全文获取类型
收费全文 | 7188篇 |
免费 | 646篇 |
国内免费 | 2篇 |
出版年
2024年 | 11篇 |
2023年 | 34篇 |
2022年 | 62篇 |
2021年 | 206篇 |
2020年 | 93篇 |
2019年 | 122篇 |
2018年 | 158篇 |
2017年 | 132篇 |
2016年 | 237篇 |
2015年 | 464篇 |
2014年 | 492篇 |
2013年 | 475篇 |
2012年 | 670篇 |
2011年 | 638篇 |
2010年 | 366篇 |
2009年 | 309篇 |
2008年 | 477篇 |
2007年 | 464篇 |
2006年 | 415篇 |
2005年 | 376篇 |
2004年 | 339篇 |
2003年 | 355篇 |
2002年 | 294篇 |
2001年 | 56篇 |
2000年 | 36篇 |
1999年 | 72篇 |
1998年 | 67篇 |
1997年 | 46篇 |
1996年 | 43篇 |
1995年 | 37篇 |
1994年 | 26篇 |
1993年 | 32篇 |
1992年 | 27篇 |
1991年 | 19篇 |
1990年 | 22篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1971年 | 6篇 |
排序方式: 共有7836条查询结果,搜索用时 203 毫秒
21.
In the UK several quantitative methods exist for the examination of bivalve molluscan shellfish for sewage contamination. These methods include roll tubes, pour plates and most probable number (MPN) techniques, but there is no national standard method. A comparative study was made of the most commonly used methods for detection of Escherichia coli in bivalve shellfish. Schemes employing solid media, such as the roll tube and pour plate methods, underestimated faecal contamination in shellfish tissue compared with a liquid MPN multiple test-tube method using minerals-modified-glutamate broth (MMGB) as primary enrichment medium. The composition of MMGB apparently permits repair of sublethally injured cells of E. coli. Incorporation of resuscitation stages into the pour plate technique did not yield higher counts. A standardized MPN technique for examination of bivalve molluscan shellfish for E. coli content is proposed as a possible national reference procedure pending further collaborative assessment. 相似文献
22.
Sex ratio and intrasexual kin competition in mammals 总被引:1,自引:0,他引:1
Summary Asymmetries in both intrasexual competition and generation overlap occur in Antechinus (Dasyuridae; Marsupialia). We show that the range of interpopulation variation in the sex ratio of pouch young spans and exceeds the range of sex ratios at birth hitherto recorded from eutherians (Clutton-Brock and Albon 1983). Although postweaning dispersal and male mortality are similar among all Antechinus populations, interpopulation variance in female longevity leads to variable duration of mother/daughter interaction. As this duration increases, parental investment is increasingly biased toward males, supporting the view that local competition among female kin for resources may influence mammalian sex ratios. 相似文献
23.
Solubilization of an Adenosine Uptake Site in Brain 总被引:1,自引:1,他引:0
Procedures are described for the solubilization of adenosine uptake sites in guinea pig and rat brain tissue. Using [3H]nitrobenzylthioinosine [( 3H]NBI) the solubilized site is characterized both kinetically and pharmacologically. The binding is dependent on protein concentration and is saturable, reversible, specific, and high affinity in nature. The KD and Bmax of guinea pig extracts are 0.13 +/- 0.02 nM and 133 +/- 18 fmol/mg protein, respectively, with linear Scatchard plots obtained routinely. Similar kinetic parameters are observed in rat brain. Adenosine uptake inhibitors are the most potent inhibitors of [3H]NBI binding with the following order of potency, dilazep greater than hexobendine greater than dipyridamole. Adenosine receptor ligands are much less potent inhibitors of binding, and caffeine is without effect. The solubilized adenosine uptake site is, therefore, shown to have virtually identical properties to the native membrane site. The binding of the adenosine A1 receptor agonist [3H]cyclohexyladenosine [( 3H]CHA) to the solubilized brain extract was also studied and compared with that of [3H]NBI. In contrast to the [3H]NBI binding site [3H]CHA binds to two apparent populations of adenosine receptor, a high-affinity site with a KD of 0.32 +/- 0.06 nM and a Bmax of 105 +/- 30 fmol/mg protein and a lower-affinity site with a KD of 5.50 +/- 0.52 nM and Bmax of 300 +/- 55 fmol/mg protein. The pharmacology of the [3H]CHA binding site is consistent with that of the adenosine receptor and quite distinct from that of the uptake [( 3H]NBI binding) site. Therefore, we show that the adenosine uptake site can be solubilized and that it retains both its binding and pharmacologic properties in the solubilized state. 相似文献
24.
25.
Michelle Dequard-Chablat Evelyne Coppin-Raynal 《Molecular & general genetics : MGG》1984,195(1-2):294-299
Summary
AS7-1 and AS7-2 are antisuppressor mutations reducing the miscoding capacity of ribosomes. Strains carrying and AS7 mutation do not sporulate. We have investigated whether the sporulation deficiency is due to the decrease of translational ambiguity. Two major findings argue in favour of this assumption. First, a significant sporulation level is restored in the presence of paromomycin. Second, three mutations which restore the sporulation of AS7-2 increase the ribosomal misreading in vitro. They define two new loci for ribosomal suppressors, su11 and su12. The two ribosomal proteins altered by su11-1 and su12-1 have been identified by electrophoresis. The results are discussed in the context of a more general hypothesis proposed by Picard-Bennoun (1982). 相似文献
26.
B. A. Prior C. P. Kenyon ‡ Michelle van der Veen J. P. Mildenhall 《Journal of applied microbiology》1987,62(2):119-128
When Pseudomonas fluorescens was grown in a glucose salts medium adjusted with NaCl to a water activity (aw ) value of 0.980, the intracellular glutamic acid concentration increased 23-fold and comprised 90% of the total amino acid pool. This increase was not observed when the aw of the medium was reduced to 0.980 with sorbitol. Sorbitol was taken up rapidly over a 30 min period and accumulated intracellularly to a level approximately two-fold greater than the concentration in the growth medium. In continuous culture, the specific rate of glutamic acid production and glucose uptake was greater at 0.980 (NaCl) than at 0.997 aw . The maintenance coefficients for glucose uptake were similar at both aw values but were 2.4-fold greater for glutamic acid production at 0.980 (NaCl) than at 0.997 aw . 相似文献
27.
The responses of ship-fouling and non-fouling isolates of Enteromorpha compressa (L.) Grev. have been compared in media containing copper at 0.0?9.6 μmol · dm?3. The responses of each isolate were found to vary, according to the conditions of the original habitat. Thus ship-fouling E. compressa was found to be tolerant of copper concentrations up to 9.6 μmol · dm?3 showing a maintenance of all of the physiological processes studied during the present research (cell viability, net photosynthesis, intracellular K+ and dimethylsulphoniopropionate content). Non-fouling plant material showed symptoms of copper toxicity at all levels of copper from 1.8 μmol · dm?3 to 9.6 μmol · dm?3. Copper tolerance in ship-fouling E. compressa appears to be genetically determined, since the progeny from ship-fouling plants are also tolerant to copper concentrations within the range 1.8 to 9.6 μmol · dm?3. The rate of accumulation of copper in ship-fouling thalli is, however, almost identical to that of non-fouling thalli, suggesting that tolerance may be due primarily to internal detoxification, rather than an exclusion mechanism. 相似文献
28.
Preliminary characterization of Thy-1.1 and Ag-B antigens from rat tissues solubilized in detergents 总被引:12,自引:4,他引:8
Michelle Letarte-Muirhead Ronald T. Acton Alan F. Williams 《The Biochemical journal》1974,143(1):51-61
1. A radioactive binding assay for Thy-1.1 alloantigen which functions in the presence of detergents was established by using glutaraldehyde-fixed thymocytes as target cells. Thy-1.1 activity in detergent extracts was then assayed by measuring inhibition of the binding assay. 2. Solubilization of Thy-1.1 from whole thymocytes, and their membranes by a large number of non-ionic detergents and deoxycholate was studied. In the same extracts Ag-B(4) histocompatibility antigenic activities were measured. With the exception of Nonidet P-40, the detergents did not affect the antigenicity of Thy-1.1, but only Lubrol-PX and deoxycholate gave effective solubilization as measured by activity remaining in the supernatant after centrifugation at 200000g for 40min. With Ag-B(4) antigen, Triton X-100, Triton X-67 and Nonidet P-40 gave effective solubilization as well as Lubrol-PX and deoxycholate. Solubilization of Thy-1.1 activity from leukaemia cells and a brain homogenate was also studied, but none of the non-ionic detergents gave satisfactory results with these tissues. 3. Extracts from thymocyte membranes were further examined by gel filtration and sucrose gradient centrifugation. The Thy-1.1 activity behaved as a single component in deoxycholate with a density similar to that of a globular protein, but in Lubrol-PX the antigen was contained in a low-density complex. In Lubrol-PX extracts Ag-B(4) was also found in aggregates not observed in deoxycholate. 4. The s(20,w) values for Thy-1.1 and Ag-B(4) antigens in deoxycholate were 2.4 and 4.4, and v values were 0.70 and 0.75 respectively. The Stokes radius observed for Thy-1.1 was 3.1nm and for Ag-B(4) 5.3nm. By using these values the molecular weights for the antigen-detergent complexes were calculated to be 28000 for Thy-1.1 and 100000 for Ag-B(4). 相似文献
29.
Jack A. Grebb Jean-Antoine Girault† Michelle Ehrlich‡ Paul Greengard 《Journal of neurochemistry》1990,55(1):204-207
DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein, Mr = 32,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) is a neuronal phosphoprotein that is enriched in neurons which possess dopamine D1 receptors, particularly striatonigral neurons. In rat brain slices, the phosphorylation state of DARPP-32 is regulated by dopamine, acting through the dopamine D1 receptor and the adenylyl cyclase system. This study reports that chronic blockade (21 days) of either dopamine D1 receptors by SCH-23390 or dopamine D2 receptors by raclopride does not affect the concentrations of DARPP-32 in specific rat brain regions (striatum, thalamus, hippocampus, frontal cerebral cortical pole). Northern blot analysis indicates that the steady-state level of DARPP-32 mRNA in striatum is also unchanged by these treatments. 相似文献
30.
The distribution of hyaluronate-binding activity was determined in the soluble and membrane fractions derived from adult mouse brain by sonication in low-ionic-strength buffer. Approximately 60% of the total activity was recovered in the soluble fraction and 33% in membrane fractions. In both cases, the hyaluronate-binding activities were found to be of high affinity (KD = 10(-9) M), specific for hyaluronate, and glycoprotein in nature. Most of the hyaluronate-binding activity from the soluble fraction chromatographed in the void volume of Sepharose CL-4B and CL-6B. Approximately 50% of this activity was highly negatively charged, eluting from diethylaminoethyl (DEAE)-cellulose in 0.5 M NaCl, and contained chondroitin sulfate chains. This latter material also reacted with antibodies raised against cartilage link protein and the core protein of cartilage proteoglycan. Thus, the binding and physical characteristics of this hyaluronate-binding activity are consistent with those of a chondroitin sulfate proteoglycan aggregate similar to that found in cartilage. A 500-fold purification of this proteoglycan-like, hyaluronate-binding material was achieved by wheat germ agglutinin affinity chromatography, molecular sieve chromatography on Sepharose CL-6B, and ion exchange chromatography on DEAE-cellulose. Another class of hyaluronate-binding material (25-50% of that recovered) eluted from DEAE with 0.24 M NaCl; this material had the properties of a complex glycoprotein, did not contain chondroitin sulfate, and did not react with the antibodies against cartilage link protein and proteoglycan. Thus, adult mouse brain contains at least three different forms of hyaluronate-binding macromolecules. Two of these have properties similar to the link protein and proteoglycan of cartilage proteoglycan aggregates; the third is distinguishable from these entities. 相似文献