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   26027篇
  免费   2655篇
  国内免费   12篇
  28694篇
  2023年   138篇
  2022年   311篇
  2021年   695篇
  2020年   394篇
  2019年   501篇
  2018年   585篇
  2017年   515篇
  2016年   809篇
  2015年   1308篇
  2014年   1431篇
  2013年   1552篇
  2012年   2053篇
  2011年   2046篇
  2010年   1280篇
  2009年   1115篇
  2008年   1476篇
  2007年   1444篇
  2006年   1294篇
  2005年   1168篇
  2004年   1115篇
  2003年   944篇
  2002年   843篇
  2001年   345篇
  2000年   296篇
  1999年   303篇
  1998年   208篇
  1997年   169篇
  1996年   125篇
  1995年   141篇
  1994年   135篇
  1993年   122篇
  1992年   208篇
  1991年   195篇
  1990年   180篇
  1989年   181篇
  1988年   180篇
  1987年   155篇
  1986年   138篇
  1985年   184篇
  1984年   141篇
  1983年   139篇
  1982年   129篇
  1981年   105篇
  1980年   104篇
  1979年   150篇
  1978年   104篇
  1977年   102篇
  1976年   107篇
  1974年   108篇
  1973年   101篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Summary Testes of the Japanese freshwater turtle Clemmys japonica Temmnick et Schlegel were fixed in 3% potassium permanganate buffered to pH 7.2 with Veronal-acetate buffer, and thin sections of the tissue, embedded in epoxy Epon resin, were studied under the electron or light microscope.At the early stage of differentiation of the spermatid, the cytoplasm contains a few mitochondria provided with cristae which are oriented transversely or longitudinally. As the differentiation of spermatids proceeds, the mitochondrion has been modified into a cupshaped body with a wall consisting of several concentric layers. Such body has been referred to the mitochondrial lamellar body. The formation of such a body is mainly attributed to the mitochondrial cristae, and subsequently to the membrane system of the endoplasmic reticulum. In a more advanced stage of differentiation, the mitochondrial lamellar bodies appear wrapped around a bundle of tail filaments, and seem to present a very wide surface available for the localization of organized enzyme systems to facilitate the motion of spermatozoa.Prior to the formation of the mitochondrial lamellar bodies, the Golgi apparatus has been reorganized into a peculiar body with a floral appearance, consisting of numerous tubular elements, and revealing to be positive in PAS-reaction. The body has been designated as the tubular body which has never been demonstrated in any spermatogenic cells through animal kingdom.One to three tubules oval in cross section, approximately 430 × 700 Å in diameter, have been found in the nucleoplasm along the longitudinal axis of a greatly elongated, cone-shaped nucleus of the spermatid. The tubules open on the apex surface of the nucleus, but they are not encountered in the acrosome. A possible physiological significance of the tubules has been discussed in view of the function of the acrosomal tubules in the decapod and other species spermatozoa as well as on the basis of the metabolism of nucleus.This study was supported by Grant GM-8327 from the United States Public Health Service.We wish to express our gratitude to Dr. B. A. Afzelius, Wenner-Gren Institute, University of Stockholm, for his valuable suggestion to the present work.  相似文献   
143.
144.
1. The stoicheiometry of the photo-oxidation of succinate by chromatophores has been investigated with [2,3-14C2]succinate. It was found that there is a stoicheiometric relationship between the amount of succinate oxidized and the NAD reduced, and that fumarate is the only product of succinate oxidation. 2. The possibility of a direct hydrogen transfer from succinate to NAD in this reaction was investigated with tritiated substrates. With tritiated succinate less than 3% of the activity expected if direct hydrogen transfer occurred was recovered in the NADH2, and this was due to contamination with the substrate. In experiments with tritiated water, NADH2 was labelled, and had half the specific activity of the water, as expected if water was the source of protons. It was also found that chromatophores catalyse an exchange reaction between NADH2 and water. 3. It is concluded that the exchange reaction makes it impossible to interpret these results as indicating either a hydrogen-transfer or an electron-transfer mechanism for the photoreduction reaction.  相似文献   
145.
Micromanipulation of yeast particles and blood granulocytes has been used to study the kinetics of single phagocytosis events. The ingestion process was quantitated by observation of sequential adhesion and encapsulation times. Both adherence and encapsulation times were found to increase greatly as the temperature was reduced below 37 degrees C; calcium in solution facilitated adhesion of the particle to the phagocyte but not encapsulation; both adhesion and encapsulation processes required a minimum level of plasma components (presumably complement). The general nature of these observations were confirmatory of previous studies, but this study is unique in that the specific time course of single particle ingestion was quantitated. It was immediately apparent that the phagocytosis process was 100% efficient above the threshold concentrations required for plasma and temperature, but variations in times from cell to cell indicated heterogeneity in the population. The total time for ingestion varied from as low as 2 sec/particle at 37 degrees C to above several min/particle below 15 degrees C. Encapsulation times for particles were normalized by estimates of particle surface areas to establish a specific time/unit area of particle surface: from 0.5 sec/10(-8) cm2 at 37 degrees C to greater than 8 sec/10(-8) cm2 at 15 degrees C. The temperature dependence of the encapsulation time correlated well with the temperature dependence of the "apparent" viscosity for granulocytes measured by micropipet aspiration. As such, the kinetic properties observed in these phagocytosis tests are consistent with a model that both assembly of the contractile system and the displacement of the surface by active contraction in phagocytosis are limited by viscous dissipation in the cell.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
146.
Enzyme electrophoresis indicated that all Schistosoma mattheei eggs passed in the urine of humans derive from S. mattheei females in copula with S. haematobium males. It appears that S. mattheei males do not reach sexual maturity in man; however, S. haematobiumxS. mattheei males possibly do.  相似文献   
147.
2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) destroys central and peripheral noradrenergic neurons, while sparing dopaminergic neurons. Previous studies indicate that 6-OH-DOPA toxicity is mediated by the formation of 6-hydroxydopamine. However, levels of 6-hydroxydopamine in brain following peripheral administration of 6-OH-DOPA have not been documented. In the current study, 6-OH-DOPA and 6-hydroxydopamine were measured in brain by HPLC with electrochemical detection after intraperitoneal injection of 6-OH-DOPA. When mice were injected with 100 mg 6-OH-DOPA/kg, 6-hydroxydopamine levels in the striatum were highest (1.9 microgram/g) at 15 min and fell slowly to 24% of the peak value at 4 h. Experiments with reserpine indicated that the relatively stability of 6-hydroxydopamine was largely dependent upon storage in synaptic vesicles. Reserpine (10 mg/kg) lowered striatal 6-hydroxydopamine levels to 21.6% of control (non-reserpine-treated) values at 1 h, and to 8.9% of control values at 4 h. Levels of 6-hydroxydopamine in the striatum at 1 h were increased 113% by pargyline (100 mg/kg), 145% by alpha-methyldopahydrazine (carbidopa; 25 mg/kg), and 261% by pargyline and carbidopa together. Levels of dopamine in the striatum were unchanged at 2.5 h after 200 mg 6-OH-DOPA/kg (with pargyline and 50 mg carbidopa/kg), whereas levels of norepinephrine in the frontal cortex fell by 77%. At the same time, 6-hydroxydopamine levels were 8.8-fold higher in the striatum (5.54 micrograms/g) than in the cortex (0.63 micrograms/g).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
148.
We have studied the growth on acetate, the metabolism of acetate enzymes, and respiration of a series of citrate synthase mutants of Saccharomyces cerevisiae. The results confirmed and extended our previous observation that cytosolic citrate synthase is not necessary for growth on acetate. Deletion of mitochondrial citrate synthase (CS1) protein resulted in changes in metabolites, decrease in the amounts of pyruvate and alpha-ketoglutarate dehydrogenase complexes, reduced mitochondrial respiration of citrate and isocitrate, and an inability to grow on acetate. Using site-directed mutagensis, we constructed two separate CS1 proteins with mutations in the enzyme's active site. The mitochondria of cells carrying either site-directed mutagenized CS1 contained the inactive citrate synthase protein. With one mutant in which His313 was replaced with a glycine (CS1/H313G), growth on acetate was restored, and mitochondrial respiration of citrate and isocitrate increased toward parental levels as did the levels of several enzymes. With the other mutant CS1 in which Asp414 was replaced with a glycine (CS1/D414G), no growth on acetate or changes in other parameters was observed. We propose that the characteristics of the strain carrying the CS1 with a H313G mutation result from the formation of an intact Krebs cycle complex by the inactive but structurally unchanged H313G protein.  相似文献   
149.
We have investigated the effects of LPS, human rTNF (hrTNF) and human rIL-1 beta (hrIL-1 beta) pretreatment on the intensity of antibody-mediated injury in vivo by using a passive model of anti-glomerular basement membrane (GBM) antibody-mediated nephritis in rats. The experiments show that all three pretreatments exacerbate injury in this model whether judged by albuminuria or the prevalence of glomerular capillary thrombi. The effect on albuminuria was dose dependent with all three treatments. The lowest effective dose of LPS was 0.025 microgram while those for hrTNF and hrIL-1 beta were 0.4 microgram and 0.5 microgram, respectively. All three pretreatments also increased the prevalence of glomerular capillary thrombi which were rare in rats injected with anti-GBM antibodies without pretreatment. LPS pretreatment appeared to be more effective in causing glomerular capillary thrombi than hrTNF or hrIL-1 beta and this was reflected in the correlations between albuminuria and the proportion of glomeruli with capillary thrombi. This relation was linear for all three pretreatments but the slope was appreciably greater for rats pretreated with LPS (0.37) when compared with results from rats given either hrTNF (0.22) or hrIL-1 beta (0.29). Pretreatment of nephritic rats with both cytokines increased the slope to 0.42 demonstrating a synergistic effect. The synergism of hrTNF with hrIL-1 beta was also demonstrated by the effective doses needed to induce albuminuria which was evident in rats treated with 0.05 microgram of IL-1 beta and 0.4 microgram of TNF. Neither the cytokines nor LPS caused clinical, morphologic, or biochemical evidence of renal toxicity when given alone in the dose used here but they did cause a transient increase in the number of neutrophils marginated in glomerular capillaries. Pretreatment of rats with LPS or cytokines increased the glomerular neutrophil influx after anti-GBM antibodies by roughly sixfold. Our experiments show that TNF and IL-1 can increase the severity of glomerular injury in nephritis; they may be important in modulating glomerular injury clinically.  相似文献   
150.
Structural requirements for florigenic activity among gibberellins (GAs) and GA derivatives, including several new ones, applied once to leaves of Lolium temulentum, were examined. The compounds were applied to plants kept either in non-inductive short days (SD) or exposed to one inductive long day (LD). Inflorescence initiation and stem-elongation responses were assessed three weeks later. Among the GAs used, the range in effective dose for inflorescence initiation was more than 1000-fold, but substantially less for stem elongation. Some GAs promoted both stem elongation and inflorescence initiation, some promoted one without the other, and some affected neither. The structural features enhancing florigenic activity were often different from those enhancing stem elongation. Except in the case of 2,2-dimethyl GA4, a double bond in the A ring at either C-1,2 or C-2,3 was essential for high florigenic activity, though not for stem elongation. A free carboxy group was needed for both. Inflorescence initiation in Lolium was enhanced by hydroxylation at C-12, ?13 and ?15, whereas hydroxylation at C-3 reduced the effect on inflorescence initiation but increased that on stem elongation. A 12β-hydroxyl was more effective than the α epimer for inflorescence initiation whereas the reverse was true for stem elongation. Although such differential effectiveness of GAs for inflorescence initiation and for stem elongation could reflect differences in uptake, transport or metabolism, we suggest that it is indicative of specific structural requirements for inflorescence initiation.  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