全文获取类型
收费全文 | 1001篇 |
免费 | 70篇 |
国内免费 | 48篇 |
出版年
2023年 | 13篇 |
2022年 | 17篇 |
2021年 | 19篇 |
2020年 | 35篇 |
2019年 | 33篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 37篇 |
2015年 | 31篇 |
2014年 | 40篇 |
2013年 | 59篇 |
2012年 | 26篇 |
2011年 | 31篇 |
2010年 | 27篇 |
2009年 | 49篇 |
2008年 | 49篇 |
2007年 | 33篇 |
2006年 | 29篇 |
2005年 | 53篇 |
2004年 | 41篇 |
2003年 | 29篇 |
2002年 | 25篇 |
2001年 | 33篇 |
2000年 | 11篇 |
1999年 | 19篇 |
1998年 | 24篇 |
1997年 | 15篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1994年 | 19篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 12篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1119条查询结果,搜索用时 31 毫秒
81.
82.
Ian Salter Mikhail V. Zubkov Phil E. Warwick & Peter H. Burkill 《FEMS microbiology letters》2009,294(2):225-231
Hydrophobic surfactants at the air–sea interface can retard evaporative and gaseous exchange between the atmosphere and the ocean. While numerous studies have examined the metabolic role of bacterioneuston at the air–sea interface, the interactions between hydrophobic surfactants and bacterioplankton are not well constrained. A novel experimental design was developed, using Vibrio natriegens and 3 H-labelled hexadecanoic acid tracer, to determine how the bacterial metabolism of fatty acids affects evaporative fluxes. In abiotic systems, >92% of the added hexadecanoic acid remained at the air–water interface. In contrast, the presence of V. natriegens cells draws down insoluble hexadecanoic acid from the air–water interface as an exponential function of time. The exponents characterizing the removal of hexadecanoic acid from the interface co-vary with the concentration of V. natriegens cells in the underlying water, with the largest exponent corresponding to the highest cell abundance. Radiochemical budgets show that evaporative fluxes from the system are linearly proportional to the quantity of hexadecanoic acid at the interface. Thus, bacterioplankton could influence the rate of evaporation and gas transfer in the ocean through the metabolism of otherwise insoluble surfactants. 相似文献
83.
Catherine E. Au Louis Hermo Elliot Byrne Jeffrey Smirle Ali Fazel Robert E. Kearney Charles E. Smith Hojatollah Vali Julia Fernandez-Rodriguez Paul H. G. Simon Craig Mandato Tommy Nilsson John J. M. Bergeron 《Open biology》2015,5(8)
Discovered in 1909 by Retzius and described mainly by morphology, the cytoplasmic droplet of sperm (renamed here the Hermes body) is conserved among all mammalian species but largely undefined at the molecular level. Tandem mass spectrometry of the isolated Hermes body from rat epididymal sperm characterized 1511 proteins, 43 of which were localized to the structure in situ by light microscopy and two by quantitative electron microscopy localization. Glucose transporter 3 (GLUT-3) glycolytic enzymes, selected membrane traffic and cytoskeletal proteins were highly abundant and concentrated in the Hermes body. By electron microscope gold antibody labelling, the Golgi trafficking protein TMED7/p27 localized to unstacked flattened cisternae of the Hermes body, as did GLUT-3, the most abundant protein. Its biogenesis was deduced through the mapping of protein expression for all 43 proteins during male germ cell differentiation in the testis. It is at the terminal step 19 of spermiogenesis that the 43 characteristic proteins accumulated in the nascent Hermes body. 相似文献
84.
Stable isotope analysis (SIA) has emerged as a common tool in ecology and has proven especially useful in the study of animal diet, habitat use, movement, and physiology. SIA has been vigorously applied to the study of marine mammals, because most species live in habitats or undergo large migrations/movements that make them difficult to observe. Our review supplies a complete list of published SIA contributions to marine mammal science and highlights informative case examples in four general research areas: (1) physiology and fractionation, (2) foraging ecology and habitat use, (3) ecotoxicology, and (4) historic ecology and paleoecology. We also provide a condensed background of isotopic nomenclature, highlight several physiological considerations important for accurate interpretation of isotopic data, and identify research areas ripe for future growth. Because it is impossible to conduct controlled laboratory experiments on most marine mammal species, future studies in marine mammal ecology must draw on isotopic data collected from other organisms and be cognizant of key assumptions often made in the application of SIA to the study of animal ecology. The review is designed to be accessible to all audiences, from students unfamiliar with SIA to those who have utilized it in published studies. 相似文献
85.
GUILLAUME TCHERKEZ RUDI SCHÄUFELE SALVADOR NOGUÉS CLÉMENT PIEL ARNOUD BOOM GARY LANIGAN CÉCILE BARBAROUX CATARINA MATA SLIMAN ELHANI DEBBIE HEMMING CHRISTINA MAGUAS DAN YAKIR FRANZ W. BADECK HOWARD GRIFFITHS HANS SCHNYDER JALEH GHASHGHAIE 《Plant, cell & environment》2010,33(6):900-913
While there is currently intense effort to examine the 13C signal of CO2 evolved in the dark, less is known on the isotope composition of day‐respired CO2. This lack of knowledge stems from technical difficulties to measure the pure respiratory isotopic signal: day respiration is mixed up with photorespiration, and there is no obvious way to separate photosynthetic fractionation (pure ci/ca effect) from respiratory effect (production of CO2 with a different δ13C value from that of net‐fixed CO2) at the ecosystem level. Here, we took advantage of new simple equations, and applied them to sunflower canopies grown under low and high [CO2]. We show that whole mesocosm‐respired CO2 is slightly 13C depleted in the light at the mesocosm level (by 0.2–0.8‰), while it is slightly 13C enriched in darkness (by 1.5–3.2‰). The turnover of the respiratory carbon pool after labelling appears similar in the light and in the dark, and accordingly, a hierarchical clustering analysis shows a close correlation between the 13C abundance in day‐ and night‐evolved CO2. We conclude that the carbon source for respiration is similar in the dark and in the light, but the metabolic pathways associated with CO2 production may change, thereby explaining the different 12C/13C respiratory fractionations in the light and in the dark. 相似文献
86.
Susan E. Crow Christopher W. Swanston Kate Lajtha J. Renée Brooks Heath Keirstead 《Biogeochemistry》2007,85(1):69-90
Soil organic matter (SOM) is often separated by physical means to simplify a complex matrix into discrete fractions. A frequent
approach to isolating two or more fractions is based on differing particle densities and uses a high density liquid such as
sodium polytungstate (SPT). Soil density fractions are often interpreted as organic matter pools with different carbon (C)
turnover times, ranging from years to decades or centuries, and with different functional roles for C and nutrient dynamics.
In this paper, we discuss the development and mechanistic basis of common density-based methods for dividing soil into distinct
organic matter fractions. Further, we directly address the potential effects of dispersing soil in a high density salt solution
on the recovered fractions and implications for data interpretation. Soil collected from forested sites at H. J. Andrews Experimental
Forest, Oregon and Bousson Experimental Forest, Pennsylvania was separated into light and heavy fractions by floatation in
a 1.6 g cm−3 solution of SPT. Mass balance calculations revealed that between 17% and 26% of the original bulk soil C and N content was
mobilized and subsequently discarded during density fractionation for both soils. In some cases, the light isotope was preferentially
mobilized during density fractionation. During a year-long incubation, mathematically recombined density fractions respired
∼40% less than the bulk soil at both sites and light fraction (LF) did not always decompose more than the heavy fraction (HF).
Residual amounts of tungsten (W) present even in well-rinsed fractions were enough to reduce microbial respiration by 27%
compared to the control in a 90-day incubation of Oa material. However, residual W was nearly eliminated by repeated leaching over the year-long incubation, and is not likely
the primary cause of the difference in respiration between summed fractions and bulk soil. Light fraction at Bousson, a deciduous
site developed on Alfisols, had a radiocarbon-based mean residence time (MRT) of 2.7 or 89 years, depending on the interpretation
of the radiocarbon model, while HF was 317 years. In contrast, both density fractions from H. J. Andrews, a coniferous site
developed on andic soils, had approximately the same MRT (117 years and 93 years for LF and HF). At H. J. Andrews the organic
matter lost during density separation had a short MRT (19 years) and can account for the difference in respired CO2 between the summed fractions and the bulk soil. Recognition and consideration of the effects of the density separation procedure
on the recovered fractions will help prevent misinterpretation and deepen our understanding of the specific role of the recovered
organic matter fractions in the ecological context of the soil studied. 相似文献
87.
Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds 总被引:4,自引:0,他引:4
When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation.
While starvation is known to induce enrichment of 15N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments
to investigate effects of a 50–55% reduction in food intake on δ15N and δ13C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to
nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in 15N and 13C compared to food-restricted chicks. Our results show that 15N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen
stable isotopes (Δ15N) need to be considered in stable isotope studies. The decrease in δ13C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from 13C-depleted lipids into proteins. Effects of nutritional restriction on δ15N and δ13C values were relatively small in both experiments (δ15N: 0.77 and 0.41‰, δ13C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of
carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological
processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these
effects to avoid drawing spurious conclusions. 相似文献
88.
Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to
advanced growing systems, studies have shown little or even negative effects on the C content of the extremely sandy soils
of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest management
on soil organic carbon (SOC) sequestration. This study was conducted to examine the C profile in a 4-year-old loblolly pine
(Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil
organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000–250 μm, 250–150 μm,
150–53 μm and <53 μm). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure
and water-soluble SOC components such as esters and amides and did not affect the N distribution. Diffuse Reflectance Infrared
Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which
showed the presence of recently added organic matter in the largest sand fraction, as well as more decomposed organic matter
in the <53 μm fraction. Intensive forest management reduced SOC in all three 2000–53 μm fractions, most likely due to reduced
root input of understory plants that were controlled using herbicides. The 2000–250 μm fractions contained nearly half of
the total SOC and showed a 23% decrease in C content due to the intensive management regime. Results from this study indicated
the significance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reductions in
SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and
the mechanisms responsible.
Responsible Editor: Barbara Wick 相似文献
89.
In spite of the rapid advances in the development of the new proteomic technologies, there are, to date, relatively fewer studies aiming to explore the neuronal proteome. One of the reasons is the complexity of the brain, which presents high cellular heterogeneity and a unique subcellular compartmentalization. Therefore, tissue fractionation of the brain to enrich proteins of interest will reduce the complexity of the proteomics approach leading to the production of manageable and meaningful results. In this review, general considerations and strategies of proteomics, the advantages and challenges to exploring the neuronal proteome are described and summarized. In addition, this article presents an overview of recent advances of proteomic technologies and shows that proteomics can serve as a valuable tool to globally explore the changes in brain proteome during various disease states. Understanding the molecular basis of brain function will be extremely useful in identifying novel targets for the treatment of brain diseases. 相似文献
90.
The partitioning of bacterial cells in a dual aqueous-solvent phase system leads to separation into 'hydrophilic' and hydrophobic functions. Sequential multistep partitioning, accompanied by successive enrichment, gives rise to several cycles of hydrophobic and hydrophilic cell populations which possess different cell-surface hydrophobicity characteristics. Characterization of the cell-surface hydrophobicity by several methods (salting-out aggregation test, bacterial adherence to hydrocarbon, polystyrene binding and hydrophobic interaction chromatography) was carried out. The cell-surface hydrophobicity varied in the order: hydrophilic fraction < parental strain < first cycle hydrophobic variant < second cycle hydrophobic variant < third cycle hydrophobic variant. Electron microscopy showed that the most hydrophobic variant was densely covered by hydrophobic structures - fimbriae - whereas the parental strain was covered by a mixture of surface structures. The hydrophilic variant was covered by an amorphous exopolymeric substance, which is a polysaccharide, shown by its reaction with Alcian blue. 相似文献