全文获取类型
收费全文 | 15190篇 |
免费 | 991篇 |
国内免费 | 3篇 |
专业分类
16184篇 |
出版年
2024年 | 21篇 |
2023年 | 106篇 |
2022年 | 190篇 |
2021年 | 390篇 |
2020年 | 311篇 |
2019年 | 316篇 |
2018年 | 385篇 |
2017年 | 354篇 |
2016年 | 533篇 |
2015年 | 813篇 |
2014年 | 874篇 |
2013年 | 1023篇 |
2012年 | 1304篇 |
2011年 | 1156篇 |
2010年 | 747篇 |
2009年 | 704篇 |
2008年 | 938篇 |
2007年 | 879篇 |
2006年 | 804篇 |
2005年 | 764篇 |
2004年 | 716篇 |
2003年 | 638篇 |
2002年 | 599篇 |
2001年 | 135篇 |
2000年 | 90篇 |
1999年 | 127篇 |
1998年 | 131篇 |
1997年 | 120篇 |
1996年 | 99篇 |
1995年 | 89篇 |
1994年 | 90篇 |
1993年 | 58篇 |
1992年 | 53篇 |
1991年 | 65篇 |
1990年 | 58篇 |
1989年 | 33篇 |
1988年 | 38篇 |
1987年 | 29篇 |
1986年 | 23篇 |
1985年 | 41篇 |
1984年 | 56篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 26篇 |
1980年 | 29篇 |
1979年 | 17篇 |
1978年 | 21篇 |
1977年 | 21篇 |
1976年 | 18篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
José Roberto Machado Cunha da Silva Laercio Ribeiro Porto-Neto João Carlos Shimada Borges Bernard Ernesto Jensch-Junior 《Polar Biology》2005,28(4):326-328
The germicide capability of the macrophage (MØ) of the Antarctic fish Notothenia coriiceps is demonstrated using fluorescence microscopy for the first time. The MØs were able to kill microorganisms by intracellular mechanism and this killing can be stimulated by oyster-derived glycogen. Although the phagocytosis index is lower than in temperate water fish species, this work demonstrates that non-specific defence mechanism plays an important role in the polar environment. There are some studies on inflammation in N. coriiceps [Silva et al. (1998) Polar Biol 20:206–212], parasite–host relation [Silva et al. (1999) Polar Biol 22:417–424] and phagocytosis [Silva et al. (2002) J Fish Biol 60:466–478]. These previous studies have shown that the MØ were able to identify biotic and abiotic factors. However, it can be of interest to study the activity of MØ in microorganism killing, and this work adds new insights of this fundamental process under Antarctic temperatures. 相似文献
2.
3.
Carlos E. Aragón Maritza Escalona Iris Capote Danilo Pina Inaudis Cejas Roberto Rodriguez Maria Jesus Cañal Jorge Sandoval Sophe Roels Pierre Debergh Justo Gonzalez-Olmedo 《In vitro cellular & developmental biology. Plant》2005,41(4):550-554
Summary The photosynthetic capacity changes and the main enzymatic systems related to carbon metabolism were investigated during the
in vitro culture of plantain shoots (Musa AAB cv. CEMSA 3/4) in temporary immersion bioreactors (TIB) and their subsequent acclimatization.
The maximal rate of photosynthesis (Pn), transpiration, and the activity of the carbon metabolism enzymes phosphoenolpyruvate
carboxylase (PEPC), acid invertase (AI), pyruvate kinase (PK) and sucrose phosphate synthase (SPS) were measured every 7 d
during the 21 d of elongation in TIB, and the following 42 d of acclimatization. Sucrose content in the liquid medium and
in the leaves was also determined. The most significant changes in plant growth were observed during acclimatization. During
the in vitro stage photosynthesis was limited (4–6 μmol CO2m−2s−1); the photosynthetic rate however increases rapidly and significantly as soon as in vitro culture is over during acclimatization. PEPC activity increased during the whole evaluation period. The highest levels were
achieved around days 42 and 56. PK and SPS activities were optimal during the first weeks in acclimatization (28–35 d), while
AI increased at the beginning of the elongation phase (7 d), and later at the end of the acclimatization (49–63 d). The relationships
between morphological parameters, photosynthetic capacity of the plantlets and the carbon metabolism enzymes during both phases
of the culture are discussed. 相似文献
4.
Comparing and evaluating process-based ecosystem model predictions of carbon and water fluxes in major European forest biomes 总被引:7,自引:0,他引:7
Pablo Morales Martin T. Sykes I. Colin Prentice† Pete Smith‡ Benjamin Smith Harald Bugmann§ Bärbel Zierl§ Pierre Friedlingstein¶ Nicolas Viovy¶ Santi Sabaté Anabel Sánchez Eduard Pla Carlos A. Gracia Stephen Sitch†† Almut Arneth Jerome Ogee¶ 《Global Change Biology》2005,11(12):2211-2233
Process‐based models can be classified into: (a) terrestrial biogeochemical models (TBMs), which simulate fluxes of carbon, water and nitrogen coupled within terrestrial ecosystems, and (b) dynamic global vegetation models (DGVMs), which further couple these processes interactively with changes in slow ecosystem processes depending on resource competition, establishment, growth and mortality of different vegetation types. In this study, four models – RHESSys, GOTILWA+, LPJ‐GUESS and ORCHIDEE – representing both modelling approaches were compared and evaluated against benchmarks provided by eddy‐covariance measurements of carbon and water fluxes at 15 forest sites within the EUROFLUX project. Overall, model‐measurement agreement varied greatly among sites. Both modelling approaches have somewhat different strengths, but there was no model among those tested that universally performed well on the two variables evaluated. Small biases and errors suggest that ORCHIDEE and GOTILWA+ performed better in simulating carbon fluxes while LPJ‐GUESS and RHESSys did a better job in simulating water fluxes. In general, the models can be considered as useful tools for studies of climate change impacts on carbon and water cycling in forests. However, the various sources of variation among models simulations and between models simulations and observed data described in this study place some constraints on the results and to some extent reduce their reliability. For example, at most sites in the Mediterranean region all models generally performed poorly most likely because of problems in the representation of water stress effects on both carbon uptake by photosynthesis and carbon release by heterotrophic respiration (Rh). The use of flux data as a means of assessing key processes in models of this type is an important approach to improving model performance. Our results show that the models have value but that further model development is necessary with regard to the representation of the some of the key ecosystem processes. 相似文献
5.
João Silva Rui Santos Carlos M. Duarte 《Journal of experimental marine biology and ecology》2005,317(1):87-95
The photosynthetic productivity of the intertidal communities dominated by the seagrass Zostera noltii and the cordgrass Spartina maritima was assessed in two contrasting situations during a tidal cycle, i.e., air exposure and water immersion. Two complementary methods were used: infra red gas analysis of CO2 flux measurements in whole communities and chlorophyll a fluorescence measurements of individual plants photosynthetic activity. Higher photosynthetic rates of Z. noltii in air were observed both at the individual plants response level determined by chlorophyll fluorescence and at the community level measured as gas exchange (CO2 uptake). S. maritima plants consistently showed low photosynthetic response when immersed. Gross community production (GCP) measured as carbon dioxide uptake was always higher in air than in water for both communities. When immersed, the GCP of both communities was similar. However, when exposed to the air, the GCP of the S. maritima community was higher than the one of Z. noltii's. The key factor in CO2 assimilation by air-exposed Z. noltii was the retention of water in sediment microdepressions. During low tide, depressions in the sediment retain a considerable amount of water, enough to maintain leaf hydration. In these conditions, rapid air-water CO2 diffusion occurs, making it readily available to plants. The community gas exchange measurements compared well with the fluorescence indications. Both Z. noltii and S. maritima were shown to be responsible for the overall pattern of photosynthetic carbon fixation within their respective communities, both during submersion and emersion periods. The short-term incubations method described in this report proved to be a valuable tool for field measurements of intertidal lagoon productivity. It provides fast and precise values of carbon dioxide fixation, both in submerged and air-exposed communities. 相似文献
6.
7.
8.
Eric Hastie Dahlia M. Besmer Nirav R. Shah Andrea M. Murphy Megan Moerdyk-Schauwecker Carlos Molestina Lopamudra Das Roy Jennifer M. Curry Pinku Mukherjee Valery Z. Grdzelishvili 《Journal of virology》2013,87(18):10283-10294
Vesicular stomatitis virus (VSV) is a promising oncolytic agent against various malignancies. Here, for the first time, we tested VSV in vitro and in vivo in a clinically relevant, immunocompetent mouse model of pancreatic ductal adenocarcinoma (PDA). Our system allows the study of virotherapy against PDA in the context of overexpression (80% of PDA patients) or no expression of human mucin 1 (MUC1), a major marker for poor prognosis in patients. In vitro, we tested three VSV recombinants, wild-type VSV, VSV-green fluorescent protein (VSV-GFP), and a safe oncolytic VSV-ΔM51-GFP, against five mouse PDA cell lines that either expressed human MUC1 or were MUC1 null. All viruses demonstrated significant oncolytic abilities independent of MUC1 expression, although VSV-ΔM51-GFP was somewhat less effective in two PDA cell lines. In vivo administration of VSV-ΔM51-GFP resulted in significant reduction of tumor growth for tested mouse PDA xenografts (+MUC1 or MUC1 null), and antitumor efficacy was further improved when the virus was combined with the chemotherapeutic drug gemcitabine. The antitumor effect was transient in all tested groups. The developed system can be used to study therapies involving various oncolytic viruses and chemotherapeutics, with the goal of inducing tumor-specific immunity while preventing premature virus clearance. 相似文献
9.
Edinaldo N. Santos-Silva Geoff A. Boxshall Carlos E.F. Rocha 《Studies on Neotropical Fauna and Environment》2013,48(2):114-128
Ribautia williamsi sp. nov., a new dwarf geophilomorph centipede from the Lower Urubamba Region, Peruvian Amazonia, is described and illustrated based on the holotype female. The new species is characterized by having the coxal organs grouped in clusters (three of these in each coxopleuron of the ultimate leg-bearing segment) and ventral pore-fields present along all the body; these two combined traits being shared by five other Neotropical species currently included in the genus Ribautia Brölemann, 1909, i.e. R. centralis (Silvestri, 1907) (from Colombia and Brazil), R. difficilis Pereira, Minelli &; Barbieri, 1995 (from Brazil), R. montana Kraus, 1954 (from Peru), R. peruana (Verhoeff, 1941) (from Peru), and R. titicacae (Turk, 1955) (from Peru). The new taxon is differentiated from the aforementioned species by the low number of leg-bearing segments and small body length; it is included in a key which will enable the identification of all known Neotropical members having coxal organs grouped in clusters. R. williamsi sp. nov. is the 14th species of Ribautia recorded from Peru.http://zoobank.org/urn:lsid:zoobank.org:pub:D9FA5FAF-7652-4A5B-AC09-8783F05A694D 相似文献
10.
Carlos Gueto-Tettay Luis Pelaez-Bedoya 《Journal of biomolecular structure & dynamics》2013,31(13):3557-3574
BACE1 is an aspartyl protease with a very relevant role in medicinal chemistry related to Alzheimer Disease since it has demonstrated to be a promising therapeutic target for inhibition and possible control for the progress of the peptide accumulation characteristic of this pathology. The enzymatic activity of this protein is given by the aspartic dyad, Asp93 and Asp289, which can adopt several protonation states depending on the chemical nature of its inhibitors, this is, monoprotonated, diprotonated and di-deprotonated states. In the present study, the analysis of the population density, for a series of protein-inhibitor molecular dynamics simulations, was carried out to identify the most feasible protonation state adopted by the catalytic dyad in the presence of tertiary carbinamine (TC) transition state analog inhibitors. The results revealed that the monoprotonated Asp289i state, in which the Asp93 and Asp289 residue side chains are deprotonated and protonated on the inner oxygen, respectively, is the most preferred in the presence of TC family inhibitors. This result was obtained after evaluating, for all 9 possible protonation state configurations, the individual and combined population densities of a set of parameters sensitive to protonation state of the Aspartic dyad, using an X-ray experimental BACE1/TC crystallographic structure as reference. This case study demonstrates again the usefulness of the concept of population density as a quantitative tool to establish the most stable system settings, among all possible, by measuring the level of occurrence of simultaneous events obtained from a sampling over time. These results will help to clear the phenomena related to the TCs inhibitory pathway, as well as assist in the design of better TC inhibitors against Alzheimer’s protease. 相似文献