全文获取类型
收费全文 | 1182275篇 |
免费 | 98756篇 |
国内免费 | 1650篇 |
专业分类
1282681篇 |
出版年
2021年 | 17601篇 |
2020年 | 12542篇 |
2019年 | 16087篇 |
2018年 | 16846篇 |
2017年 | 15599篇 |
2016年 | 27384篇 |
2015年 | 41933篇 |
2014年 | 49969篇 |
2013年 | 76183篇 |
2012年 | 30516篇 |
2011年 | 17314篇 |
2010年 | 41234篇 |
2009年 | 43341篇 |
2008年 | 17208篇 |
2007年 | 14471篇 |
2006年 | 21099篇 |
2005年 | 22157篇 |
2004年 | 21491篇 |
2003年 | 19359篇 |
2002年 | 17564篇 |
2001年 | 28076篇 |
2000年 | 24853篇 |
1999年 | 26561篇 |
1998年 | 24492篇 |
1997年 | 24294篇 |
1996年 | 23911篇 |
1995年 | 22007篇 |
1994年 | 21927篇 |
1993年 | 20865篇 |
1992年 | 22500篇 |
1991年 | 20715篇 |
1990年 | 19044篇 |
1989年 | 20270篇 |
1988年 | 18443篇 |
1987年 | 17107篇 |
1986年 | 16337篇 |
1985年 | 18309篇 |
1984年 | 19102篇 |
1983年 | 17002篇 |
1982年 | 18722篇 |
1981年 | 18115篇 |
1980年 | 16873篇 |
1979年 | 14877篇 |
1978年 | 14991篇 |
1977年 | 14570篇 |
1976年 | 13816篇 |
1975年 | 12850篇 |
1974年 | 12896篇 |
1973年 | 13394篇 |
1972年 | 11065篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Summary An improved method for separating analogues of coenzyme F420 by isocratic reversed-phase high performance liquid chromatography is described. The method offers improved resolution, shorter chromatography runs and requires less complex apparatus. 相似文献
982.
983.
P. Webb A. D. Chanana E. P. Cronkite J. A. Laissue D. D. Joel 《Cell proliferation》1980,13(3):227-237
Germ-free (GF) and conventional (CV) C3H mice received a single injection of 1 μCi [3H]thymidine and 3 μCi [125I]iododeoxyuridine to provide simultaneous labeling of DNA with the two precursors. Thymus, spleen, mesenteric lymph nodes, bone marrow (femora), small intestine, colon and skin were examined for total organ activity and rate of DNA renewal 1–8 days after injection. Precursor incorporation, assayed on day 1, was lower in the thymus, mesenteric lymph nodes and femora (and, to a lesser extent, in the spleen and colon) of GF mice as compared to CV animals. The opposite was observed in the small intestine and skin, i.e. total organ activity was higher in GF animals. Differences in precursor incorporation were partly due to differences in organ weights between the two groups of mice. In comparison to CV animals, DNA renewal rates were diminished in the mesenteric lymph nodes, bone marrow, colon (following a 3-day plateau) and spleen of GF mice. Little, if any, difference was observed between the two groups with respect to the rate of DNA turnover in the thymus and skin. Radioactivity of the small intestine remained constant for 2 days. Thereafter intestinal activity in GF mice declined at an initial slow rate between days 2 and 5 followed by a rapid decrease between days 5 and 8. In CV mice the first phase of activity loss was short with the rapid decline in intestinal activity beginning on day 3. From the slopes of the regression lines, the percentage thymidine reutilization was estimated. Reutilization varied from 0 to 63% in the various organs examined, with the greatest difference between GF and CV mice occurring in the mesenteric lymph nodes. 相似文献
984.
985.
Thomas M. Lincoln Stanley L. Keely 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,676(2):230-244
An assay method based on the ability of high concentrations of Mg2+ to stimulate phosphorylation of histone in the presence of low concentrations of ATP was developed for the measurement of cyclic GMP-dependent protein kinase activity ratios (activity -cyclic GMP/activity + cyclic GMP). In tissues which contain only trace amounts of cyclic GMP-dependent protein kinase, the basal activity ratios were high due to interference from a cyclic nucleotide-independent protein kinase. In order to study the regulation of the cardica cyclic GMP-dependent protein kinase, factors affecting the equilibrium between the active and inactive forms of the enzyme were determined. Since the rate of dissociation of cyclic GMP from its binding site(s) was relatively slow at 0–4°C at pH 7.0, the amount of time required to process tissue samples was the major limiting factor for preserving the equilibrium between active and inactive forms of the enzyme. Dilution of heart tissue extracts at 0–4°C did not significantly alter the activity ratio of the enzyme under conditions of basal or elevated cyclic GMP levels. Experiments using charcoal or exogenous cyclic GMP-dependent protein kinase in the homogenizing medium demonstrated that the release of sequestered cyclic GMP was not responsible for the elevation of the cyclic GMP-dependent protein kinase activity ratios by agents like acetylcholine. Therefore, the assay reflected in part, at least, the retention of kinase-bound cyclic GMP in the tissue extracts. The effects of acetylcholine and sodium nitroprusside on cyclic GMP levels, the cyclic GMP-dependent protein kinase activity ratios, and force of contraction were studied in the perfused rat heart. Both agents produced rapid, dose-dependent increases in cardiac cyclic GMP. Optimal concentrations of acetylcholine produced a 2–3-fold increase in the levels of cyclic GMP and an increase in the cyclic GMP-dependent protein kinase activity ratio. No significant effect of acetylcholine on cyclic nucleotide-independent protein kinase activity was observed. Associated witth the acetylcholine-induced protein kinase, factors affecting the equilibrium between the active and inactive forms of the enzyme were determined. Since the rate of dissociation of cyclic GMP from its binding site(s) was relatively slow at 0–4°C at pH 7.0, the amount of time required to process tissue samples was the major limiting factor for preserving the equilibrium between active and inactive forms of the enzyme. Dilution of heart tissue extracts at 0–4°C did not significantly alter the activity ratio of the enzyme under conditions of basal elevated cyclic GMP levels. Experiments using charcoal or exogenous cyclic GMP-dependent protein kinase in the homogenizing medium demonstrated that the release of sequestered cyclic GMP was not responsible for the elevation of the cyclic GMP-dependent protein kinase activity ratios by agents like acetylcholine. Therefore, the assay reflected in part, at least, the retention of kinase-bound cyclic GMP in the tissue extracts. The effects of acetylcholine and sodium nitroprusside on cyclic GMP levels, the cyclic GMP-dependent protein kinase activity ratios, and force of contraction were studied in the perfused rat heart. Both agents produced rapid, dose-dependent increases in cardiac cyclic GMP. Optimal concentrations of acetylcholine produced a 2–3-fold increase in the levels of cyclic GMP and an increase in the cyclic GMP-dependent protein kinase activity ratio. No significant effect of acetylcholine on cyclic nucleotide-independent protein kinase activity was observed. Associated with the acetylcholine-induced increase in cyclic GMP and the cyclic GMP-dependent protein kinase activity ratio was a reduction in the force of contraction. In contrast, nitroprusside produced little or no increase in the cyclic GMP-dependent protein kinase activity ratio despite increasing the level of cyclic GMP 8–10-fold. Nitroprusside also had no effect on contractile force. In combination, nitroprusside and acetylcholine produced additive effects on cyclic GMP levels, but protein kinase activation and force of contraction were similar to those seen with acetylcholine alone. The results suggest that the cyclic GMP produced by acetylcholine in the rat heart is coupled to activation of the cyclic GMP-dependent protein kinase, while that produced by nitroprusside is not. 相似文献
986.
Malvin C. Teich 《Biological cybernetics》1985,53(2):121-124
Approximate normalizing transformations are derived for Poisson counting systems affected by nonparalyzable and paralyzable dead time. In the nonparalyzable case the transformation takes the form of a simple inverse hyperbolic function whereas in the paralyzable case it is an inverse trigonometric function. The results are expected to find use in neural counting, photon counting, and nuclear counting, as well as in queuing theory. 相似文献
987.
L. Jerling 《Plant Ecology》1988,74(2-3):161-170
Population fluctuations ofGlaux maritima, along a transect on a Baltle sea shore meadow, were recorded between 1979 and 1983. A bimodal distribution in numbers along the transect reflects the variation in factors regulating numbers: The two maintenance systems of the species, vegetative propagation and sexual reproduction play different roles. Vegetative propagation is fast and responds quickly to variations in the environment. The seeds germinate in strongly fluctuating temperatures which are triggered by disturbances such as flooding, damaging the vegetation. 相似文献
988.
The population dynamics of Heterodera glycines as influenced by alachlor, fenamiphos, and ethoprop alone and in herbicide-nematicide combinations were studied in the field. Numbers of H. glycines juveniles and eggs were higher at midseason and harvest where nematicides were applied. Fenamiphos alone or in combination with alachlor provided better control of H. glycines and greater seed yields than treatments with ethoprop. Numbers of H. glycines eggs at harvest in 1980 were positively correlated with numbers of juveniles at planting in 1981 and negatively related to seed yield in 1981. 相似文献
989.
990.
A simple and accurate technique for the determination of the heat resistance of spores is described. The technique combines a modified capillary tube method with a solid heating block. The come-up time of spore suspensions was found to be short and simple and accurate technique is suggested for the correction of the come-up times. Experimental results are presented for the destruction of spores of Bacillus stearothermophilus at 120 which indicates the accuracy and reproducibility of the new method. 相似文献