全文获取类型
收费全文 | 29014篇 |
免费 | 3697篇 |
国内免费 | 30篇 |
专业分类
32741篇 |
出版年
2016年 | 247篇 |
2015年 | 315篇 |
2014年 | 399篇 |
2013年 | 666篇 |
2012年 | 636篇 |
2011年 | 687篇 |
2010年 | 484篇 |
2009年 | 403篇 |
2008年 | 588篇 |
2007年 | 645篇 |
2006年 | 611篇 |
2005年 | 614篇 |
2004年 | 620篇 |
2003年 | 635篇 |
2002年 | 647篇 |
2001年 | 1856篇 |
2000年 | 1829篇 |
1999年 | 1435篇 |
1998年 | 473篇 |
1997年 | 472篇 |
1996年 | 480篇 |
1995年 | 422篇 |
1994年 | 426篇 |
1993年 | 420篇 |
1992年 | 1069篇 |
1991年 | 1083篇 |
1990年 | 1065篇 |
1989年 | 1028篇 |
1988年 | 960篇 |
1987年 | 900篇 |
1986年 | 769篇 |
1985年 | 746篇 |
1984年 | 586篇 |
1983年 | 508篇 |
1982年 | 367篇 |
1981年 | 342篇 |
1980年 | 331篇 |
1979年 | 561篇 |
1978年 | 448篇 |
1977年 | 412篇 |
1976年 | 338篇 |
1975年 | 455篇 |
1974年 | 469篇 |
1973年 | 414篇 |
1972年 | 393篇 |
1971年 | 310篇 |
1970年 | 258篇 |
1969年 | 240篇 |
1968年 | 225篇 |
1967年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Locomoting cells are characterized by a pronounced external and internal anterior-posterior polarity. One of the events associated with cell polarization at the onset of locomotion is a shift of the centrosome, or MTOC, ahead of the nucleus. This position is believed to be of strategic importance for directional cell movement and cell polarity. We have used BSC-1 cells at the edge of an in vitro wound to clarify the causal relationship between MTOC position and the initiation of cell polarization. We find that pronounced cell polarization (the extension of a lamellipod) can take place in the absence of MTOC repositioning or microtubules. Conversely, MTOCs will reposition even after lamellar extension and cell polarization have occurred. Repositioning requires microtubules that extend to the cell periphery and is independent of selective detyrosination of microtubules extending towards the cell front. Significantly, MTOCs maintain, or at least attempt to maintain, a position at the cell's centroid. This is most clearly demonstrated in wounded monolayers of enucleated cells where the MTOC closely follows the centroid position. We suggest that the primary response to the would is the biased extension of a lamellipod, which can occur in the absence of microtubules and MTOC repositioning. Lamellipod extension leads to a shift of the cell's centroid towards the wound. The MTOC, in an attempt to maintain a position near the cell center, will follow. This will automatically put the MTOC ahead of the nucleus in the vast majority of cells. The nucleus as a reference for MTOC position may not be as meaningful as previously thought. 相似文献
972.
Apoplastic expression of yeast-derived invertase in potato : effects on photosynthesis, leaf solute composition, water relations, and tuber composition 总被引:11,自引:2,他引:11 下载免费PDF全文
Heineke D Sonnewald U Büssis D Günter G Leidreiter K Wilke I Raschke K Willmitzer L Heldt HW 《Plant physiology》1992,100(1):301-308
In potato plants (Solanum tuberosum), a chimeric yeast-derived invertase gene fused to a 35S cauliflower mosaic virus promoter has been expressed. The protein was targeted to the cell wall by using the signal peptide of proteinase inhibitor II fused to the amino terminus of the yeast invertase. The transformed plants had crinkled leaves, showed a reduced growth rate, and produced fewer tubers. Although in the apoplast of the leaves of the transformed plants the content of glucose and fructose rose by a factor of 20, and that of sucrose declined 20-fold, 98% of the carbohydrate in the phloem sap consisted of sucrose, demonstrating the strong specificity of phloem loading. In the leaf cells of the transformed plants, glucose, fructose, and amino acids, especially proline, were accumulated. Consequently, the osmolality of the cell sap rose from 250 to 350 mosmol/kg. Our results show that the observed 75% decrease of photosynthesis is not caused by a feedback regulation of sucrose synthesis and is accompanied by an increase in the osmotic pressure in the leaf cells. In the transformed plants, not only the amino acid to sucrose ratio in the phloem sap, but also the amino acid and protein contents in the tubers were found to be elevated. In the tubers of the transformed plants, the protein to starch ratio increased. 相似文献
973.
The highly enantioselective arylacetonitrilase of Pseudomonas putida was purified to homogeneity using a combination of (NH4)2SO4 fractionation and different chromatographic techniques. The enzyme has a molecular weight of 412 kDa and consisted of approximately nine to ten identical subunits (43 kDa). The purified enzyme exhibited a pH optimum of 7.0 and temperature optimum of 40°C. The nitrilase was highly susceptible to thiol-specific reagents and metal ions and also required a reducing environment for its activity. These reflected the presence of a catalytically essential thiol group for enzyme activity which is in accordance with the proposed mechanism for nitrilase-catalyzed reaction. The enzyme was highly specific for arylacetonitriles with phenylacetonitrile and its derivatives being the most preferred substrates. Higher specificity constant (k
cat/K
m) values for phenylacetonitrile compared to mandelonitrile also revealed the same. Faster reaction rate achieved with this nitrilase for mandelonitrile hydrolysis was possibly due to the low activation energy required by the protein. Incorporation of low concentration (<5%) of organic solvent increased the enzyme activity by increasing the availability of the substrate. Higher stability of the enzyme at slightly alkaline pH and ambient temperature provides an excellent opportunity to establish a dynamic kinetic resolution process for the production of (R)-(−)-mandelic acid from readily available mandelonitrile. 相似文献
974.
An investigation into the membrane-interactive potential of the Escherichia coli KpsE C-terminus 总被引:4,自引:0,他引:4
Phoenix DA Brandenburg K Harris F Seydel U Hammerton T Roberts IS 《Biochemical and biophysical research communications》2001,285(4):976-980
Membrane binding via C-terminal amphiphilic alpha-helical structure is a novel anchoring mechanism, which has been characterised in a number of prokaryotic carboxypeptidases. Here, we have used graphical and DWIH analyses to ascertain if a similar anchoring mechanism may be utilised by the Escherichia coli KpsE protein in its binding to the periplasmic face of the inner membrane. The results of these analyses have been compared to those obtained for similar analyses of the C-terminal sequences of E. coli penicillin-binding proteins (PBPs) PBP5 and PBP6 which, are known to function as amphiphilic alpha-helical membrane anchors, and of melittin, a known membrane-interactive toxin. We have also used FTIR spectroscopy and lipid phase transition temperature analysis to investigate the interaction of a peptide homologue of KpsE C-terminal region with membrane lipid. Our results suggest that the KpsE C-terminal sequence has the potential to form an amphiphilic alpha-helix and that this alpha-helix could feature in the membrane binding of the protein. 相似文献
975.
Thiopental (TPL) is a commonly used barbiturate anesthetic. Its binding with human serum albumin (HSA) was studied to explore the anesthetic-induced protein dysfunction. The basic binding interaction was studied by UV-absorption and fluorescence spectroscopy. An increase in the binding affinity (K) and in the number of binding sites (n) with the increasing albumin concentration was observed. The interaction was conformation-dependent and the highest for the F isomer of HSA, which implicates its slow elimination. The mode of binding was characterized using various thermodynamic parameters. Domain II of HSA was found to possess a high affinity binding site for TPL. The effect of micro-metal ions on the binding affinity was also investigated. The molecular distance, r, between donor (HSA) and acceptor (TPL) was estimated by fluorescence resonance energy transfer (FRET). Correlation between the stability of the TPL-N and TPL-F complexes and drug distribution is discussed. The structural changes in the protein investigated by circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopy reflect perturbation of the albumin molecule and provide an explanation for the heterogeneity of action of this anesthetic. 相似文献
976.
Workentine ML Harrison JJ Stenroos PU Ceri H Turner RJ 《Environmental microbiology》2008,10(1):238-250
Growth in a biofilm modulates microbial metal susceptibility, sometimes increasing the ability of microorganisms to withstand toxic metal species by several orders of magnitude. In this study, a high-throughput metal toxicity screen was initiated with the aim of correlating biological toxicity data in planktonic and biofilm cells to the physiochemical properties of metal ions. To this end, Pseudomonas fluorescens ATCC 13525 was grown in the Calgary Biofilm Device (CBD) and biofilms and planktonic cells of this microorganism were exposed to gradient arrays of different metal ions. These arrays included 44 different metals with representative compounds that spanned every group of the periodic table (except for the halogens and noble gases). The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and minimum biofilm eradication concentration (MBEC) values were obtained after exposing the biofilms to metal ions for 4 h. Using these values, metal ion toxicity was correlated to the following ion-specific physicochemical parameters: standard reduction-oxidation potential, electronegativity, the solubility product of the corresponding metal–sulfide complex, the Pearson softness index, electron density and the covalent index. When the ions were grouped according to outer shell electron structure, we found that heavy metal ions gave the strongest correlations to these parameters and were more toxic on average than the other classes of the ions. Correlations were different for biofilms than for planktonic cells, indicating that chemical mechanisms of metal ion toxicity differ between the two modes of growth. We suggest that biofilms can specifically counter the toxic effects of certain physicochemical parameters, which may contribute to the increased ability of biofilms to withstand metal toxicity. 相似文献
977.
Inhibited growth of common enteropathogenic bacteria in lactic-fermented cereal gruels 总被引:5,自引:0,他引:5
U. Svanberg E. Sjögren W. Lorri A. -M. Svennerholm B. Kaijser 《World journal of microbiology & biotechnology》1992,8(6):601-606
A natural lactic fermentation of mixtures of water and whole flour of either maize or high-tannin sorghum was obtained either before or after cooking to a weaning gruel: The preparations had a final pH of about 3.8 (range 3.67 to 4.00) and a ratio of lactic acid to acetic acid of 91 (w/w). The growth of added (about 107 c.f.u./g gruel) Gram-negative intestinal pathogenic bacteria, enterotoxigenicEscherichia coli, Campylobacter jejuni, Shigella flexneri andSalmonella typhimurium, was strongly inhibited in the sour gruels, and the effect could primarily be explained by the low pH caused by the formation of lactic and acetic acids during the fermentation process. Of the added Gram-positive bacteria,Bacillus cereus andStaphylococcus aureus showed similar inhibited growth up to 7h after inoculation in the sour gruels. The strain ofStaphylococcus, however, showed only a continued reduction in growth in the fermented gruel samples, which had a viable lactic bacteria culture indicating the presence of a bacteriocin. This implies that a low pH (< 4.0) alone is not sufficient to sustain the inhibition of the growth ofStaphylococcus aureus. The survival studies were carried out at optimal temperatures for each respective enteropathogen. 相似文献
978.
A. Jaffer W. M. U. Daniels V. A. Russell J. J. F. Taljaard 《Neurochemical research》1992,17(12):1255-1260
The aim of the present investigation was to lesion the noradrenergic system and to measure the effect on growth hormone (GH) secretion following peripheral administration of 2- and -adrenoceptor agonists. Direct injection of these agonists into the paraventricular nucleus of the hypothalamus (PVN) and its effect on GH secretion were also investigated. Systemic administration of N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4, 60 mg/kg, injected i.p. 10 days prior to experimentation) significantly decreased the noradrenaline (NA) content of the hippocampus, frontal cortex and hypothalamus but had no effect on the dopamine (DA) or serotonin (5-HT) content of these areas. Bilateral injection of 6-hydroxydopamine (6-OHDA, 10 g/l, 14 days prior to experimentation) into the medial forebrain bundle (MFB) caused a greater reduction of NA and also decreased the DA and 5-HT content of the hypothalamus. Analysis of the PVN of the hypothalami of rats following 6-OHDA lesion of the MFB showed significantly decreased NA and 5-HT content. Neither DSP4 treatment nor 6-OHDA lesion of the MFB affected the clonidine (250 g/kg, i.p.) induced stimulation of GH secretion. Injection of isoproterenol (1 mg/kg, i.p.) had varying effects on GH secretion. It stimulated GH release in control rats but not in DSP4 or MFB lesioned rats. Direct injection of clonidine (0.1 g/l) into the PVN significantly stimulated GH secretion, whereas injection of isoproterenol (2.5 g/l) into the PVN did not affect GH levels when compared to controls. The results of the present study do not support the hypothesis that hypoactivity of the central noradrenergic system may be the cause of the blunted GH response to clonidine observed in depressed patients. 相似文献
979.
U Katz 《Journal de physiologie》1987,82(3):183-187
1. Water uptake in vivo, and water fluxes across the isolated skin were studied in salt (NaCl) acclimated toads. 2. Water uptake of acclimated toads maintained in the solution of acclimation, decreased with the environmental salinity. 3. The osmotic water permeability (Pos) of the skin increased upon salt (NaCl) acclimation, both in vivo and in vitro. 4. Pos of the skin of toads acclimated to non-permeant solutes such as sucrose (230 mmol/l) or mannitol (400 nmol/l), was greatly reduced. 5. Oxytocin (syntocinon) increased the Pos both in tap water and salt acclimated toads. In high salt (greater than 200 mmol/l NaCl) acclimated toads however, the increased Pos and water flux at larger osmotic gradients, could not be stimulated further by the hormone. 6. The adaptive nature of the selective changes in the permeability properties of the skin under salt acclimation conditions is discussed. 相似文献
980.
Tugtekin I Wachter U Barth E Weidenbach H Wagner DA Adler G Georgieff M Radermacher P Vogt JA 《American journal of physiology. Endocrinology and metabolism》2002,283(6):E1223-E1231
Expired 13CO2 recovery from an oral l-[1-13C]phenylalanine ([13C]Phe) dose has been used to quantify liver function. This parameter, however, does not depend solely on liver function but also on total CO2 production, Phe turnover, and initial tracer distribution. Therefore, we evaluated the impact of these factors on breath test values. Nine ethyl-toxic cirrhotic patients and nine control subjects received intravenously 2 mg/kg of [13C]Phe, and breath and blood samples were collected over 4 h. CO2 production was measured by indirect calorimetry. The exhaled 13CO2 enrichments were analyzed by isotope ratio mass spectrometry and the [13C]Phe and l-[1-13C]tyrosine enrichments by gas chromatography-mass spectrometry. The cumulative 13CO2 recovery was significantly lower in cirrhotic patients (7 vs. 12%; P < 0.01), in part due to lower total CO2 production rates. Phe turnover in cirrhotic patients was significantly lower (33 vs. 44 micro mol. kg(-1). h(-1); P < 0.05). When these extrahepatic factors were considered in the calculation of the Phe oxidation rate, the intergroup differences were even more pronounced (3 vs. 7 micro mol. kg(-1). h(-1)) than those for 13CO2 recovery data. Also, the Phe-to-Tyr conversion rate, another indicator of Phe oxidation, was significantly reduced (0.7 vs. 3.0 micro mol. kg(-1). h(-1)). 相似文献