全文获取类型
收费全文 | 21203篇 |
免费 | 2146篇 |
国内免费 | 4篇 |
专业分类
23353篇 |
出版年
2022年 | 185篇 |
2021年 | 325篇 |
2020年 | 203篇 |
2019年 | 279篇 |
2018年 | 372篇 |
2017年 | 299篇 |
2016年 | 566篇 |
2015年 | 932篇 |
2014年 | 1032篇 |
2013年 | 1248篇 |
2012年 | 1532篇 |
2011年 | 1443篇 |
2010年 | 1021篇 |
2009年 | 841篇 |
2008年 | 1230篇 |
2007年 | 1131篇 |
2006年 | 1113篇 |
2005年 | 1007篇 |
2004年 | 1027篇 |
2003年 | 927篇 |
2002年 | 830篇 |
2001年 | 383篇 |
2000年 | 328篇 |
1999年 | 314篇 |
1998年 | 257篇 |
1997年 | 202篇 |
1996年 | 168篇 |
1995年 | 161篇 |
1994年 | 165篇 |
1993年 | 159篇 |
1992年 | 222篇 |
1991年 | 224篇 |
1990年 | 200篇 |
1989年 | 212篇 |
1988年 | 192篇 |
1987年 | 173篇 |
1986年 | 182篇 |
1985年 | 160篇 |
1984年 | 135篇 |
1983年 | 115篇 |
1982年 | 114篇 |
1981年 | 125篇 |
1980年 | 112篇 |
1979年 | 115篇 |
1978年 | 116篇 |
1977年 | 75篇 |
1976年 | 90篇 |
1975年 | 100篇 |
1974年 | 97篇 |
1973年 | 92篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The effect of carbohydrate depletion on procoagulant activity and in vivo survival of highly purified human factor VIII 总被引:1,自引:0,他引:1
P J Fay S I Chavin J E Malone D Schroeder F E Young V J Marder 《Biochimica et biophysica acta》1984,800(2):152-158
Human factor VIII procoagulant protein (factor VIII) was purified using a modification of our previously described method, in which Sephacryl S-400 elution, rather than QAE-cellulose chromatography, served as the final purification step. The protein had a specific activity of more than 2500 U/mg and consisted of a single polypeptide (Mr 100 000) when analyzed by SDS-polyacrylamide gel electrophoresis. Factor VIII was shown to be a glycoprotein by staining with periodic acid-Schiff's reagent following electrophoresis. Treatment of factor VIII with a mixture of exo- and endoglycosidases caused a reduction by about 50% in the intensity of periodic acid-Schiff staining, as determined by scanning densitometry, and an increase in electrophoretic mobility (equivalent to a new Mr 95 000). Removal of this portion of the total carbohydrate had no significant effect on factor VIII clotting activity or on thrombin potentiation of clotting activity. The in vivo survival curves of a native and sugar-depleted 125I-labeled factor VIII both showed similar patterns of initial rapid decay to 60 and 40% activity, respectively, followed by a one-half decay time of 4 h for both. These results suggest that the carbohydrate portion of human factor VIII does not contribute significantly to either clotting function in vitro or to biological turnover in vivo. 相似文献
2.
Cristian A. Acevedo Elizabeth Y. Sanchez Juan G. Reyes Manuel E. Young 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2010,878(3-4):449-455
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells. 相似文献
3.
4.
5.
Gametes of the unicellular green alga Chlamydomonas reinhardii recognize and adhere to cells of the opposite mating type by flagellar contact. Adhesion between these specialized organelles signals a rapid series of mating events which result in gamete fusion. The sequence of morphological changes (flagellar tip activation, cell wall loss, and mating structure elongation), which occur as a consequence of the sexual signalling, have been characterized. The signalling mechanisms have, however, not been defined. Calcium is known to be involved during fertilization of animal species. Increased intracellular free calcium, which can be achieved either by calcium influx or by mobilization of ions from intracellular stores, has been observed during activation of both eggs and sperm. A recent report by Bloodgood & Levin that gametes of C. reinhardii preloaded with 45Ca showed a transient increase in Ca efflux following mating, suggests that intracellular Ca redistribution may also accompany mating in this algal species. We have used X-ray microanalysis to analyze the subcellular distribution of bound calcium during mating in Chlamydomonas reinhardii. X-ray maps reveal that calcium is sequestered in discrete granules within the gamete cell body prior to mating and that during activation and cell fusion, calcium is diffuse throughout the cell. This suggests the possibility that calcium serves as a second messenger in this species. 相似文献
6.
Surface sediment diatoms from the east coast of Lake Tanganyika were analysed using ordination and classification techniques, and compared with assemblages previously described from the northern part of the lake. Grain-size analyses were performed on subsamples. Four groups of diatom assemblages were recognised. The first group clusters samples taken in the north, far from the Rusizi river mouth. The second group comprises samples taken on silty sediment along the Tanzanian coast, including one sample taken near the mouth of the Malagarazi river and those from the northernmost part of the lake. The third group comprises surface sediments along the Burundian coast (near Ramba and Magara), and the fourth is characterised by epipsammic taxa. A sample taken near the central arm of the Malagarazi river is included in the latter group. The impact of small rivers on the diatom assemblages in the surface sediments is restricted to the mouth area. 相似文献
7.
The products of mitochondrial protein synthesis have been investigated in Tetrahymena after labelling with [35S]methionine in the presence of cycloheximide. The labelled proteins were analyzed by sodium dodecyl sulfate slab polyacrylamide gel electrophoresis. We have identified 13 electrophoretically discrete bands as well as 4 other bands with a more variable occurrence. These proteins ranged in apparent molecular weight from 8100 to 57,500. The cycloheximide-resistant incorporation could be blocked with chloramphenicol. The mitochondrial proteins appeared to be in a disaggregated state and were stable to agents such as trichloroacetic acid (hot or cold) and chloroform-methanol. The pattern of proteins was similar following labelling times ranging from 30 min to 3 h. 相似文献
8.
9.
A serum- and glucocorticoid-regulated 4-kilobase mRNA encodes a cyclooxygenase-related protein. 总被引:21,自引:0,他引:21
M K O'Banion H B Sadowski V Winn D A Young 《The Journal of biological chemistry》1991,266(34):23261-23267
10.
Because the cytokine interleukin-1 beta (IL-1 beta) lacks a classical hydrophobic signal sequence, it has been unclear how it is released from cells, and whether release proceeds via a novel mechanism or through non-specific leakage. To address this issue, we have examined the secretion of the recombinant forms of human IL-1 beta from COS monkey kidney cells, which express low levels of endogenous IL-1 beta. Four proteins were expressed: precursor and mature IL-1 beta and precursor and mature IL-1 beta fused to an amino terminal hydrophobic signal sequence from human tissue plasminogen activator. By monitoring the appearance of a known cytosolic protein (ATP citrate lyase) in the medium, we find that the unmodified IL-1 beta s are non-specifically released in very small quantities from the cytosol. On the other hand, the signal sequence-modified IL-1 beta s are glycosylated and efficiently secreted by the ER/Golgi pathway. The secreted, modified-mature protein is also biologically active, suggesting that this pathway has been bypassed for reasons other than maintaining the structural integrity of IL-1 beta. More likely the alternative pathway is a critical aspect of IL-1 biology. The differences in kinetics and quantity of IL-1 beta release from monocytic and COS cells suggest that COS cells lack critical components for the rapid release seen in monocytes. 相似文献