全文获取类型
收费全文 | 38131篇 |
免费 | 2676篇 |
国内免费 | 3340篇 |
专业分类
44147篇 |
出版年
2024年 | 126篇 |
2023年 | 506篇 |
2022年 | 836篇 |
2021年 | 1114篇 |
2020年 | 1109篇 |
2019年 | 1487篇 |
2018年 | 1300篇 |
2017年 | 1002篇 |
2016年 | 1038篇 |
2015年 | 1243篇 |
2014年 | 1994篇 |
2013年 | 2416篇 |
2012年 | 1461篇 |
2011年 | 1925篇 |
2010年 | 1479篇 |
2009年 | 1828篇 |
2008年 | 1855篇 |
2007年 | 2049篇 |
2006年 | 1842篇 |
2005年 | 1630篇 |
2004年 | 1431篇 |
2003年 | 1292篇 |
2002年 | 1238篇 |
2001年 | 967篇 |
2000年 | 911篇 |
1999年 | 870篇 |
1998年 | 852篇 |
1997年 | 719篇 |
1996年 | 765篇 |
1995年 | 697篇 |
1994年 | 608篇 |
1993年 | 623篇 |
1992年 | 556篇 |
1991年 | 532篇 |
1990年 | 439篇 |
1989年 | 362篇 |
1988年 | 354篇 |
1987年 | 302篇 |
1986年 | 249篇 |
1985年 | 326篇 |
1984年 | 376篇 |
1983年 | 249篇 |
1982年 | 306篇 |
1981年 | 207篇 |
1980年 | 178篇 |
1979年 | 137篇 |
1978年 | 121篇 |
1977年 | 65篇 |
1976年 | 56篇 |
1975年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Olwin BB 《Cytotechnology》1989,2(4):351-365
Heparin-binding growth factors modulate diverse biological activities including cellular proliferation, cellular differentiation, morphogenesis, and angiogenesis. Biochemical characterization for two members of the heparin-binding growth factor family, acidic and basic fibroblast growth factors, is extensive, while characterization of the remaining five members is forthcoming. Cell surface receptors have been identified for acidic and basic fibroblast growth factors, but little is known concerning their sites of action in vivo or the mechanisms involved in transducing the energy of growth factor binding to a biological response. An understanding of the biological basis for the diversity of the heparin binding growth factor family and the in vivo actions of these factors will prove a major challenge to future research efforts. 相似文献
102.
Seasonal changes in growth and standard metabolic rate of juvenile perch, Perca fluviatilis L. 总被引:1,自引:0,他引:1
P. Karaås 《Journal of fish biology》1990,37(6):913-920
The maximum growth rate of juvenile perch, PercaJuviatilis L., at different constant temperatures and in naturally changing day-lengths was studied in the laboratory. Standard metabolic rate was studied in starvation experiments at constant temperatures under short- and long-day conditions. Growth occurred in temperatures above 8 to 10°C. In winter, from mid-October until mid-April, maximal growth was considerably reduced and was relatively slow but constant. The standard metabolic rate was reduced c . 50% under short-day conditions. The seasonal change in metabolic rates, presumably controlled by an endogenous rhythm, was considered to be an adaptation to low food availability during the short winter days. 相似文献
103.
Otolith microstructure indicating growth and mortality among plaice, Pleuronectes platessa L., post-larval sub-cohorts 总被引:1,自引:0,他引:1
Growth and mortality of post-metamorphosed plaice were studied by means of daily increments in the sagittal otoliths. The Gompertz model was the best fit to length-at-age data and there were no significant differences between length-at-age and back-calculated lengths. The microstructure pattern of the otoliths at metamorphosis was also used to estimate hatching and settlement distributions. Differential growth and mortality occurred among sub-cohorts; growth rates and mortality were higher in fish that settled earlier. In 1986, the best survival was for a sub-cohort settling in late May to early June. In contrast, in the warmer season of 1987, survival was highest for the second and third sub-cohorts settling in late April and mid May. 相似文献
104.
Plant hormone response mutants 总被引:4,自引:0,他引:4
I. M. Scott 《Physiologia plantarum》1990,78(1):147-152
A variety of plant hormone response mutants has now been described, and is surveyed in this article. In addition to hormone-insensitive mutant phenotypes with defects in hormone-related features, there exist mutants apparently constitutive for the gibberellin responses, and also a mutant hyper-responsive to gibberellin. Although there is still little biochemical evidence on the nature of these mutants, the emerging picture of their genetic dominance relationships has given rise to tentative suggestions of the involvement of represser functions in hormonal control systems. 相似文献
105.
Thymocyte growth peptide (TGP) initiates DNA synthesis in immature thymocytes and has previously been characterized as an acidic peptide isolated from calf thymus. We now report the isolation of TGP from sheep thymus and show it to be a nonapeptide with a large N-terminal blocking moiety characterized by high UV absorbance. The amino acid composition is identical to FTS, consisting of 2 Gly, 2 Ser, 2 Glx, 1 Ala, 1 Lys, 1 Asx. In contrast to FTS, TGP is acidic with an apparent isoelectric point of 4.2 and a high UV absorbance at 270–280 nm. Reverse phase chromatography of TGP at an acidic pH results in a change of the molecule and the appearance of two new compounds TGP-A and TGP-B, both with less than 50% of the original TGP activity. Full activity could be restored by the addition of ZnCl2 to TGP-A. Both TGP-A and B have some amino acid composition and high UV absorbance as native TGP. We propose that TGP consists of a non-peptide moiety bound to the N-terminal of the nonapeptide Glu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn and that the active molecule is stabilized by Zn2+. 相似文献
106.
107.
Angie Rizzino Peter Kazakoff John Nebelsick 《In vitro cellular & developmental biology. Plant》1990,26(5):537-542
Summary Previous studies have shown that cell density can regulate the binding of several growth factors. To determine whether cell
density exerts a uniform effect on the expression of epidermal growth factor (EGF) receptors, seven cell lines were examined
in detail. For each cell line, EGF binding was found to decrease as cell density increases. Scatchard analysis of the binding
data reveals that decreases in EGF binding are due to reductions in the number of cell surface EGF receptors. The only apparent
exception is the effect of cell density on the binding of EGF to A-431 cells. For these cells, increases in cell density lead
to two effects: decreases in the number of high affinity EGF receptors and increases in the total number of EGF receptors.
In addition to the effects of cell density on EGF receptors, it was determined that increases in cell density can coordinately
down-regulate receptors for as many as four different growth factors. Overall, the findings described in this report for EGF
and those previously described for transforming growth factor type-β (TGF-β) and fibroblast growth factor (FGF) demonstrate
the existence of a common mechanism for down-regulating growth factor receptors.
This work was supported by grants from the Nebraska Department of Health (89-51), the National Cancer Institute (Laboratory
Research Center Support Grant, CA36727), and the American Cancer Society (Core Grant ACS SIG-16).
EDITOR'S STATEMENT The paper by Rizzino et al. demonstrates that receptor number decreases as a function of cell density.
This may represent a mechanism by which cell proliferation is reduced as cell density increases. 相似文献
108.
109.
AP-1-Related Proteins Bind to the Enkephalin CRE-2 Element in Adrenal Chromaffin Cells 总被引:1,自引:0,他引:1
Linda MacArthur 《Journal of neurochemistry》1996,67(6):2256-2264
110.
A note on optimality in lattice square designs 总被引:1,自引:0,他引:1