全文获取类型
收费全文 | 6434篇 |
免费 | 583篇 |
专业分类
7017篇 |
出版年
2022年 | 48篇 |
2021年 | 85篇 |
2020年 | 55篇 |
2019年 | 76篇 |
2018年 | 84篇 |
2017年 | 93篇 |
2016年 | 132篇 |
2015年 | 230篇 |
2014年 | 265篇 |
2013年 | 299篇 |
2012年 | 389篇 |
2011年 | 389篇 |
2010年 | 253篇 |
2009年 | 234篇 |
2008年 | 320篇 |
2007年 | 307篇 |
2006年 | 284篇 |
2005年 | 280篇 |
2004年 | 307篇 |
2003年 | 302篇 |
2002年 | 256篇 |
2001年 | 207篇 |
2000年 | 165篇 |
1999年 | 172篇 |
1998年 | 106篇 |
1997年 | 104篇 |
1996年 | 76篇 |
1995年 | 61篇 |
1994年 | 79篇 |
1993年 | 64篇 |
1992年 | 106篇 |
1991年 | 91篇 |
1990年 | 91篇 |
1989年 | 94篇 |
1988年 | 67篇 |
1987年 | 49篇 |
1986年 | 52篇 |
1985年 | 79篇 |
1984年 | 61篇 |
1983年 | 50篇 |
1982年 | 46篇 |
1981年 | 35篇 |
1980年 | 30篇 |
1979年 | 54篇 |
1978年 | 34篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1974年 | 33篇 |
1973年 | 27篇 |
1971年 | 26篇 |
排序方式: 共有7017条查询结果,搜索用时 15 毫秒
161.
Hanne N. Rasmussen Bjarke Veierskov Jens Hansen-Møller Rikke Nørbæk Ulrik Bräuner Nielsen 《Journal of Plant Growth Regulation》2009,28(2):154-166
Conifer trees are routinely manipulated hormonally to increase flowering, branching, or adjust crown shape for production
purposes. This survey of internal cytokinin levels provides a background for such treatments in Abies nordmanniana, a tree of great economic interest. Reference points in the crown and root system were sampled destructively in 4- and 6-year-old
trees and analyzed for a range of cytokinins by LC-MS/MS. No seasonal patterns were detected in the root samples, and a major
portion of cytokinin was in conjugated forms. Dramatic and consistent seasonal changes occurred in the crown, at levels 17–65 times
higher than in the root. Predominant among crown cytokinins was ZR, except in the needles where IPR was also prominent. Within
the crown, cytokinin profiles in different organs differed consistently. The leader bud showed a pronounced mid-June minimum,
and a maximum later in summer. Subapical buds showed the same June minimum but peaked in mid autumn at a much lower level.
Maxima in these buds were preceded by peaks in the subapical stem. Parallel patterns were observed in homologous tissues on
branches.This pattern is consistent with two surges beginning in the uppermost stem tissues leading to subsequent accumulation
or stimulated production within the buds. Strong differential hormonal profiles between adjacent buds with different fates
agree with recent evidence of localized cytokinin production. The data suggest a reduced role of root-derived cytokinins in
crown development. Practical cytokinin treatments for crown-shape regulation require close attention to dosage as well as
precise timing and positioning. 相似文献
162.
SH3P7 Is a Cytoskeleton Adapter Protein and Is Coupled to Signal Transduction from Lymphocyte Antigen Receptors 总被引:3,自引:0,他引:3 下载免费PDF全文
Oliver Larbolette Bernd Wollscheid Jutta Schweikert Peter J. Nielsen Jürgen Wienands 《Molecular and cellular biology》1999,19(2):1539-1546
Lymphocytes respond to antigen receptor engagement with tyrosine phosphorylation of many cellular proteins, some of which have been identified and functionally characterized. Here we describe SH3P7, a novel substrate protein for Src and Syk family kinases. SH3P7 migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a 55-kDa protein that is preferentially expressed in brain, thymus, and spleen. It contains multiple amino acid sequence motifs, including two consensus tyrosine phosphorylation sites of the YXXP type and one SH3 domain. A region of sequence similarity, which we named SCAD, was found in SH3P7 and three actin-binding proteins. The SCAD region may represent a new type of protein-protein interaction domain that mediates binding to actin. Consistent with this possibility, SH3P7 colocalizes with actin filaments of the cytoskeleton. Altogether, our data implicate SH3P7 as an adapter protein which links antigen receptor signaling to components of the cytoskeleton. 相似文献
163.
Structure and dynamics of model pore insertion into a membrane 总被引:1,自引:0,他引:1
A cylindrical transmembrane molecule is constructed by linking hydrophobic sites selected from a coarse grain model. The resulting hollow tube assembly serves as a representation of a transmembrane channel, pore, or a carbon nanotube. The interactions of a coarse grain di-myristoyl-phosphatidyl-choline hydrated bilayer with both a purely hydrophobic tube and a tube with hydrophilic caps are studied. The hydrophobic tube rotates in the membrane and becomes blocked by lipid tails after a few tens of nanoseconds. The hydrophilic sites of the capped tube stabilize it by anchoring the tube in the lipid headgroup/water interfacial region of each membrane leaflet. The capped tube remains free of lipid tails. The capped tube spontaneously conducts coarse grain water sites; the free-energy profile of this process is calculated using three different methods and is compared to the barrier for water permeation through the lipid bilayer. Spontaneous tube insertion into an undisturbed lipid bilayer is also studied, which we reported briefly in a previous publication. The hydrophobic tube submerges into the membrane core in a carpetlike manner. The capped tube laterally fuses with the closest leaflet, and then, after plunging into the membrane interior, rotates to assume a transbilayer orientation. Two lipids become trapped at the end of the tube as it penetrates the membrane. The hydrophilic headgroups of these lipids associate with the lower tube cap and assist the tube in crossing the interior of the membrane. When the rotation is complete these lipids detach from the tube caps and fuse with the lower leaflet lipids. 相似文献
164.
Ole Frøbert Jacob Moesgaard Egon Toft Steen Hvitfeldt Poulsen Peter Søgaard 《Cardiovascular ultrasound》2004,2(1):1-8
Background
Endothelial function in hypercholesterolemic rabbits is usually evaluated ex vivo on isolated aortic rings. In vivo evaluation requires invasive imaging procedures that cannot be repeated serially.Aim
We evaluated a non-invasive ultrasound technique to assess early endothelial function in rabbits and compare data with ex vivo measurements.Methods
Twenty-four rabbits (fed with a cholesterol diet (0.5%) for 2 to 8 weeks) were given progressive infusions of acetylcholine (0.05–0.5 μg/kg/min) and their endothelial function was assessed in vivo by transcutaneous vascular ultrasound of the abdominal aorta. Ex vivo endothelial function was evaluated on isolated aortic rings and compared to in vivo data.Results
Significant endothelial dysfunction was demonstrated in hypercholesterolemic animals as early as 2 weeks after beginning the cholesterol diet (aortic cross-sectional area variation: -2.9% vs. +4% for controls, p < 0.05). Unexpectedly, response to acetylcholine at 8 weeks was more variable. Endothelial function improved in 5 rabbits while 2 rabbits regained a normal endothelial function. These data corroborated well with ex vivo results.Conclusion
Endothelial function can be evaluated non-invasively in vivo by transcutaneous vascular ultrasound of the abdominal aorta in the rabbit and results correlate well with ex vivo data. 相似文献165.
The hypothesis according to which iron overload could be harmful has been extensively and controversially discussed in the literature. One underlying pathological mechanism may be elevated oxidative stress. Thus, we studied the correlation between hemochromatosis and an established marker of oxidative stress, 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha, iPF2alpha-III, 15-F2t-IsoP). We enrolled 21 patients with hemochromatosis, positive for the homozygous C282Y mutation in the HFE gene, and 21 healthy controls frequency-matched by age and gender in a case-control study design. The objective was to show that iron overload in HFE-related hemochromatosis is associated with increased oxidative stress assessed through 8-iso-PGF(2alpha) urinary excretion, and that oxidative stress is impacted by iron-removal treatment (phlebotomy). Study parameters were transferrin saturation, 8-iso-PGF(2alpha) urine excretion, transferrin, ferritin, serum iron, and vitamins A and E for all participants. Iron concentration in the liver and non-transferrin-bound iron were measured in patients only. We found a significant difference in 8-iso-PGF2alpha in patients (245 [interquartile range 157-348] pg/mg creatinine) compared with controls (128 [106-191] pg/mg creatinine, P = 0.002). Vitamin A was significantly reduced in cases (0.34 [0.25-1.83] microg/ml compared to 3.00 [2.11-3.39] microg/ml, P < 0.001), while vitamin E did not show a significant difference in cases (14.7 [11.5-18.1] microg/ml) compared with controls (14.9 [13.1-19.2] microg/ml, P = 0.52). After phlebotomy treatment and normalization of the iron parameters in the hemochromatosis group, serum vitamin A levels were significantly increased (1.36 [1.08-1.97] microg/ml, P = 0.035 vs. baseline, P < 0.001 vs. controls) and 8-iso-PGF2alpha urinary excretion was lowered to control levels (146 [117-198] pg/mg creatinine, P = 0.38 vs. controls). In our study, HFE-related hemochromatosis was associated with increased oxidative stress and hypovitaminemia A in C282Y homozygotes. The increased oxidative stress was reversible by normalization of the iron load by phlebotomy. Thus, phlebotomy is an effective and adequate means for reducing oxidative stress in these patients. 相似文献
166.
167.
The composition of the exopolysaccharide matrix of Pseudomonas putida mt2 biofilms is relatively undefined as well as the contributions of each polymer to ecological fitness. Here, we describe the role of two putative exopolysaccharide gene clusters, putida exopolysaccharide A (pea) and bacterial cellulose (bcs) in biofilm formation and stability, rhizosphere colonization and matrix hydration under water-limiting conditions. Our findings suggest that pea is involved in the production of a novel glucose, galactose, and mannose-rich polymer that contributes to cell-cell interactions necessary for pellicle and biofilm formation and stability. In contrast, Bcs plays a minor role in biofilm formation and stability, although it does contribute to rhizosphere colonization based on a competition assay. We show that pea expression is highly induced transiently under water-limiting conditions but only slightly by high osmolarity, as determined by qRT-PCR. In contrast, both forms of water stress highly induced bcs expression. Cells deficient in making one or more exopolysaccharide experienced greater dehydration-mediated cell-envelope stress, leading to increased alginate promoter activity. However, this did not lead to increased exopolysaccharide production, except in bcs or pea mutants unable to produce alginate, indicating that P. putida compensates by producing, presumably more Pea or Bcs exopolysaccharides, to facilitate biofilm hydration. Collectively, the data suggest that Pea and Bcs contribute to biofilm formation and in turn their presence contributes to fitness under water-limiting conditions, but not to the extent of alginate. 相似文献
168.
Frans B Waldorff Volkert Siersma Ruth Ertmann Marius Brostrøm Kousgaard Anette Sonne Nielsen Peter Felding Niels Mosbæk Else Hjortsø Susanne Reventlow 《Implementation science : IS》2011,6(1):1-7
Background
Effective provider-parent communication can improve childhood vaccination uptake and strengthen immunisation services in low- and middle-income countries (LMICs). Building capacity to improve communication strategies has been neglected. Rigorous research exists but is not readily found or applicable to LMICs, making it difficult for policy makers to use it to inform vaccination policies and practice. The aim of this project is to build research knowledge and capacity to use evidence-based strategies for improving communication about childhood vaccinations with parents and communities in LMICs.Methods and design
This project is a mixed methods study with six sub-studies. In sub-study one, we will develop a systematic map of provider-parent communication interventions for childhood vaccinations by screening and extracting data from relevant literature. This map will inform sub-study two, in which we will develop a taxonomy of interventions to improve provider-parent communication around childhood vaccination. In sub-study three, the taxonomy will be populated with trial citations to create an evidence map, which will also identify how evidence is linked to communication barriers regarding vaccination. In the project's fourth sub-study, we will present the interventions map, taxonomy, and evidence map to international stakeholders to identify high-priority topics for systematic reviews of interventions to improve parent-provider communication for childhood vaccination. We will produce systematic reviews of the effects of high-priority interventions in the fifth sub-study. In the sixth and final sub-study of the project, evidence from the systematic reviews will be translated into accessible formats and messages for dissemination to LMICs.Discussion
This project combines evidence mapping, conceptual and taxonomy development, priority setting, systematic reviews, and knowledge transfer. It will build and share concepts, terms, evidence, and resources to aid the development of communication strategies for effective vaccination programmes in LMICs. 相似文献169.
Gary Gibson Pamela Nielsen Victoria Mykytyn Ken Carlson John Blass 《Neurochemical research》1989,14(1):17-24
To further elucidate the molecular basis of the selective damage to various brain regions by thiamin deficiency, changes in enzymatic activities were compared to carbohydrate flux through various pathways from vulnerable (mammillary bodies and inferior colliculi) and nonvulnerable (cochlear nuclei) regions after 11 or 14 days of pyrithiamin-induced thiamin deficiency. After 11 days,large decreases (–43 to –59%) in transketolase (TK) occurred in all 3 regions; 2-ketoglutarate dehydrogenase (KGDHC) declined (–45%), but only in mammillary bodies; pyruvate dehydrogenase (PDHC) was unaffected. By day 14, TK remained reduced by 58%–66%; KGDHC was now reduced in all regions (–48 to –55%); PDHC was also reduced (–32%), but only in the mammillary bodies. Thus, the enzyme changes did not parallel the pathological vulnerability of these regions to thiamin deficiency.14CO2 production from14C-glucose labeled in various positions was utilized to assess metabolic flux. After 14 days, CO2 production in the vulnerable regions declined severely (–46 to 70%) and approximately twice as much as those in the cochlear nucleus. Also by day 14, the ratio of enzymatic activity to metabolic flux increased as much as 56% in the vulnerable regions, but decreased 18 to 30% in the cochlear nuclei. These differences reflect a greater decrease in flux than enzyme activities in the two vulnerable regions. Thus, selective cellular responses to thiamin deficiency can be demonstrated ex vivo, and these changes can be directly related to alterations in metabolic flux. Since they cannot be related to enzymatic alterations in the three regions, factors other than decreases in the activity of these TPP-dependent enzymes must underlie selective vulnerability in this model of thiamin deficiency.Abbreviations KGDHC
2-ketoglutarate dehydrogenase complex EC 1.2.4.2., EC 2.3.1.61, EC 1.6.4.3.
- PDHC
pyruvate dehydrogenase complex EC 1.2.4.2., EC 2.3.1.12, EC 1.6.4.3
- TK
transketolase (EC 2.2.1.1)
- TPP
thiamin pyrophosphate 相似文献
170.