全文获取类型
收费全文 | 1456篇 |
免费 | 164篇 |
专业分类
1620篇 |
出版年
2022年 | 11篇 |
2021年 | 20篇 |
2020年 | 21篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 26篇 |
2016年 | 34篇 |
2015年 | 51篇 |
2014年 | 50篇 |
2013年 | 76篇 |
2012年 | 74篇 |
2011年 | 70篇 |
2010年 | 55篇 |
2009年 | 49篇 |
2008年 | 58篇 |
2007年 | 66篇 |
2006年 | 66篇 |
2005年 | 52篇 |
2004年 | 43篇 |
2003年 | 48篇 |
2002年 | 43篇 |
2001年 | 53篇 |
2000年 | 53篇 |
1999年 | 48篇 |
1998年 | 18篇 |
1997年 | 20篇 |
1996年 | 9篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 21篇 |
1992年 | 28篇 |
1991年 | 24篇 |
1990年 | 31篇 |
1989年 | 24篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 17篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 15篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1975年 | 15篇 |
1974年 | 17篇 |
1973年 | 14篇 |
1972年 | 16篇 |
1971年 | 10篇 |
排序方式: 共有1620条查询结果,搜索用时 0 毫秒
1.
Anatomical and neurophysiological findings have demonstrated neuronal connections between the diencephalic habenular nuclei and brain stem serotonergic raphe nuclei. Therefore we examined some neurochemical consequences of habenular lesions. Sixteen hours and one week after bilateral lesions serotonin metabolism (as reflected by concentrations of its metabolite, 5-hydroxyindoleacetic acid) was significantly increased in the dorsal but not the median raphe nuclei. Unilateral lesions produced a proportionally smaller augmentation. Motron locomotor activity was enhanced during the light and dark illumination phases in lesioned animals but only attained statistical significance during the day. 相似文献
2.
Stephen B. Vander Wall 《Animal behaviour》1982,30(1):84-94
Five hypotheses of cache recovery behaviour in Clark's nutcracker (Nucifraga columbiana) were examined experimentally. Most caches were made in soil within 5 cm of conspicuous large objects. Both seed-caching and non-seed-caching nutcrackers were able to locate caches. Seed-caching nutcrackers relocated caches using large objects as remembered visual cues. Soil microtopography and small (<2 cm diameter) objects may be used as cues to facilitate cache recovery but are not essential. Non-seed-caching nutcrackers located caches by using soil disturbances at cache sites as visual cues and by searching preferentially near objects where caches were concentrated. Success rates of seed-caching nutcrackers ranged from 52 to 78% and those of non-seed-caching nutcrackers ranged from 8 to 12%. Nutcrackers do not use random search or olfactory cues to locate caches. 相似文献
3.
M Lebl E E Sugg G van Binst P Vander Elst D Tourwé J Slaninová V J Hruby 《International journal of peptide and protein research》1987,30(3):318-322
Analogs of deamino-oxytocin and deamino-oxypressin containing a CH2-NH group instead of an amide bond between positions 8 and 9 were synthesized. All tested compounds exhibit significantly lowered biological activities. 相似文献
4.
Insulin-mediated modifications of myocardial lipoprotein lipase and lipoprotein metabolism 总被引:1,自引:0,他引:1
Recirculating organ perfusion in vitro was conducted with hearts from control rats, animals given a single dose of streptozotocin (65 mg/kg) 48 h earlier, and streptozotocin-treated rats administered insulin (5 units), 2 h prior to organ perfusion. During 45-min perfusions, the lipolysis of very low density lipoprotein (VLDL) triglyceride was significantly less in hearts from diabetics than in controls (41.9 +/- 7.3% of control). This was associated with significant reductions in heparin-releasable (functional) lipoprotein lipase and tissue lipoprotein lipase of perfused hearts. The decreases in VLDL triglyceride metabolism and the levels of myocardial lipoprotein lipase were completely reversed by treatment of diabetic rats with insulin 2 h prior to study. Similar improvement of VLDL triglyceride metabolism and increases in myocardial lipoprotein lipase activity were observed in hearts from diabetic rats by direct addition of 100 milliunits/ml of insulin to the recirculating perfusion media. Under these conditions, the increase in both fractions of lipoprotein lipase in response to insulin was completely inhibited, and utilization of VLDL triglyceride was partially inhibited by pre-perfusion with cycloheximide for 10 min. The data derived from either VLDL triglyceride lipolysis in organ perfusion or direct measurement of myocardial lipoprotein lipase demonstrate a direct effect of insulin on myocardial lipoprotein lipase activity, and suggest that the response to insulin may be due in part to effects on protein synthesis. 相似文献
5.
Cross-comparisons of nectar production data are complicated because different workers use bags made of various materials to exclude animal visitors. Using clonal populations of Asclepias syriaca and A. exaltata in northern Virginia, we carefully measured the effects of four bagging treatments (bridal veil, pellon, paper, plastic) on microenvironment (temperature, relative humidity) and nectar production (volume, concentration, sucrose amount) over the course of a day. In general, bridal veil bags changed the microenvironment least relative to unbagged controls. Plastic bags resulted in higher temperatures and constantly higher relative humidities. Temperature and relative humidity were also elevated, though less dramatically, in paper and pellon bags. Under more humid conditions, flowers contained larger volumes of more dilute nectar. Therefore, researchers who wish to obtain nectar production data that reflect natural field conditions should use bridal veil, or a material with similar properties, to bag inflorescences. We also performed a watering experiment, involving the addition of the equivalent of a 10-cm rain to the A. syriaca plot. After watering, nectar volumes and sucrose amounts were increased approximately twofold. 相似文献
6.
Leukoencephalomalacia similar to equine leukoencephalomalacia occurred in a white-tailed deer (Odocoileus virginianus) from North Carolina. The deer was recumbent and stuporous when presented for examination. It was believed that the disease resulted from ingestion of moldy field corn, infected with Fusarium moniliforme. 相似文献
7.
Characterization of adenine phosphoribosyltransferase from the human malaria parasite, Plasmodium falciparum 总被引:2,自引:0,他引:2
Because of their inability to synthesize purines de novo, malaria parasites rely on purine phosphoribosyltransferases (PRTases) to convert purine bases salvaged from the host cell (the erythrocyte) into the corresponding purine nucleoside monophosphates. Our studies with late trophozoites of the human malaria parasite, Plasmodium falciparum, showed that virtually all of the purine PRTase activity is accounted for by two distinct enzymes. One enzyme utilizes hypoxanthine, guanine and xanthine (Queen, S.A., Vander Jagt, D. and Reyes, P. (1988) Mol. Biochem. Parasitol. 30, 123-134). The second enzyme utilizes only adenine and is the subject of this paper. This latter enzyme exhibits a biphasic pH-activity profile and is moderately to weakly inhibited by several divalent metal ions. Several of the properties of the P. falciparum enzyme were found to differ significantly from those of human erythrocyte adenine PRTase. (1) The molecular weight (18,000) of the parasite enzyme is smaller than that of the host cell enzyme. (2) The parasite enzyme, unlike the erythrocyte enzyme, is not significantly inhibited by sulfhydryl reagents. (3) 6-Mercaptopurine and 2,6-diaminopurine proved to be competitive inhibitors of the parasite enzyme (Ki 0.70 and 1.0 mM, respectively); on the other hand, the human enzyme is not inhibited by these agents. (4) The Km for adenine (0.80 microM) and 5-phosphoribosyl-1-pyrophosphate (0.70 microM) displayed by the parasite enzyme are significantly smaller than the corresponding Km values shown by the erythrocyte enzyme. These distinctions between the parasite and host enzymes point to the possibility that adenine PRTase of P. falciparum may represent a potential target for chemotherapeutic attack. 相似文献
8.
9.
Relation of R Factor and Chromosomal β-Lactamase with the Periplasmic Space 总被引:3,自引:2,他引:1 下载免费PDF全文
The release of several R factor and chromosomal beta-lactamases by osmotic shock treatment was studied. It was found that those beta-lactamases with a molecular weight of about 20,000 were released, but those with a molecular weight of about 30,000 to 44,000 were not released during osmotic shock. This differential release did not depend on whether the structural genes were on the chromosome or on the genome of an R factor. The release or retention of the beta-lactamases appeared to be a characteristic of the enzyme rather than the host cell since the same results were obtained when the R factors were harbored by a variety of host bacteria. Studies with bacteria which produced more than one beta-lactamase showed that each enzyme reacted independently to the presence of other beta-lactamases produced by the host bacterium. 相似文献
10.