全文获取类型
收费全文 | 1455篇 |
免费 | 128篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 23篇 |
2020年 | 18篇 |
2019年 | 37篇 |
2018年 | 37篇 |
2017年 | 38篇 |
2016年 | 57篇 |
2015年 | 92篇 |
2014年 | 92篇 |
2013年 | 102篇 |
2012年 | 131篇 |
2011年 | 131篇 |
2010年 | 79篇 |
2009年 | 50篇 |
2008年 | 87篇 |
2007年 | 92篇 |
2006年 | 80篇 |
2005年 | 63篇 |
2004年 | 64篇 |
2003年 | 55篇 |
2002年 | 59篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 11篇 |
1998年 | 14篇 |
1997年 | 19篇 |
1996年 | 12篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1970年 | 2篇 |
排序方式: 共有1583条查询结果,搜索用时 15 毫秒
111.
The validity of the Michaelis–Menten–Briggs–Haldane approximation for single enzyme reactions has recently been improved by
the formalism of the total quasi-steady-state approximation. This approach is here extended to fully competitive systems,
and a criterion for its validity is provided. We show that it extends the Michaelis–Menten–Briggs–Haldane approximation for
such systems for a wide range of parameters very convincingly, and investigate special cases. It is demonstrated that our
method is at least roughly valid in the case of identical affinities. The results presented should be useful for numerical
simulations of many in vivo reactions. 相似文献
112.
Sulfate reduction rates and biogeochemical parameters of fish farm sediments across the Mediterranean were investigated in
the order to evaluate the potential effects of organic matter inputs on habitat quality for the common seagrass Posidonia oceanica. Four study sites were selected in Spain, Italy, Greece and Cyprus to represent the Mediterranean basin. P. oceanica was found in immediate vicinity of all the farms, which were located at physically exposed sites about 1 km from the shore
lines. Organic matter accumulation, sulfate reduction rates and sulfur pools were measured in depth profiles along transects
from the farms in both bare and vegetated sediments. Results show that although the organic matter accumulation was minor
at the sites (POC < 2.8% DW), the sulfate reduction rates were high, in particular at the largest farm in Italy (up to 212 mmol m−2 d−1), similar to rates found at shallower, temperate fish farm sites, where higher sedimentation rates can be expected. Sulfate
reducing bacteria in these low-organic, carbonate-rich Mediterranean sediments respond strongly to organic matter loadings
and cause habitat degradation. Sulfate reduction rates measured in the P. oceanica sediments were among the highest recorded (7.8–42.0 mmol m−2 d−1) similar to rates found in degrading meadows impacted by organic matter loadings. As sulfate reduction rates were correlated
with the sedimentation rates along the transects rather than organic matter pools this suggests mineralization processes were
controlled by organic matter loading in fish farm sediments. The vegetated sediments near the net cages were more reduced
due to accumulation of sulfides compared to control sites, which is a possible contributing factor to the observed seagrass
decline in the farm surroundings. It is recommended that Mediterranean fish farms are placed in areas with rapid dispersal
of particulate waste products to minimize organic matter loading of the sediments and thereby preserve habitat quality for
benthic fauna and flora. 相似文献
113.
A general feature of the demography of large ungulates is that many demographic traits are dependent on female body mass at
early ages. Thus, identifying the factors affecting body mass variation can give important mechanistic understanding of demographic
processes. Here we relate individual variation in autumn and winter body mass of moose calves living at low density on an
island in northern Norway to characteristics of their mother, and examine how these relationships are affected by annual variation
in population density and climate. Body mass increased with increasing age of their mother, was lower for calves born late
in the spring, decreased with litter size and was larger for males than for female calves. No residual effects of variation
in density and climate were present after controlling for annual variation in mother age and calving date. The annual variation
in adult female age structure and calving date explained a large part (71–75%) of the temporal variation in calf body mass.
These results support the hypotheses that (a) body mass of moose calves are affected by qualities associated with mother age
(e.g. body condition, calving date); and (b) populations living at low densities are partly buffered against temporal fluctuations
in the environment. 相似文献
114.
115.
Morten IJ Gosal WS Radford SE Hewitt EW 《The Journal of biological chemistry》2007,282(40):29691-29700
Dialysis related amyloidosis is a serious complication of long-term hemodialysis in which beta(2)-microglobulin (beta(2)m) forms amyloid fibrils that deposit predominantly in cartilaginous tissues. How these fibrils form in vivo, however, is poorly understood. Here we perform a systematic investigation into the role of macrophages in the formation and degradation of beta(2)m amyloid fibrils, building on observations that macrophages are found in association with beta(2)m amyloid deposits in vivo and that these cells contain intra-lysosomal beta(2)m amyloid. In live cell imaging experiments we demonstrate that macrophages internalize monomeric beta(2)m, whereupon it is sorted to lysosomes. At lysosomal pH beta(2)m self-associates in vitro to form amyloid-like fibrils with an array of morphologies as visualized by atomic force microscopy. Cleavage of the monomeric protein by both macrophages and lysosomal proteases isolated from these cells results in the rapid degradation of the monomeric protein, preventing amyloid formation. Incubation of macrophages with preformed fibrils revealed that macrophages internalize amyloid-like fibrils formed extracellularly, but in marked contrast with the monomeric protein, the fibrils were not degraded within macrophage lysosomes. Correspondingly beta(2)m fibrils were highly resistant to degradation by high concentrations of lysosomal proteases isolated from macrophages. Despite their enormous degradative capacity, therefore, macrophage lysosomes cannot ameliorate dialysis-related amyloidosis by degrading pre-existing amyloid fibrils, but lysosomal proteases may play a protective role by eliminating amyloid precursors before beta(2)m fibrils can accumulate in what may represent an otherwise fibrillogenic environment. 相似文献
116.
Marta Pérez Olga Invers Morten S. Frederiksen 《Journal of experimental marine biology and ecology》2007,344(2):149-160
Here we studied the effects of adding organic carbon and nutrients to sediment on the physiology and survival of the seagrass Posidonia oceanica in a field experiment in the Medes Islands (NE Spain). Nine randomly selected plots were established at a depth of 10 m; three were enriched with organic carbon (OM treatment), three with organic carbon and nutrients (OMN treatment), and three were kept as controls (CON). The experiment was performed over 5 months and sampling of plants and sediments was done in March, May and July 2002. Sediment sulfide pools and pore water ammonium concentrations increased significantly in OM and OMN plots, both treatments showing increased reducing conditions in the sediment. Plants in these two treatments showed higher mortality and lower biomass compared to plants from CON plots. The greatest effects on seagrass occurred in the OMN plots, indicating a synergistic effect of organic carbon and nutrient additions. Treatments had significant effects on plant nitrogen (N) and phosphorous (P) metabolism, shown by an increase in free amino acid (FAA) content, a change in FAA composition and a lack of increase in N and P tissue content. Treated plants showed higher g-aminobutyric acid (GABA) and malate concentrations and lower concentrations of non-structural carbohydrates compared to CON, indicating that anaerobic respiration in below-ground tissues occurred. Several of the physiological changes shown by P. oceanica can be interpreted as adaptations to anoxia exposure. However, the increased mortality in treated plots demonstrates that this seagrass does not tolerate highly reduced sediments. 相似文献
117.
118.
A Wetland Change Model has been developed to identify the vulnerability of coastal wetlands at broad spatial (regional to
global (mean spatial resolution of 85 km)) and temporal scales (modelling period of 100 years). The model provides a dynamic
and integrated assessment of wetland loss, and a means of estimating the transitions between different vegetated wetland types
and open water under a range of scenarios of sea-level rise and changes in accommodation space from human intervention. This
paper is an overview of key issues raised in the process of quantifying broad-scale vulnerabilities of coastal wetlands to
forcing from sea-level rise discussing controlling factors of tidal range, sediment availability and accommodation space,
identification of response lags and defining the threshold for wetland loss and transition. 相似文献
119.
Morten Skage Anders Hobæk Štĕpánka Ruthová Barbara Keller Adam Petrusek Jaromír Sed’a Piet Spaak 《Hydrobiologia》2007,594(1):19-32
A collaborative research effort was undertaken to evaluate the robustness of a recently developed genetic tool for species
identification of members in the morphologically variable Daphnia longispina species complex. This genetic method, based on restriction fragment length polymorphism (RFLP) of the internal transcribed
spacer region (ITS) of nuclear ribosomal DNA (rDNA) with restriction enzymes Mwo I and Sau96 I [Billiones et al., 2004. Hydrobiologia 526: 43–53], was applied to many different European populations. Results were
compared with two or more independently obtained characters (morphology, allozymes, mitochondrial DNA (mtDNA), or cloned rDNA-ITS
sequences). Individuals of most taxa were readily identified, but unexpected ITS-RFLP patterns were found in many individuals
indicated by other markers to be D. galeata or one of its hybrids. Among 43 investigated D. galeata populations (902 specimen analysed by ITS-RFLP), deviant RFLP fragment patterns occurred in 26 (i.e., more than half) of
the populations. The deviant patterns could be attributed to the loss of one single restriction site in the ITS2 region. This
loss made the distinction of D. galeata from other species unreliable, and F1 hybrids could not be identified. Future users should be aware of this shortcoming of
the Billions et al. [2004. Hydrobiologia 526: 43–53] protocol. As a solution to this problem, we present an improved genetic
identification protocol based on a simple double digestion of the rDNA-ITS region with the restriction enzymes BsrB I and EagI. Sequence analyses of rDNA-ITS clones and preliminary testing indicate that the new protocol is unaffected by the rDNA variation
which troubled the Mwo I/Sau96 I protocol. Further, the new protocol identifies all European species of the D. longispina complex, as well as their F1 hybrids. However, a wider screening is required to verify its general utility for all species,
since yet unknown variation may occur.
Guest editor: Piet Spaak
Cladocera: Proceedings of the 7th International Symposium on Cladocera 相似文献
120.
Using an electrical measurement known as electric cell-substrate impedance sensing (ECIS), we have recorded the dynamics of viral infections in cell culture. With this technique, cells are cultured on small gold electrodes where the measured impedance mirrors changes in attachment and morphology of cultured cells. As the cells attach and spread on the electrode, the measured impedance increases until the electrode is completely covered. Viral infection inducing cytopathic effect results in dramatic impedance changes, which are mainly due to cell death. In the current study, two different fish cell lines have been used: chinook salmonid embryonic (CHSE-214) cells infected with infectious pancreatic necrosis virus (IPNV) and epithelioma papulosum cyprini (EPC) carp cells infected with infectious hematopoeitic necrosis virus (IHNV). The impedance changes caused by cell response to virus are easily measured and converted to resistance and capacitance. An approximate linear correlation between log of viral titer and time of cell death was determined. 相似文献