全文获取类型
收费全文 | 11916篇 |
免费 | 1340篇 |
国内免费 | 3篇 |
专业分类
13259篇 |
出版年
2022年 | 77篇 |
2021年 | 136篇 |
2020年 | 142篇 |
2019年 | 222篇 |
2018年 | 241篇 |
2017年 | 258篇 |
2016年 | 331篇 |
2015年 | 528篇 |
2014年 | 533篇 |
2013年 | 727篇 |
2012年 | 715篇 |
2011年 | 638篇 |
2010年 | 535篇 |
2009年 | 470篇 |
2008年 | 571篇 |
2007年 | 587篇 |
2006年 | 572篇 |
2005年 | 618篇 |
2004年 | 588篇 |
2003年 | 634篇 |
2002年 | 638篇 |
2001年 | 127篇 |
2000年 | 109篇 |
1999年 | 156篇 |
1998年 | 203篇 |
1997年 | 144篇 |
1996年 | 143篇 |
1995年 | 142篇 |
1994年 | 123篇 |
1993年 | 91篇 |
1992年 | 131篇 |
1991年 | 101篇 |
1990年 | 93篇 |
1989年 | 95篇 |
1988年 | 74篇 |
1987年 | 88篇 |
1986年 | 78篇 |
1985年 | 95篇 |
1984年 | 102篇 |
1983年 | 83篇 |
1982年 | 107篇 |
1981年 | 120篇 |
1980年 | 90篇 |
1979年 | 92篇 |
1978年 | 90篇 |
1977年 | 80篇 |
1976年 | 83篇 |
1975年 | 53篇 |
1974年 | 73篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Dr. Paul A. Lespinat Yves M. Berlier Guy D. Fauque Rene Toci Gerard Denariaz Jean LeGall 《Journal of industrial microbiology & biotechnology》1987,1(6):383-388
Summary Hydrogenase and nitrogenase activities of sulfate-reducing bacteria allow their adaptation to different nutritional habits even under adverse conditions. These exceptional capabilities of adaptation are important factors in the understanding of their predominant role in problems related to anaerobic metal corrosion. Although the D2–H+ exchange reaction indicated thatDesulfovibrio desulfuricans strain Berre-Sol andDesulfovibrio gigas hydrogenases were reversible, the predominant activity in vivo was hydrogen uptake. Hydrogen production was restricted to some particular conditions such as sulfate or nitrogen starvation. Under diazotrophic conditions, a transient hydrogen evolution was followed by uptake when dinitrogen was effectively fixed. In contrast, hydrogen evolution proceeded when acetylene was substituted as the nitrogenase substrate. Hydrogen can thus serve as an electron donor in sulfate reduction and nitrogen metabolism. 相似文献
82.
Chardonnet Yvette Viac Jacqueline Levai Jacques Begoud Gérard Morgon Alain Thivolet Jean 《Virchows Archiv. B, Cell pathology including molecular pathology》1986,51(1):421-428
Virchows Archiv B Cell Pathology - Various parameters of the local cellular response have been studied in 16 laryngeal papillomas from ten patients with recurrent papillomas as well as normal... 相似文献
83.
Suzanne Knauf Jean Kalwas B. Frederick Helmkamp Lee W. Harwell Jackson Beecham Edith M. Lord 《Cancer immunology, immunotherapy : CII》1986,21(3):217-225
Summary Murine monoclonal antibodies (MCAs) against human ovarian tumor associated antigen NB/70K have been prepared. One of these MCAs, NB12123, was chosen for the development of a radioimmunoassay for measuring serum NB/70K levels. In this assay, the average NB/70K level in 75 normal, healthy controls was 11.9 activity units (AU) with an SD of 14.9 AU. The normal cut off value for this assay was set at 45 AU (mean +2 SD). 24 of 46 (52%) ovarian cancer patients, 7 of 18 (39%) patients with benign ovarian cysts or tumors and 3 of 85 (4%) control samples had elevated serum NB/70K levels. Comparison of NB/70K levels measured in the NB12123 assay with levels measured in an assay using a polyclonal antiNB/70K previously developed in our laboratory [13] indicated that although both assays had approximately the same percentage of positive ovarian cancer patient samples, there appeared to be no correlation between the absolute NB/70K levels measured by the two assays. The rank of ovarian cancer patient samples was also different for the two assays. Also, almost 40% of patients with benign ovarian cysts and tumors had elevated serum NB/70K levels as measured by the NB12123 assay as compared to 0% for the polyclonal assay. Reciprocal cross-blocking experiments, absorption studies, and immune precipitate analysis indicated that both the monoclonal NB12123 assay and the polyclonal antiNB/70K assay measured the same population of NB/70K molecules. However, the polyclonal antibody recognizes epitopes in addition to that recognized by NB12123. Taken together, these results suggest that the epitope recognized by NB12123 is not as specific for malignant ovarian tumors as the epitope(s) recognized by polyclonal antiNB/70K and/or that more than the one epitope detected by the MCA is responsible for the specificity for ovarian cancer of the polyclonal NB/70K assay. In spite of this, the greater sensitivity and range of the monoclonal NB12123 assay make it possible to monitor serum NB/70K levels in ovarian cancer patients. In four patients examined, the fluctuating serum NB/70K levels appeared to correlate well with clinical statusSupported in part by ACS # PDT 231 and a grant from the Elsa U. Pardee Foundation 相似文献
84.
Terry Ritchie Sheila A. Scully Jean de Vellis Ernest P. Noble 《Neurochemical research》1986,11(3):383-392
The enzymatic activities in post-mortem rat brain kept at 4°C and at 25°C were determined for a number of enzymes localized in specific cell types in the central nervous system. Choline acetyltransferase (CAT), glycerol-3-phosphate dehydrogenase (GPDH), glutamine synthetase (GS), lactate dehydrogenase (LDH) and 2,3-cyclic nucleotide phosphohydrolase (CNPase) were found to be very stable at both 4°C and 25°C with only slight, if any, losses of activity being seen even at periods as long as 72 hr. Glutamic acid decarboxylase (GAD) activity was less stable than that of the other enzymes. In brains kept at 4°C GAD activity was stable out to 24 hr after which it began to decline rapidly to 65% of control at 72 hr. In brains kept at 25°C, GAD activity was stable for 6–8 hr and then began to steadily decline to 58% of control at 24 hr and 29% of control at 72 hr. Assuming that these enzymes have similar stabilities in post-mortem human brain, the effect of post-mortem delay in processing tissues may be of lesser significance than other factors with regard to the measured enzyme activities in human brain samples. 相似文献
85.
86.
C-Glycosylation of 5,7-dihydroxy-3′,4′,5′-trimethoxyflavone was carried out with acetobromo-α-d-glucose, -α-d-galactose, -α-d-xylose, -β-l-arabinose and -αt-L-rhamnose. The respective 6-C-glycosides and 6, 8-di-C-glycosides (excepted for galactose) were isolated and permethylated. MS and TLC comparison confirmed the proposed structures of five natural tricetin-derived C-glycosides. 相似文献
87.
6-C-α-l-Arabinopyranosyl- and furanosylacacetins have been synthesized. They are isomerized by short acid treatment to give a mixture of the four anomer/ring size combinations without any Wessely-Moser isomerization. In the same conditions molludistin (8-C-α-l-arabinopyranosylgenkwanin) led only to a mixture of molludistin and 8-C-α-l-arabinofuranosylgenkwanin. This is the first demonstration of ring sugar isomerization in C-glycosylflavones. In usual solvent systems, α-anomers are easily separated from β-anomers, whereas corresponding pyranosyl and furanosyl anomers are not. However, they are easily separated after permethylation and characteristic features are found in the mass spectra of PM 6-C-arabinofuranosyl isomers. 相似文献
88.
Tn7, a transposon of 14 kb, encodes resistance to trimethoprim (Tp) and streptomycin (Sm). A cleavage site map of this transposon for twenty-two different restriction enzymes as determined by comparison of restriction enzyme cleavage patterns of the plasmids ColE1 and ColE1::Tn7 is presented. The precise localization of these sites was facilitated by the use of two deletion derivatives of ColE1::Tn7: pGB2 and ColE1::Tn7Δ6, and by the use of pOB14 and pOB15 which contain a part of Tn7 cloned into the plasmid pBR322. This map should aid in the study of the structural and genetic organization of this transposon. 相似文献
89.
90.
A study of the chromosomes of Cavia aperea and Galea musteloides has been made following the introduction into captivity of these two cavies from Argentina. The evolutionary relationships of the two genera have been considered, and the possible ancestry of C. porcellus from C. aperea was investigated in hybrids of the two species. 相似文献