全文获取类型
收费全文 | 17779篇 |
免费 | 1086篇 |
国内免费 | 39篇 |
出版年
2023年 | 185篇 |
2022年 | 578篇 |
2021年 | 933篇 |
2020年 | 499篇 |
2019年 | 660篇 |
2018年 | 786篇 |
2017年 | 541篇 |
2016年 | 766篇 |
2015年 | 949篇 |
2014年 | 1054篇 |
2013年 | 1362篇 |
2012年 | 1377篇 |
2011年 | 1255篇 |
2010年 | 771篇 |
2009年 | 626篇 |
2008年 | 754篇 |
2007年 | 776篇 |
2006年 | 652篇 |
2005年 | 598篇 |
2004年 | 518篇 |
2003年 | 425篇 |
2002年 | 391篇 |
2001年 | 216篇 |
2000年 | 179篇 |
1999年 | 141篇 |
1998年 | 105篇 |
1997年 | 67篇 |
1996年 | 77篇 |
1995年 | 73篇 |
1994年 | 55篇 |
1993年 | 55篇 |
1992年 | 108篇 |
1991年 | 84篇 |
1990年 | 104篇 |
1989年 | 91篇 |
1988年 | 86篇 |
1987年 | 70篇 |
1986年 | 70篇 |
1985年 | 73篇 |
1984年 | 79篇 |
1983年 | 64篇 |
1982年 | 56篇 |
1981年 | 65篇 |
1980年 | 60篇 |
1979年 | 39篇 |
1978年 | 52篇 |
1977年 | 41篇 |
1976年 | 38篇 |
1974年 | 36篇 |
1973年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Cretaceous and early Paleocene benthic foraminifera were studied from one section along the western Gaj River, southern Balochistan, Pakistan, to reconstruct the paleoenvironment of the Tethys Sea during the Indian-Asian contact. We recognize three lithostratigraphic units in ascending order: the Mughal Kot Formation, the Pab Sandstone, and the Jamburo Group. Both the Maastrichtian Mughal Kot Formation, which consists of shale with grey marly limestone, and the Maastrichtian Pab Sandstone, which consists of quartzose sandstone, indicate an open ocean environment as they have diversified planktic and benthic foraminiferal assemblages. The Maastrichtian-Paleocene Jamburo Group, consisting of dark grey, calcareous shale and marlstone with some sulfide grains, is characterized by low diversities of benthic assemblages. The change to the lower diversities may be associated with the development of poor circulation of deeper water that was caused by narrowing of the Tethys Sea.The Trochammina spp. Assemblage from the Jamburo Group, which can be correlated with flysch-type agglutinated foraminiferal assemblages, has a low benthic species diversity, indicating an unfavorable condition for calcareous foraminifera because of the development of oxygen-depleted water. The absolute abundance of agglutinated specimens shows a remarkable change from low numbers in the Maastrichtian to high ones in the Paleocene. The benthic foraminiferal evidence supports the hypothesis that the collision of the Asian and Indian plates occurred near the end of the Cretaceous. 相似文献
182.
Drugs of abuse, such as phencyclidine (PCP), methamphetamine (METH), and cocaine (COC) are known to affect several behaviors
in rats, such as motor activity, stereotypy, and circling. In this study, we evaluated whether these drugs produce circling
preferences in the presence or absence of unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the caudate nucleus. Adult
male CD rats were lesioned with 10 μg 6-OHDA/site. Animals were dosed with PCP (15 mg/kg, ip), its congener, (+) MK-801 (0.15
mg/kg, ip), METH (2 mg/kg, ip), COC (60 mg/kg, ip), or apomorphine (0.2 mg/kg, ip). circling preference was recorded in control
and lesioned rats for 2 h before animals were sacrificed to determine monoamine levels by HPLC/EC. In control animals, administration
of these drugs produced 60–70% left circling. In, lesioned animals, these drugs produced 78–90% ipsilateral (toward the lesion)
circling, except apomorphine, which produced 60–80% contralateral (away from the lesion) circling. Dopamine (DA) and its metabolites
3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) concentrations significantly decreased ipsilaterally in
lesioned caudate nucleus (CN) and substantia nigra (SN). However, no significant changes were observed in nucleus accumbens
(NA) and olfactory tubercles (OT). These data demonstrate that drugs of abuse like PCP, its congener (+) MK-801, METH, and
COC produce a greater preference to turn toward the left than the right, a finding similar to that found in human psychosis.
Since 6-OHDA lesions enhanced the circling bias and depleted DA and its metabolites DOPAC and HVA, it also suggests that the
dopaminergic system may be involved in the circling behavior. 相似文献
183.
Summary Glutamate (Glu) the major amino acid in mammalian brain and most dietary proteins possesses neurotransmitter as well as neurotoxic properties. We administered monosodium glutamate (MSG) 4 mg/g bwt, sc on postnatal day (PND) 1 through 10 to rats on alternate days or daily and sacrificed them on PND 45 or PND 90 respectively. The activities of glutamate dehydrogenase and aminotransferases were evaluated in the circumventricular organs of brain. Results show that neonatal MSG produces alterations in glutamate metabolism in blood-brain-barrier deficient regions. 相似文献
184.
Adherence through carbohydrate-binding adhesins is an earlystep in colonization of the lung by gram-negative organisms,and because published data indicate that binding involves mannosegroups, we tested the ability of a ß-linked acetylmannan(acemannan) to inhibit adherence of Pseudomonus aeruginosa tocultures of human lung epithelial cells. Adherence of radiolabelledP.aeruginosa to A549 cells (a type II-like pneurnocyte line)increased linearly with the duration of the incubation. Acemannaninhibited adherence of bacteria, and the extent of inhibitionwas related to the concentration of the mannan. Inhibition requiredcontinued contact between acemannan and the target epithelialcells; cells washed free of acemannan no longer discouragedbacterial binding. Comparison of binding between seven strainsof P.aeruginosa indicated that fewer mucoid than non-mucoidbacteria adhered, but binding of either phenotype was inhibitedby acemannan. Mannose methyl -D-mannopyranoside, methyl ß-D-mannopyrannosideand dextran did not affect adherence of any of the non-mucoidstrains. Mannose inhibited adherence by one mucoid strain, butnot the other, indicating differences between strains of thesame phenotype. Since prior treatment of epithelial cells withconcanavalin A did not affect acemannan-induced inhibition ofbacterial adherence, we concluded that the inhibitory effectof acemannan probably does not involve mannose-containing receptors. bacterial-host interactions lung epithelium mucoid strains non-mucoid strains 相似文献
185.
Gel filtration and thin layer chromatography were conducted on sera from uremic patients and normal subjects for the isolation of nitrogenous substances unique to uremia. Many ninhydrin-positive substances were found in greater amounts in uremic patients compared to normal subjects. Some of these ninhydrin-positive substances were also detected by staining with chlorine-tolidine. Amino acid analysis of these substances showed considerable qualitative and quantitative differences, perhaps reflecting interference with enzymatic activity by the uremic environment. 相似文献
186.
S C Wong M A Ali J R Pond S M Rubin S E Johnson J B Wilson T H Huisman 《Biochimica et biophysica acta》1984,784(2-3):187-188
A new fast-moving alpha-chain Hb variant with an Asn leads to Asp substitution at position alpha-78 was found in a French-Acadian family living in Eastern Canada. The identical substitution was reported in Hb J-Singapore, which also had an additional Ala leads to Gly substitution at position alpha-79. The new variant, which did not result in any clinical symptoms, was named accordingly, Hb J-Singa. 相似文献
187.
(1) Krebs II ascites cells were taken as a model of the neoplastic cells to investigate the transverse distribution of phospholipids in the plasma membrane. The experimental procedure was based on non-lytic degradation of phospholipids in the intact cell by Naja naja phospholipase A2 and Staphylococcus aureus sphingomyelinase C and on phopholipid analysis of purified plasma membranes. It was shown that the three major phospholipids, i.e., phosphatidylcholine, phosphatidylethanolamine and sphingomyelin, are randomly distributed between the two halves of the membranes, whereas phosphatidylserine remains located in the inner leaflet. (2) The membrane localization of phosphatidylcholine and phosphatidylethanolamine subclasses (diacyl, alkylacyl and alkenylacyl) was also examined, using a new procedure of ether-phospholipid determination. The method involves a selective removal of diacyl species by guinea pig pancreas phospholipase A1 and of alkenylacyl species by acidolysis. This analysis revealed a 50% increase of ether phospholipids in the plasma membrane as compared to the whole cell (36.5 and 23.1% of total phospholipid, respectively). Furthermore, a strong membrane asymmetry was demonstrated for the three phosphatidylcholine subclasses, since 1-alkyl-2-acyl-sn-glycerol-3-phosphocholine (alkylacyl-GPC) was entirely found in the inner leaflet, whereas both diacyl- and alkenylacyl-GPC displayed an external localization. The same pattern was observed for phosphatidylethanolamine subclasses, except for 1-alkenyl-2-acyl-sn-glycero-3-phosphoethanolamine, which was found randomly distributed. These results are discussed in relation to the process of cell malignant transformation and to the biosynthesis of platelet-activating factor (PAF-acether or 1-alkyl-2-acetyl-GPC). 相似文献
188.
Phosphorylation of the androgen receptor by a nuclear cAMP-independent protein kinase 总被引:2,自引:0,他引:2
S A Goueli J L Holtzman K Ahmed 《Biochemical and biophysical research communications》1984,123(2):778-784
The androgen receptor was purified from rat ventral prostate. The purified receptor migrated as a single band of mol. wt. 87000 on SDS-polyacrylamide gels, had a kd for R-1881 (17 beta-hydroxy-17 alpha-methyl-estra-4,9,11-trien-3-one) binding as 6 nM, and sedimentation coefficient of 4.5 S. Phosphorylation of the purified receptor was studied by incubating it with [gamma-32P]ATP in the presence of several purified protein kinases including cAMP-dependent protein kinase, and four cAMP-independent protein kinases (which were active towards substrates such as phosvitin and casein). Phosphorylation of the 87000 mol. wt. androgen receptor protein occurred only in the presence of a nuclear cAMP-independent protein kinase (of the N2 type). No auto-phosphorylation of the receptor was detected. The results indicate that the androgen receptor is a phosphoprotein. Further, phosphorylation of the androgen receptor by only a specific nuclear cAMP-independent protein kinase may be important in determining the dynamics of its function. 相似文献
189.
190.
S. H. Mahmoud J. A. Gatehouse D. Boulter 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1984,68(6):559-566
Summary Three isoenzyme systems (amylase, esterase and glutamate oxaloacetate transaminase) were examined in seeds of pea (Pisum sativum L.) and shown to give clear variation in their band patterns on gel electrophoresis between different lines. The inheritance of these isoenzyme systems, and the location of their genes on the pea genome was investigated. Reciprocal crosses were made between lines, F2 seeds were analysed for segregation in the band patterns of the isoenzymes, and F2 plants were investigated to find linkage between the genes for these isoenzymes and genes for selected morphological markers. The results obtained showed that each of the investigated isoenzyme systems is genetically controlled by co-dominant alleles at a single locus. The gene for amylase was found to be on chromosome 2, linked to the loci k and wb (wb ... 9 ... k ... 25 ... Amy). The gene for esterase was found to be linked with the gene Br (chromosome 4) but the exact location is uncertain because of the lack of the morphological markers involved in the cross. The gene for glutamate oxaloacetate transaminase was found to be on chromosome 1 and linked with the loci a and d (a... 24... Got... 41 ... d). 相似文献