全文获取类型
收费全文 | 10987篇 |
免费 | 919篇 |
国内免费 | 1420篇 |
专业分类
13326篇 |
出版年
2024年 | 52篇 |
2023年 | 239篇 |
2022年 | 558篇 |
2021年 | 737篇 |
2020年 | 529篇 |
2019年 | 661篇 |
2018年 | 568篇 |
2017年 | 377篇 |
2016年 | 507篇 |
2015年 | 765篇 |
2014年 | 880篇 |
2013年 | 878篇 |
2012年 | 1063篇 |
2011年 | 966篇 |
2010年 | 523篇 |
2009年 | 504篇 |
2008年 | 585篇 |
2007年 | 473篇 |
2006年 | 423篇 |
2005年 | 342篇 |
2004年 | 270篇 |
2003年 | 212篇 |
2002年 | 157篇 |
2001年 | 128篇 |
2000年 | 108篇 |
1999年 | 142篇 |
1998年 | 87篇 |
1997年 | 99篇 |
1996年 | 63篇 |
1995年 | 56篇 |
1994年 | 50篇 |
1993年 | 45篇 |
1992年 | 50篇 |
1991年 | 41篇 |
1990年 | 41篇 |
1989年 | 30篇 |
1988年 | 24篇 |
1987年 | 19篇 |
1986年 | 27篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Physiological integration has been documented in many clonal plants growing under resource heterogeneity. Little is still known about the response of physiological integration to heterogeneous ultraviolet-B radiation. In this paper, the changes in intensity of physiological integration and of physiological parameters under homogeneous and heterogeneous ultraviolet-B radiation (280-315 nm) were measured in order to test the hypothesis that in addition to resource integration a defensive integration in Trifolium repens might exist as well. For this purpose, homogeneous and heterogeneous ultraviolet-B radiation was applied to pairs of connected and severed ramets of the stoloniferous herb Trifolium repens. Changes in intensity of water and nutrient integration were followed with acid fuchsin dye and 15N-isotope labeling of the xylem water transport. In order to assess the patterns of physiological integration contents of chlorophyll, ultraviolet-B absorbing compounds, soluble sugar and protein were determined and activities of superoxide dismutase (SOD) and peroxidase (POD) measured. When ramets were connected and exposed to heterogeneous UV-B radiation, the velocity of water transportation from the UV-B treated ramet to its connected sister ramet was markedly lower and the percentage of 15N left in labelled ramets that suffered from enhanced UV-B radiation was higher and their transfer to unlabelled ramets lower. In comparison with clones under homogeneous ultraviolet-B radiation, the content of chlorophyll, ultraviolet-B absorbing compounds, soluble sugar and activities of SOD and POD increased notably if ultraviolet-B stressed ramets were connected to untreated ramets. Chlorophyll and UV-B absorbing compounds were shared between connected ramets under heterogeneous UV-B radiation. This indicated that physiological connection improved the performance of whole clonal plants under heterogeneous ultraviolet-B radiation. The intensity of physiological integration of T. repens for resources decreased under heterogeneous ultraviolet-B radiation in favor of the stressed ramets. Ultraviolet-B stressed ramets benefited from unstressed ramets by physiological integration, supporting the hypothesis that clonal plants are able to optimize the efficiency of their resistance maintaining their presence also in less favorable sites. The results could be helpful for further understanding of the function of heterogeneous UV-B radiation on growth regulation and microevolution in clonal plants. 相似文献
22.
Hepatitis B virus (HBV) core protein (HBc) is a major component of viral nucleocapsid and a multifunctional protein involved
in viral maturation and release. It is unstable and present in cells at low level because of K96 lysine residue, which is
a ubiquitin acceptor site. Np95/ICBP90-like RING finger protein (NIRF) has auto-ubiquitination activity which is the hallmark
of a ubiquitin ligase. In the present study, ubiquitin ligase, NIRF, binds to HBc and leads to the proteasome-mediated degradation
of HBc in vivo. NIRF down-regulates HBc protein level, resulting in the decrease of the amount of HBV particles in supernatant
of HepG2.2.15 cells. However knockdown of NIRF significantly increases endogenous HBc protein level, leading to HBV release.
The results reveal that NIRF interacts with HBc and promotes the degradation of HBc in vivo. The pathway of NIRF-mediated
ubiquitin–proteasome affects the release of HBV particles by controlling the amounts of HBc. It indicates that NIRF may participate
in the maturation of HBV. 相似文献
23.
Development of microbial biofilms and the recruitment of propagules on the surfaces of man-made structures in the marine environment cause serious problems for the navies and for marine industries around the world. Current antifouling technology is based on the application of toxic substances that can be harmful to the natural environment. For this reason and the global ban of tributyl tin (TBT), there is a need for the development of "environmentally-friendly" antifoulants. Marine microbes are promising potential sources of non-toxic or less-toxic antifouling compounds as they can produce substances that inhibit not only the attachment and/or growth of microorganisms but also the settlement of invertebrate larvae and macroalgal spores. However, so far only few antilarval settlement compounds have been isolated and identified from bacteria. In this review knowledge about antifouling compounds produced by marine bacteria and diatoms are summarised and evaluated and future research directions are highlighted. 相似文献
24.
25.
Acireductone dioxygenase (ARD) from Klebsiella ATCC 8724 is a metalloenzyme that is capable of catalyzing different reactions with the same substrates (acireductone and O2) depending upon the metal bound in the active site. A model for the solution structure of the paramagnetic Ni2+-containing ARD has been refined using residual dipolar couplings (RDCs) measured in two media. Additional dihedral restraints
based on chemical shift (TALOS) were included in the refinement, and backbone structure in the vicinity of the active site
was modeled from a crystallographic structure of the mouse homolog of ARD. The incorporation of residual dipolar couplings
into the structural refinement alters the relative orientations of several structural features significantly, and improves
local secondary structure determination. Comparisons between the solution structures obtained with and without RDCs are made,
and structural similarities and differences between mouse and bacterial enzymes are described. Finally, the biological significance
of these differences is considered. 相似文献
26.
Wan Y Wu YL Lian LH Xie WX Li X Ouyang BQ Bai T Li Q Yang N Nan JX 《Chemico-biological interactions》2012,195(3):215-223
The purpose of the study was to investigate the anti-fibrotic effect and the potential mechanisms of action of betulinic acid (BA) against hepatic fibrosis in vivo and in vitro. BA is an active compound isolated from the bark of the birch tree Betula spp. (Betulaceae). Liver fibrosis was induced by intraperitoneal injections of thioacetamide (TAA, 200mg/kg) twice weekly for 6weeks in Wistar rats. The administration of BA (20 or 50mg/kg) was started following TAA injections and was continued for 6 or 8weeks to evaluate both the preventive and the protective effects. BA demonstrated great efficacy in preventing and curing hepatic fibrosis via attenuating the TAA-mediated increases in liver tissue hydroxyproline and α-smooth muscle actin (α-SMA). In vitro, BA effectively decreased the HSC-T6 cell viability induced by TNF-α and showed low toxicity in normal human chang liver cells. Moreover, BA significantly attenuated the expression of α-SMA and tissue inhibitor of metalloproteinase-1 (TIMP-1) and increased the levels of matrix metalloprotease (MMP)-13. BA also inhibited the expression of Toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88) and the activation of nuclear factor-κB (NF-κB) in a time-dependent manner. This study provides evidence that BA exerts a significant anti-fibrosis effect by modulating the TLR4/MyD88/NF-κB signaling pathway. 相似文献
27.
Elizabeth J. Abraham Katie A. Slater Suparna Sanyal Ken Linehan Paula M. Flaherty Susan Qian 《Journal of visualized experiments : JoVE》2011,(58)
A growing number of cell-based applications require large numbers of cells. Usage of single layer T-flasks, that are adequate during small-scale expansion, may become cumbersome, laborious and time-consuming when large numbers of cells are required. To address this need, the performance of a new multi-layered cell culture vessel to facilitate easy scale up of cells from single layered T-flasks will be discussed. The flasks tested are available in 3- and 5-layer format and enable culture and complete recovery of three and five times the number of cells respectively, compared to T-175 flasks. A key feature of the BD Multi-Flask is a mix/equilibration port that allows rapid in-vessel mixing as well as uniform distribution of cells and reagents within and between layers of each vessel and consistently produce cells that can be cultured in an environment that is congruent to T-175 flasks.The design of these Multi-Flasks also allows for convenient pipette access for adding reagents and cells directly into the flasks as well as efficient recovery of valuable cells and reagents and reduces risk of contamination due to pouring. For applications where pouring is preferred over pipetting, the design allows for minimal residual liquid retention so as to reduce wastage of valuable cells and reagents. 相似文献
28.
Mammalian lung development is a complex biological process, which is temporally and spatially regulated by growth factors, hormones, and extracellular matrix proteins. Abnormal changes of these molecules often lead to impaired lung development, and thus pulmonary diseases. Epithelial-mesenchymal interactions are crucial for fetal lung development. This paper reviews two interconnected pathways, pleiotrophin and Wnt/β-catenin, which are involved in fibroblast and epithelial cell communication during fetal lung development. 相似文献
29.
Cui Liu Qian He Linlin Zeng Ling Shen Qiaomei Luo Wentao Zhang Xia Zhou Jun Wan 《化学与生物多样性》2021,18(12):e2100705
Emerging evidence suggests that a high-fat diet (HFD) can influence endoplasmic reticulum (ER) stress and gut microbiota. Crataegi Fructus is a traditional Chinese herb widely used in formulas for dyspepsia, with Dashanzha Pill composed of raw Crataegi Fructus (DR) being a representative drug. Processing products of Crataegi Fructus, however, have a stronger pro-digestive effect, and we hypothesized that Dashanzha Pill composed of charred Crataegi Fructus (DC) is more effective. We found that the contents of glucose 1-phosphate and luteolin in DR and DC were substantially different via ultra-high performance liquid chromatography-hybrid quadrupole-Orbitrap high-resolution mass spectrometry. DC outperformed DR in improving histopathological changes, increasing gastrin and motilin, and decreasing vasoactive intestinal peptides in rats with HFD induced dyspepsia. Fecal microbiota analysis revealed that DC could restore the disturbed intestinal microbiota composition, including that of Bacteroides, Akkermansia, and Intestinimonas to normal levels. Furthermore, DC significantly reduced the mRNA and protein levels of glucose-regulated protein 78, protein kinase R-like ER kinase, and eukaryotic initiation factor 2α. Taken together, DC outperformed DR in relieving dyspepsia by regulating gut microbiota and alleviating ER stress. 相似文献
30.
Zhi Qian Zeyuan Zhong Shuo Ni Dejian Li Fangxue Zhang Ying Zhou Zhanrong Kang Jun Qian Baoqing Yu 《Journal of cellular and molecular medicine》2020,24(17):10112-10127
Postmenopausal Osteoporosis (PMOP) is oestrogen withdrawal characterized of much production and activation by osteoclast in the elderly female. Cytisine is a quinolizidine alkaloid that comes from seeds or other plants of the Leguminosae (Fabaceae) family. Cytisine has been shown several potential pharmacological functions. However, its effects on PMOP remain unknown. This study designed to explore whether Cytisine is able to suppress RANKL‐induced osteoclastogenesis and prevent the bone loss induced by oestrogen deficiency in ovariectomized (OVX) mice. In this study, we investigated the effect of Cytisine on RAW 264.7 cells and bone marrow monocytes (BMMs) derived osteoclast culture system in vitro and observed the effect of Cytisine on ovariectomized (OVX) mice model to imitate postmenopausal osteoporosis in vivo. We found that Cytisine inhibited F‐actin ring formation and tartrate‐resistant acid phosphatase (TRAP) staining in dose‐dependent ways, as well as bone resorption by pit formation assays. For molecular mechanism, Cytisine suppressed RANK‐related trigger RANKL by phosphorylation JNK/ERK/p38‐MAPK, IκBα/p65‐NF‐κB, and PI3K/AKT axis and significantly inhibited these signalling pathways. However, the suppression of PI3K‐AKT‐NFATc1 axis was rescued by AKT activator SC79. Meanwhile, Cytisine inhibited RANKL‐induced RANK‐TRAF6 association and RANKL‐related gene and protein markers such as NFATc1, Cathepsin K, MMP‐9 and TRAP. Our study indicated that Cytisine could suppress bone loss in OVX mouse through inhibited osteoclastogenesis. All data provide the evidence that Cytisine may be a promising agent in the treatment of osteoclast‐related diseases such as osteoporosis. 相似文献