全文获取类型
收费全文 | 3363篇 |
免费 | 121篇 |
国内免费 | 168篇 |
专业分类
3652篇 |
出版年
2023年 | 38篇 |
2022年 | 71篇 |
2021年 | 63篇 |
2020年 | 59篇 |
2019年 | 103篇 |
2018年 | 104篇 |
2017年 | 68篇 |
2016年 | 58篇 |
2015年 | 66篇 |
2014年 | 157篇 |
2013年 | 175篇 |
2012年 | 144篇 |
2011年 | 153篇 |
2010年 | 111篇 |
2009年 | 149篇 |
2008年 | 144篇 |
2007年 | 172篇 |
2006年 | 112篇 |
2005年 | 116篇 |
2004年 | 95篇 |
2003年 | 86篇 |
2002年 | 78篇 |
2001年 | 33篇 |
2000年 | 33篇 |
1999年 | 51篇 |
1998年 | 46篇 |
1997年 | 24篇 |
1996年 | 38篇 |
1995年 | 44篇 |
1994年 | 27篇 |
1993年 | 35篇 |
1992年 | 41篇 |
1991年 | 52篇 |
1990年 | 28篇 |
1989年 | 28篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1985年 | 67篇 |
1984年 | 116篇 |
1983年 | 70篇 |
1982年 | 92篇 |
1981年 | 83篇 |
1980年 | 66篇 |
1979年 | 48篇 |
1978年 | 36篇 |
1977年 | 52篇 |
1976年 | 43篇 |
1975年 | 44篇 |
1974年 | 45篇 |
1973年 | 22篇 |
排序方式: 共有3652条查询结果,搜索用时 8 毫秒
31.
The red marine algae Microcladia borealis, M. californica and M. coulteri produce several unusual halogenated monoterpenes including violacene, plocamene-B, plocamene-C, and plocamane-D. The isolation of these terpenes along with a study of their variation in each Microcladia at different locations are described. 相似文献
32.
Joseph W. Mayo William M. Wallace Leroy W. Matthews Don M. Carlson 《Archives of biochemistry and biophysics》1976,175(2):507-513
Submandibular secretions collected from children with cystic fibrosis (CF) showed increased protein concentration (milligrams/milliliter) and increased amylase specific activity (units/milligram of protein) relative to normal secretions. These differences between normal (N) and CF secretions were as follows: protein, 1.25 ± 0.51 (N), 1.75 ± 0.35 (CF) (P < 0.02); and amylase, 58 ± 18 (N), 80 ± 19 (CF) (P < 0.001). To determine the basis for elevated protein in CF saliva, several major proteins resolved by polyacrylamide disc gel electrophoresis were quantitated by densitometry. These included four phosphoproteins (PP), serum albumin, an acid phosphatase-containing fraction, amylase, and an unidentified protein referred to as PI-7.1. Together, these proteins comprise greater than 75% of the total protein in the secretion. Differences in individual protein concentrations (milligrams/milliliter) resolved from normal and CF secretions, respectively, were as follows: PP2, 0.02 ± 0.01, 0.03 ± 0.02 (NS, not significant); PP3, 0.06 ± 0.04, 0.05 ± 0.03 (NS); acid phosphatase fraction, 0.06 ± 0.04, 0.12 ± 0.07 (P < 0.05); amylase, 0.09 ± 0.04, 0.27 ± 0.16 (P < 0.01); and pI-7.1, 0.04 ± 0.02, 0.13 ± 0.08 (P < 0.02). Amylase, the most significant contributor to the elevated protein, comprised 26% of the total protein of normal secretions and 38% of the total protein of CF secretions. Thus, our results do not support the concept of a generalized increase in all organic components in CF submandibular secretions but, rather, increases in specific proteins, namely amylase, component pI-7.1, and an acid phosphatase-containing fraction. 相似文献
33.
J.-P. Villeneuve P. Mavier J.-G. Joly 《Biochemical and biophysical research communications》1976,70(3):723-728
Cytochrome P-450 was partially purified from liver microsomes obtained from control, ethanol, phenobarbital, and 3-methylcholanthrene-treated rats. Benzphetamine demethylation, benzpyrene hydroxylation and aniline hydroxylation activities were assayed in a reconstituted system using fixed amounts of reductase and lipids, and increasing amounts of cytochrome P-450 from each source. Cytochrome P-450 from ethanol-fed rats showed substrate specificity differing from cytochrome P-450 obtained from control, phenobarbital and 3-methylcholanthrene-treated rats. 相似文献
34.
B. S. Chertow G. A. Williams Georgiann R. Baker R. D. Surbaugh G. K. Hargis 《Experimental cell research》1975,93(2):388-394
To determine the role of subcellular organelles in hormone secretion, we studied the interaction of low calcium concentration (low Ca), retinol (vitamin A, vit A), vinblastine (VB), and cytochalasin B (CB) in parathyroid hormone (PTH) secretion. Bovine parathyroid tissue pieces were incubated in media containing the above agents. Vit A stimulated PTH release to a mean of 170% of control. This effect of vit A was diminished when tissues were simultaneously stimulated with low Ca and, furthermore, absent when tissues were pre-incubated in low Ca.VB had no effect on low Ca-stimulated secretion, but did inhibit vit A-induced secretion in the presence of low Ca.CB stimulated PTH secretion to a mean of 150% of control during the second and third hours of incubation. CB had at least an additive effect with low Ca in stimulating PTH secretion, with a more prompt and greater response than seen in normal calcium. VB did not inhibit the acute effect of CB on secretion in normal calcium media, but did inhibit CB-induced secretion during the third hour of incubation.None of the agents stimulated the release of lysosomal cathepsin D, and vit A and CB did not stimulate the release of LDH.Our results suggest that; (1) vit A and low Ca stimulate PTH secretion through a common pathway involving the cell membrane; (2) CB stimulates PTH secretion through a separate effect on the cell membrane or submembrane microfilaments, which normally retards secretion of PTH; and (3) microtubular proteins may facilitate basal secretion of PTH, but are not involved in low Ca-stimulated secretion of PTH. 相似文献
35.
36.
J. Jansen M. Fedecostante M.J. Wilmer L.P. van den Heuvel J.G. Hoenderop R. Masereeuw 《Biotechnology advances》2014
With the world-wide increase of patients with renal failure, the development of functional renal replacement therapies have gained significant interest and novel technologies are rapidly evolving. Currently used renal replacement therapies insufficiently remove accumulating waste products, resulting in the uremic syndrome. A more preferred treatment option is kidney transplantation, but the shortage of donor organs and the increasing number of patients waiting for a transplant warrant the development of novel technologies. The bioartificial kidney (BAK) is such promising biotechnological approach to replace essential renal functions together with the active secretion of waste products. The development of the BAK requires a multidisciplinary approach and evolves at the intersection of regenerative medicine and renal replacement therapy. Here we provide a concise review embracing a compact historical overview of bioartificial kidney development and highlighting the current state-of-the-art, including implementation of living-membranes and the relevance of extracellular matrices. We focus further on the choice of relevant renal epithelial cell lines versus the use of stem cells and co-cultures that need to be implemented in a suitable device. Moreover, the future of the BAK in regenerative nephrology is discussed. 相似文献
37.
38.
A brief review of the genetic studies on ribonuclease P (RNase P) fromEscherichia coli is presented. Temperature-sensitive mutants ofE. coli defective in tRNA processing were isolated by screening cells which were unable to synthesize a suppressor tRNA at restrictive temperature. Structural analysis of accumulated tRNA precursors showed that the isolated mutants were defective in RNase P activity. Analyses of the mutants revealed that the enzyme is essential for the synthesis of all tRNA molecules in cells and that the enzymes consists of two subunits. Analyses of the isolated mutants revealed a possible domain structure of the RNA subunit of the enzyme.Abbreviations
E. coli
Escherichia coli
- RNase P
ribonuclease P 相似文献
39.
The most probable secondary structure of an RNA molecule, given the nucleotide sequence, can be computed efficiently if a
stochastic context-free grammar (SCFG) is used as the prior distribution of the secondary structure. The structures of some
RNA molecules contain so-called pseudoknots. Allowing all possible configurations of pseudoknots is not compatible with context-free
grammar models and makes the search for an optimal secondary structure NP-complete. We suggest a probabilistic model for RNA
secondary structures with pseudoknots and present a Markov-chain Monte-Carlo Method for sampling RNA structures according
to their posterior distribution for a given sequence. We favor Bayesian sampling over optimization methods in this context,
because it makes the uncertainty of RNA structure predictions assessable. We demonstrate the benefit of our method in examples
with tmRNA and also with simulated data. McQFold, an implementation of our method, is freely available from http://www.cs.uni-frankfurt.de/~metzler/McQFold. 相似文献
40.
V.B. Awasthi 《Journal of insect physiology》1976,22(9):1181-1186
The ultrastructure of the retrocerebral endocrine-aortal complex of the earwig, Euborellia annulipes has been studied. The space between the inner and outer stromal layers of the aorta is occupied by numerous axon terminals and pre-terminals containing large electron dense granules (NS-I) of approximately 100 to 220 nm and a few axon terminals having small granules (NS-II) of approximately 40 to 90 nm; the former appear to belong to medial neurosecretory A-cells, and the latter to the B-cells of the brain. The corpora cardiaca consist of intrinsic cells with mitochondria and multivesicular bodies. Granules of type NS-II and NS-III are observed in the axon terminals and pre-terminals in the corpora cardiaca. The NS-II are identical to those found in the aorta and are probably the secretions of the lateral B-cells. Granules of type NS-III are 40 to 120 nm and electron dense, and are intrinsic in origin. Similar granules occur in the intrinsic cells of the corpora cardiaca. E M studies have confirmed the rôle of the aorta as a neurohaemal organ for the medial neurosecretory cells, and the corpora cardiaca for the lateral neurosecretory cells of the brain. The corpora cardiaca also act as a reservoir for the intrinsic secretion. The corpus allatum is a solid body consisting of parenchymal cells with prominent nuclei, mitochondria, and endoplasmic reticulum. In between its cells are occasional glial cells and also neurosecretory as well as non-neurosecretory axons. The gland is devoid of A-cell NSM. 相似文献