全文获取类型
收费全文 | 3733篇 |
免费 | 256篇 |
出版年
2023年 | 35篇 |
2022年 | 63篇 |
2021年 | 111篇 |
2020年 | 83篇 |
2019年 | 78篇 |
2018年 | 107篇 |
2017年 | 107篇 |
2016年 | 141篇 |
2015年 | 230篇 |
2014年 | 260篇 |
2013年 | 286篇 |
2012年 | 295篇 |
2011年 | 268篇 |
2010年 | 154篇 |
2009年 | 163篇 |
2008年 | 194篇 |
2007年 | 177篇 |
2006年 | 178篇 |
2005年 | 134篇 |
2004年 | 120篇 |
2003年 | 129篇 |
2002年 | 109篇 |
2001年 | 58篇 |
2000年 | 37篇 |
1999年 | 52篇 |
1998年 | 32篇 |
1997年 | 27篇 |
1996年 | 29篇 |
1995年 | 25篇 |
1994年 | 22篇 |
1993年 | 24篇 |
1992年 | 24篇 |
1991年 | 14篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1975年 | 6篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1971年 | 7篇 |
1968年 | 7篇 |
排序方式: 共有3989条查询结果,搜索用时 15 毫秒
991.
Testing metabolic ecology theory for allometric scaling of tree size, growth and mortality in tropical forests 总被引:1,自引:0,他引:1
Muller-Landau HC Condit RS Chave J Thomas SC Bohlman SA Bunyavejchewin S Davies S Foster R Gunatilleke S Gunatilleke N Harms KE Hart T Hubbell SP Itoh A Kassim AR LaFrankie JV Lee HS Losos E Makana JR Ohkubo T Sukumar R Sun IF Nur Supardi MN Tan S Thompson J Valencia R Muñoz GV Wills C Yamakura T Chuyong G Dattaraja HS Esufali S Hall P Hernandez C Kenfack D Kiratiprayoon S Suresh HS Thomas D Vallejo MI Ashton P 《Ecology letters》2006,9(5):575-588
The theory of metabolic ecology predicts specific relationships among tree stem diameter, biomass, height, growth and mortality. As demographic rates are important to estimates of carbon fluxes in forests, this theory might offer important insights into the global carbon budget, and deserves careful assessment. We assembled data from 10 old-growth tropical forests encompassing censuses of 367 ha and > 1.7 million trees to test the theory's predictions. We also developed a set of alternative predictions that retained some assumptions of metabolic ecology while also considering how availability of a key limiting resource, light, changes with tree size. Our results show that there are no universal scaling relationships of growth or mortality with size among trees in tropical forests. Observed patterns were consistent with our alternative model in the one site where we had the data necessary to evaluate it, and were inconsistent with the predictions of metabolic ecology in all forests. 相似文献
992.
Metabolomics and the challenges ahead 总被引:1,自引:0,他引:1
Mendes P 《Briefings in bioinformatics》2006,7(2):127
Eight years have passed since the proposal that the characterizationof mutations with silent phenotypes should be carried out bya global view of metabolism [1]. Since then, the field of metabolomicshas flourished both in the development of new 相似文献
993.
Passos RM Sé AB Wolff VL Nobrega YK Hermes-Lima M 《Advances in physiology education》2006,30(2):89-93
In this article, we report on an experiment designed to improve the learning of metabolic biochemistry by nutrition and medical undergraduate students. Twelve students participated in a monitored lunch and had their blood extracted for analysis 1) before lunch, 2) 30 min after lunch, and 3) 3 h after lunch. The subjects were divided in two groups. One group had a hyperglicidic meal [pasta plus orange juice: 80% carbohydrate, 10% protein, and 10% lipid (estimated values)] and the other group had a hyperlipidic meal (calabresi pizza plus diet soda: 36% carbohydrate, 18% protein, and 46% lipid). Individual quantities of food were based on body mass index, age, and sex. The blood parameters analyzed were glucose, triglycerides (TG), and urea. Glucose remained constant in the three measurements in both groups. The TG concentration in the pasta group was constant before and after lunch but increased significantly during the evening. In the pizza group, TG increased after lunch and remained constant in the evening. Levels of urea increased only in the evening, specially in the pizza group. These results were used for the final biochemistry exam. With the maximum score set as 10, the average score was 6.0 +/- 2.4 (n = 102). We considered this activity a unique way of evaluating important issues on metabolism, because students had several hours to work on the final exam (with free access to a bibliography). It was also a good didactic experience (problem-based learning like) for the subject students, because they had to work in all phases of the experiment (idealization, realization, and analysis) and participated actively in the elaboration and correction of the exam. 相似文献
994.
Fonseca IO Magalhães ML Oliveira JS Silva RG Mendes MA Palma MS Santos DS Basso LA 《Protein expression and purification》2006,46(2):429-437
Tuberculosis (TB) poses a major worldwide public health problem. The increasing prevalence of TB, the emergence of multi-drug-resistant strains of Mycobacterium tuberculosis, the causative agent of TB, and the devastating effect of co-infection with HIV have highlighted the urgent need for the development of new antimycobacterial agents. Analysis of the complete genome sequence of M. tuberculosis shows the presence of genes involved in the aromatic amino acid biosynthetic pathway. Experimental evidence that this pathway is essential for M. tuberculosis has been reported. The genes and pathways that are essential for the growth of the microorganisms make them attractive drug targets since inhibiting their function may kill the bacilli. We have previously cloned and expressed in the soluble form the fourth shikimate pathway enzyme of the M. tuberculosis, the aroE-encoded shikimate dehydrogenase (mtSD). Here, we present the purification of active recombinant aroE-encoded M. tuberculosis shikimate dehydrogenase (mtSD) to homogeneity, N-terminal sequencing, mass spectrometry, assessment of the oligomeric state by gel filtration chromatography, determination of apparent steady-state kinetic parameters for both the forward and reverse directions, apparent equilibrium constant, thermal stability, and energy of activation for the enzyme-catalyzed chemical reaction. These results pave the way for structural and kinetic studies, which should aid in the rational design of mtSD inhibitors to be tested as antimycobacterial agents. 相似文献
995.
Isolation of Helicobacter pylori from the intestinal mucosa of patients with Crohn's disease 总被引:6,自引:0,他引:6
Oliveira AG Rocha GA Rocha AM Sanna Md Moura SB Dani R Marinho FP Moreira LS Ferrari Mde L Castro LP Queiroz DM 《Helicobacter》2006,11(1):2-9
BACKGROUND: Helicobacter species are associated with inflammatory bowel disease in rodents and in nonhuman primates. Therefore, we prospectively investigated the presence of Helicobacter species in the intestinal mucosa of patients with and without Crohn's disease by culture and polymerase chain reaction (PCR) assays. MATERIALS AND METHODS: Mucosal fragments were obtained from the ileum, different colon regions, and rectum of 43 patients with Crohn's disease and of 74 patients without inflammatory bowel disease. RESULTS: Helicobacter pylori strains, identified by 16S rRNA gene sequencing, were more frequently isolated and PCR-detected in the intestinal mucosa of patients with ulcerative colitis-like Crohn's disease than in intestinal mucosa of the control group. Otherwise, anti-H. pylori immunoglobulin G levels were significantly lower in fibrostenosing and fistulating Crohn's disease subgroups. No other Helicobacter species were found in the intestinal mucosa of the patients. CONCLUSIONS: Although our results suggest an association between the presence of H. pylori in the intestine and ulcerative colitis-like phenotype of Crohn's disease, H. pylori infection in the actual causality of Crohn's disease is still to be determined. 相似文献
996.
Rádis-Baptista G Moreno FB de Lima Nogueira L Martins AM de Oliveira Toyama D Toyama MH Cavada BS de Azevedo WF Yamane T 《Cell biochemistry and biophysics》2006,44(3):412-423
Snake venom (sv) C-type lectins encompass a group of hemorrhagic toxins that are capable of interfering with blood stasis.
A very well-studied svC-type lectin is the heterodimeric toxin, convulxin (CVX), from the venom of South American rattlesnake
Crotalus durissus terrificus. CVX is able to activate platelets and induce their aggregation by acting via p62/GPVI collagen receptor. By using polymerase
chain reaction homology screening, we have cloned several cDNA precursors of CVX subunit homologs. One of them, named crotacetin
(CTC) β-subunit, predicts a polypeptide with a topology very similar to the tridimensional conformations of other subunits of
CVX-like snake toxins, as determined by computational analysis. Using gel permeation and reverse-phase high-performance liquid
chromatography, CTC was purified from C. durissus venoms. CTC can be isolated from the venom of several C. durissus subspecies, but its quantitative predominance is in the venom of C. durissus cascavella. Functional analysis indicates that CTC induces platelet aggregation, and, importantly, exhibits an antimicrobial activity
against Gram-positive and-negative bacteria, comparable with CVX. 相似文献
997.
da Silva CV Luquetti AO Rassi A Mortara RA 《Microbes and infection / Institut Pasteur》2006,8(8):2120-2129
We examined whether the expression of Ssp-4-related carbohydrate epitopes defined by monoclonal antibodies 1D9 and 2B7 was related to cell invasion by Trypanosoma cruzi amastigotes from different isolates and whether the highest expression of the epitope defined by MAb 1D9 would confer greater infectivity. Confocal microscopy showed that both epitopes localize to the membrane of amastigotes from 569, 588, 573, 587 and SC2005 isolates, similar to the G isolate, whereas the CL isolate showed a punctate and diffuse staining. Flow cytometry revealed inter- and intra-isolate variable expression of these epitopes. Apart from the lower expression of MAb 2B7 epitope by intracellular amastigotes of the SC2005 isolate, amastigotes from chagasic patient isolates expressed both epitopes similar to the G isolate, in contrast to CL isolate, that showed lower expression of both epitopes. MAb 1D9 did not react with CL isolate on immunoblots and reacted poorly with 588 and 587 parasites. MAb 2B7 preferentially reacted with an epitope on an 84 kDa component in G and 573 isolates. Invasion assays revealed that despite the fact that amastigotes from chagasic patient isolates displayed high levels of the epitope defined by MAb 1D9, only isolate 588 invaded host cells in levels comparable to that of isolate G. Both MAbs specifically inhibited cell invasion by G and 588, but not CL. These results suggested that the highest expression of MAb 1D9 epitope was not sufficient to confer higher infectivity on the isolate, and besides the two epitopes, other factors may modulate the invasiveness of extracellular amastigotes from the different isolates. 相似文献
998.
Braga CA Pinto JR Valente AP Silva JL Sorenson MM Foguel D Suarez MC 《The international journal of biochemistry & cell biology》2006,38(1):110-122
The C-domain of troponin C, the Ca(2+)-binding subunit of the troponin complex, has two high-affinity sites for Ca(2+) that also bind Mg(2+) (Ca(2+)/Mg(2+) sites), whereas the N-domain has two low-affinity sites for Ca(2+). Two more sites that bind Mg(2+) with very low affinity (K(a)<10(3)M(-1)) have been detected by several laboratories but have not been localized or studied in any detail. Here we investigated the effects of Ca(2+) and Mg(2+) binding to isolated C-domain, focusing primarily on low-affinity sites. Since TnC has no Trp residues, we utilized a mutant with Phe 154 replaced by Trp (F154W/C-domain). As expected from previous reports, the changes in Trp fluorescence revealed different conformations induced by the addition of Ca(2+) or Mg(2+) (Ca(2+)/Mg(2+) sites). Exposure of hydrophobic surfaces of F154W/C-domain was monitored using the fluorescence intensity of bis-anilino naphthalene sulfonic acid. Unlike the changes reported by Trp, the increments in bis-ANS fluorescence were much greater (4.2-fold) when Ca(2+)+Mg(2+) were both present or when Ca(2+) was present at high concentration. Bis-ANS fluorescence increased as a function of [Ca(2+)] in two well-defined steps: one at low [Ca(2+)], consistent with the Ca(2+)/Mg(2+) sites (K(a) approximately 1.5 x 10(6)M(-1)), and one of much lower affinity (K(a) approximately 52.3M(-1)). Controls were performed to rule out artifacts due to aggregation, high ionic strength and formation of the bis-ANS-TnC complex itself. With a low concentration of Ca(2+) (0.6mM) to occupy the Ca(2+)/Mg(2+) sites, a large increase in bis-ANS binding also occurred as Mg(2+) occupied a class of low-affinity sites (K(a) approximately 59 M(-1)). In skinned fibers, a high concentration of Mg(2+) (10-44 mM) caused TnC to dissociate from the thin filament. These data provide new evidence for a class of weak binding sites for divalent cations. They are located in the C-domain, lead to exposure of a large hydrophobic surface, and destabilize the binding of TnC to the regulatory complex even when sites III and IV are occupied. 相似文献
999.
Distance to the Scaling Law: A Useful Approach for Unveiling Relationships between Crime and Urban Metrics 总被引:1,自引:0,他引:1
We report on a quantitative analysis of relationships between the number of homicides, population size and ten other urban metrics. By using data from Brazilian cities, we show that well-defined average scaling laws with the population size emerge when investigating the relations between population and number of homicides as well as population and urban metrics. We also show that the fluctuations around the scaling laws are log-normally distributed, which enabled us to model these scaling laws by a stochastic-like equation driven by a multiplicative and log-normally distributed noise. Because of the scaling laws, we argue that it is better to employ logarithms in order to describe the number of homicides in function of the urban metrics via regression analysis. In addition to the regression analysis, we propose an approach to correlate crime and urban metrics via the evaluation of the distance between the actual value of the number of homicides (as well as the value of the urban metrics) and the value that is expected by the scaling law with the population size. This approach has proved to be robust and useful for unveiling relationships/behaviors that were not properly carried out by the regression analysis, such as the non-explanatory potential of the elderly population when the number of homicides is much above or much below the scaling law, the fact that unemployment has explanatory potential only when the number of homicides is considerably larger than the expected by the power law, and a gender difference in number of homicides, where cities with female population below the scaling law are characterized by a number of homicides above the power law. 相似文献
1000.
B. M. J. Mendes S. B. Filippi C. G. B. Demétrio A. P. M. Rodriguez 《Plant cell reports》1999,18(12):967-971
The use of micropropagation to obtain large numbers of high-quality planting material has increased in recent years. Behavior in culture, mainly in terms of multiplication rate, varies among genotypes, directly affecting plant production planning. To study multiplication rates over time, suckers of banana, Musa spp., cv. Maçã, were collected in the field and the shoot apex introduced in vitro for micropropagation. The number of new shoots produced in each of the six multiplication cycles was recorded and the data analyzed statistically. Variability in total shoot production and differences in multiplication rates was considerable among families, which consisted of the initial explant and its progeny. Moreover, the adjusted Poisson regression models for the number of shoots showed that the multiplication rate in this cultivar tends to decrease with time: after the seventh subculture, new shoots may form at a very low rate. Interpretation of the first and second derivatives of the regression model allowed determination of the maximum speed of multiplication and the time at which the multiplication rate begins to decline. 相似文献