全文获取类型
收费全文 | 27122篇 |
免费 | 1485篇 |
国内免费 | 850篇 |
出版年
2023年 | 403篇 |
2022年 | 607篇 |
2021年 | 765篇 |
2020年 | 851篇 |
2019年 | 1162篇 |
2018年 | 883篇 |
2017年 | 568篇 |
2016年 | 620篇 |
2015年 | 721篇 |
2014年 | 1496篇 |
2013年 | 1997篇 |
2012年 | 1188篇 |
2011年 | 1642篇 |
2010年 | 1140篇 |
2009年 | 1210篇 |
2008年 | 1311篇 |
2007年 | 1336篇 |
2006年 | 1167篇 |
2005年 | 1073篇 |
2004年 | 995篇 |
2003年 | 791篇 |
2002年 | 704篇 |
2001年 | 485篇 |
2000年 | 399篇 |
1999年 | 420篇 |
1998年 | 377篇 |
1997年 | 310篇 |
1996年 | 319篇 |
1995年 | 246篇 |
1994年 | 252篇 |
1993年 | 214篇 |
1992年 | 207篇 |
1991年 | 175篇 |
1990年 | 151篇 |
1989年 | 137篇 |
1988年 | 146篇 |
1987年 | 137篇 |
1986年 | 129篇 |
1985年 | 218篇 |
1984年 | 318篇 |
1983年 | 252篇 |
1982年 | 277篇 |
1981年 | 224篇 |
1980年 | 247篇 |
1979年 | 239篇 |
1978年 | 188篇 |
1977年 | 152篇 |
1976年 | 142篇 |
1975年 | 118篇 |
1974年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
Levonorgestrel which binds with high affinity to SHBG, is suggested as a new radioligand for estimation of SHBG in human plasma. Using 3H-levonorgestrel as a ligand, a number of samples from men, women pregnant and non-pregnant were analysed. The SHBG content was lowest in men, low in women and rose to higher level during pregnancy. The results of the present study suggest that levonorgestrel appears to be a better ligand than dihydrotestosterone for measuring SHBG. 相似文献
242.
Antonio G. Gonzalez Jaime Bermejo Barrera Francisco J. Tolfdo Marante 《Phytochemistry》1983,22(4):1049-1050
The fruit bodies of the Basidiomycete Scleroderma polyrhizum have been shown to contain the steroids ergosta-4,6,8(14) 22-tetraen-3-one and 5α,8α-epidoxyergosta-6,22-dien-3β-ol and also palmitic and oleic acids. 相似文献
243.
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
244.
Larry E. Dillehay Larry H. Thompson Jason L. Minkler Anthony V. Carrano 《Mutation research》1983,109(2):283-296
The majority of the high (12-fold elevated) baseline sister-chromatid exchanges (SCEs) that occur in the CHO mutant line EM9 appear to be a consequence of incorporated BrdUrd, and they arise during replication of DNA containing BrdUrd in a template strand. In normal CHO cells the alkaline elution patterns of DNA newly replicated on a BrdUrd-containing template are significantly altered compared with those seen during the replication on an unsubstituted template. The nascent DNA synthesized on such an altered template is delayed in reaching mature size, possibly because replication forks are temporarily blocked at sites occurring randomly along the template. Transient blockage of replication forks may be a prerequisite for SCE. The delay in replication on BrdUrd-substituted templates was greater in EM9 cells than in parental AA8 cells and was also greater in AA8 cells treated with benzamide, an inhibitor of poly(ADPR) polymerase, than in untreated AA8 cells. Under these conditions, treatment with benzamide also produced a 7-fold increase in SCEs in AA8. An EM9-derived revertant line that has a low baseline SCE frequency showed less delay in replication on BrdUrd-substituted templates than did EM9. However, under conditions where the template strand contained CldUrd, which was shown to produce 4-fold more SCEs than BrdUrd in AA8 cells, the replication delay in AA8 was not any greater in the CldUrd-substituted cells. Thus, other factors besides the delay appear to be involved in the production of SCEs by the template lesions resulting from incorporation of the halogen-substituted pyrimidine molecules. 相似文献
245.
Antimutagenic effects of cinnamaldehyde on mutagenesis by chemical agents were investigated in Escherichia coli WP2 uvrA- trpE-. Cinnamaldehyde, when added to agar medium, greatly reduced the number of Trp+ revertants induced by 4-nitroquinoline 1-oxide (4-NQO) without any decrease of cell viability. This antimutagenic effect could not be explained by inactivation of 4-NQO caused by direct interaction with cinnamaldehyde. Mutagenesis by furylfuramide (AF-2) was also suppressed significantly. Mutations induced by methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) were slightly inhibited. However, cinnamaldehyde was not at all effective on the mutagenesis of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Two derivatives of cinnamaldehyde, cinnamyl alcohol and trans-cinnamic acid, did not have as strong antimutagenic effects on 4-NQO mutagenesis as cinnamaldehyde had. Because cinnamaldehyde showed marked antimutagenic effects against mutations induced by UV-mimic mutagens but not those induced by MNNG or EMS, it seems that cinnamaldehyde might act by interfering with an inducible error-prone DNA repair pathway. 相似文献
246.
Mutations induced by repeated EMS treatments were investigated by using mouse L5178Y cells. The frequency of TGr mutations increased linearly with the number of EMS treatments whereas the yield of BrdUrdr mutations showed a curvilinear dose-response curve. The BrdUrdr frequency was roughly proportional to the square of the TGr frequency and the results were compatible with the hypothesis that BrdUrdr cells were induced by two mutational events within a cell. Most of the BrdUrdr colonies isolated after 6 EMS treatments, however, were unstable. When BrdUrdr colonies that had arisen in BrdUrd medium after 2 weeks' incubation were isolated in normal medium, the descendant cells showed a nearly normal level of thymidine incorporation and low plating efficiencies of about 1% in BrdUrd medium. In contrast, after isolation of the same colonies in BrdUrd medium, a low level of thymidine incorporation and high plating efficiencies in BrdUrd medium were observed in the descendant cells.
Reverse selection from BrdUrdr to HATr was accomplished with frequencies of 10−6−10−3 for the descendants grown in BrdUrd medium, and AzaCyd treatment drastically increased the reversion frequency to nearly 10−1. Further re-revertants from HATr to BrdUrdr were also found with frequencies of 10−3−10−2 without treatment. These results indicate that the initial BrdUrdr cells did not result from inactivation of the thymidine-kinase gene but that the mode of gene expression was altered in some way. 相似文献
247.
Apple trees ( Malus pumila Mill . var. domestica Fuji/ Malus prunifolia rootstock) showed a high susceptibility to bitter pit when supplyed with ammonium salt instead of nitrate (control) in the nutrient solution. When apple fruit was affected by bitter pit, a lower calcium as well as a higher nitrogen and ammonium-nitrogen contents was observed in the fruit flesh near the calyx end. The activity of the mitochondrial Ca2+ -uptake of the fruit flesh near the calyx end was higher when the tree was grown with ammonium salt than when grown with nitrate. Both the activities of succinate: cytochrome c oxidoreductase and the mitochondrial Ca2+ -uptake per g of tissue were higher in affected fruit than in healthy fruit. Each of chlorpromazine, N-(6-aminohexyl)-5-chloro-l-napthalenesulfonamide (W-7) and N-(6-aminohexyl)-l-naphthalenesulfonamide (W-5), calmodulin antagonists, was infiltrated into the fruit for 20 min under reduced pressure (about 1 × 104 Pa). Few days later, numerous bitter pit-like spots were observed in both fruit treated with W-7 and chlorpromazine, while only a few spots were observed after the infiltration with W-5, a less potent calmodulin antagonist. A possible mechanism for the occurrence of bitter pit is discussed. 相似文献
248.
Incorporation of radioactivity from [6-14C]orotate into the pyrimidine constituents of shoots of Pisum sativum, Phaseolus vulgaris and Lathyrus tingitanus was examined with special reference to the unusual pyrimidine constituents. With each species, although 80% of the orotate supplied was catabolized to β-alanine, all the pyrimidine derivatives became radioactively labelled. With Pisum, the major part of the radioactivity incorporated into pyrimidines was located in UMP and the uracil derivatives, including the uracilyl amino acids willardiine and isowillardiine. With Phaseolus, UMP and the uracil derivatives were again the major radioactive products; incorporation of radioactivity into 5-ribosyluracil (pseudouridine), which accumulates in Phaseolus tissues, was comparable to the incorporation into orotidine and twice that found in cytidine. Lathyrus incorporated a substantially larger part of the presented [6-14C] orotate into pyrimidine derivatives than did the other two species. CMP was the most highly radioactive product, followed next by lathyrine and UMP. Surprisingly, 20% of the total radioactivity incorporated into pyrimidines by Lathyrus was located in the pyrimidine amino acid lathyrine. This confirms previous evidence that lathyrine is essentially a product of the orotate pathway. The overall recovery of radioactivity in all three species was 93–95%. The data emphasize the necessity of including the less common pyrimidine constituents, as well as the common ones, in quantitative studies of pyrimidine metabolism in plants. 相似文献
249.
The 1H nuclear magnetic resonance (nmr) spectra of complexes of soybean ferric leghemoglobin with 3-substituted pyridines and 5-substituted nicotinic acids have been recorded in order to determine the influence of axial ligands on heme electronic structure. The hyperfine shifted resonances of the heme group were assigned by analogy to previous assignments for the pyridine and nicotinic acid complexes of leghemoglobin. The spectra are characteristic of predominantly low-spin ferric heme complexes. For the pyridine complexes, the rate of ligand exchange was found to increase with decreasing ligand pKA. For many of the complexes, optical and nmr spectra reveal the presence of an equilibrium mixture of high- and low-spin states of the iron atom. The percentage of high-spin component increases with decreasing ligand pKA Smaller hyperfine shifts are noted for leghemoglobin complexes with ligands capable of weak ligand → metal π bonding. The pattern of hyperfine shifted resonances is similar for all complexes studied and indicates that the overall heme electronic structure is dominated by the bonding to the proximal histidine. 相似文献
250.
The accumulation of transported materials in cut axons is demonstrated by the light and electron microscopic immunocytochemical localization of substance P and enkephalin in the caudal medulla and cervical spinal cord of adult rat. Two days following unilateral knife-cuts in the caudal medulla or spinal (C2-C3) levels, substance P and enkephalin-like immunoreactivity (SPLI and ELI) are detected in lesioned axons located rostral and caudal to the transection. Rostrally, SPLI and ELI are detected in the lateral reticular region and ventrolateral fasciculus corresponding to the location of previously identified bulbospinal pathways. Caudally, previously unidentified, propriospinal pathways showing SPLI are detected in the dorsal columns and in the dorsolateral fasciculus. In contrast, ELI is found caudal to the transection only in the reticular region of the medulla. For both peptides, immunoreactivity is present throughout axons containing numerous large, dense core, and small clear vesicles. These results support the concept of both particulate and soluble modes of transport for substance P and enkephalin within axons of the central nervous system. 相似文献