首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   12298篇
  免费   1112篇
  国内免费   1篇
  2022年   111篇
  2021年   224篇
  2020年   114篇
  2019年   148篇
  2018年   188篇
  2017年   154篇
  2016年   314篇
  2015年   483篇
  2014年   580篇
  2013年   682篇
  2012年   883篇
  2011年   859篇
  2010年   575篇
  2009年   574篇
  2008年   693篇
  2007年   695篇
  2006年   618篇
  2005年   607篇
  2004年   587篇
  2003年   577篇
  2002年   529篇
  2001年   207篇
  2000年   155篇
  1999年   182篇
  1998年   142篇
  1997年   115篇
  1996年   109篇
  1995年   92篇
  1994年   86篇
  1993年   86篇
  1992年   119篇
  1991年   114篇
  1990年   92篇
  1989年   94篇
  1988年   95篇
  1987年   110篇
  1986年   80篇
  1985年   98篇
  1984年   70篇
  1983年   63篇
  1982年   51篇
  1981年   70篇
  1980年   51篇
  1979年   74篇
  1978年   73篇
  1977年   51篇
  1974年   62篇
  1972年   55篇
  1970年   51篇
  1969年   49篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
One hundred and ninety eight men seropositive for human immunodeficiency virus (HIV) antibody and 58 HIV antibody seroconverters were studied for an average of 19.3 (SEM 0.5) months to assess the relation between HIV antigenaemia and the risk of developing the acquired immune deficiency syndrome (AIDS) and AIDS related complex. Forty (20.2%) of the 198 HIV antibody seropositive men were antigen positive at entry and remained so during follow up. Eight (13.8%) of the 58 HIV antibody seroconverters and 20 (12.7%) of the remaining 158 HIV antibody seropositive men became antigen positive during follow up, resulting in an end point attack rate for HIV antigenaemia of 14.3%. AIDS related complex was diagnosed in 25 (15.8%) of the HIV antigen negative men and in 14 (20.7%) of the HIV antigen positive men. AIDS was diagnosed in 15 men, resulting in an end point attack rate for AIDS of 23.9% in the HIV antigen positive group and 1.3% in the antigen negative group. HIV antibody seropositive men without symptoms but with persistent HIV antigenaemia are at increased risk of developing AIDS and AIDS related complex.  相似文献   
32.
33.
The N-terminal part sequences of pituitary growth hormone, N-acetyl-hGH 7–13 and hGH 6–13, promoted conversion of glycogen synthase b to glycogen synthase a in skeletal muscle and adipose tissue when injected intravenously. The peptides also caused conversion of phosphorylase a to phosphorylase b in liver and adipose tissue, but not in muscle, where the peptides antagonised activation of phosphorylase. Synthase phosphatase activity in muscle and phosphorylase phosphatase activity in liver increased after injection of peptide, with time courses of change similar to those seen for muscle synthase and liver phosphorylase activities. Injection of peptide also decreased both the cyclic AMP dependent and independent synthase kinase activities in muscle. These results show that the insulin-like activities of these peptides on glycogen synthase and phosphorylase involve both increases in protein phosphatase activities and inhibition of protein kinase activities. These results are discussed in relation to the insulin-like activities of growth hormone.  相似文献   
34.
Summary Streptozotocin treatment at birth induces, in the pancreas of rats, first depletion of insulin and thyrotropin-releasing hormone and then early regeneration of cells and insulin, but not TRH. This study was undertaken to investigate whether the reduction in pancreatic TRH content can be associated with changes in the intensity and the distribution of TRH-immunoreactivity, and to follow the pattern of regeneration of cells through insulin- and TRH-immunoreactivity.In control animals, strong TRH-immunoreactivity was seen in insulin-containing cells on days 1–4 after birth. At day 7, the TRH-immunoreactivity was already decreased. In contrast, insulin-immunoreactivity was present throughout the neonatal period. A sparse population of cells near ducts also contained both TRH- and insulin-immunoreactivity at 1–2 days age.In streptozotocin-treated animals, TRH-immunoreactivity is found only in a few scattered insulin-containing cells in altered islets on days 1–4. Near the ducts, there were new insulin-containing cells which did not contain TRH. From day 7 regeneration of endocrine cells was characterized by new, typical islets, but these contained insulin-, but not TRH-immunoreactivity. These findings suggest a differential control of the biosynthesis of insulin and TRH within the pancreas.  相似文献   
35.
Summary In the locust,Locusta migratoria, the pairs of connectives between the three thoracic ganglia and in the neck were transected in all possible combinations. Each of these preparations was tested for the production of rhythmic flight motor activity, with sensory input from the wing receptors intact and after deafferentation. The motor activity elicited in these preparations was characterized by intracellular recordings from motoneurons and electromyographic analyses.The motor patterns observed in locusts with either the neck or the pro-mesothoracic connectives severed (Figs. 2, 3, and 4) were very similar to the flight motor pattern produced by animals with intact connectives. The activity recorded in mesothoracic flight motoneurons of locusts with either only the meso-metathoracic connectives cut or both the meso-metathoracic and the neck connectives transected were similar to each other. Rhythmic motor activity could be observed in these preparations only as long as sensory feedback from the wing receptors was intact. These patterns were significantly different from the intact motor pattern (Figs. 5, 6, and 7). Similar results were obtained when the mesothoracic ganglion was isolated from the other two thoracic ganglia, although the oscillations produced under these conditions were weak (Fig. 8 upper). In the isolated metathorax no rhythmic flight motor activity could be recorded (Fig. 8 lower), even when wing afferents were intact.Considering the differences between the motor patterns observed in the various preparations these results suggest that the ganglia of the locust ventral nerve cord do not contain segmental, homologous flight oscillators which are coupled to produce the intact flight rhythm. Instead they support the idea that the functional flight oscillator network is distributed throughout the thoracic ganglia (Robertson and Pearson 1984). The results also provide further evidence that sensory feedback from the wing sense organs is necessary for establishing the correct motor pattern in the intact animal (Wendler 1974, 1983; Pearson 1985; Wolf and Pearson 1987 a).Abbreviations CPG central pattern generator - EMG electromyogram  相似文献   
36.
37.
Summary The blackleg pathogen Erwinia carotovora subsp. atroseptica (Eca) causes an economically important disease of potatoes. We selected a genetically amenable Eca strain for the genetic analysis of virulence. Tn5 mutagenesis was used to generate nine mutants which exhibited reduced virulence (Rvi-) of strain SCRI1043. Following physiological characterisation, mutants were divided into three classes: (1) auxotrophs; (2) extracellular enzyme mutants; and (3) a growth rate mutant. The isolation of these Rvi- mutants has allowed us to consider some factors that affect Eca virulence.  相似文献   
38.
Modeling long-term crop response to fertilizer and soil nitrogen   总被引:1,自引:1,他引:0  
A simple nitrogen balance model to calculate long-term changes in soil organic nitrogen, nitrogen uptake by the crop and recovery of applied nitrogen, is presented. It functions with time intervals of one year or one growing season. In the model a labile and a stable pool of soil organic nitrogen are distinguished. Transfer coefficients for the various inputs of nitrogen are established that specify the fractions taken up by the crop, lost from the system, and incorporated in soil organic nitrogen. It is shown how input data, model parameters and initial pool sizes can be derived and how the model can be used for calculating long-term changes in total soil organic nitrogen and uptake by the crop. For nitrogen applied annually as fertilizer or organic material the time course of nitrogen uptake and recovery of applied nitrogen is calculated. To test the sensitivity of the model, calculations have been performed for different environmental conditions with higher or lower risks for losses. The model has also been applied to establish fertilizer recommendations for a certain target nitrogen uptake by the crop. Finally, for agricultural systems where periods of cropping alternate with peroids of green fallow the time course of nitrogen uptake by the crop is calculated.  相似文献   
39.
The two isoenzymes of NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14), previously identified in root nodules of Phaseolus vulgaris L., have both been shown to be located in root-nodule plastids. The nodule specific NADH-GOGAT II accounts for the majority of the activity in root nodules, and is present almost exclusively in the central tissue of the nodule. However about 20% of NADH-GOGAT I activity is present in the nodule cortex, at about the same specific activity as this isoenzyme is found in the central tissue. Glutamine synthetase (GS; EC 6.3.1.2) occurs predominantly as the polypeptide in the central tissue, whereas in the cortex, the enzyme is represented mainly by the polypeptide. Over 90% of both GS and NADH-GOGAT activities are located in the central tissue of the nodule and GS activity exceeds NADH-GOGAT activity by about twofold in this region. Using the above information, a model for the subcellular location and stoichiometry of nitrogen metabolism in the central tissue of P. vulgaris root nodules is presented.Abbreviations Fd-GOGAT ferredoxin-dependent glutamate synthase - GOGAT glutamate synthase - GS glutamine synthetase - NADH-GOGAT NADH-dependent glutamate synthase - IEX-HPLC ion-exchange high-performance liquid chromatography  相似文献   
40.
Summary Intact and osmotically sensitive cells of Corynebacterium glutamicum can be efficiently transformed by electroporation. This was shown by using the plasmid vector pUL-330 (5.2 kb), containing the kanamycin resistance gene of transposon Tn5. The following electric parameters yielded efficient transformation. For intact cells: one exponentially decaying field pulse with time constants and with initial field intensities of E 0=35–40 kV cm-1; prepulse temperature 20°C. Cell regeneration (survival) was 100%–80%. Transformation efficiency can be increased by an additional freeze and thaw cycle of the cells, prior to electroporation. Lysozyme treated cells (osmotically sensitive) were transformed with three successive pulses of E 0=25–30 kV cm-1. Cell regeneration under these conditions was found to be 20–30%. The optimum yield of transformants/g plasmid-DNA was 3×103 for intact cells, 2×104 for intact cells which were frozen and thawed twice and 7×104 for osmotically sensitive cells if the cell suspension was pulsed at a cell density of 1–3×108/ml and at a DNA concentration of 0.2 g/ml up to 2 g/ml. The data obtained for osmotically sensitive cells suggest that the temperature increase accompanying the electric field pulse enhances colony formation and transformation efficiency if the initial prepulse temperature is 20°C, although regeneration of electroporated C. glutamicum cells starts to decrease at temperatures20°C.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号