全文获取类型
收费全文 | 167372篇 |
免费 | 10244篇 |
国内免费 | 10817篇 |
出版年
2023年 | 2097篇 |
2022年 | 3190篇 |
2021年 | 4473篇 |
2020年 | 4661篇 |
2019年 | 6276篇 |
2018年 | 5097篇 |
2017年 | 3971篇 |
2016年 | 4340篇 |
2015年 | 5744篇 |
2014年 | 8998篇 |
2013年 | 11868篇 |
2012年 | 7252篇 |
2011年 | 9763篇 |
2010年 | 7332篇 |
2009年 | 7985篇 |
2008年 | 8427篇 |
2007年 | 8564篇 |
2006年 | 7732篇 |
2005年 | 6939篇 |
2004年 | 6093篇 |
2003年 | 5238篇 |
2002年 | 4645篇 |
2001年 | 3447篇 |
2000年 | 3016篇 |
1999年 | 3006篇 |
1998年 | 2771篇 |
1997年 | 2419篇 |
1996年 | 2195篇 |
1995年 | 2375篇 |
1994年 | 2214篇 |
1993年 | 2009篇 |
1992年 | 1953篇 |
1991年 | 1692篇 |
1990年 | 1452篇 |
1989年 | 1356篇 |
1988年 | 1298篇 |
1987年 | 1255篇 |
1986年 | 933篇 |
1985年 | 1496篇 |
1984年 | 2009篇 |
1983年 | 1453篇 |
1982年 | 1845篇 |
1981年 | 1348篇 |
1980年 | 1338篇 |
1979年 | 1192篇 |
1978年 | 753篇 |
1977年 | 624篇 |
1976年 | 530篇 |
1975年 | 407篇 |
1973年 | 408篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
In the frog intestine, both in vitro and in vivo, experiments were carried out in order to increase knowledge of the mechanism of sugar exit across the basolateral membrane of the enterocyte. The frog intestine was chosen because it lacks crypt cells and, consequently, any external fluid circuit mechanism during sugar transport can be avoided. Therefore, the sugar concentration in the absorbate collected on the serosal side is likely to be similar to that present underneath the basolateral membrane of the enterocyte. Under this condition, cell and absorbate sugar concentrations are similar; yet there is a concomitant net transintestinal sugar transport. Moreover, in in vivo experiments a net transintestinal sugar transport takes place even against a concentration difference. These results suggest that sugar exit across the basolateral membrane is not simply due to a chemically facilitated diffusion. 相似文献
212.
213.
214.
Antibodies to human amyloid precursor protein (APP695) and beta‐amyloid peptide (Aβ1‐42) were used to determine timing of amyloidosis in the brain of kokanee salmon (Oncorhynchus nerka kennerlyi) in one of four reproductive stages: immature (IM), maturing (MA), sexually mature (SM), and spawning (SP), representing a range of aging from somatically mature but sexually immature to spawning and somatic senescence. In IM fish, immunoreactive (ir) intracellular APP occurred in 18 of 23 brain regions. During sexual maturation and aging, the number of neurons expressing APP increased in 11 of these APP‐ir regions. Aβ‐ir was absent in IM fish, present in seven regions in MA fish, moderately abundant in 15 regions in SM fish, and was most abundant in all brain regions of SP fish exhibiting Aβ‐ir. Intracellular APP‐ir was observed in brain regions involved in sensory integration, olfaction, vision, stress responses, reproduction, and coordination. Intra‐ and extracellular Aβ1‐42 immunoreactivity (Aβ‐ir) was present in all APP‐ir regions except the nucleus lateralis tuberis (hypothalamus) and Purkinje cells (cerebellum). APP‐ir and Aβ deposition increase during aging. APP‐ir is present in IM fish; Aβ‐ir usually appears first in MA or SM fish and increases in SM fish as does APP‐ir. Extracellular Aβ deposition dramatically increases between SM and SP stages (1–2 weeks) in all fish, indicating an extremely rapid and synchronized process. Rapid senescence observed in pacific salmon could make them a useful model to investigate timing of amyloidosis and neurodegeneration during brain aging. © 2002 Wiley Periodicals, Inc. J Neurobiol 53: 11–20, 2002 相似文献
215.
Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH
4
+
–15N was added to the soil in the absence and the presence of NaClO3, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter. 相似文献
216.
《FEBS letters》1988,240(1-2):88-94
Four subtypes of muscarinic acetylcholine receptor (mAChR) were stably expressed in neuroblastoma-glioma hybrid cells (NG108-15). By combining fluorescent indicator dye (fura-2) studies with electrophysiological measurements it is shown that stimulation of mAChR I and mAChR III readily leads to release of calcium from intracellular stores and to associated conductance changes, whereas stimulation of mAChR II and mAChR IV exerts no such effect. Dose-response curves describing the amplitude or the delay of the calcium rise induced by acetylcholine suggest that the apparent affinity of mAChR III for its agonist is higher by about one order of magnitude than that of mAChR I. Ionic substitution experiments and current fluctuation analysis indicate that calcium activates a K+-specific conductance of ‘small’ single-channel amplitude similar to the SK type [1]. Furthermore, an outward current (M current) suppressed by activation of mAChR I and mAChR III has a single-channel amplitude corresponding to a conductance of approximately 3 pS. 相似文献
217.
Janine F. Bridges Peter C. Millard John F. Woodley 《Biochimica et Biophysica Acta (BBA)/General Subjects》1978,544(2):448-451
The uptake of free and liposome-entrapped 125I-labelled poly(vinylpyrrolidone) was measured in an intestinal sac preparation from adult rats. An an equal concentration of 125I-labelled poly(vinylpyrrolidone), the rate of uptake of the liposome-entrapped material was four times that of the free macromolecule. 相似文献
218.
When hemolymph is taken from Ascaris lumbricoides at the time the worm is collected from pigs, it contains acetic, propionic, 2-methylbutyric, n-valeric, 2-methylvaleric, and succinic acid radicals; tiglic acid is absent. 相似文献
219.
220.
Marjorie Jones Roy W. Keenan 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,678(3):403-407
A procedure was developed for the detection of 2′,3′-cyclic nucleotide 3′-phosphohydrolase in myelin. This assay was sufficiently sensitive to detect the low levels of 2′,3′-cyclic nucleotide 3′-phosphohydrolase in human erythrocytes. The 2′,3′-cyclic nucleotide 3′-phosphohydrolase of human erythrocytes was determined to be exclusively associated with the inner (cytosolic) side of the membrane. Leaky ghostsand resealed ghosts were assayed for 2′,3′-cyclic nucleotide 3′-phosphohydrolase, (Ca2+/Mg2+-ATPase, and acetylcholinesterase activity, and the 2′,3′-cyclic nucleotide 3′-phosphohydrolase profile is the same as that of the (Ca2+/Mg2+)-ATPase, an established inner membrane maker. 相似文献