全文获取类型
收费全文 | 7233篇 |
免费 | 875篇 |
国内免费 | 5篇 |
专业分类
8113篇 |
出版年
2022年 | 72篇 |
2021年 | 124篇 |
2020年 | 81篇 |
2019年 | 82篇 |
2018年 | 107篇 |
2017年 | 83篇 |
2016年 | 153篇 |
2015年 | 249篇 |
2014年 | 280篇 |
2013年 | 342篇 |
2012年 | 410篇 |
2011年 | 426篇 |
2010年 | 229篇 |
2009年 | 224篇 |
2008年 | 382篇 |
2007年 | 345篇 |
2006年 | 324篇 |
2005年 | 325篇 |
2004年 | 290篇 |
2003年 | 268篇 |
2002年 | 243篇 |
2001年 | 215篇 |
2000年 | 224篇 |
1999年 | 169篇 |
1998年 | 90篇 |
1997年 | 75篇 |
1996年 | 70篇 |
1995年 | 64篇 |
1994年 | 82篇 |
1993年 | 84篇 |
1992年 | 156篇 |
1991年 | 136篇 |
1990年 | 136篇 |
1989年 | 116篇 |
1988年 | 107篇 |
1987年 | 125篇 |
1986年 | 104篇 |
1985年 | 103篇 |
1984年 | 101篇 |
1983年 | 77篇 |
1982年 | 74篇 |
1981年 | 69篇 |
1980年 | 52篇 |
1979年 | 70篇 |
1978年 | 55篇 |
1977年 | 43篇 |
1976年 | 46篇 |
1975年 | 40篇 |
1974年 | 42篇 |
1973年 | 58篇 |
排序方式: 共有8113条查询结果,搜索用时 15 毫秒
41.
42.
43.
Conformations of the central transforming region (Ile 55-Met 67) of the p21 protein and their relationship to activation of the protein 总被引:1,自引:0,他引:1
J M Chen G Lee P W Brandt-Rauf R B Murphy K D Gibson H A Scheraga S Rackovsky M R Pincus 《International journal of peptide and protein research》1990,36(3):247-254
The GTP-binding p21 protein, encoded by the ras-oncogene, becomes transforming if amino acid substitutions are made at critical positions in the polypeptide chain, e.g., at Gly 12, Gly 13, Ala 59, Gln 61 and Glu 63. Most of these substitutions occur in two phosphate-binding loop regions, Tyr 4-Thr 20, herein designated as segment 1, and Ile 55-Met 67, herein designated, as segment 2. These two segments are homologous to two corresponding regions in the two purine nucleotide binding proteins, bacterial elongation factor (EF-tu) (Val 12-Thr 28 corresponds to segment 1; His 78-Ile 92 corresponds to segment 2) and adenylate kinase (ADK) (Lys 9-Cys 25 corresponds to segment 1 and Tyr 95-Arg 107 corresponds to segment 2). We find that the conformations of the segment 1 region in the p21 protein, EF-tu and ADK are similar to one another and that the conformation of the segment 2 region of EF-tu is superimposable on that of segment 2 of ADK. Furthermore, the relative position of the two segments in EF-tu is strikingly similar to that of the two segments in ADK. In the originally proposed X-ray structure for the p21 protein, the conformation of segment 2 in the p21 protein is not similar to that found for the other two proteins, and its disposition relative to segment 1 and the remainder of the protein is also different from that observed for the other two proteins.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
44.
45.
46.
Vincent A. Murphy Quentin R. Smith Stanley I. Rapoport 《Journal of neurochemistry》1988,51(6):1777-1782
Male Fischer-344 rats, 21 days old, were fed diets containing 0 (LOD), 2,200 (CONT), or 440,000 (HID) international units of vitamin D3 per kilogram for 12 weeks. [Ca] was measured in plasma, CSF, brain, and choroid plexus. In addition, 45Ca and 36Cl transfer coefficients (KCa and KCl) for uptake from blood into CSF and brain were determined. Although plasma ionized [Ca]s in LOD and HID rats were 50% and 136%, respectively, of values in CONT animals, CSF and brain [Ca]s ranged from only 85% to 110% of respective CONT values. Choroid plexus [Ca] was increased by 37% after HID diet, but was decreased only 10% after LOD. KCa values at CSF, parietal cortex, and pons-medulla were negatively correlated with plasma ionized [Ca], whereas KCl values at CSF and brain were not different between the diet groups. The findings demonstrate that central nervous system [Ca] is maintained during chronic hypo- or hypercalcemia by saturable transport of Ca at brain barrier membranes. This transport does not seem to involve modulation by 1,25-dihydroxyvitamin D3. 相似文献
47.
This paper illustrates the usefulness of solvent control trials and presents a statistical analysis for mouse L5178Y lymphoma data. Examination of solvent control trials establishes that the natural logarithm of mutant frequency is approximately normally distributed and that both the mean and variance of mutant frequency vary by trial. There is little evidence of downturns at higher doses in the dose-response curves studied; therefore, a trend test is proposed for the detection of an increasing dose-response curve. A Monte Carlo investigation confirms that the proposed trend test is better able to detect an increasing dose-response than 4 alternate methods of analysis. 相似文献
48.
Respiration and soluble sugar metabolism in sugar pine embryos 总被引:1,自引:0,他引:1
Embroys excised from dormant seeds of sugar pine ( Pinus lambertiana Dougl.) incubated at 25°C (non-dormancy-breaking) or stratified at 5°C (dormancy-breaking) were analyzed to determine temperature effects on the relative activities of respiration and fermentative metabolism, the levels of soluble sugers and the activities of the hydrolytic enzymes, invertase and sucrose synthase, as related to the release of dormancy and germinatio. At 25°C, despite a sharp drop in embryo oxygen uptake after 48 h, a simultaneous decline in acetaldehyde and ethanol concentrations indicated that there was not a shift to fermentative metabolism. The concentrations of soluble sugars showed no treatment effects. Embryo invertase (EC 3.2.1.26) activity changed only slightly at either temperature, while stratification was accompanied by a 4-fold increase in sucrose synthase (EC 2.4.1.13) activity (cleavage direction). Upon transfer of stratified seeds to 25°C, embryo sucrose synthase activity rapidly increased almost 10-fold, with the increase beginning prior to germination, while mvertase activity increased 20-fold, concomitant with germination. 相似文献
49.
50.
An objective structured interview is an integral part of the process of selecting and admitting applicants to study medicine at this university. During the nine years (to the end of 1986) that the interview has been used 1600 candidates were interviewed out of roughly 13 000 applicants, and from these, 584 students were admitted to the course. Analysis of the interview data was carried out based on two aspects of student progress: graduation with honours and failure to complete the course of study.The interview as a whole, and especially some of the subscales, appears to identify students who may fail to complete the course: it may also help to predict which students are likely to graduate with honours. 相似文献