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   7016篇
  免费   779篇
  国内免费   688篇
  2023年   60篇
  2022年   146篇
  2021年   250篇
  2020年   223篇
  2019年   273篇
  2018年   236篇
  2017年   204篇
  2016年   283篇
  2015年   354篇
  2014年   469篇
  2013年   489篇
  2012年   543篇
  2011年   478篇
  2010年   349篇
  2009年   335篇
  2008年   402篇
  2007年   341篇
  2006年   317篇
  2005年   271篇
  2004年   310篇
  2003年   253篇
  2002年   272篇
  2001年   187篇
  2000年   200篇
  1999年   134篇
  1998年   91篇
  1997年   93篇
  1996年   73篇
  1995年   60篇
  1994年   45篇
  1993年   44篇
  1992年   69篇
  1991年   57篇
  1990年   29篇
  1989年   39篇
  1988年   28篇
  1987年   35篇
  1986年   21篇
  1985年   20篇
  1984年   33篇
  1983年   26篇
  1982年   20篇
  1981年   22篇
  1980年   24篇
  1979年   23篇
  1978年   26篇
  1977年   18篇
  1976年   19篇
  1975年   29篇
  1974年   20篇
排序方式: 共有8483条查询结果,搜索用时 359 毫秒
61.
The effect of ABA on the membrane potential of barley (Hordeumvulgare cv. Himalaya) aleurone protoplasts was studied by measuringthe distribution of the lipophilic cation tetraphenylphosphonium(TPP+). The resting membrane potential (Em) according to ourmeasurements with TPP+ is about –53 mV and is in agreementwith membrane potential values as measured with intracellularmicroelectrodes (about –55 mV). The TPP+-measurementscould demonstrate a clear dependence of the resting Em on theexternal pH (pHe). Stimulation of the protoplasts with ABA induced a transienthyperpolarization of the membrane to –62 mV as measuredwith TPP+. The hyperpolarization was ABA-concentration dependent. Inhibition of the H+-ATPases with the specific proton pump inhibitorsdiethylstilbestrol (DES) or Micanozole effectively preventedhyperpolarization. This indicates that the hyperpolarizationis consistent with the activation of plasma membrane H+-ATPases.The K+-inward rectifier inhibitor BaCl2 was able to prolongthe hyperpolarization. This result suggests that the hyperpolarizationcauses the opening of K+-channels. The ABA-induced proton-pump activation may be involved in ABA-inducedgene-expression, as DES was able to inhibit this gene expression.BaCl2 did only show a slight inhibitory effect on ABA-inducedgene-expression. (Received January 4, 1994; Accepted April 12, 1994)  相似文献   
62.
63.
The in vitro uptake of zinc by erythrocytes was measured under near-physiological conditions, using65Zn as a radioactive tracer. Because of the presence of serum albumin—a strong zinc ligand—a low concentration of medium free zinc was maintained. Under these conditions a high-affinity carrier for zinc transport over the cell membrane was identified. With human erythrocytes, a Michaelis constant (K m ) of 0.2 nM with respect to free medium zinc was measured and aV max of 4.5 nmoles Zn transported per h/g dry wt. TheK m for medium Zn increases when the size of the internal erythrocytic Zn pool is augmented, whereasV max remains virtually unchanged. A model to explain this phenomenon is proposed. It is suggested that this phenomenon could underlie observations, confirmed here, that the in vitro uptake of Zn by animal erythrocytes depends on the Zn status of the animal.  相似文献   
64.
Inorganic cofactors (Mn, Ca2+ and Cl-) are essential for oxidation of H2O to O2 by Photosystem II. The Mn reductants NH2OH and its N-methyl derivatives have been employed as probes to further examine the interactions between these species and Mn at the active site of H2O oxidation. Results of these studies show that the size of a hydroxylamine derivative regulates its ability to inactivate O2 evolution activity, and that this size-dependent inhibition behavior arises from the protein structure of Photosystem II. A set of anions (Cl-, F- and SO4 2-) is able to slow NH2OH and CH3NHOH inactivation of intact Photosystem II membranes by exerting a stabilizing influence on the extrinsic 23 and 17 kDa polypeptides. In contrast to this non-specific anion effect, only Cl- is capable of attenuating CH3NHOH and (CH3)2NOH inhibition in salt-washed preparations lacking the 23 and 17 kDa polypeptides. However, Cl- fails to protect against NH2OH inhibition in salt-washed membranes. These results indicate that the attack by NH2OH and its N-methyl derivatives on Mn occurs at different sites in the O2-evolving complex. The small reductant NH2OH acts at a Cl--insensitive site whereas the inhibitions by CH3NHOH and (CH3)2NOH involve a site that is Cl- sensitive. These findings are consistent with earlier studies showing that the size of primary amines controls the Cl- sensitivity of their binding to Mn in the O2-evolving complex.Abbreviation MES 4-morpholinoethanesulfonic acid - PS II Photosystem II  相似文献   
65.
Abstract. A model is proposed for the fitting of species-area curves to data from the Stellenbosch region, South Africa. Basic assumptions of the model are finiteness of the number of species in a finite area, and random distribution of plant species over the region. The model involves a distribution of densities of different species, and the parameters of this distribution are useful for describing and classifying communities. The data of the Stellenbosch region suggest that the assumptions of the model break down in areas greater than 500 m2.  相似文献   
66.
人肝刺激因子对大鼠实验性慢性肝损伤的保护作用   总被引:3,自引:0,他引:3  
从健康孕妇水囊引产4─6个月龄的胎儿取肝,采用LaBrecque方法提取人肝刺激因子(hHSS)。经3H-胸腺嘧啶核苷参入肝DNA法测定其生物活性。表明此hHSS可刺激肝细胞DNA合成。采用皮下注射CCl4和饮用10%乙醇来制备慢性肝损伤动物模型,观察了hHSS的保护肝脏作用。结果表明:hHSS可使CCl4-乙醇所致慢性肝损伤大鼠的死亡率、血清谷丙转氨酶水平、肝组织中羟脯氨酸含量的升高以及肝组织中丙二醛的含量降低。肝组织切片表明:hHSS能减轻肝组织的损伤程度,促进肝细胞再生,并能明显防止肝纤维化的形成和发展。可见,hHSS对CCl4-乙醇所致的慢性肝损伤大鼠有明显的保护作用,其机制可能与促进肝细胞再生及抑制肝细胞膜的脂质过氧化有关。  相似文献   
67.
用四氯化碳(CCl4)损伤正常大鼠后,采用Western印迹法和免疫组化法观察肝细胞原癌基因(c-fos/c-jun)的表达。Western印迹法表明,当成年大鼠的静息期肝细胞受到CCl4损伤性刺激后,c-fos/c-jun产物(Fos和Jun)水平升高,在CCl4处理后30min开始升高,在4h时消失。8h后Fos/Jun再度出现,并持续24h以上。ICC法表明,Jun阳性细胞为靠近肝中央静脉区的肝实质细胞。根据上述资料推测,肝受CCl4损伤后肝细胞的原癌基因c-fos/c-jun出现即时的与滞后的两次表达,这与肝细胞进入细胞周期有关,这种基因表达也许可作为肝再生过程中识别特殊体液因子的标志。  相似文献   
68.
单纯疱疹病毒(HSV)Ⅰ型及Ⅱ型之间有很多共同抗原,能引起血清学交叉反应,鉴别诊断比较困难。本实验利用重组DNA技术,将部分HSV-2DNA的PstI片段克隆到载体质粒PSK中,并筛选出两个重组质粒(P和P)只与HSV-2反应,与HSV-1不反应,这两个重组质粒中所含的HSV-2DNA片段大小分别是3.1和4.3kb,另外,还筛选了一个重组质粒(PHSV2-1,含5.8kbHSV-2DNA片段)与HSV-1和HSV-2均反应。将4.3kb的片段用光生物素标记后作为探针检测了159份人阴道拭子,其中23份样品呈阳性反应,其余均为阴性,从23份阳性样品中挑选12价涂片用间接荧光抗体法检测也都呈阳性反应,随机挑选的几份杂交反应阴性样品在间接荧光试验中也是阴性。本实验制备的HSV通用及HSV-2型特异性探针将比常规的血清学方法诊断和鉴别HSV-1和HSV-2感染更为可靠。  相似文献   
69.
Fructan as a New Carbohydrate Sink in Transgenic Potato Plants   总被引:10,自引:0,他引:10       下载免费PDF全文
Fructans are polyfructose molecules that function as nonstructural storage carbohydrates in several plant species that are important crops. We have been studying plants for their ability to synthesize and degrade fructans to determine if this ability is advantageous. We have also been analyzing the ability to synthesize fructan in relation to other nonstructural carbohydrate storage forms like starch. To study this, we induced fructan accumulation in normally non-fructan-storing plants and analyzed the metabolic and physiological properties of such plants. The normally non-fructan-storing potato plant was modified by introducing the microbial fructosyltransferase genes so that it could accumulate fructans. Constructs were created so that the fructosyltransferase genes of either Bacillus subtilis (sacB) or Streptococcus mutans (ftf) were fused to the vacuolar targeting sequence of the yeast carboxypeptidase Y (cpy) gene. These constructs were placed under the control of the constitutive cauliflower mosaic virus 35S promoter and introduced into potato tissue. The regenerated potato plants accumulated high molecular mass (>5 [times] 106 D) fructan molecules in which the degree of polymerization of fructose units exceeded 25,000. Fructan accumulation was detected in every plant tissue tested. The fructan content in the transgenic potato plants tested varied between 1 and 30% of dry weight in leaves and 1 and 7% of dry weight in microtubers. Total nonstructural neutral carbohydrate content in leaves of soil-grown plants increased dramatically from 7% in the wild type to 35% in transgenic plants. Our results demonstrated that potato plants can be manipulated to store a foreign carbohydrate by introducing bacterial fructosyltransferase genes. This modification affected photosynthate partitioning in microtubers and leaves and increased nonstructural carbohydrate content in leaves.  相似文献   
70.
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