全文获取类型
收费全文 | 41880篇 |
免费 | 3700篇 |
国内免费 | 205篇 |
专业分类
45785篇 |
出版年
2018年 | 498篇 |
2017年 | 480篇 |
2016年 | 625篇 |
2015年 | 704篇 |
2014年 | 905篇 |
2013年 | 1077篇 |
2012年 | 1222篇 |
2011年 | 1261篇 |
2010年 | 855篇 |
2009年 | 863篇 |
2008年 | 1179篇 |
2007年 | 1168篇 |
2006年 | 1179篇 |
2005年 | 1007篇 |
2004年 | 964篇 |
2003年 | 912篇 |
2002年 | 886篇 |
2001年 | 2777篇 |
2000年 | 2454篇 |
1999年 | 1800篇 |
1998年 | 612篇 |
1997年 | 525篇 |
1996年 | 470篇 |
1995年 | 401篇 |
1994年 | 387篇 |
1992年 | 1280篇 |
1991年 | 1159篇 |
1990年 | 1095篇 |
1989年 | 1043篇 |
1988年 | 929篇 |
1987年 | 926篇 |
1986年 | 802篇 |
1985年 | 794篇 |
1984年 | 563篇 |
1983年 | 513篇 |
1982年 | 367篇 |
1979年 | 632篇 |
1978年 | 451篇 |
1977年 | 416篇 |
1976年 | 368篇 |
1975年 | 495篇 |
1974年 | 575篇 |
1973年 | 554篇 |
1972年 | 572篇 |
1971年 | 548篇 |
1970年 | 522篇 |
1969年 | 527篇 |
1968年 | 427篇 |
1967年 | 373篇 |
1966年 | 409篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
N-caffeoyl-4-amino-n-butyric acid, a new flower-specific metabolite in cultured tobacco cells and tobacco plants 总被引:1,自引:0,他引:1
PUT cells were selected from the XD line of cultured tobacco cells (Nicotiana tabacum L. cv. Xanthi-nc) for the ability to utilize putrescine as sole nitrogen source. Previous work had indicated that hydroxycinnamoylputrescines (principally caffeoylputrescine) and 4-amino-n-butyric acid (GABA) are obligatory intermediates in the assimilation of putrescine by PUT cells. The apparent absence in these cells of diamine or polyamine oxidase and pyrroline dehydrogenase, enzymes which catalyze putrescine oxidation in some plant species, led us to propose the following pathway for putrescine oxidation in PUT cells: putrescine----hydroxycinnamoylputrescine----hydroxycinnamoyl - 4-aminobutyraldehyde----hydroxycinnamoyl-GABA----GABA. We tested the hypothesis by looking for the predicted compound, caffeoyl-GABA. A chemical synthesis was developed, and chromatographic and mass spectroscopic procedures were devised for identifying the compound in extracts of cells and plant tissues. Caffeoyl-GABA was found in extracts of PUT cells in micromolar concentrations but was not present in XD cells. Thus, its occurrence in PUT cells appears to be a direct result of selection for the ability to catabolize putrescine. Caffeoyl-GABA has the same distribution in tobacco plants as caffeoylputrescine, i.e. flower buds greater than open flowers greater than floral leaves, green fruit; absent in vegetative tissues. 相似文献
102.
103.
104.
A L Lautsiavichius R Meshig A M Martsinkiavichius Z I Montvila G Arnol'd 《Biulleten' eksperimental'no? biologii i meditsiny》1989,108(11):519-522
In 12 anaesthetized dogs acute effects of serotonin on the atrioventricular conduction system were studied by obtaining intracardiac electrograms. The significant increase in the heart rate and decrease in aortic pressure elicited by an intravenous infusion of 50 mg/kg.min serotonin for 30 minutes were comparable to those observed in previous investigations. Our data indicate a serotonin induced acceleration of the sinus node pacemaker and atrioventricular conduction by direct and indirect mechanisms. 相似文献
105.
1H-Detected 15N-edited NMR in solution was used to study the conformational differences between the GDP- and GTP gamma S-bound forms of human N-p21ras. The amide protons of 15N-labeled glycine and isoleucine were observed. Resonances were assigned to residues of particular interest, glycines-60 and -75 and isoleucines-21 and -36, by incorporating various 13C-labeled amino acids in addition to [15N]glycine and [15N]iosleucine and by replacing Mg2+ by Co2+. When GTP gamma S replaced GDP in the active site of p21ras, only 5 of the 14 glycine amide resonances show major shifts, indicating that the conformational effects are fairly localized. Responsive glycines-10, -12, -13, and -15 are in the active site. Gly-75, located at the far end of a conformationally-active loop and helix, also responds to substitution of GTP gamma S for GDP, while Gly-77 does not, supporting a role for Gly-75 as a swivel point for the conformational change. The amide proton resonances of isoleucines-36 and -21 and a third unidentified isoleucine also undergo major shifts upon replacement of GDP by GTP gamma S. Thus, the effector-binding loop containing Ile-36 is confirmed to be involved in the conformational change, and the alpha-helix containing Ile-21 is also shown to be affected. 相似文献
106.
T P Chernobel' T I Randmaa Z Ia Maksimova A A Shinkorenko V N Migunov 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1987,(11):88-90
A total of 218 batches of blood preparations produced from different raw materials have been studied by means of enzyme immunoassay kits (Abbott Laboratories, USA). The assays have revealed that the preparations under study are nonstandard with respect to the content and isolation rate of HBsAg, the marker of hepatitis B virus. These data necessitate search for the ways of improving the quality of blood preparations. 相似文献
107.
Kh S Khamitov E I Bogdanov F A Khabirov V Z Khamidullina 《Biulleten' eksperimental'no? biologii i meditsiny》1983,96(11):24-26
Experiments with 22 rats have shown that the anterior tibial muscle in the stage of incomplete reinnervation is marked by decreased force and retardation of the semi-relaxation of an isometric contraction. In completely reinnervated muscles, the changes in the contractility are determined by the degree of nerve damage. The group of animals with the sciatic nerve injury demonstrated the contractility characteristic of a slower muscle, in contrast to the group with the fibular nerve damage. 相似文献
108.
109.
110.