全文获取类型
收费全文 | 572975篇 |
免费 | 51653篇 |
国内免费 | 1093篇 |
专业分类
625721篇 |
出版年
2018年 | 17247篇 |
2017年 | 16211篇 |
2016年 | 14059篇 |
2015年 | 8722篇 |
2014年 | 9664篇 |
2013年 | 13944篇 |
2012年 | 20590篇 |
2011年 | 30485篇 |
2010年 | 24216篇 |
2009年 | 19883篇 |
2008年 | 25583篇 |
2007年 | 28037篇 |
2006年 | 14361篇 |
2005年 | 14647篇 |
2004年 | 14431篇 |
2003年 | 14220篇 |
2002年 | 13514篇 |
2001年 | 19804篇 |
2000年 | 19790篇 |
1999年 | 16195篇 |
1998年 | 6260篇 |
1997年 | 6301篇 |
1996年 | 6078篇 |
1995年 | 5544篇 |
1994年 | 5647篇 |
1993年 | 5567篇 |
1992年 | 13476篇 |
1991年 | 12838篇 |
1990年 | 12730篇 |
1989年 | 12672篇 |
1988年 | 11609篇 |
1987年 | 11047篇 |
1986年 | 10253篇 |
1985年 | 10339篇 |
1984年 | 8596篇 |
1983年 | 7517篇 |
1982年 | 5951篇 |
1981年 | 5347篇 |
1980年 | 5211篇 |
1979年 | 8078篇 |
1978年 | 6374篇 |
1977年 | 5728篇 |
1976年 | 5487篇 |
1975年 | 5891篇 |
1974年 | 6173篇 |
1973年 | 6056篇 |
1972年 | 6011篇 |
1971年 | 5372篇 |
1970年 | 4281篇 |
1969年 | 3959篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Claudia Raedig Carsten F. Dormann Anke Hildebrandt Sven Lautenbach 《Biodiversity and Conservation》2010,19(6):1523-1546
Monographic data rely on specimens deposited in herbaria and museums, which have been thoroughly revised by experts. However,
monographic data have been rarely used to map species richness at large scale, mainly because of the difficulties caused by
spatially heterogeneous sampling effort. In this paper we estimate patterns of species richness and narrow endemism, based
on monographic data of 4,055 Neotropical angiosperm species. We propose a geometric interpolation method to derive species
ranges at a 1° grid resolution. To this we apply an inverse distance-weighted summation scheme to derive maps of species richness
and endemism. In the latter we also adjust for heterogeneous sampling effort. Finally, we test the robustness of the interpolated
species ranges and derived species richness by applying the same method but using a leave-one-out-cross-validation (LOOCV).
The derived map shows four distinct regions of elevated species richness: (1) Central America, (2) the Northern Andes, (3)
Amazonia and (4) the Brazilian Atlantic coast (‘Mata Atlantica’). The region with the highest estimated species richness is
Amazonia, with Central America following closely behind. Centers of narrow endemism are located over the entire Neotropics,
several of them coinciding with regions of elevated species richness. Sampling effort has a minor influence on the interpolation
of overall species richness, but it substantially influences the estimation of regions of narrow endemism. Thus, in order
to improve maps of narrow endemism and resulting conservation efforts, more collection and identification activity is required. 相似文献
22.
P C de Visser N M A J Kriek P A V van Hooft A Van Schepdael D V Filippov G A van der Marel H S Overkleeft J H van Boom D Noort 《The journal of peptide research》2003,61(6):298-306
As part of a program towards the development of novel antibiotics, a convenient method for solid-phase synthesis of the cyclic cationic peptide polymyxin B1 and analogues thereof is described. The methodology, based on cleavage-by-cyclization using Kenner's safety-catch linker, yields crude products with purities ranging from 37-67%. Antibacterial assays revealed that analogues 23-26, in which the (S)-6-methyloctanoic acid moiety is replaced with shorter acyl chains, exhibit distinct antimicrobial activity. The results suggest that the length of the acyl chain is rather critical for antimicrobial activity. On the other hand, substitution of the hydrophobic ring-segment D-Phe-6/Leu-7 in polymyxin B1 with dipeptide mimics (i.e. analogues 27-33) resulted in almost complete loss of antimicrobial activity. 相似文献
23.
24.
Elevated levels of intracellular calcium are a major cause of myocardial dysfunction. To find possible mediators of the deregulated calcium we searched for EF-hand calcium-binding proteins of the S100 family. By PCR technology we identified three members of the S100 protein family (S100 alpha, CACY, and CAPL) in the human heart. We cloned the corresponding cDNAs and examined their expression levels in various human tissues by Northern blot analysis. All three proteins are expressed at high levels in the human heart. Whereas CACY and CAPL mRNAs are expressed ubiquitously, S100 alpha mRNA is restricted to heart, skeletal muscle, and brain. Interestingly, the expression pattern of S100 alpha, CACY, and CAPL in human tissues differs significantly from that in rodent tissues. 相似文献
25.
26.
Results are presented on the analysis of three-dimensional motion of compound cilia or cirri in voltage-clamped specimens of the protozoan Stylonychia mytilus. Time series of three-dimensional data were obtained by using the anaxial illumination method for simultaneous recording of stereoscopic video images. Data processing involved the following steps: determination of a reference coordinate system based solely on features present in each stereo-pair; tracing of cirral axes in digitized images, conversion to parameter curves by means of least-squares polynomial approximation, conversion of pairs of two-dimensional data to a series of three-dimensional data; correction for distortion due to projective shortening and conversion to a series of polynomial triplets, and analysis of the periodical components of the motion pattern in the frequency domain. Reconstructed beating cycles show typical differences between hyperpolarization-induced ciliary activity and depolarization-induced ciliary activity. Reconstructions of the motion of the basal segment of a cirrus are in agreement with existing data. Analysis of the curvature and torsion of a cirral axis during beating does not reveal any simple pattern of propagated activity within the axoneme. The return stroke may be subdivided into two phases. First, a curvature peak develops proximally. Secondly, a region with increased torsion arises more distally and spreads out in proximal direction. Both curvature and torsion return to minimal values by the beginning of the power stroke. 相似文献
27.
28.
U Mura A M Osman A S Mohamed D Di Martino P L Ipata 《Comparative biochemistry and physiology. B, Comparative biochemistry》1987,87(1):157-160
The preservation of purine ring as purine bases appears to be a common feature of camel liver. Hepatic guanine appears to be actively converted into GMP in the camel rather than further degraded. The limiting step of guanine degradation appears to be the lack of hepatic guanase activity. Higher purine bases over uric acid ratios were found in camel urine with respect to those of zebu. 相似文献
29.
Purification of smooth-muscle myosin free of calmodulin and myosin light-chain kinase. Susceptibility to oxidation. 下载免费PDF全文
Smooth-muscle myosin purified as described by Persechini & Hartshorne [(1983) Biochemistry 22, 470-476] contains trace amounts of calmodulin and myosin light-chain kinase, which can be removed by Ca2+-dependent hydrophobic-interaction chromatography followed by calmodulin-Sepharose affinity chromatography. The resultant column-purified myosin exhibits properties similar to those of the non-purified myosin, e.g. actin activation of the Mg2+-ATPase requires Ca2+/calmodulin-dependent phosphorylation of the two 20 kDa light chains. However, unlike the non-purified myosin, the column-purified myosin undergoes a time-dependent transition to a form which no longer requires phosphorylation for actin activation of the myosin Mg2+-ATPase. This transition is identified as a time-dependent change in conformation of the column-purified myosin from a 10 S to 6 S form and is caused by slow oxidation of the column-purified myosin, since it could be prevented by storage under N2 and reversed by 5 mM-dithiothreitol. 相似文献
30.