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   17733篇
  免费   1580篇
  国内免费   23篇
  2022年   127篇
  2021年   256篇
  2020年   125篇
  2019年   184篇
  2018年   240篇
  2017年   188篇
  2016年   370篇
  2015年   619篇
  2014年   690篇
  2013年   922篇
  2012年   1146篇
  2011年   1168篇
  2010年   770篇
  2009年   731篇
  2008年   994篇
  2007年   977篇
  2006年   950篇
  2005年   945篇
  2004年   968篇
  2003年   969篇
  2002年   979篇
  2001年   264篇
  2000年   194篇
  1999年   223篇
  1998年   260篇
  1997年   199篇
  1996年   156篇
  1995年   140篇
  1994年   156篇
  1993年   176篇
  1992年   152篇
  1991年   155篇
  1990年   142篇
  1989年   112篇
  1988年   127篇
  1987年   128篇
  1986年   119篇
  1985年   153篇
  1984年   125篇
  1983年   137篇
  1982年   153篇
  1981年   162篇
  1980年   133篇
  1979年   104篇
  1978年   109篇
  1977年   100篇
  1976年   106篇
  1975年   75篇
  1974年   111篇
  1973年   89篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
212.
Assessing functional multidrug resistance (MDR) status in clinical biopsy material using drug autofluorescence has potential applications to clinical management. The small size of many cystoscopy specimens has led us to develop, as an alternative to flow cytometry, a protocol for studying epirubicin accumulation in adherent colonies of primary bladder cancer cells viewed live andin situ by confocal microscopy. The limitations to quantitation inherent in this technique are compensated for by preservation of cellular organisation and the elimination of non-malignant cells. Biopsy material is disaggregated and explanted into culture-grade petri dishes. After incubation for three to seven days plaques of epithelial cells have developed. Classical patterns of sensitive and resistant drug distribution are observed. Cells of the rolled edges of the colony accumulate more drug than those of the inner epithelial monolayer. Some central areas of larger colonies give the appearance of drug arrested at the intercellular junctions to give a fenestrated pattern. These observations contribute to the understanding of mechanisms in MDR as well as forming the basis for a clinical urological MDR evaluation protocol.  相似文献   
213.
Summary The use of DNA amplification fingerprinting (DAF) as a tool for monitoring mixed microbial populations in bioreactors was evaluated. Short (8-mer or 10-mer) oligonucleotides were used to prime DNA extracts from various biological reactors during polymerase chain reaction (PCR) amplification. The reactors examined in this study included two sets of anaerobic stirred tank continuous flow bioreactors. One set of anaerobic reactors was operated under methanogenic conditions and one set was operated under sulfate-reducing conditions. The anaerobic reactor communities in the methanol-fed reactors showed extensive DAF homology. DAF was also applied to a fixed-film azo dye degrading reactor to examine the degree of uniformity of colonization of the substratum in representative regions of the reactor. This method is a quick and relatively inexpensive means of monitoring microbial community structure during biological processes. Since no cultivation of the sample is involved, the genetic profile of the community is not biased by outgrowth conditions. DAF profiles may be useful for comparisons of population changes over time or of bench-scale vs pilot-scale reactors but not adequate for assessing community diversity.  相似文献   
214.
The effects of surfactant concentration on the hydrolytic activity of Candida rugosa lipase in AOT/isooctane reverse micelles with olive oil as the substrate has been investigated. A noncompetitive inhibition by the surfactant on the enzyme was observed. Strong dependences of the kinetic constants kcat and kM, but not kI on the water-to-surfactant ratio (R value) have been identified. The benefits of carrying out the hydrolysis at higher surfactant and water concentrations were demonstrated from the improvement of the initial rate and time course of conversion.  相似文献   
215.
GAPs for rho-related GTPases   总被引:26,自引:0,他引:26  
Ras-related GTP-binding proteins (GTPases) of the rbo subfamily play important roles in regulating the organization of the actin cytoskeleton. A large number of multifunctional proteins that can stimulate their intrinsic GTPase activity have been identified. Here, we discuss the nature of such GTPase-activating proteins (GAPs) and their potential importance for cell signalling.  相似文献   
216.
Summary In a prior study we combined game theory and inclusive fitness models to examine whether the guarded altruism that can evolve among non-relatives (tit for tat, TFT) might also evolve among close relatives, supplanting unconditional altruism. In most cases, TFT replaced unconditional altruism in family-structured models. Even when TFT is selected at a single locus, however, by withholding altruism from non-reciprocating relatives it may qualify as an outlaw from the standpoint of modifier genes at other loci. Here we examine this possibility with a series of haploid, two-locus models in which a modifier gene transforms TFT into unconditional altruism. The modifier allele spreads to fixation whenever Hamilton's Rule is satisfied, resulting in an unconditional altruist replacing the TFT strategy. As such, TFT may be regarded as an outlaw vulnerable to suppression by alleles at other loci.  相似文献   
217.
Iturin A: A potential new fungicide for stored grains   总被引:6,自引:0,他引:6  
The removal of many synthetic fungicides from the market has created a demand for new, environmentally safe fungicides. Iturin A, a cyclic lipopeptide produced byBacillus subtilis, has strong antifungal properties and low mammalian toxicity. To determine the efficacy of this compound as a potential fungicide on stored feed grains, lots of corn, peanuts and cottonseed were treated with varying concentrations of iturin A. The mycoflora of treated seed was assayed along with that of untreated seed and seed treated with fungicides used commercially for planting seed. Fungal species varied considerably in their sensitivity to iturin A. Significant reductions in total mycoflora occurred in most seed lots tested at iturin A concentrations of 50 to 100 ppm.  相似文献   
218.
The distribution of neurones immunoreactive to antisera raised against the undecapeptide C-terminal fragment of drosulfakinin II (DrmSKII), Asp-Gln-Phe-Asp-Asp-Tyr(SO3H)-Gly-His-Met-Arg-Phe-NH2, has been studied in the blowfly Calliphora vomitoria. Antisera were preabsorbed with combinations of the parent antigen, the tetrapeptide Phe-Met-Arg-Phe-NH2 and cholecystokinin, the vertebrate sulfated octapeptide (CCK-8), Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2, in order to ensure specificity for the sulfakinin peptides of C. vomitoria (the nonapeptide callisulfakinin I is identical to drosulfakinin I and callisulfakinin II differs from DrmSK II only by the presence of -Glu3-Glu4- in place of -Asp3-Asp4-). Only four pairs of sulfakinin-immunoreactive neurones have been visualised in the entire nervous system. These occur in the brain: two pairs of cells situated medially in the caudo-dorsal region close to the roots of the ocellar nerve and two other pairs at the same level but positioned more laterally. Despite the small number of sulfakinin-immunoreactive cells, there are extensive projections to many areas of neuropile in the brain and the thoracic ganglion. The pathway of the medial sulfakinin cells extends into each of the three thoracic ganglia and a metameric arrangement of sulfakinin neuronal projections is also seen in the abdominal ganglia. Neither the dorsal neural sheath of the thoracic ganglion, nor the abdominal nerves contain sulfakinin-immunoreactive material. These observations suggest that the sulfakinins of the blowfly function as neurotransmitters or neuromodulators. They do not appear to have a direct role in gut physiology, as has been shown by in vitro bioassays for the sulfakinins of orthopterans and blattodeans. In addition to the neurones that display specific sulfakinin immunoreactivity, other cells within the brain and thoracic ganglion are immunoreactive to cholecystokinin/gastrin antisera. There are, therefore, at least two types of dipteran neuropeptides with amino acid sequences that are similar to the vertebrate molecules cholecystokinin and gastrin.  相似文献   
219.
Chaos in three species food chains   总被引:7,自引:0,他引:7  
We study the dynamics of a three species food chain using bifurcation theory to demonstrate the existence of chaotic dynamics in the neighborhood of the equilibrium where the top species in the food chain is absent. The goal of our study is to demonstrate the presence of chaos in a class of ecological models, rather than just in a specific model. This work extends earlier numerical studies of a particular system by Hastings and Powell (1991) by showing that chaos occurs in a class of ecological models. The mathematical techniques we use are based on work by Guckenheimer and Holmes (1983) on co-dimension two bifurcations. However, restrictions on the equations we study imposed by ecological assumptions require a new and somewhat different analysis.  相似文献   
220.
Spinach plants were grown in bowls of aerated nutrient solutionin a controlled environment chamber for 24 h, and harvestedevery 3·5-5 h to record their growth, nitrate and wateruptake, and plant nitrate concentration. Twelve such experimentsare described, either with a 14/10 h dark/light regime, or continuouslight or darkness. The irradiance was either 110, 320, or 510µmol m-2 s-1 (PPFD). All these regimes began at the endof the light period of a 14/10 h dark/light regime (510 µmolm-2 s-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almostunabated at about 17 µmol h-1 through the initial 14-hdark period, and then fell away sharply if the light was notrestored, but increased slightly when it was. With continuouslight at 510 µmol m-2 s-1, uptake rate rose steadily forthe first 24 h of light, and then fell sharply for about 6 h.Shoot nitrate concentration increased about three-fold in thedark phase, and declined in the light at a rate which was positivelyrelated to the irradiance. Root nitrate concentration was severaltimes higher than that of the shoot: its diurnal change wassmaller (relative to the mean) than that of the shoot. Nitratereduction occurred to a small extent in the dark, and increasedrapidly as soon as the lights came on, to remain at a roughlyconstant rate (related to the irradiance) throughout the lightphase. Dry matter increase in the light was related to irradiance,but with little increase above 320 µmol m-2 s-1. Respiratoryweight loss in the dark was not detectable. Rate of fresh weightincrease was approximately constant throughout light and darkperiods. The results compare quite well with the predictions of a simplesimulation model, based on the pump/leak principle.Copyright1994, 1999 Academic Press Spinacia oleracea, nitrate, uptake, reduction, influx, efflux, diurnal, regulation, model, simulation  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