochemical staining (p53 mRNA, protein) and RFLP analysis
using several probes including p53 and nm23. RFLP analysis using a total of seven polymorphic markers located on 17p and 17q
arms showed allelic losses aroundthe p53 locus in 16 out of the 20 cell lines (80%), four of which were losses of thep53 locus itself. In addition, seven cell lines (out of nine informative cases) also showed losses of thenm23 gene, four with concurrent losses of thep53 locus, while the remaining three were homozygous. In addition, five out of seven cell lines withnm23 deletions were derived from hepatic metastatic tumours, and one cell line was obtained from recurrent tumour. A comparison
between allelic deletions of 1p and functional loss ofnm23 gene revealed a close association between these two events in cell lines derived from hepatic metastasis. Following immunohistochemical
staining, nine out of the twenty cell lines showed high levels (25–80%) of mutant p53, four showed intermediate levels (>20%),
and seven had undetectable levels of the protein. Of these seven, four showed complete absence of mRNA. Of the remaining three
cell lines one showed aberrant mRNA due to germline rearrangement of thep53 gene, whereas in two cell lines normal levels of mRNA were present. Nineteen of the 20 cell lines had normal germline configurations
for thep53 gene, while one showed a rearrangement. These data suggest that functional loss ofp53 andnm23 genes accomplished by a variety of mechanisms may be associated with poor prognosis and survival. In addition, concurrent
deletions of chromosome regions 17p, 17q and 1p were closely associated with high-stage hepatic metastatic disease. These
cell lines with well-characterized genetic alterations and known clinical history provide an invaluable source of material
for various biological and clinical studies relating to multistep colorectal tumorigenesis. 相似文献
68.
B Müller D Besser P Kleinw?chter O Arad S Reissmann 《The journal of peptide research》1999,54(5):383-393
To improve the assembly of backbone cyclic peptides, N-functionalized dipeptide building units were synthesized. The corresponding N-aminoalkyl or N-carboxyalkyl amino acids were formed by alkylation or reductive alkylation of amino acid benzyl or tert-butyl esters. In the case of N-aminoalkyl amino acid derivatives the aldehydes for reductive alkylation were obtained from N,O-dimethyl hydroxamates of N-protected amino acids by reduction with LiAlH4. N-carboxymethyl amino acids were synthesized by alkylation using bromoacetic acid ester and the N-carboxyethyl amino acids via reductive alkylation using aldehydes derived from formyl Meldrums acid. Removal of the carboxy protecting group leads to free N-alkyl amino acids of very low solubility in organic solvents, allowing efficient purification by extraction of the crude product. These N-alkyl amino acids were converted to their tetramethylsilane-esters by silylation with N,O-bis-(trimethylsilyl)acetamide and could thus be used for the coupling with Fmoc-protected amino acid chlorides or fluorides. To avoid racemization the tert-butyl esters of N-alkyl amino acids were coupled with the Fmoc-amino acid halides in the presence of the weak base collidine. Both the N-aminoalkyl and N-carboxyalkyl functionalized dipeptide building units could be obtained in good yield and purity. For peptide assembly on the solid support, the allyl type protection of the branching moiety turned out to be most suitable. The Fmoc-protected N-functionalized dipeptide units can be used like any amino acid derivative under the standard conditions for Fmoc-solid phase synthesis. 相似文献
69.
70.
Ross A. Robinson Samuel C. Griffiths Lieke L. van de Haar Tomas Malinauskas Eljo Y. van Battum Pavol Zelina Rebekka A. Schwab Dimple Karia Lina Malinauskaite Sara Brignani Marleen H. van den Munkhof Özge Düdükcü Anna A. De Ruiter Dianne M.A. Van den Heuvel Benjamin Bishop Jonathan Elegheert A. Radu Aricescu R. Jeroen Pasterkamp Christian Siebold 《Cell》2021,184(8):2103-2120.e31