全文获取类型
收费全文 | 43259篇 |
免费 | 3877篇 |
国内免费 | 31篇 |
专业分类
47167篇 |
出版年
2023年 | 166篇 |
2022年 | 310篇 |
2021年 | 599篇 |
2020年 | 396篇 |
2019年 | 491篇 |
2018年 | 642篇 |
2017年 | 556篇 |
2016年 | 1047篇 |
2015年 | 1760篇 |
2014年 | 1882篇 |
2013年 | 2461篇 |
2012年 | 2984篇 |
2011年 | 3059篇 |
2010年 | 1945篇 |
2009年 | 1658篇 |
2008年 | 2519篇 |
2007年 | 2510篇 |
2006年 | 2273篇 |
2005年 | 2338篇 |
2004年 | 2297篇 |
2003年 | 2194篇 |
2002年 | 2134篇 |
2001年 | 508篇 |
2000年 | 413篇 |
1999年 | 511篇 |
1998年 | 609篇 |
1997年 | 462篇 |
1996年 | 413篇 |
1995年 | 393篇 |
1994年 | 359篇 |
1993年 | 346篇 |
1992年 | 391篇 |
1991年 | 327篇 |
1990年 | 273篇 |
1989年 | 276篇 |
1988年 | 277篇 |
1987年 | 255篇 |
1986年 | 235篇 |
1985年 | 306篇 |
1984年 | 327篇 |
1983年 | 260篇 |
1982年 | 356篇 |
1981年 | 276篇 |
1980年 | 257篇 |
1979年 | 190篇 |
1978年 | 224篇 |
1977年 | 210篇 |
1976年 | 182篇 |
1974年 | 190篇 |
1973年 | 196篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Populations of the vernal cladoceranDaphniopsis ephemeralis are found in woodland ponds throughout southern Ontario. The species reproduces by cyclic parthenogenesis, and genotype frequencies
at allozyme loci are ordinarily in good agreement with Hardy-Weinberg expectations. Occasional heterozygote deficiencies are
apparently the consequence of admixture of ephippial hatchlings produced in temporally separated bouts of sexual reproduction.
Considerable heterogeneity in genotypic frequencies exists among local populations in southwestern Ontario, indicating that
gene flow among populations is restricted. Inbreeding coefficients suggest that populations receive an average of 0.3 migrants
per generation. The completion of a sexual life cycle is made possible despite the brief persistence of populations by the
emergence of males from ephippial eggs and by the production of equal numbers of male and female progeny in the first parthenogenetic
brood. 相似文献
112.
The defence reaction of operculum closing in response to the presence of the molluscivorous leech Glossiphonia complanata (L.) and the non-molluscivorous Erpobdella octoculata (L.) was studied in four species of freshwater prosobranch gastropod. Bithynia tentaculata (L.) and Valvata piscinalis (Müller) can distinguish between the leeches, reacting only to G. complanata. V. piscinalis is capable of a greater degree of distance chemoreception of the leech ‘scent’. Valvata cristata Müller and Potamopyrgus jenkinsi (Smith) did not react to either leech. V. cristata may not be a potential prey item for G. complanata, while P. jenkinsi is fed on by the leech, but is a relative newcomer to the freshwater fauna.
Animal Ecology Research Group, Department of Zoology, University of Oxford
Commonwealth Forestry Institute, Department of Plant Sciences, University of Oxford 相似文献
113.
C R Paul C A Belfi A V Arakali H C Box 《International journal of radiation biology and related studies in physics, chemistry, and medicine》1987,51(1):103-114
The mediation of radiation-induced damage to dinucleoside monophosphate by oxygen and by glutathione was studied. The sequence isomers d(TpA) and d(ApT) were X-irradiated in aqueous solutions and the products isolated by reverse-phase high-performance liquid chromatography. The main products were characterized by proton NMR spectroscopy. In the presence of oxygen the principal products are the formamido derivative formed by breakdown of thymine and the aldehyde derivative formed at the 5' end of the dinucleoside monophosphate, both nucleoside monophosphates and free bases. In the presence of glutathione, the two stereoisomers of the 5,6-dihydrothymine derivatives are prominent. Radiation-induced damage to d(TpA) and d(ApT) in the solid state was also studied. 相似文献
114.
Demonstration of up-regulated IL 2 receptor expression on an in vitro cloned BCL1 subline 总被引:1,自引:0,他引:1
K Nakanishi T Hashimoto K Hiroishi K Matsui T Yoshimoto H C Morse J Furuyama T Hamaoka K Higashino W E Paul 《Journal of immunology (Baltimore, Md. : 1950)》1987,138(6):1817-1825
We have established BCL1 CL-3 cells capable of responding to B15-TRF and interleukin 2 (IL 2). This clone has both high affinity and low affinity receptors for IL 2 (IL 2R), but IL 2 by itself did not stimulate either proliferation or immunoglobulin (Ig) secretion. B15-TRF, which possesses both growth and differentiation activity, causes an increase in size of CL-3 cells and renders CL-3 cells responsive to IL 2, including an increased expression of IL 2R (eight-fold to 10-fold) and the differentiation of CL-3 cells into Ig secretion (60 to 80% of cultured cells). CL-3 cells pretreated with B15-TRF for 12 hr become competent to respond to IL 2 by up-regulation of IL 2R within 12 hr. In contrast CL-3 cells pretreated with IL 2 for 12 hr required 24 hr B15-TRF stimulation to result in IL 2R up-regulation. Thus the ordered action of B15-TRF and IL 2 is the most effective operational pathway for the up-regulation of IL 2R. This IL 2-mediated IL 2R up-regulation and induction of Ig synthesis depends upon the concentration of IL 2 in the culture. Both responses seem to be caused by IL 2 molecules bound to high affinity IL 2R. However, the possibility of involvement of low affinity IL 2R can not be vigorously excluded. In fact the level of IL 2 required for a response is far higher than that needed for activated T cell proliferation. This cloned BCL1 subline promises to be a useful tool for studying the regulation and mechanisms of B cell responses. 相似文献
115.
High-efficiency purification and chemical characterization of B cell stimulatory factor-1/interleukin 4 总被引:5,自引:0,他引:5
J Ohara J E Coligan K Zoon W L Maloy W E Paul 《Journal of immunology (Baltimore, Md. : 1950)》1987,139(4):1127-1134
B cell stimulatory factor-1/interleukin 4, a lymphokine produced by phorbol ester activated-EL-4 thymoma cells, was purified to homogeneity in good yield by a two-step purification procedure, using affinity chromatography and a single subsequent round of reverse-phase high-performance liquid chromatography. The N-terminal sequence of the first 24 amino acids was consistent with that inferred from the nucleotide sequence of BSF-1 cDNA clones. Amino acid composition analysis also agreed well with that predicted from the nucleotide sequence. A rabbit antibody to a peptide corresponding to positions 100 to 113 inferred from the nucleotide sequence of the cDNA clones bound to BSF-1 purified from EL-4 cells. Purified BSF-1 possesses complex N-linked glycosidic side chains as shown by reduction in Mr from 20,000 to approximately 15,000 by endoglycosidase F but not by endoglycosidase H treatment. Removal of these N-linked sugars does not diminish the activity of BSF-1 as a costimulant in the response of B cells to anti-IgM. By contrast, the reduction of disulfide bonds completely destroyed biologic activity. A monoclonal antibody to BSF-1 blocks its binding to cellular receptors and inhibits biological activities, whereas antibody to the BSF-1 peptide (100-113) has neither effect. 相似文献
116.
Summary Patch-clamp techniques were used to study a K channel in the cell membrane of MDCK cells. This cell line derives from the kidney of a normal dog, presumably from the distal nephron, a region involved in potassium secretion. The cells were cultured in confluent monolayers and approached from the apical side. The K channel we describe is Ca2+ and voltage activated, has a conductance of 221±7 pS, and can be inhibited by 10mm tetraethylammonium and by 1mm quinidine, but not by 4-aminopyridine, nor by 1mm Ba2+ added to the outer side. Using the whole-cell configuration, we find that most of the cationic conductance of the membrane is constituted by a K-specific one (maximum K conductance 32.1±3.9 nSvs. a leak conductance of 1.01±0.17 nS). Comparisons of the maximum K conductance with that of a single K channel indicates that an MDCK cell has an average of 145 such channels. The membrane capacity is 24.5±1.4 pF. 相似文献
117.
Summary DNA from the kangaroo rat,Dipodomys ordii, contains a 3.3-kb, highly repeated sequence that is interspersed throughout the genome in small tandem clusters. One 3.3-kb unit has been cloned into pBR322 and the nucleotide sequence determined. The clone used was shown to be representative of the bulk of such sequences found in the genomic DNA. The sequence contains 10 homologous subunits each ca. 260 bp in length. Comparison of these to one another yielded a 258-bp consensus sequence containing a 35-bp terminal inverted repeat. Two unique stretches also occur. One of these contains a region that could serve as a promoter for RNA polymerase III; the other contains a sequence related to the ARS sequences of yeast. It is proposed that an ancestral sequence similar to the consensus sequence was amplified to 10 or more units, and that, subsequently, two other sequences were inserted. The properties of these insertions may have led to the dispersal of the sequence throughout the genome. 相似文献
118.
The embryonic development of total specific activities as well as of molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) and of butyrylcholinesterase (BChE, EC 3.1.1.8) have been studied in the chick brain. A comparison of the development in different brain parts shows that cholinesterases first develop in diencephalon, then in tectum and telencephalon; cholinesterase development in retina is delayed by about 2-3 days; and the development in rhombencephalon [not studied until embryonic day 6 (E6)] and cerebellum is last. Both enzymes show complex and independent developmental patterns. During the early period (E3-E7) first BChE expresses high specific activities that decline rapidly, but in contrast AChE increases more or less constantly with a short temporal delay. Thereafter the developmental courses approach a late phase (E14-E20), during which AChE reaches very high specific activities and BChE follows at much lower but about parallel levels. By extraction of tissues from brain and retina in high salt plus 1% Triton X-100, we find that both cholinesterases are present in two major molecular forms, AChE sedimenting at 5.9S and 11.6S (corresponding to G2 and G4 globular forms) and BChE at 2.9S and 10.3S (G1 and G4, globular). During development there is a continuous increase of G4 over G2 AChE, the G4 form reaching 80% in brain but only 30% in retina. The proportion of G1 BChE in brain remains almost constant at 55%, but in retina there is a drastic shift from 65% G1 before E5 to 70% G4 form at E7.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
119.
Beth J. Hoffman Ursula Scheffel† John R. Lever† Michael D. Karpa Paul R. Hartig† 《Journal of neurochemistry》1987,48(1):115-124
Methylation of 2-125I-lysergic acid diethylamide (125I-LSD) at the N1 position produces a new derivative, N1-methyl-2-125I-lysergic acid diethylamide (125I-MIL), with improved selectivity and higher affinity for serotonin 5-HT2 receptors. In rat frontal cortex homogenates, specific binding of 125I-MIL represents 80-90% of total binding, and the apparent dissociation constant (KD) for serotonin 5-HT2 receptors is 0.14 nM (using 2 mg of tissue/ml). 125I-MIL also displays a high affinity for serotonin 5-HT1C receptors, with an apparent dissociation constant of 0.41 nM at this site. 125I-MIL exhibits at least 60-fold higher affinity for serotonin 5-HT2 receptors than for other classes of neurotransmitter receptors, with the dopamine D2 receptor as its most potent secondary binding site. Studies of the association and dissociation kinetics of 125I-MIL reveal a strong temperature dependence, with very slow association and dissociation rates at 0 degree C. Autoradiographic experiments confirm the improved specificity of 125I-MIL. Selective labeling of serotonin receptors was observed in all brain areas examined. In vivo binding studies in mice indicate that 125I-MIL is the best serotonin receptor label yet described, with the highest frontal cortex to cerebellum ratio of any serotonergic radioligand. 125I-MIL is a promising ligand for both in vitro and in vivo labeling of serotonin receptors in the mammalian brain. 相似文献
120.
In the mouse neuroblastoma x dorsal root ganglion hybrid cell line F-11, bradykinin receptor stimulation induced the release of inositol-1,4,5-trisphosphate (IP3) and inositol-1,4-bisphosphate (IP2). Maximal stimulation of [2-3H]IP3 and [2-3H]IP2 release by bradykinin in the absence of LiCl occurred at 7 (or less) and 15 s, respectively, with average levels of 5.7-(IP3) and 3.4-(IP2) fold of control values. The EC50 for bradykinin was 33 +/- 5 nM. IP3 and IP2 concentrations returned to basal levels approximately 1 min after bradykinin addition. Bradykinin-induced IP3 release was blocked by several novel bradykinin analogues. In particular, [D-Arg0]-Hyp3-Thi5,8-[D-Phe7]-bradykinin [Hyp, hydroxyproline; Thi, beta-(2-thienyl)-L-alanine] blocked IP3 production in a dose-dependent fashion. Several of these analogues alone showed little or no agonist activity. The bradykinin receptor may be coupled to phospholipase C via a GTP-sensitive protein (Gi or Go), as preincubation for 18-20 h with pertussis toxin decreased IP3 concentrations by 45%. Bradykinin is also known to modulate the concentrations of other second messengers in neurons, increasing the concentrations of Ca2+, diacylglycerol (DG), and cyclic GMP and decreasing the concentration of cyclic AMP. These second messengers modulated bradykinin-dependent IP3 release to varying degrees. A23187, a Ca2+ ionophore, produced a 37% decrease in IP3 concentration. 12-O-Tetradecanoylphorbol-13-acetate, which mimics the effects of DG and activates protein kinase C, inhibited IP3 release by 80%. Dibutyryl cyclic GMP produced little or no inhibition of IP3. [D-Ala2,D-Leu5]Enkephalin (DADLE), an opioid peptide that decreases cyclic AMP concentrations, likewise had no effect.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献