首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1453篇
  免费   110篇
  国内免费   2篇
  1565篇
  2024年   1篇
  2023年   8篇
  2022年   21篇
  2021年   42篇
  2020年   25篇
  2019年   23篇
  2018年   38篇
  2017年   27篇
  2016年   51篇
  2015年   106篇
  2014年   96篇
  2013年   107篇
  2012年   182篇
  2011年   140篇
  2010年   88篇
  2009年   58篇
  2008年   98篇
  2007年   88篇
  2006年   77篇
  2005年   59篇
  2004年   61篇
  2003年   54篇
  2002年   38篇
  2001年   4篇
  2000年   5篇
  1999年   7篇
  1998年   10篇
  1997年   6篇
  1996年   4篇
  1995年   8篇
  1994年   8篇
  1993年   4篇
  1991年   3篇
  1989年   1篇
  1988年   2篇
  1986年   5篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1975年   1篇
  1973年   1篇
  1971年   1篇
  1969年   1篇
排序方式: 共有1565条查询结果,搜索用时 14 毫秒
31.
The beetle family Lampyridae (fireflies) encompasses ~100 genera worldwide with considerable diversity in life histories and signaling modes. Some lampyrid males use reproductive accessory glands to produce spermatophores, which have been shown to increase female lifetime fecundity. Sexual dimorphism in the form of neotenic and flightless females is also common in this family. A major goal of this study was to test a hypothesized link between female flight ability and male spermatophore production. We examined macroevolutionary patterns to test for correlated evolution among different levels of female neoteny (and associated loss of flight ability), male accessory gland number (and associated spermatophore production), and sexual signaling mode. Trait reconstruction on a molecular phylogeny indicated that flying females and spermatophores were ancestral traits and that female neoteny increased monotonically and led to flightlessness within multiple lineages. In addition, male spermatophore production was lost multiple times. Our evolutionary trait analysis revealed significant correlations between increased female neoteny and male accessory gland number, as well as between flightlessness and spermatophore loss. In addition, female flightlessness was positively correlated with the use of glows as female sexual signal. Transition probability analysis supported an evolutionary sequence of female flightlessness evolving first, followed by loss of male spermatophores. These results contribute to understanding how spermatophores have evolved and how this important class of seminal nuptial gifts is linked to other traits, providing new insights into sexual selection and life-history evolution.  相似文献   
32.
The cultivation of perennial wild plant mixtures (WPMs) in biogas cropping systems dominated by maize (Zea mays L.) restores numerous ecosystem functions and improves both spatial and temporal agrobiodiversity. In addition, the colorful appearance of WPM can help enhance landscape beauty. However, their methane yield per hectare (MYH) varies greatly and amounts to only about 50% that of maize. This study aimed at decreasing MYH variability and increasing accumulated MYH of WPM by optimizing the establishment method. A field trial was established in southwest Germany in 2014, and is still running. It tested the effects of three WPM establishment procedures (E1: alone [without maize, in May], E2: undersown in cover crop maize [in May], E3: WPM sown after whole‐crop harvest of spring barley [Hordeum vulgare L.] in June) on both MYH and species diversity of two WPMs [S1, S2]). Mono‐cropped maize and cup plant (Silphium perfoliatum L.) were used as reference crops. Of the WPM treatments tested, S2E2 achieved the highest (19,296 , 60.5% of maize) and S1E1 the lowest accumulated MYH (8,156 , 25.6% of maize) in the years 2014–2018. Cup plant yielded slightly higher than S2E2 (19,968 , 62.6% of maize). In 2014, the WPM sown under maize did not significantly affect the cover crop performance. From 2015 onward, E1 and E2 had comparable average annual MYH and average annual number of WPM species. With a similar accumulated MYH but significantly higher number of species (3.5–10.2), WPM S2E2 outperformed cup plant. Overall, the long‐term MYH performance of WPM cultivation for biogas production can be significantly improved by undersowing with maize as cover crop. This improved establishment method could help facilitate the implementation of WPM cultivation for biogas production and thus reduce the trade‐off between bioenergy and biodiversity.  相似文献   
33.
34.
In this video publication a standardized mouse model of subarachnoid hemorrhage (SAH) is presented. Bleeding is induced by endovascular Circle of Willis perforation (CWp) and proven by intracranial pressure (ICP) monitoring. Thereby a homogenous blood distribution in subarachnoid spaces surrounding the arterial circulation and cerebellar fissures is achieved. Animal physiology is maintained by intubation, mechanical ventilation, and continuous on-line monitoring of various physiological and cardiovascular parameters: body temperature, systemic blood pressure, heart rate, and hemoglobin saturation. Thereby the cerebral perfusion pressure can be tightly monitored resulting in a less variable volume of extravasated blood. This allows a better standardization of endovascular filament perforation in mice and makes the whole model highly reproducible. Thus it is readily available for pharmacological and pathophysiological studies in wild type and genetically altered mice.  相似文献   
35.
IL-7 delivers survival signals to cells at an early stage in lymphoid development. In the absence of IL-7, pro-T cells undergo programmed cell death, which has previously been associated with a decline in Bcl-2 and translocation of Bax from cytosol to mitochondria. A new, earlier feature of IL-7 withdrawal was identified using an IL-7-dependent thymocyte line. We observed that withdrawal of IL-7 induced increased expression of jun and fos family member genes including c-jun, junB, junD, c-fos and fra2. This transient response peaked 3-4 h after IL-7 was withdrawn and resulted in increased DNA-binding activity of AP-1 and in a change in the composition of the Jun/Fos family dimers shown by electrophoretic mobility shift and supershift assays. Induction of jun and fos genes and the increased DNA-binding activity of AP-1 were attributable to the phosphorylation-induced activation of the stress kinases p38 and JNK and were blocked by the chemical kinase inhibitors SB203580 and SB202190. The stress response contributed to cell death following IL-7 withdrawal as shown by blocking the activity of the stress (MAP) kinases or by blocking the production of c-Jun and c-Fos using antisense oligonucleotides.  相似文献   
36.
Dittmann K  Riese U  Hamburger M 《Phytochemistry》2004,65(21):2885-2891
An assay for the HPLC-based search for monoamine oxidase-A (MAO-A) inhibitors in plant extracts was established. It combines human recombinant MAO-A, expressed as GST-fusion protein in yeast, with a kinetic measurement of the conversion of kynuramine to 4-hydroxyquinoline. Substrate selectivity and kinetic parameters of the GST-fusion protein were comparable to the wild-type enzyme. The applicability of the assay to HPLC-based activity profiling was tested with plant extracts spiked with small amounts of known MAO inhibitors.  相似文献   
37.
Cardiac diseases such as myocardial infarction and heart failure are among the leading causes of death in western societies. Therapeutic angiogenesis has been suggested as a concept to combat these diseases. The biology of angiogenic factors expressed in the heart such as vascular endothelial growth factor (VEGF) is well studied, whereas data on anti-angiogenic mediators in the heart are scarce. Here we study the expression of the anti-angiogenic factor pigment epithelium-derived factor (PEDF) in the human heart and in human cardiac cells. PEDF expression could be detected in human cardiac tissue on the protein and mRNA levels. PEDF mRNA levels were significantly lower in explanted human ischemic hearts as compared to healthy hearts. Our in vitro experiments showed that human adult cardiac myocytes and fibroblasts constitutively secrete PEDF. In addition to anoxic conditions, cobalt chloride, 2,2'dipyridyl and dimethoxally glycine, which stabilize hypoxia inducible factor-α decreased PEDF expression. Furthermore we show that PEDF inhibits VEGF-induced sprouting. We have identified PEDF in healthy and ischemic human hearts and we show that PEDF expression is down-regulated by low oxygen levels. Therefore, we suggest a role for PEDF in the regulation of angiogenesis in the heart and propose PEDF as a possible therapeutic target in heart disease.  相似文献   
38.
Leuciscine fishes represent an important component of freshwater ichthyofauna endemic to northern Mediterranean areas. This lineage shows high intra-specific morphological variability and exhibits high levels of hybridization, two characteristics that contribute to systematic uncertainties, misclassification of taxa and, potentially, the mismanagement of biodiversity. This study focused on brook chub, Squalius lucumonis, an endemic taxon of Central Italy. The taxonomic status of this species has long been questioned, and a hybrid origin from sympatric leusciscines (S. squalus x Rutilus rubilio, or S. squalus x Telestes muticellus) has been hypothesised. A phenotypic (evaluating shape and meristic counts) and genetic (using mitochondrial and nuclear markers) investigation of these four taxa was conducted to test species delimitation in sympatric areas and to evaluate the taxonomic status of S. lucumonis. One hundred and forty-five individuals of all four taxa were collected within streams of the lowest portion of the Tiber River basin and analysed; this region encompasses a large portion of the S. lucumonis distribution. The different morphological and genetic approaches were individually examined, compared, and then combined in a quantitative model to both investigate the limits of each approach and to identify cases of misclassification. The results obtained confirm the cladogenetic non-hybrid origin of S. lucumonis, highlight the need for immediate conservation actions and emphasise the value of an integrated approach in the study of leuciscines evolution.  相似文献   
39.
The production of hydrogen peroxide (H2O2) drives tumourigenesis in ulcerative colitis (UC). Recently, we showed that H2O2 activates DNA damage checkpoints in human colonic epithelial cells (HCEC) through c‐Jun N‐terminal Kinases (JNK) that induces p21WAF1. Moreover, caspases circumvented the G1/S and intra‐S checkpoints, and cells accumulated in G2/M. The latter observation raised the question of whether repeated H2O2 exposures alter JNK activation, thereby promoting a direct passage of cells from G2/M arrest to driven cell cycle progression. Here, we report that increased proliferation of repeatedly H2O2‐exposed HCEC cells (C‐cell cultures) was associated with (i) increased phospho‐p46 JNK, (ii) decreased total JNK and phospho‐p54 JNK and (iii) p21WAF1 down‐regulation. Altered JNK activation and p21WAF1 down‐regulation were accompanied by defects in maintaining G2/M and mitotic spindle checkpoints through adaptation, as well as by apoptosis resistance following H2O2 exposure. This may cause increased proliferation of C‐cell cultures, a defining initiating feature in the inflammation‐carcinoma pathway in UC. We further suggest that dysregulated JNK activation is attributed to a non‐apoptotic function of caspases, causing checkpoint adaptation in C‐cell cultures. Additionally, loss of cell‐contact inhibition and the overcoming of senescence, hallmarks of cancer, contributed to increased proliferation. Furthermore, there was evidence that p54 JNK inactivation is responsible for loss of cell‐contact inhibition. We present a cellular model of UC and suggest a sinusoidal pattern of proliferation, which is triggered by H2O2‐induced reactive oxygen species generation, involving an interplay between JNK activation/inactivation, p21WAF1, c‐Fos, c‐Jun/phospho‐c‐Jun, ATF2/phospho‐ATF2, β‐catenin/TCF4‐signalling, c‐Myc, CDK6 and Cyclin D2, leading to driven cell cycle progression.  相似文献   
40.
Data availability and data quality are still critical factors for successful LCA work. The SETAC-Europe LCA Working Group ‘Data Availability and Data Quality’ has therefore focused on ongoing developments toward a common data exchange format, public databases and accepted quality measures to find science-based solutions than can be widely accepted. A necessary prerequisite for the free flow and exchange of life cycle inventory (LCI) data and the comparability of LCIs is the consistent definition, nomenclature, and use of inventory parameters. This is the main subject of the subgroup ‘Recommended List of Exchanges’ that presents its results and findings here:
•  Rigid parameter lists for LCIs are not practical; especially, compulsory lists of measurements for all inventories are counterproductive. Instead, practitioners should be obliged to give the rationale for their scientific choice of selected and omitted parameters. The standardized (not: mandatory!) parameter list established by the subgroup can help to facilitate this.
•  The standardized nomenclature of LCI parameters and the standardized list of measurement bases (units) for these parameters need not be appliedinternally (e.g. in LCA software), but should be adhered to inexternal communications (data for publication and exchange). Deviations need to be clearly stated.
•  Sum parameters may or may not overlap - misinterpretations in either direction introduce a bias of unknown significance in the subsequent life cycle impact assessments (LCIA). The only person who can discriminate unambiguously is the practitioner who measures or calculates such values. Therefore, a clear statement of independence or overlap is necessary for every sum parameter reported.
•  Sum parameters should be only used when the group of emissions as such is measured. Individually measured emission parameters should not be hidden in group or sum parameters.
•  Problematic substances (such as carcinogens, ozone depleting agents and the like) maynever be obscured in group emissions (together with less harmful substances or with substances of different environmental impact), butmust be determined and reported individually, as mentioned in paragraph 3.3 of this article.
•  Mass and energy balances should be carried out on a unit process level. Mass balances should be done on the level of the entire mass flow in a process as well as on the level of individual chemical elements.
•  Whenever possible, practitioners should try to fill data gaps with their knowledge of analogous processes, environmental expert judgements, mass balance calculations, worst case assumptions or similar estimation procedures.
  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号