全文获取类型
收费全文 | 2517篇 |
免费 | 228篇 |
国内免费 | 1篇 |
出版年
2023年 | 10篇 |
2022年 | 25篇 |
2021年 | 35篇 |
2020年 | 25篇 |
2019年 | 42篇 |
2018年 | 35篇 |
2017年 | 39篇 |
2016年 | 82篇 |
2015年 | 127篇 |
2014年 | 140篇 |
2013年 | 154篇 |
2012年 | 216篇 |
2011年 | 162篇 |
2010年 | 117篇 |
2009年 | 92篇 |
2008年 | 142篇 |
2007年 | 145篇 |
2006年 | 145篇 |
2005年 | 140篇 |
2004年 | 119篇 |
2003年 | 120篇 |
2002年 | 132篇 |
2001年 | 39篇 |
2000年 | 23篇 |
1999年 | 29篇 |
1998年 | 35篇 |
1997年 | 18篇 |
1996年 | 22篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 23篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 16篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 17篇 |
1975年 | 6篇 |
1973年 | 9篇 |
1967年 | 5篇 |
排序方式: 共有2746条查询结果,搜索用时 18 毫秒
41.
Freeze cleaving electron microscopy has shown that fusion of isolated secretory vesicles from bovine neurohypophyses was induced by Ca2+ in micromolar concentrations. Mg2+ and Sr2+ were ineffective. Mg2+ inhibited Ca2+-induced fusion.In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca2+. Mg2+ and Sr2+ were ineffective in replacing Ca2+ as trigger for fusion or vasopressin release.Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosome were exposed to cold. 相似文献
42.
Niels H. Andersen Shoji Imamoto Donald H. Picker 《Prostaglandins & other lipid mediators》1977,14(1):61-101
With the report given herein all diastereomers of PGF2, PGE2, and PGD2 which bear the naturally recognized 15-S hydroxylated center, whether in the natural or -prostanoic acid skeleton, have been prepared by a route involving initial introduction of the carboxyl (α) chain (1). A major advantage of the initial α-ylation+ route is the facile reduction of the 13,14-en-15-one system with methanolic NaBH4 which proceeds without competing 1,4-reduction. The products are thus free of 13,14-dihydro-PG2 contaminants (2). The initial pharmaco logical evaluation of these diastereomers will be submitted for publication in this journal (3). 相似文献
43.
Refinement of the three-dimensional solution structure of barley serine proteinase inhibitor 2 and comparison with the structures in crystals. 总被引:6,自引:0,他引:6
S Ludvigsen H Y Shen M Kjaer J C Madsen F M Poulsen 《Journal of molecular biology》1991,222(3):621-635
The three-dimensional structure of barley serine proteinase inhibitor, CI-2, has been determined using nuclear magnetic resonance spectroscopy. The present structure determination is a refinement of the structure previously determined by us, using in the present case stereo-specific assignments, and a virtually complete set of assignments of the two-dimensional nuclear Overhauser spectrum. The structure determination is based on the identification of more than 1300 nuclear Overhauser effects, of which 961 were used in the structure calculation as distance restraints, and on 94 dihedral angle restraints, of which 31 are for chi 1 angles in defined chiral centers. These have been used to calculate a series of 20 three-dimensional structures using a combination of distance geometry, simulated annealing and restrained molecular dynamics. Each of the 20 structures was in agreement within less than 0.5 A of each of the distance restraints and with all dihedral angle restraints. When compared to the geometric average structure of the 20 refined structures the root-mean-square differences for the backbone atoms were 0.8 (+/- 0.2) A and for all atoms were 1.6 (+/- 0.2) A. By comparison, the values obtained for the structures determined previously were 1.4 (+/- 0.2) A and 2.1 (+/- 0.1) A, respectively. The structures were also compared to the structure determined in the crystalline state by X-ray diffraction showing root-mean-square differences of 1.6 (+/- 0.2) A and 2.8 (+/- 0.2) A for the backbone and all atoms, respectively. Common features of the solution structure and the two crystal structures are the four-stranded beta-structure, composed of a pair of parallel strands, and three pairs of antiparallel beta-strands flanked on one side by a 12-residue alpha-helix and on the other side by a loop containing the serine proteinase binding site. The new analysis of the structure has revealed an additional pair of antiparallel beta-strands, consisting of residues 65 to 67 and 81 to 83, that was not seen in either of the crystal structures or the previous solution structure. Identification of this was based on nuclear magnetic resonance evidence for the hydrogen bond (67HN to 81CO) not reported previously. Also the presence of a bifurcated hydrogen bond involving Phe69 CO and HN atoms of Ala77 and Gln78 was observed in solution but not in crystals. Minor differences between the two structures were observed in the phi-angles of residues Met59 and Glu60 in the inhibitory site. 相似文献
44.
Niels C. Krejci Lynne Smith Rebecca Rudd Robert Langdon Joseph McGuire 《In vitro cellular & developmental biology. Animal》1991,27(12):933-938
Summary To investigate the regulation of epithelial differentiation, normal human epidermal keratinocytes were cultured floating on
the surface of culture medium without attachment to a solid substrate. Keratinocytes spread out on the surface of the medium,
proliferated and differentiated either into several flat lacy sheets 1 to 3 cells thick (on medium containing 0.15 mM calcium) or formed one single aggregate of cells from 5 to 15 cells in thickness on medium containing 1.15 mM calcium. The cell aggregates demonstrated a pattern of ordered epithelial differentiation. Levels of progressive differentiation
resembling the structure of normal human epidermis were identified by light microscopy, immunohistochemistry, and electron
microscopy. Differentiation proceeded from cells at the air side toward cells at the medium side with basal appearing cells
on the air side and keratinocytes expressing filaggrin and involucrin on the side toward the medium. These results demonstrate
that organized epithelial differentiation can occur in the absence of extracellular matrix. In contrast with other air-liquid
interface cultures, epithelial differentiation in the absence of extracellular matrix progresses from air towards medium. 相似文献
45.
46.
Constance Vind Niels Grunnet 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1982,720(3):295-302
This paper describes the transfer of tritium from [2-3H]xylitol or (1R)-[1-3H]ethanol into lactate in cells from fed rats either untreated or triiodothyronine-treated. The labelling pattern of lactate during the metabolism of [2-3H]xylitol or (1R)-[1-3H]ethanol follows the equation (μmol tritium/μmol lactate). The yield in lactate together with the minimum value of the total flux of reducing equivalents are used to estimate the specific radioactivity of NADH. We have calculated the lactate dehydrogenase-catalysed oxidation rate of NADH from the experimental values of lactate labelling and the specific radioactivity of NADH. We found the calculated flux of reducing equivalents from NADH to pyruvate to be of the same order of magnitude whether labelled ethanol or labelled xylitol was metabolized. We found the flux to be only a few percent of the maximal activity of lactate dehydrogenase. The results obtained suggest that the cytoplasm can be regarded as one compartment, containing a single pool of NAD(H). 相似文献
47.
Elizabeth M.K. Leovey Niels H. Andersen Pamela Bissonette 《Prostaglandins & other lipid mediators》1979,17(1)
, originally introduced as an inadvertent contaminant in solutions used for evaluating the stability of prostaglandins, proved to lead to the rapid disappearance of the cyclopentenone unit of PGA2 (as monitored by circular dichroic spectroscopy). The cyclopentenone unit is converted, in various metabolites, to a 9-keto, 9α or 9β-hydroxy group lacking the ring unsaturation. The major EtoAc-soluble 9-hydroxy metabolite (Compound-I) was shown to be 9α, 15α-dihydroxy-2,3,4,5-tetranor-13-
-prostenoic acid. Similar tetranor 9-hydroxy metabolites with one additional degree of unsaturation, and with a 9β-hydroxy group, also occur but these have not been fully characterized. Only two of the wide range of 9-keto metabolites are fully characterized by mass spectral (MS) data: 9,15-oxo-2,3,4,5-tetranorprostanoic acid and 9,15-oxo-2,3,4,5-tetranor-13-
-prostenoic acid. The water soluble metabolites have not been characterized further.The fully characterized metabolites together with MS data from mixtures of minor metabolites indicate that
can perform the following transformations: β-oxidation, dehydrogenation at C-15, reduction of the enone carbon-carbon double bonds (both Δ10,11 and Δ13,14), reduction of the 9-ketone, and possibly migration of the cyclopentyl double bond (Δ10,11 → Δ11,12).
metabolizes 15-epimeric PGA2 equally readily with the production of similar products. PGA1 affords less 9-keto metabolites with compound I constituting 33% of the product by HPLC analysis.
displays some enantioselectivity, PGA2 and 15-epi-PGA2 are each metabolized more rapidly than their enantiomers. Other prostaglandins appear to be less readily metabolized. 相似文献
48.
Steen Pedersen Solveig V. Reeh Jack Parker Robert J. Watson James D. Friesen Niels P. Fiil 《Molecular & general genetics : MGG》1976,144(3):339-343
Summary The presence of EF-Tu, RNA polymerase subunit , and EF-G on the dfus-3 genome and EF-Tu, ribosomal proteins L7/L12, and RNA polymerase subunit on the drif
d
18 genome has been confirmed using a two-dimensional gel electrophoresis technique sensitive to changes in isoelectric point and molecular weight. In this system two EF-Tu gene products could not be resolved. Following infection of ultraviolet light-irradiated Escherichia coli with either dfus-3 or drif
d
18, the EF-Tu gene, tufA, near 65 minutes on the genetic map is expressed as 3–4 copies per EF-G molecule. The EF-Tu gene, tufB, near 79 minutes on the genetic map, is expressed at about one-third of this rate. is expressed as 1 copy per EF-G molecule, as 0.14 per EF-G molecule and L7/L12 as 2.5 per EF-G. These figures compare well with the relative amounts found in exponentially-growing cells, in which the ratio of EF-Tu to EF-G is approximately 5. Almost 90% of the total number of proteins (calculated on a molecular weight basis) which theoretically can be encoded on the drif
d
18 have been identified on the two-dimensional gel. 相似文献
49.
CYTODYNAMICS IN THE THYMUS OF YOUNG ADULT MICE: 总被引:1,自引:0,他引:1
Cell proliferation and cell loss in the thymic blast cell population were studied in young adult mice by (1) stathmokinetic methods combined with an analysis of the PLMe-curve after a pulse 3H-TdR, and (2) nigrosin-dye exclusion combined with 3H-TdR-autoradiography. It was calculated that about 17% of the blast cells do not progress into mitosis within the period of an average cell cycle. The dye exclusion studies indicated a rate of blast cell death of about 2–5 %/hr. The two methods of assessing blast cell loss (death) support each other very well. In spite of these findings scintillation countings on thymuses removed from 1 to 17 hr after 3H-TdR injection showed fairly constant levels of thymic radioactivity. This suggests a very extensive reutilization of 3H-labelled break-down products from dying blast cells. The very sparse labelling of pyknotic thymocytes strongly suggests that thymic blast cells do not become pyknotic. The rate of small thymocyte production and disappearance was studied by pulse and repeated 3H-TdR labelling techniques combined with dye exclusion studies and pyknotic counts. The data from the repeated labelling experiment were analysed by use of a model based on the assumption of first order kinetics of small viable, dead, and pyknotic thymocytes. The rate of cell production was estimated to 1–6 %/hr whereas the rates of cell loss due to disintegration, i.e. supravital stainability and nuclear pyknosis, were calculated to 0–02 %/hr and 0–0006 %/hr respectively. Cell loss due to disintegration was less than 2 % of the total loss of small thymocytes. It was concluded that pyknotic counts are a useless method of assessing the cell death in the population of thymic blast cells and small thymocytes. On the basis of a model for thymocyte proliferation, production and loss it is suggested that about 45 % of the small viable thymocytes re-enter the generative cell pool, whereas about 55% disappear by emigration. 相似文献
50.