全文获取类型
收费全文 | 3620篇 |
免费 | 284篇 |
国内免费 | 1篇 |
专业分类
3905篇 |
出版年
2023年 | 12篇 |
2022年 | 31篇 |
2021年 | 47篇 |
2020年 | 44篇 |
2019年 | 43篇 |
2018年 | 47篇 |
2017年 | 40篇 |
2016年 | 99篇 |
2015年 | 154篇 |
2014年 | 167篇 |
2013年 | 233篇 |
2012年 | 283篇 |
2011年 | 253篇 |
2010年 | 147篇 |
2009年 | 138篇 |
2008年 | 202篇 |
2007年 | 221篇 |
2006年 | 210篇 |
2005年 | 202篇 |
2004年 | 185篇 |
2003年 | 166篇 |
2002年 | 168篇 |
2001年 | 52篇 |
2000年 | 40篇 |
1999年 | 45篇 |
1998年 | 35篇 |
1997年 | 41篇 |
1996年 | 27篇 |
1995年 | 26篇 |
1994年 | 31篇 |
1993年 | 30篇 |
1992年 | 24篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 28篇 |
1987年 | 16篇 |
1986年 | 26篇 |
1985年 | 15篇 |
1984年 | 24篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 20篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 18篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 24篇 |
排序方式: 共有3905条查询结果,搜索用时 0 毫秒
61.
Ingrid Lombeck K. Kasperek L. E. Feinendegen H. J. Bremer 《Biological trace element research》1980,2(3):193-198
The rubidium content of whole blood was estimated by instrumental neutron activation analysis. In 46 healthy children it amounts
to {ie193-1} g/g dry weight. There was no difference between the values found for infants, toddlers, and school children.
In 29 dietetically treated patients with phenylketonuria and maple-syrup-urine disease the values were significantly lower
than in healthy children. During the first three months of diet therapy the rubidium levels remained in the lower range of
the normal values, decreasing to about 60% of the mean of normal values later on. With increasing length of diet therapy these
values tended to decrease. It remains questionable whether these decreased levels reflect only an induced biochemical phenomenon
without biological importance, or whether they are the first signs of a deficiency syndrome. 相似文献
62.
Lars-Åke Fransson Anders Malmström Ingrid Sjöberg Thomas N. Huckerby 《Carbohydrate research》1980,80(1):131-145
Heparin, heparan sulphate, and various derivatives thereof have been oxidised with periodate at pH 3.0 and 4° and at pH 7.0 and 37°. Whereas oxidation under the latter conditions destroys all of the nonsulphated uronic acids, treatment with periodate at low pH and temperature causes selective oxidation of uronic acid residues. The reactivity of uronic acid residues depends on the nature of neighbouring 2-amino-2-deoxyglucose residues. d-Glucuronic acid residues are susceptible to oxidation when flanked by N-acetylated amino sugars, but resistant when adjacent residues are either unsubstituted or N-sulphated. L-Iduronic acid residues in their natural environment (2-deoxy-2-sulphoamino-d-glucose) are resistant to oxidation, whereas removal of N-sulphate groups renders a portion of these residues periodate-sensitive. Oxidised uronic acid residues in heparin-related glycans may be cleaved by alkali, producing a series of oligosaccharide fragments. Thus, periodate oxidation-alkaline elimination provides an additional method for the controlled degradation of heparin. 相似文献
63.
Summary Membrane-bounded spherical vesicles found in rat Sertoli cells have been examined quantitatively during the cycle of the seminiferous epithelium. Most of the vesicles were localized to the basal and columnar portions of the Sertoli cell cytoplasm. The thin lateral projections of the Sertoli cells contained very few vesicles. Morphometric analysis of the basal portion of the Sertoli cell cytoplasm revealed that the volume density (V
v
) of the vesicles changed markedly during the cycle. The V
v
was at its minimum (0.036) at stage VII and maximum (0.117) at stages XI-I. The vesicles were also smaller at stage VII compared to the vesicles at stages IX-V. The stage-dependent difference in the size of the vesicles was found both in the basal and the columnar portions of the Sertoli cells. At stage VII some of the vesicles appeared to be elongated much like the tubular elements of the smooth endoplasmic reticulum (SER) from which they are probably derived. The stage-dependent differences in volume density and size of the Sertoli cell vesicles may be related to cyclic biochemical variations in the Sertoli cells, and are further indications of a variation in Sertoli cell function during the cycle of the seminiferous epithelium. Whether or not this is due to an internal cycle of the Sertoli cell or to influences from adjacent germ cells remains to be determined. 相似文献
64.
Virginia E. Papaioannou John D. West Theodor Bücher Ingrid M. Linke 《Genesis (New York, N.Y. : 2000)》1981,2(3):305-315
We have used a sensitive electrophoretic technique for estimating the activity, or ratio, of two allozymes of the X-chromosome-linked enzyme phosphoglycerate kinase (PGK-1), in order to investigate the randomness of X-chromosome expression in the derivatives of the three primary cell lineages of the early mouse conceptus. The maternally derived Pgk-1 allele is preferentially expressed in the derivatives of the primitive endoderm and trophectoderm lineages at 6 1/2 days post coitum in Pgk-1a/Pgk-1b heterozygous conceptuses, and in the one informative 5 1/2-day heterozygous conceptus analysed. This evidence for preferential expression of the maternally derived X chromosome (Xm), so soon after the time of X-chromosome inactivation, favors the possibility that the preferential expression of Xm is a consequence of primary non-random X-chromosome inactivation, rather than a secondary selection phenomenon. The majority of embryos analysed at 4 1/2 and 5 1/2 days pc produced only a single PGK-1 band, corresponding to the allozyme produced by the Pgk-1 allele on Xm, although 50% of these embryos should have been heterozygous females. Possible explanations are discussed. 相似文献
65.
A simple and selective assay for monomeric and filamentous actin is presented, based on the inhibition of DNAase I by actin. In mixtures of monomeric and filamentous actin, only the monomeric form is measured as DNAase inhibitor. The total amount of actin in a sample can be determined after depolymerization of F actin with guanidine hydrochloride. The assay is rapid enough to detect changes in the polymerization state of actin in vitro over time intervals as short as 3 min. Data characterizing unpolymerized and filamentous actin pools in extracts of human platelets, lymphocytes and HeLa cells are presented. 相似文献
66.
67.
68.
The nonproteinogenic amino acid, cyclopentenylglycine, is found in certain Flacourtiaceae. This compound may be synthesized by two C1-chain elongations of -ketoglutarate via -ketopimelate (C5+2C1) or by condensation of C4 and C3 units (C4+C3), a pathway not involving -ketopimelate. The following experimental design elucidated the biosynthetic pathway: Idesia polycarpa callus cultures were freshly established from leaf petioles; synthetic -[1,2-14C]ketopimelate was added to the medium and cultures were incubated for 3 weeks. After isolation and separation of free amino acids from the tissues, the radioactivity incorporated into cyclopentenylglycine was determined. The results establish -ketopimelate as a precursor for cyclopentenylglycine, thus providing evidence for the C5+2C1 biosynthetic path. 相似文献
69.
70.
Young Atlantic salmon, Salmo salar , brown trout, Salmo trutta , and Arctic char, Salvelinus alpinus , from eight rivers in North and Mid Norway were examined for Gyrodactylus. The fish were collected from 1975 to 1980. A Gyrodactylus salaris type was observed infecting salmon from six of these rivers. No trout or char were infected. A high frequency and intensity of infection of salmon were observed in all but one of the rivers surveyed. In the Saltdalselva, only one specimen of Gyrodactylus infecting one fish was observed. No subsequent mortality of salmon was observed in this river while there were signs of a high mortality of salmon in the other rivers. The salmon parr were more frequently attacked than the fry, the mortality of salmon seemed to have happened in most of the rivers one year after the first observations of Gyrodactylus were made. The mortality of salmon and the high infection rate of Gyrodactylus in these rivers appears unique, and as far as known to the authors there are no other described cases of mortality due to Gyodactylus in Atlantic salmon in natural waters. The reasons for the outbreak of Gyrodactylus in these rivers are not known. Two theories are discussed: one that the fish were weakened by environmental factors and the other that Gyrodactylus was introduced from some of the infected salmon hatcheries in Scandinavia. 相似文献