全文获取类型
收费全文 | 65631篇 |
免费 | 1166篇 |
国内免费 | 10014篇 |
出版年
2022年 | 339篇 |
2021年 | 438篇 |
2020年 | 406篇 |
2019年 | 977篇 |
2018年 | 1302篇 |
2017年 | 715篇 |
2016年 | 798篇 |
2015年 | 990篇 |
2014年 | 1485篇 |
2013年 | 2383篇 |
2012年 | 1660篇 |
2011年 | 1186篇 |
2010年 | 966篇 |
2009年 | 1537篇 |
2008年 | 1786篇 |
2007年 | 2465篇 |
2006年 | 2695篇 |
2005年 | 2765篇 |
2004年 | 2981篇 |
2003年 | 2702篇 |
2002年 | 2669篇 |
2001年 | 2392篇 |
2000年 | 1771篇 |
1999年 | 2055篇 |
1998年 | 1791篇 |
1997年 | 1350篇 |
1996年 | 1312篇 |
1995年 | 1380篇 |
1994年 | 1462篇 |
1993年 | 1304篇 |
1992年 | 1121篇 |
1991年 | 1114篇 |
1990年 | 911篇 |
1989年 | 767篇 |
1988年 | 805篇 |
1987年 | 621篇 |
1986年 | 540篇 |
1985年 | 2018篇 |
1984年 | 3043篇 |
1983年 | 1998篇 |
1982年 | 2378篇 |
1981年 | 2023篇 |
1980年 | 1894篇 |
1979年 | 1680篇 |
1978年 | 1546篇 |
1977年 | 1415篇 |
1976年 | 1303篇 |
1975年 | 1101篇 |
1974年 | 1087篇 |
1973年 | 1021篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
Phenotypic dissections of the Blastocladiella emersonii zoospore's developmental choice 总被引:1,自引:0,他引:1
A developmentally homogeneous neural crest cell population has been used to assay the role of environmental factors in regulating crest cell differentiation. If cultured on tissue culture plastic, virtually all of the cells of this population differentiate into melanocytes. In contrast, when these cells are cultured for 3 or more days on substrata “conditioned” by somite fibroblasts, the proportion of cells undergoing melanogenesis decreased and the proportion expressing formaldehyde-induced fluorescence (FIF), characteristic of catecholamine-containing cells, increased. For a limited period of culture on somite-conditioned substrata, some cells in the population exhibit both pigment granules and fluorescence. Collagen-coated substrata decreased the number of cells that formed pigment but did not stimulate FIF. In contrast, optimum doses of exogenous cellular fibronectin mimicked the effect of somite-conditioned substrata, suppressing melanogenesis and promoting FIF. Glycosaminoglycan-derivatized substrata (i.e., hyaluronic acid, various chondroitin sulfate preparations, and heparin) did not alter the differentiative homogeneity of the cultured crest cell populations. The choice and expression of phenotype by some members of a cultured crest cell population can, therefore, be affected by environmental stimuli provided in the form of certain substrate-attached macromolecules. We suggest that optimal concentrations of some extracellular matrix components produced by embryonic tissue and normally encountered by migrating crest cells may elicit the expression of FIF in crest cells that would otherwise follow a different developmental pathway. 相似文献
122.
The determination of purine levels in human and mouse plasma 总被引:2,自引:0,他引:2
Variable levels of acetic anhydride have been recommended for addition to one of two reagents used in the glyoxylic acid method for the determination of tryptophan. For use of this reagent immediately after preparation it was shown that a minimum of 16% (v/v) of acetic anhydride should be included in the formulation to obtain near-maximum sensitivity. It was further demonstrated that reagent formulations with and without acetic anhydride changed with exposure to light. The observed changes are manifest as changes in the relative sensitivities of the assay. Several modifications are recommended to improve the sensitivity and stability of the acetic anhydride-containing reagent in this assay. 相似文献
123.
Sanda Clejan Platon J. Collipp Vaddanahally T. Maddaiah 《Archives of biochemistry and biophysics》1980,203(2):744-752
The effects of hypophysectomy and subsequent administration of growth hormone, thyroxine, insulin, and testosterone were examined in rat liver for the relationship between the thermotropic effects on State 3 respiration (ADP induced) and fatty acid composition of the phospholipid fraction of intact mitochondria as well as of inner membrane vesicles. The Arrhenius profile for energy-linked (succinate) State 3 respiration of mitochondria from hypophysectomized rats lacked the discontinuity at 23.5 °C seen with mitochondria from normal rats. After injections of the hormones the discontinuity representing the transition temperature from gel to liquid crystalline state of lipids occurred at different temperatures: 18.5 °C for growth hormone, 26.0 °C for thyroxine, 19.5 °C for growth hormone + thyroxine, 27.6 °C for insulin, and 25.3 °C for testosterone. The energy of activation between 37.5 and 23.5 °C was 1.9 times greater for hypophysectomy than for controls. Growth hormone was the most effective in restoring the energy of activation to normal, above as well as below transition temperature. The effect of thyroxine appears to be due to a larger stimulation of the State 4 respiration than that of growth hormone, insulin, or testosterone, especially at higher temperatures. Phospholipids extracted from intact mitochondria or inner membrane vesicles of hypophysectomized rats contained less arachidonic acid (20:4) and more linoleic acid (18:2) than those of normal rats. In addition, the contents of some of the minor fatty acids were also changed. Calculated unsaturation index showed an 18.8 and 14.9% depletion in unsaturation in whole mitochondria and inner membranes, respectively. Among the different hormones used to treat the hypophysectomized rats, growth hormone was the most effective in restoring the transition temperature and fatty acid composition to normal levels and increasing the gain in body weight. Although the other hormones increased total unsaturation index to some extent, some of the individual fatty acids were affected differently. Good correlation exists between the unsaturation index of mitochondrial fatty acids and transition temperature of State 3 respiration. These results strongly suggest a role for the hormones, particularly growth hormone, in the control of mitochondrial membrane fluidity of hypophysectomized rat liver, through fatty acid composition of phospholipids. 相似文献
124.
Multiple forms of amylase in leaf extracts: electrophoretic transfer of the enzyme forms into amylose-containing polyacrylamide gels 总被引:1,自引:0,他引:1
A method for the analysis of multiple forms of glucan-degrading enzymes is described. The procedure consists of the separation of the proteins by electrophoresis or isoelectric focusing in glucan-free polyacrylamide gels followed by the nondenaturing electrophoretic transfer into a second polyacrylamide layer which contains immobilized glucans. The method combines the resolving power of electrophoretic separations in glucan-free media with the sensitivity of amylase activity detection in amylose-containing polyacrylamide gels. The procedure is especially useful when samples containing low amylase activity, but a large number of multiple enzyme forms, are to be analyzed. 相似文献
125.
Using the imidate procedure, 2,3,4,6-tetra-O-benzyl-1-O-(N-methylacetimidoyl)-β-d-galactopyranose was condensed with various monosaccharides to provide, in good yield and with high stereoselectivity, α-linked disaccharides. 相似文献
126.
Activation of the sympathetic system by phencyclidine (PCP) should result in catecholamine release from the adrenals. However, adrenalectomy does not reduce PCP-induced hypertension. In an attempt to rectify this inconsistency, the direct effects of PCP on the bovine adrenal medulla were examined. At (3×10?6M), PCP reduced the acetylcholine-(ACh)-induced catecholamine release by 50%. Surprisingly, barium-induced secretion of catecholamines was also reduced by PCP. ACh-induced catecholamine release was not altered by 10?3M 4-aminopyridine (4 AP), the potassium channel blocker. Thus, calcium antagonist actions of PCP and consequent block of catecholamine secretion from adrenal medulla may explain the lack of effect of adrenalectomy on PCP-induced hypertension. Possible contributions of calcium and/or potassium channel blockade to other manifestations of PCP overdosage are discussed. 相似文献
127.
The interaction of several N-acetyl-d-glucosamine analogs and of sialyl lactose with the lectin wheat germ agglutinin was studied by nuclear magnetic resonance. N-2H3-acetyl-d-gluocosamine was synthesized and found to displace the N-acetyl methyl signal toward its free chemical shift in N-acetylglucosamine and N-acetylneuraminic acid demonstrating common binding sites for the latter two compounds. The N-acetyl methyl signal of the α-methylglucoside of N-acetylglucosamine could be titrated but a 3-deoxy analog could not, the latter exhibiting very weak binding and demonstrating the importance of the 3-OH group in the binding process. Sialyl lactose (an N-acetylneuraminic acid analog) was rather tightly bound to the lectin. N-F3-acetyl-d-glucosamine was synthesized and its binding to the lectin was studied at pH 4, 4.5, 5.1 by 19F NMR. The two anomers were found to bind with nearly equal Kd′s but exhibited a pH and anomer dependent Δ (total bound chemical shift). The -CF3 analog was found to bind considerably stronger to the lectin than the -CH3 compound. The clear resolution of the α and β anomers of this molecule make it a very useful probe of the lectin binding site. 相似文献
128.
MARTIN G. M. RANDALL 《Ecological Entomology》1982,7(2):177-185
Abstract. 1. The parasitization of the larvae of Coleophora alticolella . feeding on Juncus squarrosus , was investigated at a series of altitudes from 15 to 520m above sea-level in northern England during 1977 and 1978.
2. Six species of primary parasitoid and one hyperparasitoid were reared from this host. Five of the primary parasitoids were ectophagous; only two specimens of the endoparasitoid, Gonotypus melanostoma , were reared.
3. All of the parasitoid species were recorded at 15 m but fewer at sites of higher altitude. Only one species, Scambus brevicomis , was recorded above 305 m, and none above 395 m. The hyperparasitoid, Tetrastichus endemus, was present only at 15 m.
4. Percentage parasitization was highest at 15 m; it was reduced from 51% to only 2% between 215 and 305 m in 1978. There was an increase in host density over this altitudinal range.
5. Three species, Scambus brevicomis. Elachertus olivaceus and Euderus viridis , accounted for most of the parasitization, but their relative proportions vaned at different altitudes.
6. The sex-ratios of the parasitoids reared from Coleophora alticolella ranged from 3.2% females for Scambus brevicomis , which is considered to also use larger hosts, to 99.4% females for Elachertus olivaceus , which develops by thelytokous parthenogenesis.
7. Euderus viridis and Scambus brevicomis started to attack the Coleophora alticolella larvae at a later date at 245 m than at 15 m, but attack by Elachertus olivacats was not delayed at the higher site. 相似文献
2. Six species of primary parasitoid and one hyperparasitoid were reared from this host. Five of the primary parasitoids were ectophagous; only two specimens of the endoparasitoid, Gonotypus melanostoma , were reared.
3. All of the parasitoid species were recorded at 15 m but fewer at sites of higher altitude. Only one species, Scambus brevicomis , was recorded above 305 m, and none above 395 m. The hyperparasitoid, Tetrastichus endemus, was present only at 15 m.
4. Percentage parasitization was highest at 15 m; it was reduced from 51% to only 2% between 215 and 305 m in 1978. There was an increase in host density over this altitudinal range.
5. Three species, Scambus brevicomis. Elachertus olivaceus and Euderus viridis , accounted for most of the parasitization, but their relative proportions vaned at different altitudes.
6. The sex-ratios of the parasitoids reared from Coleophora alticolella ranged from 3.2% females for Scambus brevicomis , which is considered to also use larger hosts, to 99.4% females for Elachertus olivaceus , which develops by thelytokous parthenogenesis.
7. Euderus viridis and Scambus brevicomis started to attack the Coleophora alticolella larvae at a later date at 245 m than at 15 m, but attack by Elachertus olivacats was not delayed at the higher site. 相似文献
129.
The influence on the productive value of diets for broiler chickens based on barley harvested at two stages of ripeness, when including β-glucanase in increasing amounts, was evaluated in a production experiment over three weeks. Feed consumption and live weight increased (P < 0.001) and feed conversion was improved (P < 0.001) by including β-glucanase in the diets. Dry matter content of excreta increased significantly as a result of β-glucanase inclusion. Broiler chickens receiving diets based on barley harvested at combine ripeness were slightly heavier (P < 0.05) at three weeks than those fed on barley harvested at early yellow ripeness. 相似文献
130.
Acholeplasma laidlawii cells were grown in cholesterol-enriched medium and exposed continuously to either air (control), 4.0 vol.% halothane in air at 1 atm pressure (4% atm halothane), or 80% cyclopropane in oxygen for 24 h at 37°C. Cells grown in the presence of 4% atm halothane or 80% cyclopropane had approximately twice as much membrane cholesterol content/mg protein as the control cells. Cells grown in an anesthetic environment also tended to have a higher membrane cholesterol/phospholipid molar ratio compared to control cells. Membranes isolated from halothane-exposed cells grown in a cholesterol-enriched medium were more ordered at 37°C (measurements were made with no anesthetic present) than membranes from control cells grown in an identically enriched medium. This difference in membrane physical state between control and anesthetic-exposed cells decreased as the temperature decreased, and disappeared at approx. 23°C. Continuous exposure of A. laidlawii to 4% atm halothane or 80% cyclopropane for 24 h did not markedly affect membrane fatty acid composition, either in cells grown on an unsupplemented medium or in cells grown in a medium enriched in myristic, palmitic or stearic acids. These results further support the hypothesis that an increased membrane cholesterol content may play a role in the tolerance or dependence that develops after chronic exposure to anesthetic agents. 相似文献