全文获取类型
收费全文 | 6498篇 |
免费 | 788篇 |
国内免费 | 3篇 |
专业分类
7289篇 |
出版年
2023年 | 18篇 |
2022年 | 40篇 |
2021年 | 107篇 |
2020年 | 53篇 |
2019年 | 76篇 |
2018年 | 60篇 |
2017年 | 71篇 |
2016年 | 140篇 |
2015年 | 248篇 |
2014年 | 272篇 |
2013年 | 303篇 |
2012年 | 456篇 |
2011年 | 478篇 |
2010年 | 252篇 |
2009年 | 255篇 |
2008年 | 393篇 |
2007年 | 428篇 |
2006年 | 386篇 |
2005年 | 417篇 |
2004年 | 391篇 |
2003年 | 411篇 |
2002年 | 368篇 |
2001年 | 69篇 |
2000年 | 42篇 |
1999年 | 92篇 |
1998年 | 115篇 |
1997年 | 86篇 |
1996年 | 83篇 |
1995年 | 69篇 |
1994年 | 76篇 |
1993年 | 71篇 |
1992年 | 56篇 |
1991年 | 52篇 |
1990年 | 47篇 |
1989年 | 42篇 |
1988年 | 42篇 |
1987年 | 57篇 |
1986年 | 34篇 |
1985年 | 47篇 |
1984年 | 57篇 |
1983年 | 50篇 |
1982年 | 81篇 |
1981年 | 51篇 |
1980年 | 52篇 |
1979年 | 38篇 |
1978年 | 26篇 |
1977年 | 38篇 |
1976年 | 19篇 |
1975年 | 20篇 |
1974年 | 18篇 |
排序方式: 共有7289条查询结果,搜索用时 0 毫秒
91.
Optically active diastercoisomers of the brominated monoterpene quinols, cyclocymopol and cyclocymopol monomethyl ether, were isolated from the green marine alga Cymopolia barbata and charac- terized. 相似文献
92.
Computer simulations showed that reproductive synchrony in the greater investing sex can evolve as a strategy for increasing the investment of mates. The potential for synchrony to increase in frequency for this reason decreased with (a) increasing population size, (b) decreasing importance of synchrony-enforced investment, (c) increasing variance in quality among members of the lesser investing sex, and (d) decreasing male: female sex ratio. By reducing the payoffs to desertion, synchrony may result in the evolution of uniquely effective forms of parental care by the lesser investing sex, making desertion unprofitable regardless of the availability of potential mates. Thus relative levels of optimal parental investment are subject to change through the action of sexual selection itself. 相似文献
93.
The assembly of tetrameric prolyl hydroxylase in tendon fibroblasts from newly synthesized α-subunits and from preformed cross-reacting protein 下载免费PDF全文
Embryonic-chick tendon cells were incubated in suspension for 4h with (14)C-labelled amino acids, cell extracts were subjected to gel filtration, and the effluent was examined by rocket immunoelectrophoresis by using antibodies specific for the beta-subunit of chick prolyl hydroxylase. Two peaks of immunoreactive protein were found. The first peak contained 40% of the immunoreactive protein eluted from the column and 100% of the enzyme activity. Polyacrylamide-slab-gel electrophoresis in sodium dodecyl sulphate of an immunoprecipitate of this peak demonstrated that it consisted of the tetrameric form of prolyl hydroxylase, subunit composition alpha(2)beta(2) where alpha and beta are non-identical subunits. Only the alpha-subunits were labelled, indicating that they were synthesized during the 4h labelling period. The beta-subunits were unlabelled, indicating that they had been synthesized before the labelling period. The second peak eluted from the gel-filtration column contained 60% of the immunoreactive protein eluted from the column and was enzymically inactive. Polyacrylamide-slab-gel electrophoresis of an immunoprecipitate of this peak indicated that it consisted of a single labelled polypeptide chain, identified as cross-reacting protein, which was related to, but not identical with, the beta-subunit of prolyl hydroxylase. Pulse-chase experiments were performed on cultured chick tendon cells to demonstrate that alpha-subunits and cross-reacting protein had half-lives of about 60h. The half-life of beta-subunits was considerably longer, and the kinetic pattern was consistent with their being derived from a labelled precursor such as cross-reacting protein. The data presented here indicate that the active tetrameric form of prolyl hydroxylase in cells is assembled from alpha-subunits which are newly synthesized, and from beta-subunits which are derived from cross-reacting protein. 相似文献
94.
When the thymidine analog BrdU was incorporated into the DNA of a fibroblast cell line derived from the cactus mouse Peromyscus eremicus, a chromosome region with an increased frequency of gaps and breaks was observed. Nearly a third of the chromatid aberrations found at this site were associated with a sister-chromatid exchange (SCE) although this chromosome region showed no increase in sister-chromatid exchange in the absence of a gap or break. SCEs were significantly decreased in the remainder of the chromosome arm when it contained an aberration at the unstable site. This BrdU-sensitive region, unlike others reported, was found not to be late-replicating. — In this chromosome complement, the frequency of sisterchromatid exchange in C-band positive regions was significantly lower than that in C-band negative regions. 相似文献
95.
96.
Samuel B. Jones Jr. William C. Burnett Jr. Nancy C. Coile Tom J. Mabry M. F. Betkouski 《Oecologia》1979,39(1):71-77
Summary Sesquiterpene lactone glaucolide-A from Vernonia, incorporated in the rearing diets of five species of Lepidoptera, significantly reduced the rate of growth of larvae of the southern armyworm, Spodoptera eridania; fall armyworm, S. frugiperda; and yellowstriped armyworm, S. ornithogalli. Quantitative feeding tests demonstrated that decreased feeding levels and reduced growth resulted from ingestion of a sesquiterpene lactone. Ingestion of glaucolide-A increased the number of days to pupation in four of the species. In the southern armyworm, it significantly reduced pupal weight. Glaucolide-A decidedly reduced percentage of survival of southern and fall armyworms. Yellow woollybear, Diacrisia virginica, and cabbage looper, Trichoplusia ni, larvae were essentially uneffected by the ingestion of the sesquiterpene lactone. Sesquiterpene lactones adversely affect growth rate and survival of certain insects that feed upon plants containing them. They apparently function as defensive products, screening out a portion of the potential herbivores.Vernonieae: Compositae 相似文献
97.
A review of the literature on macaque play, emphasizing the social correlates of such behavior, is presented and summarized.
In general, males are more playful than females, adults are relatively nonplayful, and nurturant behavior is negatively correlated
with play. However, the currently inadequate definitions, conceptualizations, and experimental treatments of play make generalizations
tentative.
Supported in part by faculty research grant D-552, University of California, Davis, and by RR00169 to the California Primate
Research Center. 相似文献
98.
The 8-hydroxy derivative of adenine (6-amino-1,7-dihydro-8H-purin-8-one) is produced from adenine by two Oerskovia xanthineolytica strains. This transformation by a microorganism has not been reported previously. No novel products of dissimilation of xanthine (3,7-dihydro-1H-purine-2,6-dione) or hypoxanthine (1,7-dihydro-6H-purin-6-one) were found. Xanthine was oxidized to uric acid, but intermediates in the breakdown of hypoxanthine could not be demonstrated. 相似文献
99.
A protein which contains 2-aminoethylphosphonic acid (AEP) has been isolated from the ciliate protozoan Tetrahymena thermophila. The protein contains about 30% carbohydrate with both N- and O-glycosidic linkages to the polypeptide and 8% AEP which is attached only to the O-linked glycoside. The amino group of AEP is unreactive to dansyl chloride as is the amino terminus of the protein. The polypeptide portion of the molecule, Mr 22,500, contains 22% glycine, 5.5% hydroxyproline, and is quite acidic. The phosphoprotein is found in the cell membranes. Its synthesis is inhibited by tunicamycin to the same extent which the antibiotic inhibits cell division. 相似文献
100.