全文获取类型
收费全文 | 4854篇 |
免费 | 376篇 |
国内免费 | 1篇 |
出版年
2023年 | 20篇 |
2022年 | 41篇 |
2021年 | 103篇 |
2020年 | 46篇 |
2019年 | 66篇 |
2018年 | 95篇 |
2017年 | 81篇 |
2016年 | 140篇 |
2015年 | 240篇 |
2014年 | 216篇 |
2013年 | 328篇 |
2012年 | 390篇 |
2011年 | 346篇 |
2010年 | 243篇 |
2009年 | 197篇 |
2008年 | 272篇 |
2007年 | 290篇 |
2006年 | 269篇 |
2005年 | 259篇 |
2004年 | 211篇 |
2003年 | 241篇 |
2002年 | 203篇 |
2001年 | 91篇 |
2000年 | 78篇 |
1999年 | 72篇 |
1998年 | 54篇 |
1997年 | 32篇 |
1996年 | 17篇 |
1995年 | 33篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 41篇 |
1991年 | 36篇 |
1990年 | 33篇 |
1989年 | 28篇 |
1988年 | 25篇 |
1987年 | 31篇 |
1985年 | 26篇 |
1984年 | 28篇 |
1983年 | 24篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 15篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1973年 | 18篇 |
1972年 | 12篇 |
排序方式: 共有5231条查询结果,搜索用时 13 毫秒
71.
Strand scission of DNA by bound adriamycin and daunorubicin in the presence of reducing agents. 总被引:7,自引:0,他引:7
J W Lown S K Sim K C Majumdar R Y Chang 《Biochemical and biophysical research communications》1977,76(3):705-710
Adriamycin and daunorubicin bound to covalently closed circular DNA nick the latter when reduced by sodium borohydride as demonstrated using an ethidium bromide fluorescence assay. The degradation, dependent on oxygen, is strongly inhibited by (i) superoxide dismutase (ii) catalase and (iii) sodium benzoate indicating the intermediacy in the cleavage of superoxide radical anion, hydrogen peroxide and hydroxyl radicals respectively. Less nicking of the DNA is observed by the reduced aglycones, so binding to the DNA by the aminosugar moiety assists the cleavage process. Adriamycin, daunorubicin and both ring C reduced forms bind intercalatively and completely relax supercoiled DNA. The results provide a possible rationale for the degradation of DNA which accompanies anthracycline administration. 相似文献
72.
G. Simán 《Plant and Soil》1982,64(1):35-41
Summary The applicability of the Electro-Ultra-Filtration (EUF) method in soil analyses was studied. The reproducibilities of the
amounts of soil extracts, of ion concentrations in the extracts and of the distribution of cations and anions over the cathode
and anode extracts by use of fully automatic EUF equipment were tested.
The degree of variability among replicates was expressed as coefficient of variation (CV) and as the highest percentual divergence
of an individual analytical measurement from the mean (L).
The extraction volumes of five replicates of six different soils were found to vary between 1.1–7.1% with an average of 3.8%,
as CV and between 1.5–11.3% as L. The reproducibility of desorbed P in the anode extract varied between 2.7–31.7% with an
average of 8.7%, as CV and between 3.2–37.9% as L. Corresponding values for CV and L of K desorbed varied between 1.3–13.9%
and 1.6–23.8%, respectively.
Variations among replicates of desorbed P were especially high in the first 1–2 sub-fractions of a total of seven fractions
in a single extraction run. Low K concentrations in the extract had a slightly negative influence on the reproducibility of
K desorption.
Furthermore, it was found that a portion of the cations is collected in the anode extract and a portion of the anions in the
cathode extract, especially at the beginning of an extraction run. Pooling of anode and cathode extracts before analysis is
therefore recommended. 相似文献
73.
Georgirene D. Vladutiu Richard M. Fike Valerie T. Amigone 《In vitro cellular & developmental biology. Plant》1981,17(7):588-592
Summary Fibroblasts derived from patients with I-cell disease have been shown to accumulate many natural substrates including a three
to fourfold increase in sialic acid content compared to that found in normal fibroblasts. This diverse accumulation of storage
material is due to a massive deficiency of multiple lysosomal hydrolases as they are preferentially excreted into the culture
fluid. There is evidence that the I-cell plasma membrane itself is abnormal with respect to certain transferase activities
and in its sensitivity to freezing and Triton X-100. In this study, we have shown that a neuraminidase-sensitive substrate,
and perhaps others in I-cell fibroblasts, contribute to an increased electronegativity of the I-cell fibroblast surface and
to the cells' sensitivity to freezing. We also found that neuraminidase treatment of I-cell fibroblasts before preservative
freezing in liquid nitrogen enables the cells to adapt more easily to subculture upon thawing.
This project was supported in part by National Institutes of Health (NIH) BRSG Grant RR-05493, NIH Grant 1-R01-HD-11453-01-A1,
National Science Foundation Grant PCM 77-05733, and Maternal and Child Health Service Project 417. Georgirene D. Vladutiu
is the recipient of Research Career Development Award 1K04 HD 00312-01A1 from the National Institutes of Health. 相似文献
74.
The structure and enzymic activities of the C1r and C1s subcomponents of C1, the first component of human serum complement. 总被引:22,自引:14,他引:8
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The subcomponents C1r and C1s and their activated forms C-1r and C-1s were each found to have mol.wts. in dissociating solvents of about 83000. The amino acid compositions of each were similar, but there were significant differences in the monosaccharide analyses of subcomponents C1r and C1s, whether activated or not. Subcomponents C1r and C1s have only one polypeptide chain, but subcomponents C-1r and C-1s each contain two peptide chains of approx. mol.wts. 56000 ("a" chain) and 27000 ("b" chain). The amino acid analyses of the "a" chains from each activated subcomponent are similar, as are those of the "b" chains. The N-terminal amino acid sequence of 29 residues of the C-1s "a" chain was determined, but the C-1r "a" chain has blocked N-terminal amino acid. The 20 N-terminal residues of both "b" chains are similar, but not identical, and both show obvious homology with other serine proteinases. The difference in polysaccharide content of the subcomponents C-1r and C-1s is most marked in the 'b' chains. When tested on synthetic amino acid esters, subcomponent C-1r hydrolysed both lysine and tyrosine ester bonds, but subcomponent C-1r did not hydrolyse any amino acid esters tested nor any protein substrate except subcomponent C1s. The lysine esterase activity of subcomponent C1s provides a rapid and sensitive assay of the subcomponent. 相似文献
75.
Seed maturation of Pisum sativum cv. Progress No. 9 proceeds more slowly in winter than in summer even when the parent plants are grown in greenhouse conditions with light-and heat-supplementation. For parent plants grown under summer and winter conditions the metabolism of [3H]GA9 in cultured seeds is qualitatively different in seeds of equivalent age and qualitatively the same in seeds of equivalent weight. 13-Hydroxylation of [3H]GA9[3H]GA20 is restricted to early stages of seed development. 2-Hydroxylation of [3H]GA92-OH-[3H]GA9 has only been observed at a stage of development after endogenous GA9 has accumulated. 2-OH-GA9 has been shown to be endogenous to pea and is named GA51. H2-GA31 and its conjugate have not been shown to be present in pea and may be induced metabolites of [3H]GA9. The metabolism of GA20GA29 is used to illustrate a technique of feeding [2H][3H]GAs in order to distinguish a metabolite from the same endogenous compound. The in vitro conversion of [3H]GA20[3H]GA29, and the virtual non-metabolism of [3H]GA29 have been confirmed for seeds in intact fruits. These results are discussed in relation to the apparent absence of conjugated GAs in mature pea seeds.Abbreviations GAn
gibberellin An
- GC
gas chromatography
- GC-MS
combined gas chromatography-mass spectrometry
- GC-RC
combined gas chromatography-radio counting
- Me
methyl ester
- RT
etention time
- SICM
selected ion current monitoring
- TLC
thin layer chromatography
- TMS
trimethyl silyl ether
The author is née Frydman 相似文献
76.
The patterns of termination of DNA replication in human embryonic MRC-5 fibroblasts at four passage levels have been examined by autoradiography. Only chromosome 9 showed statistically significant differences in the time of replication among cultures of different ages. This chromosome terminated replication earlier at later passages than at earlier passages, primarily because of differences in the time of replication of the centromere region. Because very few differences were observed at different passage levels, we conclude that changes in the order of chromosome replication are unlikely to contribute to the phenomenon of in vitro senescence. 相似文献
77.
M Sedegah B K Sim C Mason T Nutman A Malik C Roberts A Johnson J Ochola D Koech B Were 《Journal of immunology (Baltimore, Md. : 1950)》1992,149(3):966-971
In rodent malaria model systems, protective immunity induced by immunization with irradiated sporozoites is eliminated by in vivo depletion of CD8+ T cells, and adoptive transfer of CTL clones against the circumsporozoite protein protects against malaria. We recently demonstrated that volunteers immunized with irradiated Plasmodium falciparum sporozoites produce CTL against peptide 368-390 of the P. falciparum circumsporozoite protein. To determine whether natural exposure to malaria induced similar CTL, we studied 11 adult, male, life-long residents of a highly malarious area of Kenya, who were selected because their lymphocytes had been shown to proliferate after stimulation with peptides 361-380, 371-390, or 368-390 and because nine had been resistant to malaria in previous studies. In four of the 11 individuals there was peptide-specific, genetically restricted, CTL activity. In all four individuals, this activity was unaffected by depletion of CD4+ T cells. In three volunteers the activity was eliminated or reduced by depletion of CD8+ T cells; in the fourth volunteer the CD8+ T cell depletion was uninterpretable. This first demonstration of CD8+ T cell, genetically restricted, Ag-specific CTL against a malaria protein among individuals exposed to endemic malaria provides a foundation for studying the relationship between circulating CTL and resistance to malaria infection. 相似文献
78.
Leslie M. Loew Lawrence B. Cohen James Dix Eric N. Fluhler Valerie Montana Guy Salama Wu Jian-young 《The Journal of membrane biology》1992,130(1):1-10
The fast potentiometric indicator di-4-ANEPPS is examined in four different preparations: lipid vesicles, red blood cells, squid giant axon, and guinea pig heart. The dye gives consistent potentiometric responses in each of these systems, although some of the detailed behavior varies. In lipid vesicles, the dye displays an increase in fluorescence combined with a red shift of the excitation spectrum upon hyperpolarization. Similar behavior is found in red cells where a dual wavelength radiometric measurement is also demonstrated. The signal-to-noise ratio of the potentiometric fluorescence response is among the best ever recorded on the voltage-clamped squid axon. The dye is shown to be a faithful and persistent monitor of cardiac action potentials with no appreciable loss of signal or deterioration of cardiac activity for periods as long as 2 hr with intermittent illumination every 10 min. These results, together with previously published applications of the dye to a spherical lipid bilayer model and to cells in culture, demonstrate the versatility of di-4-ANEPPS as a fast indicator of membrane potential. 相似文献
79.
80.
Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH4 oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH4) headspace, (b) estimating CH4 oxidation rates from soil CH4 distributions and (c)222Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH4) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH4) headspaces, and (g) jar experiments on core sections involving tracer additions of14CH4. Short-term unamended chamber measurements,222Rn-calibrated flux measurements, and soil CH4 distributions show independently that the soils are capable of oxidizing atmospheric CH4 at rates ranging to < 2 mg m–2 d–1. Jar experiments with free-air headspaces and soil CH4 profiles show that CH4 oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH4 oxidation occurs at low (< 0.9 ppm) thresholds. The14CH4-amended jar experiments show the distribution of end products of CH4 oxidation; 60% is transformed to CO2 and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH4 oxidation and indicate potential CH4 oxidation rates as high as 867 mg m–2 d–1. Methane oxidation in moist soils modulates CH4 emission and can serve as a negative feedback on atmospheric CH4 increases. 相似文献