全文获取类型
收费全文 | 156391篇 |
免费 | 17905篇 |
国内免费 | 2329篇 |
出版年
2021年 | 2454篇 |
2020年 | 1635篇 |
2019年 | 1958篇 |
2018年 | 2330篇 |
2017年 | 1938篇 |
2016年 | 2783篇 |
2015年 | 3804篇 |
2014年 | 4566篇 |
2013年 | 5357篇 |
2012年 | 6344篇 |
2011年 | 6076篇 |
2010年 | 3894篇 |
2009年 | 3523篇 |
2008年 | 4379篇 |
2007年 | 4226篇 |
2006年 | 4045篇 |
2005年 | 3644篇 |
2004年 | 3434篇 |
2003年 | 3313篇 |
2002年 | 3167篇 |
2001年 | 11682篇 |
2000年 | 11508篇 |
1999年 | 8788篇 |
1998年 | 2271篇 |
1997年 | 2436篇 |
1996年 | 2224篇 |
1995年 | 2008篇 |
1994年 | 1840篇 |
1993年 | 1645篇 |
1992年 | 5655篇 |
1991年 | 5225篇 |
1990年 | 4630篇 |
1989年 | 4489篇 |
1988年 | 3987篇 |
1987年 | 3409篇 |
1986年 | 3031篇 |
1985年 | 2898篇 |
1984年 | 2146篇 |
1983年 | 1866篇 |
1982年 | 1328篇 |
1981年 | 1083篇 |
1979年 | 1852篇 |
1978年 | 1431篇 |
1977年 | 1267篇 |
1976年 | 1088篇 |
1975年 | 1208篇 |
1974年 | 1226篇 |
1973年 | 1236篇 |
1972年 | 1091篇 |
1971年 | 999篇 |
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
101.
In the course of studies on the oxygenation of steroids by purified P-450 cytochromes, particularly rabbit liver microsomal cytochrome P-450 form 3b, a rapid and reliable radiometric assay has been devised for progesterone 16 alpha-hydroxylation. In view of the lack of a commercially available, suitably tritiated substrate, [1,2,6,7,16,17-3H]progesterone was treated with alkali to remove the label from potential hydroxylation sites other than the 16 alpha position. The resulting [1,7,16-3H]progesterone was added to a reconstituted enzyme system containing cytochrome P-450 form 3b, NADPH-cytochrome P-450 reductase, and NADPH, and the rate of 16 alpha-hydroxylation was measured by the formation of 3H2O. This reaction was shown to be linear with respect to time and to the cytochrome P-450 concentration. An apparent tritium isotope effect of 2.1 was observed by comparison of the rates of formation of tritium oxide and 16 alpha-hydroxyprogesterone, and the magnitude of the isotope effect was confirmed by an isotope competition assay in which a mixture of [1,7,16-3H]progesterone and [4-14C]progesterone was employed. 相似文献
102.
J H Huang D P Hsieh 《Proceedings of the National Science Council, Republic of China. Part B, Life sciences》1988,12(1):34-42
Production of aflatoxins M1 (AFM) and B1 (AFB) by Aspergillus flavus NRRL 3251 in solid-state and shaking liquid cultures using rice as the carbon source was compared. In general, solid-state cultures produced more aflatoxins than shaking liquid cultures on an equal rice weight basis. Solid-state cultures with continuous shaking yielded higher levels of toxins than those with intermittent shaking. However, intermittent shaking is a feasible replacement for the continuous shaking method for AFM production. A typical solid rice culture supplemented with yeast extract produced 30 and 2600 mg per kg rice of AFM and AFB, respectively, in 8 days at 29 degrees C. The optimal culture conditions for toxin production in a shaking liquid culture were also studied. Parameters under consideration included the amount of carbon (rice) and nitrogen source, initial medium pH, and aeration rate. At optimum conditions, a representative shaking liquid culture produced 18 and 1680 mg per kg rice of AFM and AFB, respectively, in 5 days at 29 degrees C. This shaking liquid culture appears feasible for scaling up and routine production of AFM and AFB for toxicological investigations. 相似文献
103.
104.
Yunfan Sun Liang Wu Yu Zhong Kaiqian Zhou Yong Hou Zifei Wang Zefan Zhang Jiarui Xie Chunqing Wang Dandan Chen Yaling Huang Xiaochan Wei Yinghong Shi Zhikun Zhao Yuehua Li Ziwei Guo Qichao Yu Liqin Xu Jia Fan 《Cell》2021,184(2):404-421.e16
- Download : Download high-res image (233KB)
- Download : Download full-size image
105.
106.
K Nomura H Imai T Koumura M Arai Y Nakagawa 《The Journal of biological chemistry》1999,274(41):29294-29302
Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is a key enzyme in the protection of biomembranes exposed to oxidative stress. We investigated the role of mitochondrial PHGPx in apoptosis using RBL2H3 cells that overexpressed mitochondrial PHGPx (M15 cells), cells that overexpressed non-mitochondrial PHGPx (L9 cells), and control cells (S1 cells). The morphological changes and fragmentation of DNA associated with apoptosis occurred within 15 h in S1 and L9 cells upon exposure of cells to 2-deoxyglucose (2DG). The release of cytochrome c from mitochondria was observed in S1 cells after 4 h and was followed by the activation of caspase-3 within 6 h. Overexpression of mitochondrial PHGPx prevented the release of cytochrome c, the activation of caspase-3, and apoptosis, but non-mitochondrial PHGPx lacked the ability to prevent the induction of apoptosis by 2DG. An ability to protect cells from 2DG-induced apoptosis was abolished when the PHGPx activity of M15 cells was inhibited by diethylmalate, indicating that the resistance of M15 cells to apoptosis was indeed due to the overexpression of PHGPx in the mitochondria. The expression of members of the Bcl-2 family of proteins, such as Bcl-2, Bcl-xL, Bax, and Bad, was unchanged by the overexpression of PHGPx in cells. The levels of hydroperoxides, including hydrogen and lipid peroxide, in mitochondria isolated from S1 and L9 cells were significantly increased after the exposure to 2DG for 2 h, while the level of hydroperoxide in mitochondria isolated from M15 cells was lower than that in S1 and L9 cells. M15 cells were also resistant to apoptosis induced by etoposide, staurosporine, UV irradiation, cycloheximide, and actinomycin D, but not to apoptosis induced by Fas-specific antibodies, which induces apoptosis via a pathway distinct from the pathway initiated by 2DG. Our results suggest that hydroperoxide, produced in mitochondria, is a major factor in apoptosis and that mitochondrial PHGPx might play a critical role as an anti-apoptotic agent in mitochondrial death pathways. 相似文献
107.
P Kischel L Stevens V Montel F Picquet Y Mounier 《Journal of applied physiology》2001,90(5):1825-1832
We examined the changes in functional properties of triceps brachii skinned fibers from monkeys flown aboard the BION 11 satellite for 14 days and after ground-based arm immobilization. The composition of myosin heavy chain (MHC) isoforms allowed the identification of pure fibers containing type I (slow) or type IIa (fast) MHC isoforms or hybrid fibers coexpressing predominantly slow (hybrid slow; HS) or fast (hybrid fast) MHC isoforms. The ratio of HS fibers to the whole slow population was higher after flight (28%) than in the control population (7%), and the number of fast fibers was increased (up to 86% in flight vs. 12% in control). Diameters and maximal tensions of slow fibers were decreased after flight. The tension-pCa curves of slow and fast fibers were modified, with a decrease in pCa threshold and an increase in steepness. The proper effect of microgravity was distinguishable from that of immobilization, which induced less marked slow-to-fast transitions (only 59% of fast fibers) and changed the tension-pCa relationships. 相似文献
108.
Levels of genetic diversity at different stages of the domestication cycle of interior spruce in British Columbia 总被引:3,自引:0,他引:3
M. U. Stoehr Y. A. El-Kassaby 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1997,94(1):83-90
Concerns over the reductionist nature of the domestication of forest-tree species focus on the possibility of potential genetic
erosion during this process. To address these concerns, genetic diversity assessments in a breeding zone the Province of British
Columbia “interior” spruce (Picea glauca×engelmanni) program was conducted using allozyme markers. Genetic-variation comparisons were made between natural and production (seed
orchard) populations as well as seed and seedling crops produced from the same breeding zone’s seed orchard. The natural population
sample consisted of a total of 360 trees representing three stands within each of three watersheds present in the Shuswap-Adams
low-elevation zone of interior British Columbia. Small amounts of genetic differentiation were observed among the nine natural
populations (4%) and this was attributable to extensive gene flow Consequently, the sum of these nine populations was considered as a baseline for the genetic variation present in the breeding
zone. The comparisons between the seed orchard and the breeding zone produced a similar percentage of polymorphic loci while the expected hetrozygosity (0.207 vs 0.210) and the average number of alleles per locus (2.7 vs 2.4) were slightly lower in the seed orchard. A total
of seven natural populations’ rare alleles were not present in the orchard population, while one allele was unique to the orchard. The %P increased to 70.6% in the seedlot, but dropped to the natural populations level (64.7%) in the plantation. The observed increase
in %P was a result of pollen contamination in the orchard. It is suspected that the reduction in the plantation was caused by an
unintentional selection in the nursery. Simulated roguing in the orchard did not drastically reduce even if up to 50% of the orchard’s clones were rogued. However, roguing was associated with a reduction in the average number
of alleles per locus (i.e., sampling effect).
Received: 2 January 1996 / Accepted: 24 May 1996 相似文献
109.