全文获取类型
收费全文 | 83059篇 |
免费 | 7333篇 |
国内免费 | 44篇 |
出版年
2021年 | 1099篇 |
2020年 | 702篇 |
2019年 | 845篇 |
2018年 | 1080篇 |
2017年 | 1032篇 |
2016年 | 1729篇 |
2015年 | 3026篇 |
2014年 | 3191篇 |
2013年 | 4276篇 |
2012年 | 5265篇 |
2011年 | 5289篇 |
2010年 | 3572篇 |
2009年 | 3085篇 |
2008年 | 4343篇 |
2007年 | 4434篇 |
2006年 | 4199篇 |
2005年 | 4230篇 |
2004年 | 4258篇 |
2003年 | 3983篇 |
2002年 | 3907篇 |
2001年 | 1438篇 |
2000年 | 1282篇 |
1999年 | 1397篇 |
1998年 | 1189篇 |
1997年 | 938篇 |
1996年 | 933篇 |
1995年 | 900篇 |
1994年 | 824篇 |
1993年 | 871篇 |
1992年 | 1122篇 |
1991年 | 957篇 |
1990年 | 912篇 |
1989年 | 926篇 |
1988年 | 826篇 |
1987年 | 762篇 |
1986年 | 723篇 |
1985年 | 771篇 |
1984年 | 851篇 |
1983年 | 680篇 |
1982年 | 740篇 |
1981年 | 682篇 |
1980年 | 606篇 |
1979年 | 562篇 |
1978年 | 550篇 |
1977年 | 569篇 |
1976年 | 444篇 |
1975年 | 458篇 |
1974年 | 477篇 |
1973年 | 431篇 |
1971年 | 305篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
Olga Maslova Oleg Fedevych Nadiia Shuvalova Olena Deryabina Volodymyr Zhovnir Miroslav Novak Peter Kruzliak 《Cell and tissue banking》2016,17(2):335-339
The need for selection of the optimal material for the manufacturing of cardio-patches can be resolved by the use of cryostored autologous pericardial tissue. This short communication is a concise fragment of a large-scale research and demonstrates only the efficiency of cell culturing before and after pericardial preservation in the low temperature conditions. 相似文献
72.
Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
73.
74.
Transmembrane signaling implies that peripheral protein binding to one leaflet be detected by the opposite leaflet. Therefore, protein recruitment into preexisting cholesterol and sphingolipid rich platforms may be required. However, no clear molecular picture has evolved about how these rafts in both leaflets are connected. By using planar lipid bilayers, we show that the peripheral binding of a charged molecule (poly-lysine, PLL) is detected at the other side of the bilayer without involvement of raft lipids. The diffusion coefficient, DP, of PLL differed by a factor of √2 when PLL absorbed to one or to both leaflets of planar membranes. Fluorescence correlation spectroscopy showed that the changes of the lipid diffusion coefficient, DM, were even more pronounced. Although DM remained larger than DP on PLL binding to the first membrane leaflet, DM dropped to DP on PLL binding to both leaflets, which indicated that the lipids sandwiched between two PLL molecules had formed a nanodomain. Due to its small area of ∼20 nm2 membrane electrostriction or leaflet interaction at bilayer midplane can only make a small contribution to interleaflet coupling. The tendency of the system to maximize the area where the membrane is free to undulate seems to be more important. As a spot with increased bending stiffness, the PLL bound patch in one leaflet attracts a stiffening additive on the other leaflet. That is to say, instead of suppressing undulations in two spots, two opposing PLL molecules migrate along a membrane at matching positions and suppress these undulations in a single spot. The gain in undulation energy is larger than the energy required for the alignment of two small PLL domains in opposite leafs and their coordinated diffusion. We propose that this type of mechanical interaction between two membrane separated ligands generally contributes to transmembrane signaling. 相似文献
75.
Corythalia canosa from Florida is an unusual salticid because it is known to eat ants. This species' specialized behaviour for catching ants is described in detail for the first time and compared to its behaviour for catching other insects. Pystira orbiculata from Queensland is shown to be another ant-eating salticid, although its behaviour for catching ants seems less specialized than that of C. canosa. Three different types of tests of prey preference were carried out. In each type of test C. canosa and P. orbiculata took ants in preference to other insects. Another species of salticid, Trite planiceps from New Zealand, failed to eat ants in these tests, although T. planiceps often attacked then released the ants. Corythalia canosa's and P. orbiculata's preference for ants, and their prey-specific predatory behaviour for catching ants, are shown not to depend on prior experience with ants. 相似文献
76.
Prokaryotic and ciliate communities of healthy and aquarium White Syndrome (WS)-affected coral fragments were screened using denaturing gradient gel electrophoresis (DGGE). A significant difference (R = 0.907, p < 0.001) in 16S rRNA prokaryotic diversity was found between healthy (H), sloughed tissue (ST), WS-affected (WSU) and antibiotic treated (WST) samples. Although 3 Vibrio spp were found in WS-affected samples, two of these species were eliminated following ampicillin treatment, yet lesions continued to advance, suggesting they play a minor or secondary role in the pathogenesis. The third Vibrio sp increased slightly in relative abundance in diseased samples and was abundant in non-diseased samples. Interestingly, a Tenacibaculum sp showed the greatest increase in relative abundance between healthy and WS-affected samples, demonstrating consistently high abundance across all WS-affected and treated samples, suggesting Tenacibaculum sp could be a more likely candidate for pathogenesis in this instance. In contrast to previous studies bacterial abundance did not vary significantly (ANOVA, F2, 6 = 1.000, p = 0.422) between H, ST, WSU or WST. Antimicrobial activity (assessed on Vibrio harveyi cultures) was limited in both H and WSU samples (8.1% ±8.2 and 8.0% ±2.5, respectively) and did not differ significantly (Kruskal-Wallis, χ2 (2) = 3.842, p = 0.146). A Philaster sp, a Cohnilembus sp and a Pseudokeronopsis sp. were present in all WS-affected samples, but not in healthy samples. The exact role of ciliates in WS is yet to be determined, but it is proposed that they are at least responsible for the neat lesion boundary observed in the disease. 相似文献
77.
Household‐level correlates of children's physical activity levels in and across 12 countries 下载免费PDF全文
Deirdre M. Harrington Fiona Gillison Stephanie T. Broyles Jean‐Philippe Chaput Mikael Fogelholm Gang Hu Rebecca Kuriyan Anura Kurpad Allana G. LeBlanc Carol Maher Jose Maia Victor Matsudo Timothy Olds Vincent Onywera Olga L. Sarmiento Martyn Standage Mark S. Tremblay Catrine Tudor‐Locke Pei Zhao Peter T. Katzmarzyk for the ISCOLE Research Group 《Obesity (Silver Spring, Md.)》2016,24(10):2150-2157
78.
I. J. Pickering Graham N. George Verena Van Fleet-Stalder Thomas G. Chasteen Roger C. Prince 《Journal of biological inorganic chemistry》1999,4(6):791-794
Received: 2 April 1999 / Accepted: 17 September 1999 相似文献
79.
80.
K Haya B A Waiwood L Van Eeckhaute 《Comp. Biochem. Physiol. C, Comp. Pharmacol. Toxicol.》1985,82(2):323-329
Juvenile Atlantic salmon were held for 76 days at pH 4.7 during the period when the final stages of smoltification normally occur. Control salmon (pH 6.5) had significant increases in weight, length and liver somatic index which were not observed in those held at low pH. Condition factor decreased in both groups but to a significantly greater extent in the low pH group. After 15 days levels of ADP and glucose were higher and AEC, CP and glycogen were lower in muscles of salmon exposed to low pH. These qualitative differences were maintained until the end of the experiment. ATP and total adenylate concentrations in muscle were lower after 62 days of exposure to low pH compared to controls. The levels of ATP, total adenylates, AEC and glucose were consistently higher in livers of salmon exposed to low pH than those of controls. There were no differences in liver concentrations of AMP, CP and glycogen between the two treatments or with time of exposure within each treatment. The results suggest that exposure of juvenile Atlantic salmon to low pH increased gluconeogenesis and decreased food intake to the detriment of growth. 相似文献