全文获取类型
收费全文 | 2068篇 |
免费 | 234篇 |
国内免费 | 273篇 |
出版年
2024年 | 7篇 |
2023年 | 52篇 |
2022年 | 43篇 |
2021年 | 71篇 |
2020年 | 61篇 |
2019年 | 87篇 |
2018年 | 84篇 |
2017年 | 89篇 |
2016年 | 82篇 |
2015年 | 79篇 |
2014年 | 102篇 |
2013年 | 158篇 |
2012年 | 103篇 |
2011年 | 103篇 |
2010年 | 100篇 |
2009年 | 91篇 |
2008年 | 98篇 |
2007年 | 115篇 |
2006年 | 111篇 |
2005年 | 78篇 |
2004年 | 68篇 |
2003年 | 74篇 |
2002年 | 78篇 |
2001年 | 52篇 |
2000年 | 50篇 |
1999年 | 51篇 |
1998年 | 40篇 |
1997年 | 32篇 |
1996年 | 39篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 30篇 |
1992年 | 21篇 |
1991年 | 29篇 |
1990年 | 15篇 |
1989年 | 24篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 19篇 |
1985年 | 27篇 |
1984年 | 25篇 |
1983年 | 23篇 |
1982年 | 16篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 14篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1972年 | 3篇 |
排序方式: 共有2575条查询结果,搜索用时 265 毫秒
31.
本文报道了从腐梨上分离到的一株未见描述的毕赤酵母属新种(Pichia hangzhou-ana Lu et Li),并讨论了与其近似种的区别。同时进行了DNA G+C百分比值的测定。 相似文献
32.
Normal and mutant human adenosine deaminase genes 总被引:2,自引:0,他引:2
33.
Versieck Jacques Vanballenberghe Lidia Wittoek Ann Vermeir Gerda Vandecasteele Carlo 《Biological trace element research》1990,(1):683-689
A method is described for the determination of mercury in human blood serum and packed blood cells employing neutron activation
analysis. Great attention was devoted to the collection and manipulation of the samples. The accuracy and precision of the
method were tested by analyzing biological reference materials and by comparing the concentrations measured in a number of
serum samples to those obtained by another, independent technique (cold vapor atomic absorption spectrometry) in the same
samples. The article reports the levels measured in blood serum and packed blood cells samples from 15 adult volunteers, as
well as the figures determined in a “second-generation” biological reference material (freeze-dried human serum), prepared
and conditioned at the University of Ghent. 相似文献
34.
Agar yield and quality of Gracilaria chilensis (Gigartinales,Rhodophyta) in tank culture using fish effluents 总被引:1,自引:0,他引:1
Tank cultivation of Gracilaria using fish effluents has permitted a production of 48 kg m–2 yr–1 and can reduce the dissolved nitrogen loads in the seawater. We report the yield, gel strength, gelling and melting point of agar from Gracilaria cultivated in tanks with seawater previously utilized in intensive, land-based salmon cultures and compared to a control using directly pumped seawater, over a study period of 22 months. The results show that the highest agar yield (20 to 22%) was obtained when Gracilaria was cultivated with pure seawater as compared to the fish effluents. The gel strength, gelling and melting point were higher in the agar obtained from algae cultured with fish effluents. During the spring, the gel strength, gelling and melting point increased in tanks with fish effluents and decreased in tanks with a supply of pure seawater. 相似文献
35.
The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in 13C values along a rainfall gradient (50–600 mm) and in different habitat conditions. However, there are significant differences in the 13C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest 13C values (–11.7 ) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower 13C values (–13.4 ) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (–12.5 ) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative 13C values than PCK species and 13C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, 13C values decreased from –11 in the inland region (600 mm precipitation) to –15 near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed. 相似文献
36.
37.
Josiane Arnaud Pierre Bourlard Bernard Denis Alain E. Favier 《Biological trace element research》1996,53(1-3):129-136
This study was carried out to assess manganese (Mn) status after an acute episode of myocardial infarction. Plasma and erythrocyte
Mn concentrations were measured from admission to hospital to day 15 postadmission in 21 patients suffering from acute myocardial
infarction and in three control groups. The determination of Mn in these biological fluids was performed by electrothermal
atomic absorption spectrometry. Plasma Mn was higher (p<0.01) and erythrocyte Mn was similar in the acute myocardial infarction group compared to healthy age-matched control group.
Plasma and erythrocyte Mn remained unchanged during the 2 wk after acute myocardial infarction and were not correlated to
enzyme activities. A decrease of erythrocyte Mn with age, expressed in nmol/L, was noted (p<0.02). These results suggest that plasma and erythrocyte Mn do not provide an indication of myocardial damage. Nonetheless,
Mn status in elderly merits further attention. 相似文献
38.
Tetsuo Ajiki Takahiro Fujimori Hideo Ikehara Yoichi Saitoh Sakan Maeda 《Biotechnic & histochemistry》1995,70(2):90-94
To investigate the relationship of oncogene analysis to morphology, we analyzed K-ras gene mutations by dot-blot hybridization with and without consideration of histological atypias in individual colorectal adenomas. Each of 54 colon polyps were divided into two parts after fixation. One part was used as a mass to assess point mutations; the remaining portion of each polyp was paraffin-embedded, stained with hematoxylin and eosin, and examined for point mutations related to histological atypias. In the first part of our study, K-ras gene mutations at codon 12 were detected in 13 cases (24%). In the second part of our study, 12 cases had distinctly different histological atypias. From each of these 12 cases, two areas, one with higher or one with lower grade atypia in the same polyp were excised to analyze for K-ras gene mutation. Two of these 12 cases (17%) had the mutation in different areas of the same tumor. These two cases contained the mutation only in the areas with higher grade atypia, and only one case added information regarding ras mutation upon microdissection when compared to the entire biopsy. These results suggest that oligonucleotide hybridization can identify the majority of cases containing ras mutations despite regional morphologic variation. Individual cases, however, may contain clonal subpopulations within adenomas with different ras sequences from other regions within the same adenoma. 相似文献
39.
John H. Gillespie 《Evolution; international journal of organic evolution》1994,48(4):1101-1113
Substitution processes are of two sorts: origination processes record the times at which nucleotide mutations that ultimately fix in the population first appear, and fixation processes record the times at which they actually fix. Substitution processes may be generated by combining models of population genetics—here the symmetrical-neutral, overdominance, underdominance, TIM, and SAS-CFF models—with the infinite-sites, no-recombination model of the gene. This paper is mainly concerned with a computer simulation study of these substitution processes. The rate of substitution is shown to be remarkably insensitive to the strength of selection for models with strong balancing selection caused by the genealogical drift of mutations through alleles held in the population by selection. The origination process is shown to be more regular than Poisson for the overdominance, TIM, and SAS-CFF models but more clustered for the underdominance model. A class of point processes called Sawyer processes is introduced to help explain these observations as well as the observation that the times between successive originations are nearly uncorrelated. Fixation processes are shown to be more complex than origination processes, with regularly spaced bursts of multiple fixations. An approximation to the fixation process is described. One important conclusion is that protein evolution is not easily reconciled with any of these models without adding perturbations that recur on a time scale that is commensurate with that of molecular evolution. 相似文献
40.
从假菠萝麻叶汁中用乙醇分部沉淀法得到一种蛋白酶,对酪蛋白有强烈的水解活性,经Sephadex G-100柱层析和SP-Sephadex C-50离子交换柱层析等步骤纯化,可得到六角形结晶。结晶酶液经PAGE圆盘电泳,每条胶柱上只显示一条蛋白带,其活性在pH4。5-10、55℃范围内较稳定,最适pH8.5最适温度50℃,Km(酪蛋白)值为0.185%(W/V)。用SDS-PAGE和Sephadex 相似文献