全文获取类型
收费全文 | 120540篇 |
免费 | 2921篇 |
国内免费 | 934篇 |
专业分类
124395篇 |
出版年
2021年 | 319篇 |
2019年 | 287篇 |
2018年 | 12095篇 |
2017年 | 10874篇 |
2016年 | 7862篇 |
2015年 | 1443篇 |
2014年 | 1180篇 |
2013年 | 1582篇 |
2012年 | 5677篇 |
2011年 | 14275篇 |
2010年 | 12847篇 |
2009年 | 9050篇 |
2008年 | 11096篇 |
2007年 | 12719篇 |
2006年 | 1591篇 |
2005年 | 1830篇 |
2004年 | 2206篇 |
2003年 | 2226篇 |
2002年 | 1989篇 |
2001年 | 669篇 |
2000年 | 534篇 |
1999年 | 446篇 |
1998年 | 373篇 |
1997年 | 310篇 |
1996年 | 303篇 |
1995年 | 269篇 |
1994年 | 280篇 |
1993年 | 278篇 |
1992年 | 310篇 |
1991年 | 336篇 |
1990年 | 260篇 |
1989年 | 280篇 |
1988年 | 320篇 |
1987年 | 267篇 |
1986年 | 237篇 |
1985年 | 285篇 |
1984年 | 343篇 |
1983年 | 296篇 |
1982年 | 301篇 |
1981年 | 327篇 |
1980年 | 278篇 |
1979年 | 251篇 |
1978年 | 271篇 |
1977年 | 239篇 |
1976年 | 211篇 |
1975年 | 208篇 |
1974年 | 250篇 |
1973年 | 242篇 |
1972年 | 385篇 |
1971年 | 438篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Takashi Kitagawa Shingo Kimura Hideaki Nakata Harumi Yamada Akira Nitta Yoshikazu Sasai Hideharu Sasaki 《Environmental Biology of Fishes》2009,84(2):193-196
The habitat and movements of a Pacific bluefin tuna were investigated by reanalyzing archival tag data with sea surface temperature
data. During its trans-Pacific migration to the eastern Pacific, the fish took a direct path and primarily utilized waters,
in the Subarctic Frontal Zone (SFZ). Mean ambient temperature during the trans-Pacific migration was 14.5 ± 2.9 (°C ± SD),
which is significantly colder than the waters typically inhabited by bluefin tuna in their primary feeding grounds in the
western and eastern Pacific (17.6 ± 2.1). The fish moved rapidly through the colder water, and the heat produced during swimming
and the thermoconservation ability of bluefin tuna likely enabled it to migrate through the cold waters of the SFZ. 相似文献
993.
Toll-like receptor expression in normal ovary and ovarian tumors 总被引:1,自引:0,他引:1
Mingfu Zhou Molly M. McFarland-Mancini Holly M. Funk Nader Husseinzadeh Taofic Mounajjed Angela F. Drew 《Cancer immunology, immunotherapy : CII》2009,58(9):1375-1385
Recent studies have implicated inflammation in the initiation and progression of ovarian cancer, though the mechanisms underlying
this effect are still not clear. Toll-like receptors (TLRs) allow immune cells to recognize pathogens and to trigger inflammatory
responses. Tumor cell expression of TLRs can promote inflammation and cell survival in the tumor microenvironment. Here we
sought to characterize the expression of TLRs in normal human ovaries, benign and malignant ovarian tumors from patients,
and in established ovarian tumor cell lines. We report that TLR2, TLR3, TLR4, and TLR5 are strongly expressed on the surface
epithelium of normal ovaries. In contrast to previous studies of uterus and endocervix, we found no cyclic variation in TLR
expression occurred in murine ovaries. TLR2, TLR3, TLR4, and TLR5 are expressed in benign conditions, epithelial tumors, and
in ovarian cancer cell lines. Variable expression of TLR6 and TLR8 was seen in benign and malignant epithelium of some patients,
while expression of TLR1, TLR7, and TLR9 was weak. Normal and malignant ovarian stroma were negative for TLR expression. Vascular
endothelial cells, macrophages, and occasional fibroblasts in tumors were positive. Functional activity for TLRs was demonstrated
by stimulation of cell lines with specific ligands and subsequent activation and translocation of NFκB and release of the
proinflammatory cytokines interleukin-6 and CCL-2. These studies demonstrate expression of multiple TLRs in the epithelium
of normal ovaries and in ovarian tumor cells, and may indicate a mechanism by which epithelial tumors manipulate inflammatory
pathways to facilitate tumor progression. 相似文献
994.
Steffen Harzsch Carsten H. G. Müller Verena Rieger Yvan Perez Silvia Sintoni Christian Sardet Bill Hansson 《Zoomorphology》2009,128(1):53-73
The enigmatic arrow worms (Chaetognatha) are marine carnivores and among the most abundant planktonic organisms. Their phylogenetic
position has been heavily debated for a long time. Most recent molecular studies still provide a diverging picture and suggest
arrow worms to be some kind of basal protostomes. In an effort to understand the organization of the nervous system in this
clade for a broad comparison with other Metazoa we analysed the ultrastructure of the ventral nerve centre in Spadella cephaloptera by transmission electron microscopy. We were able to identify six different types of neurons in the bilateral somata clusters
by means of the cytoplasmic composition (regarding the structure of the neurite and soma including the shape and eu-/heterochromatin
ratio within the nucleus) as well as the size and position of these neurons. Furthermore, our study provides new insights
into the neuropil composition of the ventral nerve centre and several other fine structural features. Our second goal was
to examine if individually identifiable neurons are present in the ventral nerve centres of four chaetognath species, Sagitta setosa, Sagitta enflata, Pterosagitta draco, and Spadella cephaloptera. For that purpose, we processed whole mount specimens of these species for immunolocalization of RFamide-related neuropeptides
and analysed them with confocal laser-scanning microscopy. Our experiments provide evidence for the interspecific homology
of individual neurons in the ventral nerve centres of these four chaetognath species suggesting that the potential to generate
serially arranged neurons with individual identities is part of their ground pattern. 相似文献
995.
Chen-Xi Hu Hui Huang Li Zhang You Huang Zhu-Fang Shen Ke-Di Cheng Guan-Hua Du Ping Zhu 《Biotechnology letters》2009,31(7):979-984
Dipeptidyl peptidase (DPP) IV inhibitors provide a new strategy for the treatment of type 2 diabetes. Human DPP-IV gene was
cloned from differentiated Caco-2 cells and expressed in Pichia pastoris. The recombinant enzyme was used in a new system for screening of DPP-IV inhibitors. By high throughput screening, a novel
compound (W5188) was identified from 75,000 compounds with an IC50 of 6.5 μM. This method is highly reproducible and reliable for discovery of DPP-IV inhibitors as shown by Z′ value of 0.73
and S/N ratio of 6.89. 相似文献
996.
Microbial production of 2,3-butanediol from Jerusalem artichoke tubers by <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis> 总被引:1,自引:0,他引:1
Li-Hui Sun Xu-Dong Wang Jian-Ying Dai Zhi-Long Xiu 《Applied microbiology and biotechnology》2009,82(5):847-852
2,3-Butanediol is one of the promising bulk chemicals with wide applications. Its fermentative production has attracted great
interest due to the high end concentration. However, large-scale production of 2,3-butanediol requires low-cost substrate
and efficient fermentation process. In the present study, 2,3-butanediol production by Klebsiella pneumoniae from Jerusalem artichoke tubers was successfully performed, and various technologies, including separate hydrolysis and fermentation
(SHF) and simultaneous saccharification and fermentation (SSF), were investigated. The concentration of target products reached
81.59 and 91.63 g/l, respectively after 40 h in batch and fed-batch SSF processes. Comparing with fed-batch SHF, the fed-batch
SSF provided 30.3% higher concentration and 83.2% higher productivity of target products. The results showed that Jerusalem
artichoke tuber is a favorable substrate for 2,3-butanediol production, and the application of fed-batch SSF for its conversion
can result in a more cost-effective process. 相似文献
997.
Raimondi S Roncaglia L De Lucia M Amaretti A Leonardi A Pagnoni UM Rossi M 《Applied microbiology and biotechnology》2009,81(5):943-950
Twenty-two strains of Bifidobacterium, representative of eight major species of human origin, were screened for their ability to transform the isoflavones daidzin
and daidzein. Most of the strains released the aglycone from daidzin and 12 gave yields higher than 90%. The kinetics of growth,
daidzin consumption, and daidzein production indicated that the hydrolytic activity occurred during the growth. The supernatant
of the majority of the strains did not release the aglycone from daidzin, suggesting that cell-associated β-glucosidases (β-Glu)
are mainly responsible for the metabolism of soybean glyco-conjugates. Cell-associated β-Glu was mainly intracellular and
significantly varied among the species and the strains. The lack of β-Glu was correlated with the inability to hydrolyze daidzin.
Although S-equol production by anaerobic intestinal bacteria has been established, information on S-equol-producing bifidobacteria
is contradictory. In this study, 22 bifidobacteria failed to transform daidzein into reduced metabolites under all the experimental
conditions, excluding any role in the reductive pathway of daidzein toward the production of S-equol. These results suggest
that selected probiotic strains of Bifidobacterium can be used to speed up the release of daidzein, improving its bioavailability for absorption by colonic mucosa and/or biotransformation
to S-equol by other intestinal microorganisms. 相似文献
998.
999.
In the analysis of human movement, researchers often sum individual joint kinetics to obtain a single measure of lower extremity function. The extent to which these summed measures relate to the mechanical objectives of the task has not been formally validated. The criterion validity of these measures was established with comparisons to the mechanical objective of two multiple-joint tasks. For the Work task 18 participants performed a loaded barbell squat using 4 resistances while instrumented for biomechanical analysis. For the Power they performed 2 predetermined amounts of work at both self-selected and fast speeds. Using inverse dynamics techniques, the peak net joint moment (PM) was calculated bilaterally in the sagittal plane at the ankle, knee, and hip and was summed into a single measure. This measure was correlated with the task objectives using simple linear regression. Similar procedures were used for the average net joint moment (AM), peak (PP), and average (AP) net joint moment power, and the net joint moment impulse (IM) and work (IP). For the Work task all 6 measures were significantly correlated with the task objective, but only AM, PM, and IP had correlation coefficients above 0.90. For the Power task, IM was not significantly correlated with the task objective, and only AP had a correlation coefficient above 0.90. These findings indicate that the validity of summing individual kinetic measures depends on both the measure chosen and the mechanical objective of the task. 相似文献
1000.
Fernando Valladares José Chico Ismael Aranda Luis Balaguer Pierre Dizengremel Esteban Manrique Erwin Dreyer 《Trees - Structure and Function》2002,16(6):395-403
The responses of Quercus robur (oak) and Fagus sylvatica (beech) seedlings to four different light environments (full, 50%, 40% and 15% sunlight) and to a rapid increase in irradiance were explored during the summer, after 2 years of growth in a forest nursery at Nancy (France). Significant differences between the two species were found for most variables. Phenotypic plasticity for morphological variables (root-shoot ratio, leaf size, leaf weight ratio) was higher in beech than in oak, while the reverse was true for anatomical (stomatal density, epidermis thickness, exchange surface area of the palisade parenchyma) and physiological (maximum photosynthetic rate, stomatal conductance, Rubisco activity) variables. Predawn photochemical efficiency (Fv/Fm) was higher in oak than in beech in all light environments except in 15% sunlight. Fv/Fm was significantly lower in 100% sunlight than in the other light environments in beech but not in oak. Maximum photosynthetic rates (Amax) increased with increasing light availability in the two species but they were always higher in oak than in beech. Oak exhibited higher Rubisco activity than beech in full sunlight. The transfer of shade-adapted seedlings to the open caused a decrease of Fv/Fm, which was larger for beech than for oak. Transferred oak but not beech plants recovered gradually to the control Fv/Fm values. The decreased chlorophyll content and the increased non-photochemical quenching observed in high-light beech seedlings were not enough to avoid photoinhibition. The results suggest that a greater tolerance of strong irradiance is linked to an enhanced physiological plasticity (variables related to photosynthesis), while shade tolerance relies on an enhanced plasticity in light-harvesting variables (crown morphology and chlorophyll content). 相似文献