全文获取类型
收费全文 | 72516篇 |
免费 | 5043篇 |
国内免费 | 2622篇 |
专业分类
80181篇 |
出版年
2024年 | 138篇 |
2023年 | 1219篇 |
2022年 | 1809篇 |
2021年 | 2414篇 |
2020年 | 2350篇 |
2019年 | 3263篇 |
2018年 | 2837篇 |
2017年 | 2035篇 |
2016年 | 2010篇 |
2015年 | 2513篇 |
2014年 | 4741篇 |
2013年 | 5893篇 |
2012年 | 3653篇 |
2011年 | 4691篇 |
2010年 | 3572篇 |
2009年 | 3868篇 |
2008年 | 3940篇 |
2007年 | 3967篇 |
2006年 | 3518篇 |
2005年 | 3053篇 |
2004年 | 2703篇 |
2003年 | 2146篇 |
2002年 | 1927篇 |
2001年 | 1228篇 |
2000年 | 951篇 |
1999年 | 972篇 |
1998年 | 976篇 |
1997年 | 764篇 |
1996年 | 684篇 |
1995年 | 613篇 |
1994年 | 565篇 |
1993年 | 431篇 |
1992年 | 432篇 |
1991年 | 356篇 |
1990年 | 293篇 |
1989年 | 241篇 |
1988年 | 211篇 |
1987年 | 184篇 |
1986年 | 161篇 |
1985年 | 272篇 |
1984年 | 455篇 |
1983年 | 336篇 |
1982年 | 347篇 |
1981年 | 264篇 |
1980年 | 201篇 |
1979年 | 194篇 |
1978年 | 172篇 |
1977年 | 143篇 |
1976年 | 115篇 |
1975年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A procedure is described that enables use of the p-phenylphenol color reaction to determine acetaldehyde in the presence of formaldehyde. The sample is first treated with an acidic 2,4-pentanedione reagent, which selectively removes formaldehyde. The method is applicable to blochemical reactions using tissue preparations. 相似文献
992.
The conformations of the major coat protein of a filamentous bacteriophage can be described by nuclear magnetic resonance spectroscopy of the protein and the virus. The NMR experiments involve detection of the 13C and 1H nuclei of the coat protein. Both the 13C and 1H nuclear magnetic resonance (NMR) spectra show that regions of the polypeptide chain have substantially more motion than a typical globular protein. The fd coat protein was purified by gel chromatography of the SDS solubilized virus. Natural abundance 13C NMR spectra at 38 MHz resolve all of the nonprotonated aromatic carbons from the three phenylalanines, two tyrosines, and one tryptophan of the coat protein. The α carbons of the coat protein show at least two different classes of relaxation behavior, indicative of substantial variation in the motion of the backbone carbons in contrast to the rigidity of the α carbons of globular proteins. The 1H spectrum at 360 MHz shows all of the aromatic carbons and many of the amide protons. Titration of a 1H spectra gives the pKas for the tyrosines. 相似文献
993.
The pharmacological effects of PGE1 (6 and 9 days, 21,250 μg/kg per day subcutaneously) upon the growth and the bone resorption of mammals were studied using the proximal tibia and upper incisor of immature rats along with lead acetate as a time marker, and upon the serum calcium and inorganic phosphorus levels. The following results were obtained. 1. PGE1 hardly affected the body weight or the weight of organs of the rats but apparently inhibited the longitudinal growth of proximal tibia in a dose related manner. 2. PGE1 clearly inhibited not only the longitudinal growth (incisor growth) but also the appositional growth (dentin formation) of incisal dentin. 3. The grade of the inhibitory effect on the growth was in the order of bone growth >dentin formation >incisor growth. 4. The occurrence of osteoporosis due to a low calcium diet was inhibited by the simultaneous administration of PGE1, the mechanism being considered to be mainly due to the inhibitory effect on the bone resorption. 5. PGE1 lowered the level of serum calcium and the lowering effect was not observed in the thyro-parathyroidectomized rat. From the facts that the above effects were exactly the same as those of calcitonin (1), the possibility that the subcutaneous injection of PGE1 may induce a calcitonin-like action, a part of which may dependent on the calcinonin secretion is suggested. 相似文献
994.
Warren I. Schaeffer A. H. L. Chuang Nicholas Heintz Edward Bresnick 《In vitro cellular & developmental biology. Plant》1979,15(6):437-440
Summary We have tested the sensitivity of a cloned rat hepatocyte line, RL-PR-C, to aflatoxin B1 and benzo(a)pyrene as a function of population-doubling level. The cells were much more sensitive to the cytotoxic action
of these agents subsequent to 230 population doublings. This sensitivity corresponded to the enhanced inducibility of arylhydrocarbon
hydroxylase activity by 3-methylcholanthrene.
Supported by Grants CA 21258 and CA 12056 from the National Cancer Institute. 相似文献
995.
An electrophoretic variant of the X-linked enzyme phosphoglycerate kinase (PGK-1) has been used to study regulation of X chromosome expression in the diploid derivatives of the trophectoderm at 8–8.5 days post coitum in the mouse. These derivatives included the chorionic ectoderm and the polar trophoblast. The biochemical analysis suggests that only the maternally derived X chromosome (Xm) is expressed in the diploid trophectoderm derivatives. Cell selection and maternal tissue contamination were ruled out as possible causes of the observed Xm expression. From these and other results, we conclude that all derivatives of the trophectoderm, along with the primitive endoderm, express only Xm, whereas derivatives of the primitive ectoderm show random X chromosome expression. 相似文献
996.
Short illumination with white light of dark-maintained Anacystis nidulans prior to immersion in liquid nitrogen resulted in a marked change of fluorescence emission characteristics at 77 K. The fluorescence of Photosystem II-associated membrane bound pigments increases, while the emission due to phycobilins decreases. This effect seems to be due to a light-dependent alteration in the extent of contact between phycobilisomes and thylakoids, since the effect is reversible in the dark and is abolished by short glutaraldehyde fixation. The preillumination effect is not inhibited by DCMU. Emission spectra obtained with actively growing and CO2-starved cells indicate that the light-dependent increase in energy transfer from phycobilins to chlorophyll depends upon the physiological state of the cells. 相似文献
997.
998.
999.
The relationship between the binding patterns of soybean agglutinin, peanut agglutinin (both in their native (unaggregated) form and in their polymerized form), and of Phaseolus vulgaris leucoagglutinin, to neuraminidase-treated lymphocytes from different sources, and the mitogenic activity of these lectins, was studied. In all cases investigated, binding of a lectin to lymphocytes which resulted in stimulation was a positive cooperative process. Our findings support the assumption that clustering of receptors and conformational changes in membrane structure are prerequisites for mitogenic stimulation. 相似文献
1000.
Tatsuro Koike 《生物化学与生物物理学报:生物膜》1978,509(3):429-439
A clonal cell line of mouse neuroblastoma cells was found to undergo morphological differentiation in the presence of a K+ ionophore, valinomycin, in the assay medium. This effect was blocked by increasing the concentration of KCl of the medium, suggesting that the changes in resting membrane potential and ion fluxes may be involved in the mechanism of the formation of neurites. No enhancement of the neurite formation was observed in salines containing high concentrations of KCl in the absence of valinomycin. Depolarizing agents including veratridine, gramicidin and ouabain did not stimulate the outgrowth of neurites. Neither electrophoretic mobility of the cells nor molecular anisotropy of fluorescence probes in the membranes was modified by the treatment of valinomycin. Instead, it modified the slow binding phase in kinetics of the interaction of 1-anilinonaphthalene-8-sulfonate (ANS) with the cells, which is related to the penetration process of the probe into membranes. Valinomycin also enhanced the fluorescence intensity of ANS by increasing the binding sites in neuroblastoma cells. 相似文献