全文获取类型
收费全文 | 4226篇 |
免费 | 406篇 |
国内免费 | 1篇 |
出版年
2023年 | 25篇 |
2022年 | 40篇 |
2021年 | 93篇 |
2020年 | 63篇 |
2019年 | 75篇 |
2018年 | 57篇 |
2017年 | 61篇 |
2016年 | 105篇 |
2015年 | 205篇 |
2014年 | 205篇 |
2013年 | 247篇 |
2012年 | 303篇 |
2011年 | 270篇 |
2010年 | 168篇 |
2009年 | 144篇 |
2008年 | 196篇 |
2007年 | 224篇 |
2006年 | 177篇 |
2005年 | 162篇 |
2004年 | 117篇 |
2003年 | 118篇 |
2002年 | 101篇 |
2001年 | 80篇 |
2000年 | 85篇 |
1999年 | 77篇 |
1998年 | 47篇 |
1997年 | 43篇 |
1996年 | 40篇 |
1995年 | 34篇 |
1994年 | 23篇 |
1993年 | 23篇 |
1992年 | 46篇 |
1991年 | 51篇 |
1990年 | 51篇 |
1989年 | 59篇 |
1988年 | 45篇 |
1987年 | 51篇 |
1986年 | 42篇 |
1985年 | 38篇 |
1984年 | 39篇 |
1983年 | 49篇 |
1982年 | 35篇 |
1981年 | 23篇 |
1980年 | 25篇 |
1979年 | 43篇 |
1977年 | 30篇 |
1975年 | 25篇 |
1974年 | 25篇 |
1972年 | 33篇 |
1968年 | 25篇 |
排序方式: 共有4633条查询结果,搜索用时 250 毫秒
121.
Deoxycytidine triphosphate deaminase (EC 3.5.4., dCTP aminohydrolase) of Salmonella typhimurium LT2 has been pruified 500-fold. The reaction requires the presence of Mg-2plus, Mn-2plus, Ca-2lus, or Co-2plus. Kinetics of the reaction with varying Mg-2plus concentrations, keeping the concentration of dCTP constant, suggests that the true substrate of the reaction is MgdCTP. The dependence of the rate of reaction on dCTP concentration in the presnece of 5-fold excess of Mg-2plus is sigmoid, with a Hill coefficient of 1.7. The enzyme is specifically inhibited by dTTP and dUTP. In the presence of increasing dTTP concentrations the sigmoidicity of the substrate saturation curves increases. With 0.2 and 0.4 mM dTTP the Hill coefficients are 2.6 and 3.0, respectively. Despite several attempts no dissociation of the substrate site and the inhibitor site of the enzyme was achieved. 相似文献
122.
Summary Previous use of hemolysis time measurement to determine permeability coefficients for the red blood cell membrane rested on the assumption that cells swelling in a hypotonic medium hemolyzed immediately on reaching critical volume. By preswelling red cells to various volumes prior to immersion in hemolytic solutions we extrapolate to the hemolysis time of red cells immersed at critical volume and thereby find a significant period of time during which the cells apparently remain in a spherical form prior to release of hemoglobin. Revised estimates of permeability coefficients follow from including this spherical (nonswelling) phase. In addition, the appreciation of a characteristic time period during which the membrane is under tension provides new opportunity to study physical and chemical properties of the membrane.Presented in part at the 1974 joint meeting of the Biophysical Society and the American Society of Biological Chemists. 相似文献
123.
Previous use of hemolysis time measurement to determine permeability coefficients for the red blood cell membrane rested on the assumption that cells swelling in a hypotonic medium hemolyzed immediately on reaching critical volume. By preswelling red cells to various volumes prior to immersion in hemolytic solutions we extrapolate to the hemolysis time of red cells immersed at critical volume and thereby find a significant period of time during which the cells apparently remain in a spherical form prior to release of hemoglobin. Revised estimates of permeability coefficients follow from including this spherical (nonswelling) phase. In addition, the appreciation of a characteristic time period during which the membrane is under tension provides new opportunity to study physical and chemical properties of the membrane. 相似文献
124.
The sensory organs on the tarsi of the antenniform first legs of the whip spider Admetus pumilio C. L. Koch (Amblypygi, Arachnida) were examined with the scanning and transmission electron microscope. At least four different types of hair sensilla were found: (1) thick-walled bristles, which have the characteristics of contact chemoreceptors (several chemoreceptive dendrites in the lumen plus two mechanoreceptors at the base); (2) short club sensilla, innervated by 4-6 neurons which terminate in a pore on the tip; they are possibly humidity receptors; (3) porous sensilla, which are either innervated by 20-25 neurons and have typical pore tubules, or they have 40-45 neurons but no pore tubules; both types are considered to be olfactory; (4) rod sensilla occur in clusters near segmental borders; they are innervated by only one large dendrite which branches inside the lumen. Other tarsal receptors are the claws, which correspond to contact chemoreceptors, and the pit organ which resembles the tarsal organ of spiders. Compared to other arthropod sensilla, the contact chemoreceptors are very similar to those of spiders, while the porous sensilla correspond structurally to olfactory receptors in insects; the club and rod sensilla seem to be typical for amblypygids. 相似文献
125.
126.
127.
The cellular levels of O-glucosides of 3H-(diH)Z and 3H-(diH)[9R]Z, the major short-term metabolites of 3H-(diH)Z having been exogenously supplied to photoautotrophically growing suspension cell cultures of Chenopodium rubrum, decreased significantly during further culture, irrespective of whether the cells were maintained in the stationary phase or were transferred to conditions restoring cell divison. Metabolism of both compounds was more pronounced during the active growth phase than during the stationary phase. The O-glucosides were converted preferentially to polar compounds of as yet unknown nature, which were partly excreted into the medium. The cellular pools of both glycosides remained compartmented within the vacuole. In contrast to the O-glycosides, the small cellular pools of the aglycones 3H-(diH)Z and 3H-(diH)[9R]Z maintained their level during the experimental period of 30 days. Small amounts of the glucosides, as well as of the aglycones, were recovered from the medium and could have resulted from the lysis of a few cells. The results demonstrate, for the first time, that O-glucosides of cytokinins are not irreversibly deposited within the vacuole of plant cells but may serve to maintain a small, but more or less constant pool of extra-vacuolar, presumably cytosolic, aglycones. (DiH)Z and its derivatives could be demonstrated to be endogenous cytokinins of Chenopodium rubrum suspension cultured cells occurring along with those of the isopentenyladenine and zeatin types. 相似文献
128.
Washed lamellae from isolated spinach chloroplasts exhibited peroxidative activity with 3,3'-diaminobenzidine or ascorbate as electron donors. By heat treatment or by incubation of the chloroplasts with pronase a heat-labile enzymic activity (system A) and a heat-stable non-enzymic peroxidative activity (system B) could be differentiated. System A is membrane-bound, reacts with 3,3'-diaminobenzidine and with ascorbate as electron donors, shows a sharp pH optimum between 7.5 and 8.0 with both substrates and is inhibited competitively by cyanide. The heat-stable factor can be extracted from the chloroplast lamellae by heat treatment, reacts only with ascorbate as electron donor, shows increasing activity with higher pH values but no optimum and is not inhibited by cyanide. Both peroxidative systems in connection with a relatively high concentration of ascorbate in chloroplasts should represent an important tool for the detoxification of H2O2 which is produced in these organelles by photosynthetic O2 reduction. 相似文献
129.
130.
Light microscope immunofluorescence was used to localize the membrane binding of Bacillus thuringiensis kurstaki 63-kDa delta-endotoxin in Heliothis virescens midgut and Malpighian tubules. Staining was observed along all exposed mucosal (apical microvillar) plasma membranes. Interpretation of the serosal (basal) plasma membrane staining was complicated because the basal lamina also stained. The results suggest that the toxin binds to all exposed plasma membranes without apparent specificity for particular membrane domains. 相似文献