全文获取类型
收费全文 | 570篇 |
免费 | 74篇 |
出版年
2019年 | 3篇 |
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 9篇 |
2013年 | 16篇 |
2012年 | 16篇 |
2011年 | 22篇 |
2010年 | 8篇 |
2009年 | 10篇 |
2008年 | 12篇 |
2007年 | 33篇 |
2006年 | 7篇 |
2005年 | 12篇 |
2004年 | 14篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 15篇 |
2000年 | 20篇 |
1999年 | 25篇 |
1998年 | 8篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 19篇 |
1986年 | 14篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 14篇 |
1981年 | 14篇 |
1980年 | 13篇 |
1979年 | 24篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 11篇 |
1972年 | 10篇 |
1971年 | 11篇 |
1970年 | 10篇 |
1969年 | 12篇 |
1968年 | 4篇 |
1967年 | 6篇 |
1966年 | 6篇 |
1965年 | 3篇 |
排序方式: 共有644条查询结果,搜索用时 31 毫秒
101.
Radioactive metabolites present in bone, blood, liver, and feces of rats given (3)H vitamin D(3) have been isolated. Of these the aqueous soluble metabolite(s) from tissue and all those isolated from feces did not cure rickets in rats, while all the others were at least partially active in this regard. One of the metabolites proved to be as active as the parent vitamin in curing rickets and was found in large amounts in liver, blood, and bone. As much as 50-80% of the radioactivity in bone was found in this metabolite after a 500 IU oral dose of (3)H vitamin D(3). With 10 IU doses of 1,2-(3)H vitamin D(3), most of the radioactivity of the organs examined was found in this metabolite fraction. This metabolite appears to be more polar than vitamin D and is not an esterified form of the vitamin nor a complex of the vitamin with tissue lipids. Its possible role as the metabolically active form of the vitamin is discussed. 相似文献
102.
A Emami-Khoyi DA Hartley RH Cruickshank LJ Boren JG Ross 《New Zealand journal of zoology.》2016,43(4):322-335
New Zealand fur seals are one of many pinniped species that survived the commercial sealing of the eighteenth and nineteenth centuries in dangerously low numbers. After the enforcement of a series of protection measures in the early twentieth century, New Zealand fur seals began to recover from the brink of extinction. We examined the New Zealand fur seal populations of Banks Peninsula, South Island, New Zealand using the mitochondrial DNA control region. We identified a panmictic population structure around Banks Peninsula. The most abundant haplotype in the area showed a slight significant aggregated structure. The Horseshoe Bay colony showed the least number of shared haplotypes with other colonies, suggesting a different origin of re-colonisation of this specific colony. The effective population size of the New Zealand fur seal population at Banks Peninsula was estimated at approximately 2500 individuals. The exponential population growth rate parameter for the area was 35, which corresponds to an expanding population. In general, samples from adjacent colonies shared 4.4 haplotypes while samples collected from colonies separated by between five and eight bays shared 1.9 haplotypes. The genetic data support the spill-over dynamics of colony expansion already suggested for this species. Approximate Bayesian computations analysis suggests re-colonisation of the area from two main clades identified across New Zealand with a most likely admixture coefficient of 0.41 to form the Banks Peninsula population. Approximate Bayesian computations analysis estimated a founder population size of approximately 372 breeding individuals for the area, which then rapidly increased in size with successive waves of external recruitment. The population of fur seals in the area is probably in the late phase of maturity in the colony expansion dynamic. 相似文献
103.
104.
Purification and characterization of Dolichos lablab lectin 总被引:1,自引:0,他引:1
The mannose/glucose-binding Dolichos lablab lectin (designated DLL) has
been purified from seeds of Dolichos lablab (hyacinth bean) to
electrophoretic homogeneity by affinity chromatography on an ovalbumin-
Sepharose 4B column. The purified lectin gave a single symmetric protein
peak with an apparent molecular mass of 67 kDa on gel filtration
chromatography, and five bands ranging from 10 kDa to 22 kDa upon SDS-PAGE.
N-Terminal sequence analysis of these bands revealed subunit heterogeneity
due to posttranslational proteolytic truncation at different sites mostly
at the carboxyl terminus. The carbohydrate binding properties of the
purified lectin were investigated by three different approaches:
hemagglutination inhibition assay, quantitative precipitation inhibition
assay, and ELISA. On the basis of these studies, it is concluded that the
Dolichos lablab lectin has neither an extended carbohydrate combining site,
nor a hydrophobic binding site adjacent to it. The carbohydrate combining
site of DLL appears to most effectively accommodate a nonreducing terminal
alpha-d-mannosyl unit, and to be complementary to the C-3, C-4, and C-6
equatorial hydroxyl groups of alpha-d-mannopyranosyl and
alpha-d-glucopyranosyl residues. DLL strongly precipitates murine IgM but
not IgG, and the recent finding that this lectin interacts specifically
with NIH 3T3 fibroblasts transfected with the Flt3 tyrosine kinase receptor
and preserves human cord blood stem cells and progenitors in a quiescent
state for prolonged periods in culture, make this lectin a valuable tool in
biomedical research.
相似文献
105.
106.
Shimizu M Miyamoto Y Takaku H Matsuo M Nakabayashi M Masuno H Udagawa N DeLuca HF Ikura T Ito N 《Bioorganic & medicinal chemistry》2008,16(14):6949-6964
107.
Nitrogen fixation in mixed <Emphasis Type="Italic">Hylocomium splendens</Emphasis> moss communities 总被引:1,自引:1,他引:0
The pleurocarpus feather moss, Hylocomium splendens, is one of two co-dominant moss species in boreal forest ecosystems and one of the most common mosses on earth, yet little
is known regarding its capacity to host cyanobacterial associates and thus contribute total ecosystem N. In these studies,
we evaluated the N-fixation potential of the H. splendens–cyanobacteria association and contrasted the N-fixation activity with that of the putative N-fixing moss–cyanobacteria association
of Pleurozium schreberi. Studies were conducted to: quantify N-fixation in H. splendens and P. schreberi in sites ranging from southern to northern Fennoscandia; assess N and P availability as drivers of N-fixation rates; contrast
season-long N-fixation rates for both mosses; and characterize the cyanobacteria that colonize shoots of H. splendens. Nitrogen-fixation rates were generally low at southern latitudes and higher at northern latitudes (64–69°N) potentially
related to anthropogenic N deposition across this gradient. Nitrogen fixation in H. splendens appeared to be less sensitive to N deposition than P. schreberi. The season-long assessment of N-fixation rates at a mixed feather moss site in northern Sweden showed that H. splendens fixed a substantial quantity of N, but about 50% less total N compared to the contribution from P. schreberi. In total, both species provided 1.6 kg fixed N ha−1 year−1. Interestingly, H. splendens demonstrated somewhat higher N-fixation rates at high fertility sites compared to P. schreberi. Nostoc spp. and Stigonema spp. were the primary cyanobacteria found to colonize H. splendens and P. schreberi. These results suggest that H. splendens with associated Nostoc or Stigonema communities contributes a significant quantity of N to boreal forest ecosystems, but the contribution is subordinate to that
of P. schreberi at northern latitudes. Epiphytic cyanobacteria are likely a key factor determining the co-dominant presence of these two
feather mosses across the boreal biome. 相似文献
108.
Pascale Simard Hugo Galarneau Sébastien Marois Daniel Rusu Caroline D Hoemann Patrice E Poubelle Hani El-Gabalawy Maria JG Fernandes 《Arthritis research & therapy》2009,11(3):R74-10
Introduction
Osteoarthritis is characterized by the progressive destruction of cartilage in the articular joints. Novel therapies that promote resurfacing of exposed bone in focal areas are of interest in osteoarthritis because they may delay the progression of this disabling disease in patients who develop focal lesions. Recently, the addition of 80% deacetylated chitosan to cartilage microfractures was shown to promote the regeneration of hyaline cartilage. The molecular mechanisms by which chitosan promotes cartilage regeneration remain unknown. Because neutrophils are transiently recruited to the microfracture site, the effect of 80% deacetylated chitosan on the function of neutrophils was investigated. Most studies on neutrophils use preparations of chitosan with an uncertain degree of deacetylation. For therapeutic purposes, it is of interest to determine whether the degree of deacetylation influences the response of neutrophils to chitosan. The effect of 95% deacetylated chitosan on the function of neutrophils was therefore also investigated and compared with that of 80% deacetylated chitosan. 相似文献109.
Background and aims
The feather moss Pleurozium schreberi (Brid.) Mitt. is colonized by cyanobacteria, which fix substantial amounts of atmospheric nitrogen (N) in pristine and N-poor ecosystems. Cyanobacterial N2 fixation is inhibited by N deposition. However, the threshold of N input that leads to the inhibition of N2 fixation has not been adequately investigated. Further, the ability of N2 fixation to recover in mosses from high N deposition areas has not been studied to date.Methods
We conducted two laboratory studies in which we (1) applied a range of concentrations of N as NH4NO3 to mosses from low N-deposition areas, and (2) we deprived mosses from a high N-deposition area of N to test their ability to recover N2 fixation.Results
Higher addition rates (up to 10 kg N ha?1) did not systematically inhibit N2 fixation in P. schreberi. Conversely, upon weeks of N deprivation of mosses from a high N environment, N2 fixation rates increased.Conclusions
The threshold of total N deposition above which N2 fixation in P. schreberi is inhibited is likely to be > 10 kg N ha?1. Further, cyanobacteria are able to recover from high N inputs and are able to fix atmospheric N2 after a period of N deprivation. 相似文献110.
Grubbs J Rahmanian S DeLuca A Padmashali C Jackson M Duff MR Howell EE 《Biochemistry》2011,50(18):3673-3685
Chromosomal dihydrofolate reductase from Escherichia coli catalyzes the reduction of dihydrofolate to tetrahydrofolate using NADPH as a cofactor. The thermodynamics of ligand binding were examined using an isothermal titration calorimetry approach. Using buffers with different heats of ionization, zero to a small, fractional proton release was observed for dihydrofolate binding, while a proton was released upon NADP(+) binding. The role of water in binding was additionally monitored using a number of different osmolytes. Binding of NADP(+) is accompanied by the net release of ~5-24 water molecules, with a dependence on the identity of the osmolyte. In contrast, binding of dihydrofolate is weakened in the presence of osmolytes, consistent with "water uptake". Different effects are observed depending on the identity of the osmolyte. The net uptake of water upon dihydrofolate binding was previously observed in the nonhomologous R67-encoded dihydrofolate reductase (dfrB or type II enzyme) [Chopra, S., et al. (2008) J. Biol. Chem. 283, 4690-4698]. As R67 dihydrofolate reductase possesses a nonhomologous sequence and forms a tetrameric structure with a single active site pore, the observation of weaker DHF binding in the presence of osmolytes in both enzymes implicates cosolvent effects on free dihydrofolate. Consistent with this analysis, stopped flow experiments find betaine mostly affects DHF binding via changes in k(on), while betaine mostly affects NADPH binding via changes in k(off). Finally, nonadditive enthalpy terms when binary and ternary cofactor binding events are compared suggest the presence of long-lived conformational transitions that are not included in a simple thermodynamic cycle. 相似文献