全文获取类型
收费全文 | 664篇 |
免费 | 17篇 |
国内免费 | 21篇 |
出版年
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 20篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 19篇 |
2016年 | 17篇 |
2015年 | 17篇 |
2014年 | 37篇 |
2013年 | 36篇 |
2012年 | 13篇 |
2011年 | 37篇 |
2010年 | 21篇 |
2009年 | 37篇 |
2008年 | 49篇 |
2007年 | 36篇 |
2006年 | 27篇 |
2005年 | 25篇 |
2004年 | 21篇 |
2003年 | 13篇 |
2002年 | 17篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 12篇 |
1996年 | 18篇 |
1995年 | 13篇 |
1994年 | 16篇 |
1993年 | 12篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有702条查询结果,搜索用时 15 毫秒
101.
Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community 总被引:11,自引:0,他引:11
We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado.
The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant
removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C4 shrub) and Bouteloua gracilis (a C4 grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and
leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ18O value of plant stem water (for comparison with potential water sources) and the δ13C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water
storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased
water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized
by the shrubs. Based on stem water δ18O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees
were exclusively using groundwater, and the consistent δ13C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal
water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ18O, shrubs may have also had access to groundwater. However, their overall δ18O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil
layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function
of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently,
our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition
by shrubs.
Received: 23 December 1997 / Accepted: 16 September 1998 相似文献
102.
Atsuko Takagi Yasuyuki Ikeda Kunihisa Kobayashi Kazuhiro Kobayashi Yoshihiko Ikeda Junji Kozawa Hideyuki Miyauchi Ming Li Chikako Hashimoto Yasuhiro Hara Satoshi Yamaguchi Akira Suzuki Tatsushi Toda Hironori Nagasaka Ken-ichi Hirano 《Biochemical and biophysical research communications》2018,495(1):646-651
Triglyceride deposit cardiomyovasculopathy (TGCV) is a rare and newly identified disease among patients requiring cardiac transplantation. TGCV is characterized by cardiomyocyte steatosis and triglyceride (TG)-deposit atherosclerosis, resulting from the abnormal intracellular metabolism of TG. TGCV is classified into primary and idiopathic types. Primary TGCV carries ultra-rare genetic mutations in the adipose triglyceride lipase (ATGL), a rate-liming enzyme that hydrolyzes intracellular TG in adipose and non-adipose tissues. Idiopathic TGCV, first identified among autopsied individuals with diabetes mellitus (DM) with severe heart diseases, shows no ATGL mutations and its causes and underlying mechanisms are still unknown. TGCV is difficult to diagnose in daily clinics, thereby demanding feasible diagnostic procedures. We aimed to develop an assay to measure ATGL activity using peripheral leucocytes. Human his6-ATGL was expressed in COS1 cells, purified to homogeneity, and used to raise a polyclonal antibody neutralizing TG-hydrolyzing activity of ATGL. We developed a selective immunoinactivation assay (SIIA) for the quantitation of ATGL activity in cell lysates of leucocytes by the antibody neutralizing ATGL activities. ATGL activity was measured in 13 idiopathic TGCV patients, with two patients with primary TGCV as the negative control. Healthy (non-DM) and DM controls without heart diseases were also subjected. The developed SIIA assay revealed significant reduction in ATGL activity in leucocytes from patients with idiopathic TGCV who did not carry ATGL mutations as compared with non-DM and DM controls. Thus, ATGL in leucocytes may be an important biomarker for the diagnosis of TGCV and our assay may provide insights into pathophysiology and elucidate the underlying mechanism of TGCV and related disorders. 相似文献
103.
Lysine specific demethylase 1 (LSD1) plays a vital role in epigenetic regulation of gene activation and repression in several human cancers and is recognized as a promising antitumor therapeutic target. In this paper, a series of 4-(4-benzyloxy)phenoxypiperidines were synthesized and evaluated. Among the tested compounds, compound 10d exhibited the potent and reversible inhibitory activity against LSD1 in vitro (IC50 = 4 μM). Molecular docking was conducted to predict its binding mode. Furthermore, 10d displayed it could inhibit migration of HCT-116 colon cancer cells and A549 lung cancer cells. Taken together, 10d deserves further investigation as a hit-to-lead for the treatment of LSD1 associated tumors. 相似文献
104.
H. C. Gerhardt J. Schul 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1999,185(1):33-40
The selectivity of female phonotactic responses to synthetic advertisement calls was tested in choice situations. Preferences
based on differences in the linear rise-time of synthetic pulses depended on intensity and carrier frequency. When the carrier
frequency was 1.1 kHz, simulating the low-frequency peak in the advertisement call, females preferred alternatives with slower
rise-time pulses that differed by 5 ms at playback levels of 75 dB SPL and higher. A rise-time difference of 10 ms was discriminated
at 65 dB SPL. When the carrier frequency was 2.2 kHz, simulating the high-frequency peak in the call, females discriminated
a 5-ms difference in rise-time only at 85 dB SPL. Females showed no preference when the difference was 10 ms at lower playback
levels. The difference in the thresholds (about 15–20 dB) for discriminating differences in rise-time at the two carrier frequencies
was greater than the difference in behavioral thresholds for these two frequencies (about 10 dB). This result suggests that
rise-time discrimination can be mediated solely by the neural channel mainly tuned to the low-frequency peak in the call.
Females probably assess differences in rise-time by comparing the first few pulses of each call rather than by averaging over
the entire call.
Accepted: 30 March 1999 相似文献
105.
Xiang-Yu Qi Yang Cao Ya-Li Li Ming-Guang Mo Lu Zhou De-Yong Ye 《Bioorganic & medicinal chemistry letters》2017,27(15):3511-3515
Sphingomyelin synthase (SMS) is a key enzyme in sphingomyelin biosynthetic pathway, whose activity is highly related to the atherosclerosis progression. SMS2 could serve as a promising therapeutic target for atherosclerosis. Based on the structure of lead compound D2, a series of oxazolopyridine derivatives were designed, synthesized, and their inhibitory activities against purified SMS1 and SMS2 enzymes were evaluated respectively. The representative molecules QY4 and QY16 possess micromolar inhibitory activities against SMS2 and excellent isoform preferences over SMS1, qualified to be selected as potential molecules in further discovery of specific SMS2 inhibitors. 相似文献
106.
G. Trevor Cottrell 《生物化学与生物物理学报:生物膜》2005,1711(2):126-141
The capacity of multiple connexins to hetero-oligomerize into functional heterogeneous gap junction channels has been demonstrated in vivo1, in vitro2, and in nonmammalian expression systems. These heterogeneous channels display gating activity, channel conductances, selectivity and regulatory behaviors that are sometimes not predicted by the behaviors of the corresponding homogeneous channels. Such observations suggest that heteromerization of gap junction proteins offers an efficient cellular strategy for finely regulating cell-to-cell communication. The available evidence strongly indicates that heterogeneous gap junction assembly is important to normal growth and differentiation, and may influence the appearance of several disease states. Definitive evidence that heterogeneous gap junction channels differentially regulate electrical conduction in excitable cells is absent. This review examines the prevalence, regulation, and implications of gap junction channel hetero-oligomerization. 相似文献
107.
Question: Does grazing by large herbivores affect species composition or community‐wide variation in plant functional traits? Location: Dune grasslands at the Belgian coast. Methods: Plant cover and soil data were collected in 146 plots that were randomly selected at 26 grazed and ungrazed grassland sites. Plant community composition was assessed by Detrended Correspondence Analysis and mean values of plant trait categories were calculated across the plots. Results: Differentiation of plant composition and community‐wide plant trait characteristics was largely determined by grazing, soil acidity and their interaction. In ungrazed situations, a clear floristic distinction appears between acidic (non‐calcareous) and alkaline (calcareous) grasslands. In grazed situations, these floristic differences largely disappeared, indicating that grazing results in a decrease of natural variation in species composition. At higher soil pH, a larger difference in plant community composition and community‐wide plant traits was observed between grazed and ungrazed plots. In ungrazed situations, shifts in plant functional traits along the acidity gradient were observed. Conclusions: Grazing is responsible for shifts in plant community composition, and hence a decrease in plant diversity among grasslands at opposing acidity conditions in coastal dune grasslands. Therefore, care should be taken when introducing grazing as a system approach for nature conservation in dune grasslands as it may eliminate part of the natural variation in plant diversity along existing abiotic gradients. 相似文献
108.
Jain KS Bariwal JB Kathiravan MK Phoujdar MS Sahne RS Chauhan BS Shah AK Yadav MR 《Bioorganic & medicinal chemistry》2008,16(9):4759-4800
Hypertension is one of the most serious health problems of the modern world with a continuous rise in the number of patients. Selective α1-adrenoreceptor antagonists though have many advantages and uses in the management of arterial hypertension, their lack of specificity at the level of α1-adr subtypes leads to multiple side effects. Existence of multiple α1-adr subtypes holds great promise for the discovery and development of more specific and selective drug molecules, targeting only one α1-adr subtype at a time and thus relative freedom from side effects. Herein, the research done on the discovery and evaluation of a variety of chemically diverse structures as selective antagonists of α1-adr and α1-adr subtypes in recent years has been reviewed. 相似文献
109.
Fungi play a crucial role in the decomposition of lignin in fallen leaves but few studies have examined the functional roles
of ligninolytic fungi associated with the decomposition of fallen leaves on tropical forest soils. This study examined fungal
populations responsible for lignin decomposition in Castanopsis sieboldii leaves in a subtropical evergreen broad-leaved forest in southern Japan. Fallen leaves of C. sieboldii are characterized by the occurrence of bleached portions attributable to fungal colonization of leaf tissues and decomposition
of lignin. The bleached area accounted for 29.7%, on average, of the total area of C. sieboldii fallen leaves in the study site. Leaf mass per unit area (LMA) and lignin content were lower in the bleached area than in
the surrounding nonbleached area of the same leaves, indicating that removal of lignin enhanced mass loss from leaf tissues
and created small-scale heterogeneity of decomposition within single leaves. An unidentified species of Lachnocladiaceae (Basidiomycetes)
was isolated frequently from the bleached area and caused selective decomposition of lignin in leaves under pure culture conditions,
indicating that this fungus was responsible for the bleaching. The greater hyphal length of basidiomycetes in the bleached
area than in the nonbleached area supported the finding that this Lachnocladiaceae sp. was associated with the bleaching.
The relatively rapid decomposition of C. sieboldii leaves on the subtropical forest soil is partly attributable to colonization of the litter by this Lachnocladiaceae sp. 相似文献
110.
Here we report a highly selective stoichiometric decarbonylation reaction for alkylformates and alkynyl aldehydes by a rhodium-dinitrogen complex (PCP-Rh-N2) at room temperature. While electronic effects of the substrates cannot be completely ruled out, the selectivity is rationalized by a steric effect, consistent with the results of an X-ray crystallographic study and density functional theory (DFT) modeling. 相似文献