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   3050篇
  免费   252篇
  国内免费   2篇
  2021年   33篇
  2020年   21篇
  2019年   14篇
  2018年   22篇
  2017年   25篇
  2016年   61篇
  2015年   90篇
  2014年   110篇
  2013年   144篇
  2012年   146篇
  2011年   150篇
  2010年   117篇
  2009年   117篇
  2008年   147篇
  2007年   152篇
  2006年   134篇
  2005年   132篇
  2004年   134篇
  2003年   113篇
  2002年   136篇
  2001年   64篇
  2000年   54篇
  1999年   43篇
  1998年   42篇
  1997年   44篇
  1996年   36篇
  1995年   37篇
  1994年   29篇
  1993年   24篇
  1992年   32篇
  1991年   23篇
  1990年   24篇
  1989年   37篇
  1988年   27篇
  1987年   22篇
  1985年   26篇
  1984年   31篇
  1983年   21篇
  1982年   28篇
  1981年   32篇
  1980年   16篇
  1979年   19篇
  1978年   19篇
  1977年   23篇
  1976年   20篇
  1973年   14篇
  1968年   18篇
  1963年   15篇
  1960年   16篇
  1951年   14篇
排序方式: 共有3304条查询结果,搜索用时 31 毫秒
71.
A lectin which agglutinates Zajdela hepatoma cells; rat red cells and lymphocytes, but no normal rat liver cells, was detected in the mucus, yielded by simple saline extraction, of the two snail species Arion empiricorum (Fér.) and Arion lusitanicus (MAB). The agglutination spectrum involves also human erythrocytes and red cells of several animal species.  相似文献   
72.
Characterization of ionomycin as a calcium ionophore.   总被引:12,自引:0,他引:12  
The ionophorous properties of a new antibiotic, ionomycin, have been studied. It was found that the antibiotic is capable of extracting calcium ion from the bulk of an aqueous phase into an organic phase. The antibiotic also acts as a mobile ion carrier to transport the cation across a solvent barrier. The divalent cation selectivity order for ionomycin as determined by ion competition experiments was found to be: Ca greater than Mg greater than Sr = Ba, where the binding of strontium and barium by the antibiotic is insignificant. The antibiotic also binds La3+ to some extent, but its complexation with monovalent alkali metal ions is negligible. Measurement of the binding of ionomycin with Ca2+ indicates that ionomycin complexes and transports calcium ion in a one to one stoichiometry.  相似文献   
73.
Die Seitenzweige von Cupressus sempervirens L. sind auf vier Orthostichen in scheinbar mehr oder weniger zufälligen, voneinander unabhängigen Mustern angeordnet. Eine auf L-Systeme sich berufende mathematische Konstruktion gestattet die Definition eines Morphismus. Acht Parameter, Periodizität der absoluten Wachstumsrate der zugrundeliegenden theoretischen Serien, die Väriation dieser Raten in komplementaren Unterserien und eine Schwellenreaktion sind die Hauptargumente.  相似文献   
74.
75.
76.
Rhodopsin in bovine photoreceptor disk membranes was subjected to limited proteolysis by thermolysin, removing twelve amino acids from rhodopsin's carboxyl terminus. (1) The rate of proteolysis is significantly faster with rhodopsin following exposure to light than with unbleached rhodopsin, provided that the incubation conditions (pH, temperature) favor the formation of metarhodopsin II. (2) If the disk membranes are illuminated under conditions in which metarhodopsin I is the predominant photoproduct (pH 8.5, 0°C), no increase in the rate of proteolysis is observed compared to unilluminated membranes. (3) The light-induced increase in the rate of proteolysis is transient: it slowly decays in the dark to the original rate found for unbleached rhodopsin. The enhanced susceptibility to proteolysis appears to measure a conformational change at rhodopsin's cytoplasmic surface which is first exhibited at the metarhodopsin II stage. This and possibly other light-dependent changes may allow rhodopsin to mediate its signal as a light-receptor protein by binding to and activating certain rod cell enzymes.  相似文献   
77.
Ultra-small colloidal gold (less than 1 nm), bound to Fab fragments provides the shortest practical specific marker system to date and can be used in concert with field emission scanning electron microscopes to precisely locate antigenic sites. An "in-lens" FE-SEM equipped with a highly sensitive single crystal YAG-detector for backscattered electrons, as well as the use of advanced specimen preparation techniques based on cryofixation, are among the indispensible prerequisites. A T-even type Escherichia coli bacteriophage, Tu II*-46, was chosen to study properties of the immunogold labeling system. Distinct regions on the tail fibers of this phage were labeled with Fab fragments derived from antibodies against the related phage Tu II*-6. The tail fibers are composed of pairs of homologous proteins, thus offering two identical antigenic sites at the same locus on the tail fibers. Fab fragments can be visualized in the SEM at high accelerating voltage (30 kV) without any additional marker. This permits comparison of the labeling characteristics of unmarked and colloidal gold-marked Fab fragments. Unmarked Fab fragments often bind by pairs (two singlet Fab fragments bound opposed to each other along the axis of the tail fiber). The labeling efficiency of unmarked Fab fragments was greater than that of ultra-small gold-labeled Fab fragments. Binding by pairs was not seen after labeling with ultra-small gold-Fab fragments. The conjugates used in this study exhibited one colloidal gold per Fab fragment.  相似文献   
78.
High levels of Lp[a] in blood form an independent risk factor for atherosclerosis. Oxidative modification of Lp[a] may be involved in the suggested atherogenic action of Lp[a]. After Cu(2+)-mediated oxidative modification of the 440 kDa and 610 kDa apo[a] isoforms of lipoprotein[a] (Ox-Lp[a]), the in vivo fate was investigated in rats. Ox-Lp[a], when injected into rats, was rapidly removed from the blood circulation by the liver, in which the intrahepatic fate is dependent on the degree of oxidation of the isoforms. Upon oxidation to a slightly increased negative charge of Lp[a], the high molecular weight form of Lp[a] is recognized more efficiently by the Kupffer cells than by the endothelial cells. When the liver uptake of Ox-Lp[a] is blocked by preinjection of polyinosinic acid (poly I), the association of Ox-Lp[a] with the rat heart is increased 20-fold. In vitro studies show that the association and degradation of 125I-labeled Ox-Lp[a] with liver endothelial and Kupffer cells was inhibited by oxidized LDL (Ox-LDL), poly I, or Ox-Lp[a] itself by 60-90%, while only a partial competition was found with acetylated-LDL (up to 25%). In conclusion, after oxidative modification of Lp[a], there is recognition of Ox-Lp[a] by specific oxidized-lipoprotein receptors on liver endothelial and Kupffer cells; the relative importance at low degrees of oxidation of Lp[a] is dependent on the molecular weight of the apo[a] isoforms. Under conditions in which liver uptake is not adequate, the deposition of Ox-Lp[a] in the heart may be of potential pathological importance.  相似文献   
79.
The gene cluster that codes for feedback-resistant aspartate kinase (lysCα and lysCβ) and aspartate semialdehyde dehydrogenase (asd) was cloned from a mutant strain of Corynebacterium glutamicum. Its functional analysis by subcloning, enzyme assays, and type of aspartate kinase regulation enabled the isolation of a fragment for separate expression of the feedback-resistant kinase without aspartate semialdehyde dehydrogenase expression. This was used together with other clones constructed (J. Cremer, L. Eggeling, and H. Sahm, Mol. Gen. Genet. 220:478-480, 1990) to overexpress individually each of the six genes that convert aspartate to lysine. Analysis of lysine formation revealed that overexpression of the feedback-resistant kinase alone suffices to achieve lysine formation (38 mM). Also, sole overexpression of wild-type dihydrodipicolinate synthase resulted in lysine formation but in a lower amount (11 mM). The other four enzymes had no effect on lysine secretion. With a plasmid overexpressing both relevant enzymes together, a further increase in lysine yield was obtained. This shows that of the six enzymes that convert aspartate to lysine the kinase and the synthase are responsible for flow control in the wild-type background and can be useful for construction of lysine-producing strains.  相似文献   
80.
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