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   2062篇
  免费   175篇
  国内免费   124篇
  2361篇
  2023年   53篇
  2022年   36篇
  2021年   78篇
  2020年   60篇
  2019年   93篇
  2018年   86篇
  2017年   51篇
  2016年   73篇
  2015年   72篇
  2014年   119篇
  2013年   160篇
  2012年   82篇
  2011年   97篇
  2010年   71篇
  2009年   74篇
  2008年   89篇
  2007年   115篇
  2006年   82篇
  2005年   71篇
  2004年   51篇
  2003年   55篇
  2002年   48篇
  2001年   45篇
  2000年   40篇
  1999年   26篇
  1998年   28篇
  1997年   21篇
  1996年   15篇
  1995年   17篇
  1994年   22篇
  1993年   16篇
  1992年   21篇
  1991年   25篇
  1990年   31篇
  1989年   15篇
  1988年   16篇
  1987年   21篇
  1986年   21篇
  1985年   23篇
  1984年   18篇
  1983年   18篇
  1982年   33篇
  1981年   25篇
  1980年   25篇
  1979年   27篇
  1978年   23篇
  1977年   17篇
  1975年   12篇
  1973年   10篇
  1972年   11篇
排序方式: 共有2361条查询结果,搜索用时 0 毫秒
61.
Dipankar Sen 《Phytochemistry》1975,14(7):1505-1506
For measurement of cytochrome c oxidase activity in intact plant mitochondria the optimum concentration of K-Pi buffer and pH in the reaction was found to be 75 mM and 7.4 respectively. The suitable concentration of K-Pi buffer for suspending and storing mitochondria, however, was found to be 20 mM or lower. These requirements applied equally well for mitochondria from wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), maize (Zea mays L.), and snap bean (Phaseolus vulgaris L.).  相似文献   
62.
E. K. J. Risse, J. P. Holierhoek, E. M. Meijer‐Marres, E. Ouwerkerk‐Noordam and M. E. Boon Increased diagnostic accuracy of atypical glandular cells in cervical liquid‐based cytology using cell blocks Objective: The purpose of this study was to reduce the number of diagnoses of atypical glandular cells (AGC). Residual material from the cervical ThinPrep® samples (Hologic, Marlboruogh, MA, USA) was used for cell blocks (CB) and immunohistochemistry (IHC). Methods: In 2007 there were 87 patients (0.12% of tests) with AGC on liquid‐based cytology (LBC) in the Leiden Cytology and Pathology Laboratory (LCPL) using the Bethesda System 2001 (TBS). CB with IHC was used for 26 of these cases. The vials still containing the brush (Cervex‐Brush® Combi) were placed in a shaker for 10 minutes to dislodge the material trapped between the bristles. The residual sampling fluid was used to prepare paraffin sections (Shandon Cytoblock®) stained with Papanicolaou and immunostaining. Results: Four of five cases with AGC not otherwise specified (NOS) were diagnosed with CB/IHC as benign mimics (endometrium, tubal metaplasia, follicular cervicitis, microglandular hyperplasia) and one of four with AGC‐favour neoplasia (FN) (endocervical polyp). In one of five cases with AGC‐NOS and in two of seven with AGC‐FN, CIN3 was found on subsequent histological biopsy. Of six cases diagnosed as adenocarcinoma in situ (AIS) on LBC with CB/IHC the diagnosis was confirmed in four; one was adenocarcinoma and one glandular atypia. Of eight cases diagnosed as adenocarcinoma on cytology and CB/IHC, the diagnosis was confirmed in three. The other five cases were found to be one each of AIS, squamous cell carcinoma, CIN3, CIN2 with glandular atypia, and cervical endometriosis. Conclusions: By reducing the number of benign mimics of AGC, we achieved a high proportion (16/26; 61.5%) of neoplastic or preneoplastic lesions (glandular or squamous) on histological outcome potentially avoiding colposcopy. Histological biopsy verification by the gynaecologist is needed for final diagnosis of AGC‐FN, AIS and adenocarcinoma.  相似文献   
63.
目的:探讨光动力疗法(PDT)辅助治疗轻中度牙周炎患者的临床疗效。方法:选取我院2016年1月-2017年8月收治的轻中度牙周炎患者46例为研究对象,共选取患牙276颗,随机分为观察组与对照组,每组138颗患牙,对照组予以龈下刮治术和根面平整术(SRP)治疗,观察组在对照组的基础上联合PDT治疗,比较治疗前、治疗后1个月、2个月、3个月观察两组患者的牙周袋探诊深度(PD)、探诊出血(BOP)阳性率、出血指数(BI)。结果:治疗后1个月、2个月、3个月,两组PD均随着时间的推移逐渐变浅(P0.05),且观察组治疗后各时间点均浅于对照组(P0.05);两组BOP阳性率均较治疗前降低,且观察组治疗后各时间均低于对照组(P0.05);两组BI随着时间的推移呈逐渐下降趋势(P0.05),且观察组治疗后各时间点BI均低于对照组(P0.05)。结论:PDT辅助治疗轻中度牙周炎患者的临床疗效较好,其能减小PD,降低BI和BOP阳性率。  相似文献   
64.
Parental care is a major component of reproduction in social organisms, particularly during the foundation steps. Because investment into parental care is often costly, each parent is predicted to maximize its fitness by providing less care than its partner. However, this sexual conflict is expected to be low in species with lifelong monogamy, because the fitness of each parent is typically tied to the other's input. Somewhat surprisingly, the outcomes of this tug‐of‐war between maternal and paternal investments have received important attention in vertebrate species, but remain less known in invertebrates. In this study, we investigated how queens and kings share their investment into parental care and other social interactions during colony foundation in two termites with lifelong monogamy: the invasive species Reticulitermes flavipes and the native species R. grassei. Behaviors of royal pairs were recorded during six months using a non‐invasive approach. Our results showed that queens and kings exhibit unbalanced investment in terms of grooming, antennation, trophallaxis, and vibration behavior. Moreover, both parents show behavioral differences toward their partner or their descendants. Our results also revealed differences among species, with R. flavipes exhibiting shorter periods of grooming and antennation toward eggs or partners. They also did more stomodeal trophallaxis and less vibration behavior. Overall, this study emphasizes that despite lifelong monogamy, the two parents are not equally involved in the measured forms of parental care and suggests that kings might be specialized in other tasks. It also indicates that males could play a central, yet poorly studied role in the evolution and maintenance of the eusocial organization.  相似文献   
65.
水面和水下长航对艇员肠道菌群影响的比较及意义探讨   总被引:4,自引:3,他引:4  
比较研究和水下两种长期航行条件对航艇和核潜艇艇员肠道菌群的影响并对其意义进行初步探讨。结果显示两种长航条件均对人体肠道菌群产生显著影响,可打破肠道微生态平衡,但两者所致的肠道菌群构成变化的特点不同。提示不同长航条件可造成不同的微生态失调,需区别对待。  相似文献   
66.
PurposeThis study compared the positioning accuracy between cone-beam CT (CBCT) and ExacTrac (ETX) for a single-isocenter multiple target stereotactic radiosurgery (SRS) on two TrueBeam STx systems.MethodsA single-isocenter treatment plan was simulated on an anthropomorphic head phantom with six spherical steel ball bearings (BBs). One of the BBs was chosen to be the isocenter. The five off-isocenter targets were located at various distances from the isocenter. MV portal images were generated to evaluate the deviations between the expected and the real center of the targets after CBCT and ETX positioning, respectively.ResultsThe evaluation of the positioning accuracy for the isocenter target showed that CBCT and ETX positioning provided comparable, sub-millimetric results. Deviations in positioning accuracy were also calculated for all other targets, also showing comparable results for CBCT and ETX. Moreover, our study showed that the deviation between CBCT and ETX positioning were in better agreement for TBSTx1 and deviated slightly higher on TBSTx2 (maximum: 1.23 mm at S/I direction), due to a less perfect alignment between the CBCT coordinate system and the ETX coordinate system on TBSTx2 compared to TBSTx1. This study also showed a correlation between the target positioning accuracy and the distance to the isocenter.ConclusionThe positioning accuracy of ETX and CBCT for targets located at isocenter and off-isocenter locations was compared on two treatment machines and found comparable. Our study highlights the importance of a proper calibration procedure, to ensure correct alignment between the CBCT, ETX and machine coordinate systems.  相似文献   
67.
Restoration efforts will be taking place over the next decade(s) in the largest scope and capacity ever seen. Immense commitments, goals, and budgets are set, with impactful wide‐reaching potential benefits for people and the environment. These are ambitious aims for a relatively new branch of science and practice. It is time for restoration action to scale up, the legacy of which could impact over 350 million hectares targeted for the U.N. Decade on Ecosystem Restoration. However, restoration still proceeds on a case‐by‐case, trial by error basis and restoration outcomes can be variable even under similar conditions. The ability to put each case into context—what about it worked, what did not, and why—is something that the synthesis of data across studies can facilitate. The link between data synthesis and predictive capacity is strong. There are examples of extremely ambitious and successful efforts to compile data in structured, standardized databases which have led to valuable insights across regional and global scales in other branches of science. There is opportunity and challenge in compiling, standardizing, and synthesizing restoration monitoring data to inform the future of restoration practice and science. Through global collation of restoration data, knowledge gaps can be addressed and data synthesized to advance toward a more predictive science to inform more consistent success. The interdisciplinary potential of restoration ecology sits just over the horizon of this decade. Through truly collaborative synthesis across foci within the restoration community, we have the opportunity to rapidly reach that potential and achieve extraordinary outcomes together.  相似文献   
68.
Maternal adrenalectomy at 7 or 14 days of gestation produced increased cell necrosis within zona reticularis cells on the day of birth and at 24 or 48 h after birth. Small remnants or large portions of adrenocortical cells were present within macrophages. In otherwise normal adrenocortical cells, lipid droplets were incorporated within some mitochondria. Autophagocytosis of single mitochondria was observed within adrenocortical cells. Undoubtedly ultrastructural changes represent stimulation of adrenocortical cells in neonatal rats in response to maternal adrenalectomy.  相似文献   
69.
In polyandrous social insects such as honey bees, a worker’s affinity for a particular task may be genetically infl uenced and so some patrilines may have lower stimulus thresholds for commencing a task than others. We used simulation models to investigate the effects of intracolonial diversity in the task thresholds that stimulate workers to engage in heating and cooling during nest thermoregulation. First, we simulated colonies comprised of one or 15 patrilines that were engaged in heating the brood nest, and observed that single patriline colonies maintained, on average, less stable brood nest temperatures than multiple patriline colonies. Second we simulated colonies with five patrilines that were engaged in cooling their nest, recording the proportions of bees of different patrilines that engaged in nest cooling in response to changing temperatures. Both of our simulations show remarkably similar qualitative patterns to those that we have previously observed empirically. This provides further support for the hypothesis that geneticallybased variability in task thresholds among patrilines within honey bee colonies is an important contributor to the ability of colonies to precisely thermoregulate their nests, and we suggest that diversity is important for optimal expression of a range of other colony-level phenotypes. Received 17 June 2005; revised 27 October 2005; accepted 23 December 2005.  相似文献   
70.
Subtilisin 72 was immobilized on cryogel of poly(vinyl alcohol), the macroporous carrier prepared by the freeze-thaw-treatment of concentrated aqueous solution of the polymer. The obtained biocatalyst was active and stable in aqueous, aqueous-organic, as well as in low water media. The stability of immobilized biocatalyst was substantially higher than that of native enzyme in all mixtures especially in aqueous buffer containing 5–8 M Urea and in acetonitrile/60–90%DMF mixtures. The ability of native and immobilized subtilisin to catalyze peptide bond formation between Z-Ala-Ala-Leu-OMe and Phe-pNA was studied in non-aqueous media. Considerable enzyme stabilization in acetonitrile/90%DMF mixture, induced by the immobilization, resulted in higher product yield (57%) than in case of native subtilisin suspension (32%). Detailed study of synthesis reaction revealed that notable increase in product yield could be reached using increase in both substrate concentrations up to 200 mM.  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