全文获取类型
收费全文 | 4765篇 |
免费 | 348篇 |
专业分类
5113篇 |
出版年
2023年 | 26篇 |
2022年 | 46篇 |
2021年 | 81篇 |
2020年 | 60篇 |
2019年 | 60篇 |
2018年 | 70篇 |
2017年 | 79篇 |
2016年 | 112篇 |
2015年 | 242篇 |
2014年 | 264篇 |
2013年 | 309篇 |
2012年 | 427篇 |
2011年 | 377篇 |
2010年 | 230篇 |
2009年 | 199篇 |
2008年 | 304篇 |
2007年 | 313篇 |
2006年 | 304篇 |
2005年 | 231篇 |
2004年 | 232篇 |
2003年 | 221篇 |
2002年 | 230篇 |
2001年 | 43篇 |
2000年 | 44篇 |
1999年 | 57篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 37篇 |
1995年 | 48篇 |
1994年 | 20篇 |
1993年 | 36篇 |
1992年 | 23篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 20篇 |
1983年 | 20篇 |
1982年 | 22篇 |
1981年 | 10篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 20篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1974年 | 11篇 |
1971年 | 6篇 |
排序方式: 共有5113条查询结果,搜索用时 0 毫秒
41.
42.
Johan Stjernschantz Dale Gregerson Larry Bausher Marvin Sears 《Journal of neurochemistry》1982,38(5):1323-1328
A solid phase enzyme-linked immunosorbent assay for quantitation of substance P is presented. The assay measures the capacity of soluble substance P to compete with the solid phase antigen for a limited quantity of specific substance P antibody. The solid-phase antigen consists of a synthetic substance P.poly-D-glutamic acid conjugate coated to polystyrene micro-ELISA plate wells. Soluble substance P and antibodies to substance P are first preincubated together and then added to the wells containing solid-phase antigen. Subsequently the wells are incubated with anti-antibodies conjugated to alkaline phosphatase. The wells are finally incubated with p-nitrophenyl phosphate an the absorbance is read in a spectrophotometer 16--24 hr after the start of the assay. The threshold for detection of substance P was 5--10 pg per well (0.25 ml). Substance P was extracted from rabbit eyes and the values obtained with the present method are compared with previously reported values based on radioimmunoassay. 相似文献
43.
The green alga Stichococcus bacillaris Naeg. is able to take up at least eleven amino acids. All of these except glutamic and aspartic acids are transported by carrier systems that obey saturation kinetics. The acidic amino acids enter the cell by passive diffusion. Michaelis-Menten parameters (Ks and Vmax) were calculated for several amino acids. All obey simple Michaelis-Menten behavior except for 2-methylalanine and leucine which may have double carrier systems of different affinities. Interactions between pairs of amino acids suggest that there is at least one carrier system specific for basic amino acids and probably several systems specific for neutral amino acids. Further analysis of neutral amino acid interactions reveal that the uptake of several amino acids is incompletely inhibited by competitor uptake at infinite concentration. The simplest interpretation of the data is the operation of three carrier systems for neutral amino acids, one of which has higher affinity and broader specificity than the other two. The amino acid carrier systems appear to operate by an active mechanism. The metabolic poison DCCD inhibits uptake up to 99%. The capacities of the neutral amino acid carrier systems are increased when cells are grown in medium containing suboptimal concentrations of nitrogen. 相似文献
44.
Alain B. Schreiber Johan Hoebeke Bernard Vray A. Donny Strosberg 《Experimental cell research》1981,132(2)
The association and dissociation mechanisms of lectin membrane receptor microclustering on HeLa cells have been studied by measuring resonance energy transfer between fluoresceinated and rhodaminated lentil lectin. Compounds known to affect membrane receptor mobility, such as Ca2+ ions, methylamine, cytochalasin D and nocodazole, did not modify the association kinetics nor the maximal energy transfer values at 4 and 37 °C. Dissociation of the membrane receptor microclusters was followed by measuring the temporal decrease in energy transfer values at 4 °C after preincubation for different time intervals at 37 °C. The rate of dissociation of the lectin receptors decreased in the presence of Ca2+ ions (10−3 M) and after cross-linking with anti-lectin antibodies. An increase was observed in the presence of cytochalasin D (10−6 M) and, to a lesser extent, of methylamine (10−2 M). When cytochalasin D and methylamine were combined at subliminal concentrations, a partial synergistic effect was observed. Nocodazole (10−6 M) had no effect. The results suggest that the association of lectin membrane receptors in microclusters is mediated only by physicochemical parameters. Ca2+ ions, cytochalasin D (microfilaments) and methylamine (transglutaminase)-sensitive components appear, however, to play an important role in the stabilization of the receptor microclusters. 相似文献
45.
Hjördis Thor Peter Moldéus Rolf Hermanson Johan Högberg Donald J. Reed Sten Orrenius 《Archives of biochemistry and biophysics》1978,188(1):122-129
Hepatocytes freshly isolated from diethylmaleate-treated rats exhibited a markedly decreased concentration of reduced glutathione (GSH) which increased to the level present in hepatocytes from nontreated rats upon incubation in a complete medium. When bromobenzene was present in the medium, however, this increase in GSH concentration upon incubation was reversed and a further decrease occurred that resulted in GSH depletion and cell death. This was prevented by metyrapone, an inhibitor of the cytochrome P-450-linked metabolism of bromobenzene. Bromobenzene metabolism in hepatocytes was accompanied by a fraction of metabolites covalently binding to cellular proteins. The size of this fraction, relative to the amount of total metabolites, was increased in hepatocytes isolated from diethylmaleate-treated rats and in hepatocytes from phenobarbital-treated rats incubated with bromobenzene in the presence of 1,2-epoxy-3,3,3-trichloropropane, an inhibitor of microsomal epoxide hydrase which, however, also acted as a GSH-depleting agent. In addition, the metabolism of bromobenzene by hepatocytes was associated with a marked decrease in various coenzyme levels, including coenzyme A, NAD(H), and NADP(H). Cysteine and cysteamine inhibited the formation of protein-bound metabolites of bromobenzene in microsomes, but did not prevent bromobenzene toxicity in hepatocytes when added at higher concentrations to the incubation medium (containing 0.4 mm cysteine). Methionine, on the other hand, did not cause a significant effect on bromobenzene metabolism in microsomes and prevented toxicity in hepatocytes, presumably by stimulating GSH synthesis and thereby decreasing the amount of reactive metabolites available for interaction with other cellular nucleophiles. It is concluded that, in contrast to hepatocytes with normal levels of GSH, hepatocytes from diethylmaleate-treated rats were sensitive to bromobenzene toxicity under our incubation conditions. In this system, bromobenzene metabolism led to GSH depletion and was associated with a progressive decrease in coenzyme A and nicotinamide nucleotide levels and a moderate increase in the formation of metabolites covalently bound to protein. Methionine was a potent protective agent which probably acted by enhanced GSH synthesis via the formation of cystathionine. 相似文献
46.
47.
Johan Zwaan 《Developmental biology》1975,44(2):306-312
Aphakia, an autosomal recessive single gene mutation in the mouse, seriously affects the development of the ocular lens. Up to advanced stages of lens invagination morphogenesis proceeds normally. In the late lens cup and early lens vesicle stage, however, the epithelium of the lens rudiment becomes disorganized and the lumen of the vesicle fills up with rounded cells, apparently released from the epithelium. The lens stalk persists frequently. Probably as a consequence of the aphakic state other parts of the eye secondarily become abnormal.Immunofluorescent studies were done on embryonic normal and aphakia eyes with antisera against adult mouse crystallins. In the normal embryo the first positive reactions were found in the late lens cup stage ( days of gestation). By Day 12 all cells of the lens vesicle were brightly fluorescent. A day later the cells of the posterior wall, now lens fibers, had elongated sufficiently to obliterate the lumen of the vesicle. The entire organ was highly fluorescent, indicating that all of its cells contained large amounts of crystallins. The mutant lens, studied over the same time span, showed no reaction at all. The most likely explanation is, that the multiple structural genes, which normally must be involved in the production of the crystallins, are not expressed up to this time in the mutant.The combination of morphological and biochemical defects suggests that the gene involved in the mutation somehow functions in the control of lens differentiation. 相似文献
48.
49.
50.
FranOis Blanc Michel Leveau Marie-claude Bonin 《International Review of Hydrobiology》1975,60(3):359-378
In the areas studied, an unusual structure and dynamic behavior was exhibited by the plankton ecosystems, due for the most part to industrial and natural effluents from the Rhǒne and Durance. The ecosystem was kept at a low state of maturity, which is characterized by frequent periods of intense multiplication by small species with high metabolic rates, such as the diatom, Skeletonema costatum, and the dinoflagellates, Exuviaella and Prorocentrum. Brackish water seems congenial to that type of proliferation. The turnover rate of these populations decreases as they become older and cell size increases. A lack of competition by species of the same genus is a characteristic of the photo-autotrophic organisms in these environments. Secondary production follows the same cycle as the primary production by the dinoflagellate population. Zooplankton species of the genus Acartia have periods of intensive development in these areas. 相似文献