全文获取类型
收费全文 | 4139篇 |
免费 | 352篇 |
国内免费 | 1篇 |
出版年
2023年 | 13篇 |
2022年 | 48篇 |
2021年 | 67篇 |
2020年 | 34篇 |
2019年 | 43篇 |
2018年 | 70篇 |
2017年 | 56篇 |
2016年 | 82篇 |
2015年 | 172篇 |
2014年 | 175篇 |
2013年 | 249篇 |
2012年 | 319篇 |
2011年 | 311篇 |
2010年 | 242篇 |
2009年 | 183篇 |
2008年 | 227篇 |
2007年 | 235篇 |
2006年 | 227篇 |
2005年 | 225篇 |
2004年 | 210篇 |
2003年 | 202篇 |
2002年 | 201篇 |
2001年 | 61篇 |
2000年 | 31篇 |
1999年 | 59篇 |
1998年 | 69篇 |
1997年 | 41篇 |
1996年 | 44篇 |
1995年 | 37篇 |
1994年 | 43篇 |
1993年 | 40篇 |
1992年 | 40篇 |
1991年 | 38篇 |
1990年 | 24篇 |
1989年 | 25篇 |
1988年 | 27篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1975年 | 12篇 |
1974年 | 13篇 |
1973年 | 17篇 |
排序方式: 共有4492条查询结果,搜索用时 31 毫秒
961.
B A Prior C P Kenyon M van der Veen J P Mildenhall 《The Journal of applied bacteriology》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
962.
963.
Dr. Edward B. Blanchard Cynthia L. Radnitz Donald D. Evans Shirley P. Schwarz Debra F. Neff Maryrose A. Gerardi 《Applied psychophysiology and biofeedback》1986,11(3):221-230
Comparisons were made among patients with IBS (n=55), tension headache (n=69), or migraine headache (n=68) and nonpatient controls (n=64) on the MMPI and several other psychological tests, including BDI, STAI, Life Events, and Psychosomatic Symptom Checklist. With two nonsignificant exceptions (MMPI scale F and Life Events) the groups were consistently ordered, in terms of increasing psychological distress: Normals < Migraine Headache < Tension Headache < IBS. The IBS patients were more like the tension headache patients than any other group. Subgroups of IBS patients, on the basis of presence or absence of diarrhea or constipation in addition to abdominal pain, were generally not significantly different on the psychological tests. 相似文献
964.
Mark D. Johnson Alberto Grignolo Cynthia M. Kuhn Saul M. Schanberg 《Life sciences》1983,33(2):169-180
Chronic continuous infusion of norepinephrine (NE) or epinephrine (Epi) subcutaneously in rats resulted in a rapid elevation of systolic blood pressure (SBP) by 40 mm Hg. Pressure remained high for the duration of the infusion but rapidly returned to control levels after its termination. Pronounced hypertrophy of the thoracic aorta, abdominal aorta and heart was evident within 2 days of the initiation of NE infusion and a plateau was attained by 5 days of infusion. The activity of tissue ornithine decarboxylase (ODC), the rate limiting enzyme in polyamine biosynthesis, was elevated preceeding the onset of tissue hypertrophy, and returned to control levels coincidentally with the cessation of accelerated tissue growth. It is concluded that high blood pressure in this animal model of hypertension is dependent upon the continued presence of exogenous catecholamine, and that pronounced cardiovascular hypertrophy is not sufficient to maintain the hypertension. The elevation and decline of vascular ODC activity is consistent with reports that in other tissues an elevation of ODC activity is an obligatory early event in hypertrophy. 相似文献
965.
Evaluation of the overland runoff mode of land wastewater treatment for virus removal. 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The removal of enteric and tracer viruses by the overland runoff mode of domestic wastewater treatment was evaluated. Raw and primary and secondary treated wastewaters were sprayed onto grass-covered, 36-m soil plots of fine, sandy loam overlying an impermeable clay subsoil. Tracer bacteriophage f2 was seeded into the applied wastewaters, which were subsequently sampled at several points along the length of the plots. Assay of effluent samples revealed modest tracer virus removals of 30 to 60%. Data from timed experiments indicated that advancement of tracer virus to the bottom of the slopes proceeded at the same rate as wastewater, reaching the plot effluents within 50 to 90 min after application. Indigenous enteric virus levels were reduced by approximately 68 to 85% during migration down the treatment slopes. Soil sampling revealed that, although some f2 virus was found associated with the wastewater-saturated topsoil, little penetration of virus into the soil profile occurred. Laboratory soil adsorption studies revealed that poliovirus I was adsorbed much more readily than f2 virus. Comparison of virus removal characteristics during overland runoff with chemical removal characteristics of wastewater did not reveal any obvious correlations that could be used to predict virus removal. 相似文献
966.
Biswanath De Niels H. Andersen Robert M. Ippolito Cynthia H. Wilson Walter D. Johnson 《Prostaglandins & other lipid mediators》1980,19(2):221-247
In the mercuri- and halo-cyclizations of PGF2α methyl ester and its 11,15-bis(α-ethoxyethyl)-ether (or other protected forms) the exo-PGI1 derivative predominates independent of reagent and degree of protective of the PGF2α sample used. Diastereomerically pure samples of exo- and endo-PGI1 and prostacyclin (PGI2) were prepared. PGI0 epimers were prepared: catalytic hydrogenation of PGI2 Me ester provides exclusively the endo isomer. PGI2 methyl ester was found to be stable to extensive chromatography on silica, and to storage for at least a year in anhydrous ethanol at −20°C. At pH 7.4 in 2:1 H2O:EtOH, the ester has a half-life in excess of 5 hr at 25°C. A reproducible small scale (0.4–3 mg) synthesis of prostacyclin uses a modification of Whittaker's iodocyclization followed by DBN treatment. This procedure, developed with 15-3H-PGF2α, proved widely applicable to PGF2α analogs and diastereomers. The following prostacyclins (in the Me ester and Na salt forms) bearing the 5-en-6-yl ether unit were prepared in this way: ent-PGI2, rac-PGI2, 15-epi-PGI2, ent-15-epi-PGI2, 11-epi-PGI2, 8,9,12-epi-PGI2,
-PGI2, 13,14-dihydro-PGI2, and 13,14-dihydro-15-epi-PGI2. NMR comparisons for the methyl esters reveal that of the resonances (H-5,9,11,15) that appear at δ4.0±0.6 ppm, the most deshielded is H-9 so long as the 5,6-olefin is
. The 8
,9
-6,9-oxido-
-5,6-ene unit is most readily characterized by its strong positive dichroic absorption at 210–230 nm. CD spectroscopy not only serves to confirm the presence of this unit in analogs, but also can be used for quantitative analysis of PGI2 solutions and for monitoring the rate of hydrolytic cleavage of these enol ethers. 相似文献
967.
The rapid isolation of vacuoles from leaves of crassulacean Acid metabolism plants 总被引:4,自引:4,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A technique is presented for the isolation of vacuoles from Sedum telephium L. leaves. Leaf material is digested enzymically to produce protoplasts rapidly which are partially lysed by gentle osmotic shock and the inclusion of 5 millimolar ethyleneglycol-bis (β-aminoethyl ether)N,N′-tetraacetic acid in the wash medium. Vacuoles are isolated from the partially lysed protoplasts by brief centrifugation on a three-step Ficoll-400 gradient consisting of 5, 10, and 15% (w/v) Ficoll-400. A majority of the vacuoles accumulate at the 5 to 10% Ficoll interface, whereas a smaller proportion sediments at the 10 to 15% Ficoll-400 interface. The total time required for vacuole isolation is 2 to 2.5 hours, beginning from leaf harvest. 相似文献
968.
Observations of age-sex class associations of three young female and five young male bonnet macaques born in the spring, 1975,
were taken semiannually over three years beginning in the fall, 1975. Analysis reveals several patterns: (1) both males and
females show continuous decline in association with age mates; (2) solitary time of females peaks at 1.5 years after which
they begin to associate with adult females; (3) solitary time of males peaks at 2.5 years after which they associate predominantly
with sub-adult males; (4) both males and females showed higher association with juveniles of the same sex than of opposite
sex; and (5) both males and females showed annual cycles of association with some age-sex classes. These results are best
understood with reference to the natural breeding cycle of the animals and the different life histories of male and female
macaques in the wild. 相似文献
969.
Diurnal Changes in Metabolite Levels and Crassulacean Acid Metabolism in Kalanchoë daigremontiana Leaves
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Diurnal changes in levels of selected metabolites associated with glycolysis, the C3 cycle, C4-organic acids, and storage carbohydrates were analyzed in active Kalanchoë daigremontiana Crassulacean acid metabolism leaves. Three metabolic transition periods occurred each day. During the first two hours of light, nearly all of the metabolite pools underwent transient changes. Beginning at daylight, stomata opened transiently and closed again within 30 minutes; malate synthesis continued for about 1 hour into the light; C3 photosynthesis began within 30 minutes; and net quantities of starch and glucan began to accumulate after 2 hours, continuing linearly throughout the rest of the day. 相似文献
970.
Summary The fatty acids of 32 axenic strains of filamentous blue-green algae have been analyzed. As an aid to the interpretation of the results, the strains have been assigned to provisional typological groups based upon their morphology and certain physiological characters. The latter are the ability to grow heterotrophically in the dark with glucose as carbon and energy source, the ability to grow in the light at the expense of glucose in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and the ability to synthesize nitrogenase under anaerobic conditions in the light. Each typological group has been given an appropriate generic name.The strains examined for fatty acid composition can be divided into groups according to the major fatty acid of highest degree of unsaturation found in each strain as was done for the unicellular strains examined previously in this laboratory. Four metabolic groups of strains of unicellular and filamentous blue-green algae can be recognized: 1. those in which there is little or no desaturation of oleate; 2. those in which linoleate is desaturated toward the -end of the molecule to give -linolenate; 3. those in which linoleate is desaturated toward the carboxyl end of the molecule to give -linolenate; 4. those in which octadecatetraenoate is synthesized. The nature of the major cellular fatty acids of two of the strains examined is the same whether growth is in the light or in the dark on glucose. All filamentous strains contain glycolipids with the properties of mono- and digalactosyldiglycerides.Abbreviation DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea 相似文献