全文获取类型
收费全文 | 106549篇 |
免费 | 10035篇 |
国内免费 | 48篇 |
专业分类
116632篇 |
出版年
2022年 | 872篇 |
2021年 | 1819篇 |
2020年 | 1029篇 |
2019年 | 1352篇 |
2018年 | 1650篇 |
2017年 | 1517篇 |
2016年 | 2465篇 |
2015年 | 4201篇 |
2014年 | 4723篇 |
2013年 | 5683篇 |
2012年 | 7579篇 |
2011年 | 7340篇 |
2010年 | 4699篇 |
2009年 | 4428篇 |
2008年 | 6234篇 |
2007年 | 6277篇 |
2006年 | 5871篇 |
2005年 | 5690篇 |
2004年 | 5475篇 |
2003年 | 5276篇 |
2002年 | 4987篇 |
2001年 | 1317篇 |
2000年 | 1094篇 |
1999年 | 1324篇 |
1998年 | 1458篇 |
1997年 | 1063篇 |
1996年 | 986篇 |
1995年 | 892篇 |
1994年 | 790篇 |
1993年 | 866篇 |
1992年 | 920篇 |
1991年 | 800篇 |
1990年 | 809篇 |
1989年 | 774篇 |
1988年 | 722篇 |
1987年 | 687篇 |
1986年 | 609篇 |
1985年 | 833篇 |
1984年 | 853篇 |
1983年 | 764篇 |
1982年 | 868篇 |
1981年 | 791篇 |
1980年 | 774篇 |
1979年 | 556篇 |
1978年 | 595篇 |
1977年 | 519篇 |
1976年 | 511篇 |
1975年 | 409篇 |
1974年 | 485篇 |
1973年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
A method for the isolation and concentration of the monoglutamate forms of folate cofactors from tissues and for their subsequent separation and quantitation using HPLC coupled with uv detection at 284 nm is described. A chromatographic procedure utilizing Dowex 50 has been developed for the separation of the folate monoglutamates from a large portion of the nonfolate-related material following digestion of the polyglutamated froms with a highly purified preparation of rat liver conjugase. This chromatographic procedure combined with concentration of the Dowex eluate by lyophilization eliminates uv-absorbing material, which interferes with the detection and quantitation of the folate cofactors and makes possible uv measurement of the individual folates. Reverse-phase paired-ion chromatography on μBondapak C18 coupled with uv detection allows direct quantitation of the folates in the nanogram range. 相似文献
132.
Anne M. Berry Eric M. Glare David Hansman James C. Paton 《FEMS microbiology letters》1989,65(3):275-278
Twenty-four of 63 enteric Gram-negative organisms (38.1%) which were isolated from 35 apparently healthy Nigerian students were found to have low trimethoprim resistance (MIC less than 1000 mg/l). These isolates were also found to be resistant to several other antibiotics and trimethoprim resistance was found to be transferable from 15 (62.5%) of the trimethoprim resistant organisms into E. coli EC 1005. It is likely that the high percentage of trimethoprim resistance encountered in this study is related to the high rate of resistance transfer which was observed. 相似文献
133.
John M. Robinson Sylvia A. Larrimore David W. Craft H.E. Heath Gary L. Sloan 《Biochemical and biophysical research communications》1982,109(3):730-737
The extracellular protease, endopeptidase, and hexosaminidase produced by were neither induced nor repressed by amino acids but required a tryptic digest of casein for their production. Catabolite repression of exoenzyme production by glucose was not affected by exogenous cyclic adenosine 3′, 5′-monophosphate but was partially relieved by di- or monobutyryl derivatives of this compound. 相似文献
134.
135.
A bioassay was developed by Rittschof el al. (1983) to examinedistance chemoreception in the predatory marine gastropod, Urosalpinxcinerea. This bioassay was used to test the effect of a senesof low mol. wt. organics on the ability of newly hatched oysterdrills to detect a prey odor released from barnacles, Balanusbalanoides. Two series of low mol. wt. organics were testedusing methanol as the reference compound. In one series, R-OH,the carbon chain length was varied from 1 to 4. In the secondseries, CH3-R, the chain length was held constant while thefunctional group, R, was varied. When these compounds were presentin the rnM range, they inhibited the creeping response of oysterdrills towards barnacle prey odor. In the CH3-R series, inhibitionincreased in the following order: sodium acetate > ethylacetate > acetonitnle > methanol; and, in the alcoholseries C1 to C4, inhibition increased with increasing chainlength. No creeping response was observed when these compoundswere tested in the absence of prey odor. 相似文献
136.
Gillian D Alton David L Pearl Ken G Bateman W Bruce McNab Olaf Berke 《BMC veterinary research》2010,6(1):42
Background
Ontario provincial abattoirs have the potential to be important sources of syndromic surveillance data for emerging diseases of concern to animal health, public health and food safety. The objectives of this study were to: (1) describe provincially inspected abattoirs processing cattle in Ontario in terms of the number of abattoirs, the number of weeks abattoirs process cattle, geographical distribution, types of whole carcass condemnations reported, and the distance animals are shipped for slaughter; and (2) identify various seasonal, secular, disease and non-disease factors that might bias the results of quantitative methods, such as cluster detection methods, used for food animal syndromic surveillance. 相似文献137.
C P Taylor 《Biochimica et biophysica acta》1977,491(1):137-148
A simple method is presented for calculating the parameters of the hole model for distorted octahedral, low spin (t2g)5 complexes. In the case of negligible covalent bonding explicit formulas for the coefficients of the Kramer's doublet, (formula: see text). The two numerically largest g values define the plane and orientation of the orbital with the largest coefficient, which in turn indicates the directions of maximal unpaired spin density. The energy of eta with respect to xi (in units of lambda, the spin-orbit coupling constant) is (formula: see text). The product gzgygx, independent of axes, and positive for free electrons, is shown to be positive for tetragonal and negative for nearly octahedral complexes. It is considered positive for hemes. In this method coefficients will only be normalized when there is no covalency. For the majority of published cases they are, to about 1%. Since this discrepancy is larger than can be caused by propagated errors, covalency must be the rule. For comparative purposes A and B, uncorrected for covalency, should still be useful. Examination of published complete g tensors for five hemes shows that the largest g value is nearly normal to the heme plane. If the g values are taken positive and labelled so that gz greater than gy greater than gx, then the proper tetragonal axis is roughly normal to plane of the ring in hemes, but not in chlorins. 相似文献
138.
139.
High hydrostatic pressure applied between sperm attachment and the onset of cortical granule exocytosis will inhibit this exocytotic event in sea urchin eggs. Such pressure-treated zygotes, nevertheless, are activated and capable of development. Thus, this technique can be used as a tool to study the relationship between cortical granule breakdown and other fertilization-related responses. We have studied whether the exocytosis of cortical granules is necessary for proton efflux (acid release) to occur. Our results indicate that although Ca2+ is released while the eggs are under pressure (a prerequisite for the following events to take place), cortical granule exocytosis and acid release are pressure-sensitive and completely inhibited at pressures above 400 atm (6000 psi) and 275 atm (4000 psi), respectively. However, upon decompression, acid release is initiated which amounts to 65–70% of that seen in the unpressurized controls, suggesting that the efflux mechanism does not require cortical granule exocytosis and must result from some modification of the original plasma membrane of the egg. The remaining 30–35% of the acid release is related to cortical granule exocytosis, since it can be obtained upon induction of the cortical granule fusion 30 min later under atmospheric pressure. The initiation of acid release after decompression indicates that the efflux mechanism is not transiently turned on at fertilization, but undergoing long-term modification; the recovery of the ability to induce cortical granule fusion after fertilization under pressure suggests a refilling of cytoplasmic Ca2+ stores within this time course. 相似文献
140.
G Mertens J J Cassiman H Van den Berghe J Vermylen G David 《The Journal of biological chemistry》1992,267(28):20435-20443
Human aortic endothelial cells (HAEC) and human umbilical vein endothelial cells (HUVEC) were labeled with 35SO(4)2- for 48 h. The membrane-associated proteoglycans were solubilized from these monolayers with detergent and purified by ion-exchange chromatography on Mono Q, incorporation in liposomes, and gel filtration. The liposome-intercalated proteoglycans were 125I-iodinated and treated with heparitinase before SDS-polyacrylamide gel electrophoresis. Radio-labeled proteins with apparent molecular masses of 130, 60, 46, 35, and 30 kDa (HAEC) and 180, 130, 62, 43, and 35 kDa (HUVEC) were detected by autoradiography. Further characterization by affinity chromatography on immobilized monoclonal antibodies and by Northern blot analysis provided evidence for the expression of syndecan, glypican, and fibroglycan in human endothelial cells. Most of the heparan sulfate which accumulated in the subendothelial matrix was implanted on a 400-kDa core protein. This protein was immunologically related to perlecan and bound to fibronectin. Binding studies on immobilized antithrombin III suggested that all membrane-associated heparan sulfate proteoglycan forms had the capacity to bind to antithrombin III but that high affinity binding was more typical for glypican. Most of the proteoglycans isolated from the extracellular matrix also bound only with low affinity to antithrombin III. These results imply that glypican may specifically contribute to the antithrombotic properties of the vascular wall. 相似文献