全文获取类型
收费全文 | 93096篇 |
免费 | 266篇 |
国内免费 | 884篇 |
专业分类
94246篇 |
出版年
2023年 | 2篇 |
2022年 | 5篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 11842篇 |
2017年 | 10674篇 |
2016年 | 7450篇 |
2015年 | 586篇 |
2014年 | 283篇 |
2013年 | 293篇 |
2012年 | 4199篇 |
2011年 | 12798篇 |
2010年 | 11995篇 |
2009年 | 8220篇 |
2008年 | 9775篇 |
2007年 | 11355篇 |
2006年 | 250篇 |
2005年 | 503篇 |
2004年 | 959篇 |
2003年 | 1014篇 |
2002年 | 777篇 |
2001年 | 253篇 |
2000年 | 159篇 |
1999年 | 23篇 |
1998年 | 9篇 |
1997年 | 24篇 |
1996年 | 11篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 37篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1984年 | 7篇 |
1983年 | 17篇 |
1982年 | 3篇 |
1975年 | 5篇 |
1972年 | 246篇 |
1971年 | 274篇 |
1970年 | 5篇 |
1969年 | 1篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Shunhua Ding Qingtao Lu Yan Zhang Zhipan Yang Xiaogang Wen Lixin Zhang Congming Lu 《Plant molecular biology》2009,69(5):577-592
To investigate the possible mechanisms of glutathione reductase (GR) in protecting against oxidative stress, we obtained transgenic
tobacco (Nicotiana tabacum) plants with 30–70% decreased GR activity by using a gene encoding tobacco chloroplastic GR for the RNAi construct. We investigated
the responses of wild type and transgenic plants to oxidative stress induced by application of methyl viologen in vivo. Analyses
of CO2 assimilation, maximal efficiency of photosystem II photochemistry, leaf bleaching, and oxidative damage to lipids demonstrated
that transgenic plants exhibited enhanced sensitivity to oxidative stress. Under oxidative stress, there was a greater decrease
in reduced to oxidized glutathione ratio but a greater increase in reduced glutathione in transgenic plants than in wild type
plants. In addition, transgenic plants showed a greater decrease in reduced ascorbate and reduced to oxidized ascorbate ratio
than wild type plants. However, there were neither differences in the levels of NADP and NADPH and in the total foliar activities
of monodehydroascorbate reductase and dehydroascorbate reductase between wild type and transgenic plant. MV treatment induced
an increase in the activities of GR, ascorbate peroxidase, superoxide dismutase, and catalase. Furthermore, accumulation of
H2O2 in chloroplasts was observed in transgenic plants but not in wild type plants. Our results suggest that capacity for regeneration
of glutathione by GR plays an important role in protecting against oxidative stress by maintaining ascorbate pool and ascorbate
redox state. 相似文献
942.
This study examines the impact of the exotic nitrogen-fixing legume Melilotus officinalis (L.) Lam. on native and exotic species cover in two Great Plains ecosystems in Badlands National Park, South Dakota. Melilotus is still widely planted and its effects on native ecosystems are not well studied. Melilotus could have direct effects on native plants, such as through competition or facilitation. Alternatively, Melilotus may have indirect effects on natives, e.g., by favoring exotic species which in turn have a negative effect on native species.
This study examined these interactions across a 4-year period in two contrasting vegetation types: Badlands sparse vegetation
and western wheatgrass (Pascopyrum smithii) mixed-grass prairie. Structural equation models were used to analyze the pathways through which Melilotus, native species, and other exotic species interact over a series of 2-year time steps. Melilotus can affect native and exotic species both in the current year and in the years after its death (a lag effect). A lag effect
is possible because the death of a Melilotus plant can leave an open, potentially nitrogen-enriched site on the landscape. The results showed that the relationship between
Melilotus and native and exotic species varied depending on the habitat and the year. In Badlands sparse vegetation, there was a consistent,
strong, and positive relationship between Melilotus cover and native and exotic species cover suggesting that Melilotus is acting as a nurse plant and facilitating the growth of other species. In contrast, in western wheatgrass prairie, Melilotus was acting as a weak competitor and had no consistent effect on other species. In both habitats, there was little evidence
for a direct lag effect of Melilotus on other species. Together, these results suggest both facilitative and competitive roles for Melilotus, depending on the vegetation type it invades. 相似文献
943.
Siddhartha Sankar Mishra Sankar Kumar Acharjee Susanta Kumar Chakraborty 《Environmental Biology of Fishes》2009,84(4):395-407
Twenty-five species of fishes belonging to 18 genera, 7 families under the order siluriformes have been recorded from different
freshwater and brackish water wetlands of South West Bengal, India. Fishery potential of 17 commercially important siluroid
fishes were noted especially from market surveys as well as from information collected from local fishermen communities. Following
the IUCN guidelines, and also through Participatory Rural Appraisal (P.R.A.) methods, an attempt has been made to assess the
conservation status of such fishes, many of which are thought to be threatened. Trend analysis, Fish Magnitude Value and Rank
based Quotient revealed the threatened status of 17 siluroid fishes. 相似文献
944.
Ramachandra M. Bhaskara C. M. Brijesh Saveer Ahmed Renee M. Borges 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2009,195(4):409-417
The perception of ultraviolet (UV) light by spiders has so far been only demonstrated in salticids. Crab spiders (Thomisidae)
hunt mostly on flowers and need to find appropriate hunting sites. Previous studies have shown that some crab spiders that
reflect UV light use UV contrast to enhance prey capture. The high UV contrast can be obtained either by modulation of body
colouration or active selection of appropriate backgrounds for foraging. We show that crab spiders (Thomisus sp.) hunting on Spathiphyllum plants use chromatic contrast, especially UV contrast, to make themselves attractive to hymenopteran prey. Apart from that,
they are able to achieve high UV contrast by active selection of non-UV reflecting surfaces when given a choice of UV-reflecting
and non-UV reflecting surfaces in the absence of odour cues. Honeybees (Apis cerana) approached Spathiphyllum plants bearing crab spiders on which the spiders were high UV-contrast targets with greater frequency than those plants on
which the UV contrast of the spiders was low. Thus, crab spiders can perceive UV and may use it to choose appropriate backgrounds
to enhance prey capture, by exploiting the attraction of prey such as honeybees to UV. 相似文献
945.
During an adaptive immune response, lymphocytes proliferate for five to twenty-five cell divisions, then stop and die over
a period of weeks. Based on extensive flow cytometry data, Hawkins et al. (Proc Natl Acad Sci USA 104:5032–5037, 2007) introduced
a cell-level stochastic model of lymphocyte population dynamics, called the Cyton Model, that accurately captures mean lymphocyte
population size as a function of time. In Subramanian et al. (J Math Biol 56(6):861–892, 2008), we performed a branching process
analysis of the Cyton Model and deduced from parameterizations for in vitro and in vivo data that the immune response is predictable
despite each cell’s fate being highly variable. One drawback of flow cytometry data is that individual cells cannot be tracked,
so that it is not possible to investigate dependencies in the fate of cells within family trees. In the absence of this information,
while the Cyton Model abandons one of the usual assumptions of branching processes (the independence of lifetime and progeny
number), it adopts another of the standard branching processes hypotheses: that the fates of progeny are stochastically independent.
However, new experimental observations of lymphocytes show that the fates of cells in the same family tree are not stochastically
independent. Hawkins et al. (2008, submitted) report on ciné lapse photography experiments where every founding cell’s family
tree is recorded for a system of proliferating lymphocytes responding to a mitogenic stimulus. Data from these experiments
demonstrate that the death-or-division fates of collaterally consanguineous cells (those in the same generation within a founding
cell’s family tree) are strongly correlated, while there is little correlation between cells of distinct generations and between
cells in distinct family trees. As this finding contrasts with one of the assumptions of the Cyton Model, in this paper we
introduce three variants of the Cyton Model with increasing levels of collaterally consanguineous correlation structure to
incorporate these new found dependencies. We investigate their impact on the predicted expected variability of cell population
size. Mathematically we conclude that while the introduction of correlation structure leaves the mean population size unchanged
from the Cyton Model, the variance of the population size distribution is typically larger. Biologically, through comparison
of model predictions for Cyton Model parameterizations determined by in vitro and in vivo experiments, we deduce that if collaterally
consanguineous correlation extends beyond cousins, then the immune response is less predictable than would be concluded from
the original Cyton Model. That is, some of the variability seen in data that we previously attributed to experimental error
could be due to intrinsic variability in the cell population size dynamics.
相似文献
946.
Jennifer L. Bankers-Fulbright Kathleen R. Bartemes Gail M. Kephart Hirohito Kita Scott M. O’Grady 《The Journal of membrane biology》2009,228(2):99-109
Human eosinophils spontaneously adhere to various substrates in the absence of exogenously added activators. In the present
study a method was developed for characterizing eosinophil adhesion by measuring changes in impedance. Impedance measurements
were performed in HCO3-buffered HybriCare medium maintained in a humidified 5% CO2 incubator at 37°C. Impedance increased by more than 1 kΩ within minutes after eosinophils made contact with the substrate,
reaching a peak within 20 min. Blocking mobilization of intracellular [Ca2+] that precedes adhesion with BAPTA-AM (10 μM) completely inhibited the rise in impedance as well as the changes in cell shape
typically observed in adherent cells. However, lowering the extracellular [Ca2+] with 2.5 mM EGTA did not inhibit the increase in impedance. Pretreatment with anti-CD18 antibody to block substrate interactions
with β2-integrins, or jasplakinolide (2 μM) to block actin reorganization, abolished the increase in impedance and adherent morphology
of the cells. Exposure of eosinophils to the phosphatidylinositol 3 kinase inhibitor LY294002 (5 μM) or treatment with protein
kinase C zeta pseudosubstrate to competitively inhibit activity of the enzyme significantly reduced the increase in impedance
and inhibited the cell spreading associated with adhesion. These results demonstrate a novel method for measuring eosinophil
adhesion and showed that, following formation of a tethered attachment, a rapid increase in intracellular [Ca2+] precedes the cytoskeletal rearrangements required for cell shape changes and plasma membrane-substrate interactions associated
with adhesion. 相似文献
947.
Dale E. Tronrud Jianzhong Wen Leslie Gay Robert E. Blankenship 《Photosynthesis research》2009,100(2):79-87
The absorbance spectrum of the Fenna-Matthews-Olson protein—a component of the antenna system of Green Sulfur Bacteria—is always one of two types, depending on the species of the source organism. The FMO from Prosthecochloris aestuarii 2K has a spectrum of type 1 while that from Chlorobaculum tepidum is of type 2. The previously reported crystal structures for these two proteins did not disclose any rationale that would explain their spectral differences. We have collected a 1.3 Å X-ray diffraction dataset of the FMO from Prosthecochloris aestuarii 2K, which has allowed us to identify an additional Bacteriochlorophyll-a molecule with chemical attachments to both sides of the central magnesium atom. A new analysis of the previously published X-ray data for the Chlorobaculum tepidum FMO shows the presence of a Bacteriochlorophyll-a molecule in an equivalent location but with a chemical attachment from only one side. This difference in binding is shown to be predictive of the spectral type of the FMO. 相似文献
948.
Low-temperature (77 K) steady-state fluorescence emission spectroscopy and dynamic light scattering were applied to the main
chlorophyll a/b protein light harvesting complex of photosystem II (LHC II) in different aggregation states to elucidate the mechanism of
fluorescence quenching within LHC II oligomers. Evidences presented that LHC II oligomers are heterogeneous and consist of
large and small particles with different fluorescence yield. At intermediate detergent concentrations the mean size of the
small particles is similar to that of trimers, while the size of large particles is comparable to that of aggregated trimers
without added detergent. It is suggested that in small particles and trimers the emitter is monomeric chlorophyll, whereas
in large aggregates there is also another emitter, which is a poorly fluorescing chlorophyll associate. A model, describing
populations of antenna chlorophyll molecules in small and large aggregates in their ground and first singlet excited states,
is considered. The model enables us to obtain the ratio of the singlet excited-state lifetimes in small and large particles,
the relative amount of chlorophyll molecules in large particles, and the amount of quenchers as a function of the degree of
aggregation. These dependencies reveal that the quenching of the chl a fluorescence upon aggregation is due to the formation of large aggregates and the increasing of the amount of chlorophyll
molecules forming these aggregates. As a consequence, the amount of quenchers, located in large aggregates, is increased,
and their singlet excited-state lifetimes steeply decrease. 相似文献
949.
Ruxandra F. Sîrbulescu Iulian Ilieş Günther K. H. Zupanc 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2009,195(7):699-714
In contrast to mammals, teleost fish exhibit an enormous potential to regenerate adult spinal cord tissue after injury. However,
the mechanisms mediating this ability are largely unknown. Here, we analyzed the major processes underlying structural and
functional regeneration after amputation of the caudal portion of the spinal cord in Apteronotus leptorhynchus, a weakly electric teleost. After a transient wave of apoptotic cell death, cell proliferation started to increase 5 days
after the lesion and persisted at high levels for at least 50 days. New cells differentiated into neurons, glia, and ependymal
cells. Retrograde tract tracing revealed axonal re-growth and innervation of the regenerate. Functional regeneration was demonstrated
by recovery of the amplitude of the electric organ discharge, a behavior generated by spinal motoneurons. Computer simulations
indicated that the observed rates of apoptotic cell death and cell proliferation can adequately explain the re-growth of the
spinal cord.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
950.
Mass mortalities of larval cultures of Chilean scallop Argopecten purpuratus have repeatedly occurred in northern Chile, characterized by larval agglutination and accumulation in the bottom of rearing
tanks. The exopolysaccharide slime (EPS) producing CAM2 strain was isolated as the primary organism from moribund larvae in
a pathogenic outbreak occurring in a commercial hatchery producing larvae of the Chilean scallop Argopecten purpuratus located in Bahía Inglesa, Chile. The CAM2 strain was characterized biochemically and was identified by polymerase chain reaction
amplification of 16S rRNA as Halomonas sp. (Accession number DQ885389.1). Healthy 7-day-old scallop larvae cultures were experimentally infected for a 48-h period
with an overnight culture of the CAM2 strain at a final concentration of ca. 105 cells per milliliter, and the mortality and vital condition of larvae were determined by optical and scanning electron microscopy
(SEM) to describe the chronology of the disease. Pathogenic action of the CAM2 strain was clearly evidenced by SEM analysis,
showing a high ability to adhere and detach larvae velum cells by using its “slimy” EPS, producing agglutination, loss of
motility, and a posterior sinking of scallop larvae. After 48 h, a dense bacterial slime on the shell surface was observed,
producing high percentages of larval agglutination (63.28 ± 7.87%) and mortality (45.03 ± 4.32%) that were significantly (P < 0.05) higher than those of the unchallenged control cultures, which exhibited only 3.20 ± 1.40% dead larvae and no larval
agglutination. Furthermore, the CAM2 strain exhibited a high ability to adhere to fiberglass pieces of tanks used for scallop
larvae rearing (1.64 × 105 cells adhered per square millimeters at 24 h postinoculation), making it very difficult to eradicate it from the culture
systems. This is the first report of a pathogenic activity on scallop larvae of Halomonas species, and it prompts the necessity of an appraisal on biofilm-producing bacteria in Chilean scallop hatcheries. 相似文献