全文获取类型
收费全文 | 24699篇 |
免费 | 728篇 |
国内免费 | 1020篇 |
出版年
2023年 | 1236篇 |
2022年 | 833篇 |
2021年 | 618篇 |
2020年 | 470篇 |
2019年 | 855篇 |
2018年 | 820篇 |
2017年 | 777篇 |
2016年 | 345篇 |
2015年 | 340篇 |
2014年 | 456篇 |
2013年 | 544篇 |
2012年 | 244篇 |
2011年 | 1575篇 |
2010年 | 481篇 |
2009年 | 625篇 |
2008年 | 681篇 |
2007年 | 742篇 |
2006年 | 685篇 |
2005年 | 656篇 |
2004年 | 804篇 |
2003年 | 577篇 |
2002年 | 778篇 |
2001年 | 1142篇 |
2000年 | 1081篇 |
1999年 | 1126篇 |
1998年 | 1166篇 |
1997年 | 1039篇 |
1996年 | 546篇 |
1995年 | 294篇 |
1994年 | 202篇 |
1993年 | 188篇 |
1992年 | 152篇 |
1991年 | 160篇 |
1990年 | 171篇 |
1989年 | 127篇 |
1988年 | 138篇 |
1987年 | 109篇 |
1985年 | 216篇 |
1984年 | 481篇 |
1983年 | 462篇 |
1982年 | 380篇 |
1981年 | 359篇 |
1980年 | 366篇 |
1979年 | 336篇 |
1978年 | 210篇 |
1977年 | 184篇 |
1976年 | 176篇 |
1975年 | 152篇 |
1974年 | 127篇 |
1973年 | 98篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Understanding the state of forest resource utilization in China and correctly evaluating the role and function of forest resources in national economic development are essential for realizing balanced development of forests, the environment, and the economy. This is especially true given the present situation of increasingly scarce forest resources. Using data from Chinese forest industry statistical yearbooks, forestry development reports, and other documents, this paper examines the current state of forest resource utilization in China from the angle of combining quantities and values based on input–output tables. We show that demand for and input use of forest resources varies greatly across industrial sectors; the paper products and furniture manufacturing industries have the greatest direct consumption coefficient for timber use. When considering direct and indirect demand, it is clear that forest resources restrict different industrial sectors in diverse ways. These results provide an important set of reference values regarding the utilization of forest resources and coordinated industrial development in China. 相似文献
3.
The effects of variation of intramuscular temperature (T) on conduction velocity (CV) of the action potential along single human muscle fibers of the biceps brachii was studied in situ in 15 normal volunteers (mean age 39 years, range 21–62 years). Cooling was obtained by direct application of ice over a rectangular skin region including the stimulating and recording area. The intramuscular T was monitored by a needle thermocouple (copperconstantane). In all the 24 muscle fibers studied, a linear relationship was observed between CV and T. The slopes of the regression lines, ranging between 0.190 and 0.079 m/s, were positively correlated with the starting CV at 36°C ranging between 2.2 and 5.2 m/s. If conduction changes are expressed as a percentage of the basal CV at 36°C, the CV/T coefficient is the same for all the fibers and independent of the individual CV: 3.4% of CV/°C. 相似文献
4.
《Bioorganic & medicinal chemistry》2020,28(22):115739
N-phenyl ureidobenzenesulfonates (PUB-SOs) is a new class of promising anticancer agents inducing replication stresses and cell cycle arrest in S-phase. However, the pharmacological target of PUB-SOs was still unidentified. Consequently, the objective of the present study was to identify and confirm the pharmacological target of the prototypical PUB-SO named 2-ethylphenyl 4-(3-ethylureido)benzenesulfonate (SFOM-0046) leading to the cell cycle arrest in S-phase. The antiproliferative and the cytotoxic activities of SFOM-0046 were characterized using the NCI-60 screening program and its fingerprint was analyzed by COMPARE algorithm. Then, human dihydroorotate dehydrogenase (hDHODH) colorimetric assay, uridine rescuing cell proliferation and molecular docking in the brequinar-binding site were performed. As a result, SFOM-0046 exhibited a mean antiproliferative activity of 3.5 μM in the NCI-60 screening program and evidenced that leukemia and colon cancer cell panels were more sensitive to SFOM-0046. COMPARE algorithm showed that the SFOM-0046 cytotoxic profile is equivalent to the ones of brequinar and dichloroallyl lawsone, two inhibitors of hDHODH. SFOM-0046 inhibited the hDHODH in the low nanomolar range (IC50 = 72 nM) and uridine rescued the cell proliferation of HT-29, HT-1080, M21 and MCF-7 cancer cell lines in the presence of SFOM-0046. Finally, molecular docking showed a binding pose of SFOM-0046 interacting with Met43 and Phe62 present in the brequinar-binding site. In conclusion, PUB-SOs and notably SFOM-0046 are new small molecules hDHODH inhibitors triggering replication stresses and S-phase arrest. 相似文献
5.
《Bioorganic & medicinal chemistry》2020,28(11):115492
Effective chemotherapy for solid cancers is challenging due to a limitation in permeation that prevents anticancer drugs from reaching the center of the tumor, therefore unable to limit cancer cell growth. To circumvent this issue, we planned to apply the drugs directly at the center by first collapsing the outer structure. For this, we focused on cell–cell communication (CCC) between N-glycans and proteins at the tumor cell surface. Mature N-glycans establish CCC; however, CCC is hindered when numerous immature N-glycans are present at the cell surface. Inhibition of Golgi mannosidases (GMs) results in the transport of immature N-glycans to the cell surface. This can be employed to disrupt CCC. Here, we describe the molecular design and synthesis of an improved GM inhibitor with a non-sugar mimic scaffold that was screened from a compound library. The synthesized compounds were tested for enzyme inhibition ability and inhibition of spheroid formation using cell-based methods. Most of the compounds designed and synthesized exhibited GM inhibition at the cellular level. Of those, AR524 had higher inhibitory activity than a known GM inhibitor, kifunensine. Moreover, AR524 inhibited spheroid formation of human malignant cells at low concentration (10 µM), based on the disruption of CCC by GM inhibition. 相似文献
6.
7.
Thymic B cells are a constituent of normal human thymic medulla. They are supposed to play a role in T cell maturation. Thymic
B cells have been characterized morphologically and immunohistochemically at the light-microscopic level. Their ultrastructural
appearance in vivo has not been demonstrated. Six normal infantile thymi were immunolabelled with the pan-B cell marker CD20
using a pre-embedding technique and viewed at the electron-microscopic level. Cells expressing CD20 had long cytoplasmic processes.
They were all ”asteroid” in shape and in close contact with thymocytes. Also, their long cytoplasmic processes intermingled
with cytoplasmic processes of cells that were presumed to be interdigitating reticulum cells (IDC) based on morphological
criteria. Thymic B cells may act in concert with IDC during T cell maturation.
Received: 20 October 1995 / Accepted: 10 January 1996 相似文献
8.
9.
《Cryobiology》2020
Cryopreservation, the freezing and later warming of biological samples with minimal loss of viability, is important in many scientific disciplines. For some applications, particularly those where there is limited available material, it is critical to ensure the maximal survival rates of cryopreserved materials. Most of the challenges encountered with such techniques take place after the warming process where cryodamage affects cell viability and future development. Here we have used the nematode Caenorhabditis elegans to investigate the effects of cryodamage caused by slow-freezing. We find that freezing results in the death of some worms, with an approximately 40% reduction in the number of worms that develop in the frozen populations, but that the effects on worms that survive are limited. For example, there are no differences in the lifetime fecundity or in lifespan between frozen and control worms, although early fecundity and body size was reduced in frozen worms. Similarly, analyses of body wall muscle structure and of pharyngeal function indicates that muscle development and function are not significantly affected by freezing. We do however determine that freezing increases the rates of matricidal hatching, where progeny hatch within the mother. Overall, these results indicate that, for worms that survive, cryopreservation produces limited long-term effects, but do indicate that some phenotypes could be used in further analyses of the cellular damage induced by cryopreservation. 相似文献
10.
The main ecosystem services (ES) central European mountain forests provide are: protection against gravitational hazards, timber production, recreation, biodiversity conservation and carbon storage, which are all in high demand. These demands make managing mountain forests a challenging task, involving manifold synergies and conflicts between the different ES. There is therefore an urgent need for appropriate concepts and tools for support decisions in forest management and planning (FMP) to take into consideration all ES and to manage the wide variety of information types, parameters and uncertainties involved in assessing the sustainability of ES. Multi-criteria decision analysis (MCDA) provides a suitable set of methods for sustainability evaluations. In this study sustainability means the persistent fulfilment of the required ES. To address all the phases of the FMP process, MCDA and forest models should be applied together, with indicators providing the main interfaces to combine them. This paper aims to: i) review assessment approaches in order to select appropriate and widely accepted indicators for measuring and assessing the effects of different silvicultural management alternatives on forest ES, and ii) present additional standardisation approaches (value functions) for each indicator. Standardisations are necessary to make the different ES comparable and to study synergies and trade-offs between different management objectives in MCDA. The main ES in central European mountain regions are considered, with a clear focus on those indicators that are directly derivable from forest model outputs and that can refer to sustainable forest management practices. The scales considered are that of the single forest stand and of the larger forest management unit. A holistic indicator-based analysis framework for FMP in mountain forests can be built using the indicators and value functions described. The influence of different management alternatives on ES can then be evaluated, taking into consideration the instruments and information on forest management (forest models, inventory) available. All indicators are selected according to existing and approved approaches that only require data that is normally available in operational forest management. The framework can thus be an important element in developing a decision support system for FMP in mountain forests. 相似文献