全文获取类型
收费全文 | 193984篇 |
免费 | 12827篇 |
国内免费 | 28篇 |
出版年
2023年 | 984篇 |
2022年 | 916篇 |
2021年 | 2495篇 |
2020年 | 2076篇 |
2019年 | 2291篇 |
2018年 | 4631篇 |
2017年 | 4161篇 |
2016年 | 5837篇 |
2015年 | 8219篇 |
2014年 | 8668篇 |
2013年 | 11461篇 |
2012年 | 13477篇 |
2011年 | 12464篇 |
2010年 | 8195篇 |
2009年 | 6513篇 |
2008年 | 10199篇 |
2007年 | 9955篇 |
2006年 | 9418篇 |
2005年 | 8361篇 |
2004年 | 8083篇 |
2003年 | 7339篇 |
2002年 | 6751篇 |
2001年 | 4376篇 |
2000年 | 4142篇 |
1999年 | 3570篇 |
1998年 | 1655篇 |
1997年 | 1286篇 |
1996年 | 1207篇 |
1995年 | 1173篇 |
1994年 | 1137篇 |
1993年 | 941篇 |
1992年 | 2267篇 |
1991年 | 2109篇 |
1990年 | 1962篇 |
1989年 | 1945篇 |
1988年 | 1697篇 |
1987年 | 1592篇 |
1986年 | 1468篇 |
1985年 | 1575篇 |
1984年 | 1328篇 |
1983年 | 1156篇 |
1982年 | 951篇 |
1979年 | 1271篇 |
1978年 | 949篇 |
1977年 | 902篇 |
1975年 | 1053篇 |
1974年 | 1122篇 |
1973年 | 1082篇 |
1972年 | 932篇 |
1969年 | 925篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
H Vaer?y F Nyberg H Franzén L Terenius 《Biochemical and biophysical research communications》1987,148(1):24-30
Enzyme activity capable of converting the glycine-extended substance P precursor, substance P-Gly12, into substance P was purified from human cerebrospinal fluid. The conversion reaction was monitored by radioimmunoassay measurement of substance P formation. The chemical identity of the product was verified by reversed-phase HPLC. The enzyme reaction was stimulated by Cu(II) ion and ascorbic acid and inhibited by the presence of diethyldithiocarbamate. By HPLC molecular sieving, the major enzyme activity appeared as a protein of 26,000 molecular weight. 相似文献
72.
Rosalem Gabriel Santos Las Casas Estevam Barbosa Lima Thiago Parente González-Torres Libardo Andrés 《Biomechanics and modeling in mechanobiology》2020,19(5):1537-1549
Biomechanics and Modeling in Mechanobiology - Cell migration is a process of crucial importance for the human body. It is responsible for important processes such as wound healing and tumor... 相似文献
73.
Mélody Porlier Marc Bélisle Dany Garant 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1523):1543-1554
Improving our knowledge of the links between ecology and evolution is especially critical in the actual context of global rapid environmental changes. A critical step in that direction is to quantify how variation in ecological factors linked to habitat modifications might shape observed levels of genetic variability in wild populations. Still, little is known on the factors affecting levels and distribution of genetic diversity at the individual level, despite its vital underlying role in evolutionary processes. In this study, we assessed the effects of habitat quality on population structure and individual genetic diversity of tree swallows (Tachycineta bicolor) breeding along a gradient of agricultural intensification in southern Québec, Canada. Using a landscape genetics approach, we found that individual genetic diversity was greater in poorer quality habitats. This counter-intuitive result was partly explained by the settlement patterns of tree swallows across the landscape. Individuals of higher genetic diversity arrived earlier on their breeding grounds and settled in the first available habitats, which correspond to intensive cultures. Our results highlight the importance of investigating the effects of environmental variability on individual genetic diversity, and of integrating information on landscape structure when conducting such studies. 相似文献
74.
75.
76.
77.
L Chrobák D Radochová K Smetana F Mat?ja M Kout J Polák P Dít? J Pribrosky I Dvorácková 《Folia haematologica (Leipzig, Germany : 1928)》1980,107(4):628-640
These siblings of a Czech family aged 21, 19 and 6 years, respectively, with congenital dyserythropoietic anemia, type II, (HEMPAS) are reported. In two elder siblings ferrokinetic studies revealed a rapid plasma 59Fe clearance, markedly decreased erythrocyte incorporation and shortened 51Cr red-cell survival. Direct anti-globulin test was found positive in one of them. Further investigations revealed low values of blood plasma cholesterol, total lipids, beta-lipoproteins, beta-carotine and vitamin E and A as well as low values of the prothrombin complex. Liver biopsy demonstrated siderosis and disseminated intravascular coagulation in the liver in both patients. The possible reasons for these humoral aberrations are discussed. 相似文献
78.
79.
In Escherichia coli, two enzymes catalyze the synthesis of methionine from homocysteine using methyltetrahydrofolate as the donor of the required methyl group: cobalamin-dependent and cobalamin-independent methionine synthases. Comparison of the mechanisms of these two enzymes offers the opportunity to examine two different solutions to the same chemical problem. We initiated the research described here to determine whether the two enzymes were evolutionarily related by comparing the deduced amino acid sequences of the two proteins. We have determined the nucleotide sequence for the metE gene, encoding the cobalamin-independent methionine synthase. Our results reveal an absence of similarity between the deduced amino acid sequences of the cobalamin-dependent and cobalamin-independent proteins and suggest that the two have arisen by convergent evolution. We have developed a rapid one-step purification of the recombinant cobalamin-independent methionine synthase (MetE) that yields homogeneous protein in high yield for mechanistic and structural studies. In the course of these studies, we identified a highly reactive thiol in MetE that is alkylated by chloromethyl ketones and by iodoacetamide. We demonstrated that alkylation of this residue, shown to be cysteine 726, results in complete loss of activity. While we are unable to deduce the role of cysteine 726 in catalysis at this time, the identification of this reactive residue suggests the possibility that this thiol functions as an intermediate methyl acceptor in catalysis, analogous to the role of cobalamin in the reaction catalyzed by the cobalamin-dependent enzyme. 相似文献
80.
D J Méwissen J H Rust J Rust 《Comptes rendus des séances de la Société de biologie et de ses filiales》1987,181(4):439-444
A total of 2,377 C 57 Bl/6M mice were assigned to control groups and experimental groups exposed to tritiated water administered as a pulse injection or in drinking water, at a dose of 1.0 microCi per injection or per ml of drinking water. Weanlings were observed for the duration of life span. Data analysis was based on two coefficient estimates (1) individual carcinogenic induction coefficient and (2) specific tumorigenic induction coefficient. The carcinogenic potency of tritium was found to be dual in nature in enhancing the absolute induction of lymphocytic lymphomas in both sexes as well as their relative induction in competition with reticulo-endothelial tumors of other types. 相似文献