首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6020篇
  免费   383篇
  国内免费   118篇
  2023年   48篇
  2022年   56篇
  2021年   78篇
  2020年   130篇
  2019年   182篇
  2018年   204篇
  2017年   135篇
  2016年   122篇
  2015年   114篇
  2014年   265篇
  2013年   299篇
  2012年   202篇
  2011年   209篇
  2010年   132篇
  2009年   166篇
  2008年   216篇
  2007年   263篇
  2006年   208篇
  2005年   202篇
  2004年   128篇
  2003年   144篇
  2002年   113篇
  2001年   83篇
  2000年   70篇
  1999年   73篇
  1998年   80篇
  1997年   70篇
  1996年   54篇
  1995年   52篇
  1994年   57篇
  1993年   66篇
  1992年   63篇
  1991年   43篇
  1990年   50篇
  1989年   60篇
  1988年   46篇
  1987年   46篇
  1986年   44篇
  1985年   144篇
  1984年   283篇
  1983年   223篇
  1982年   274篇
  1981年   254篇
  1980年   171篇
  1979年   169篇
  1978年   134篇
  1977年   108篇
  1976年   64篇
  1975年   42篇
  1974年   34篇
排序方式: 共有6521条查询结果,搜索用时 46 毫秒
961.
Exposure of human erythrocyte ghosts (pH 8, 10°C) to visible light in the presence of the photosensitizer, methylene blue, results in a relatively rapid loss of spectrin (bands 1 and 2 on sodium dodecyl sulfate gel electropherograms) and the appearance of high molecular weight cross-linked derivatives. Isolated spectrin also undergoes photosensitized cross-linking, indicating that the reaction is not lipid-dependent.Extensive cross-linking was neither reversed by dithiothreitol nor prevented by prior blocking of SH groups with N-ethylmaleimide, suggesting that cysteine residues are not crucial bridging sites. The possible requirement for NH2 groups, as suggested by previous model studies (Dubbelman, T.M.A.R., de Goeij, A.F.P.M. and van Steveninck, J. (1978) Biochim. Biophys. Acta 511, 141–151), was tested. Succinylation of spectrin protected against cross-linking, but this effect is attributed to the disruption of quaternary structure, as deduced from sedimentation measurements. However, virtually complete blocking of NH2 groups by amidination perturbed overall structure relatively little, and had no effect on cross-linking. Moreover, exogenous amines such as ethylamine, added in large excess to spectrin prior to irradiation, did not interfere with cross-link formation. These results suggest that NH2 groups are not involved in the reaction.  相似文献   
962.
Following administration of phenobarbital to rats, liver ligandin content, bilirubin binding, glutathione-S-transferase and steriod isomerase activities by 150% and the 22 000-dalton subunit was selectively increased. Following adminstration of 3′-methyl-N,N-dimethyl-4-aminoazobenzene, rat liver ligandin content and steroid isomerase decrased by 65%, glutathione-S-transferase incrased by 100%, bilirubin binding was abolished, and the relative proportion of the 22 000- and 25 000-dalton subunits remained unchanged.  相似文献   
963.
Clearance studies were performed in mice using α2-macroglobulin (α2M), α2M-trypsin comlex and α2M-CH3NH2 complex. All three species were incubated with cis-dichlorodiamine platinum(II) (cis-DDPt) at concentrations between 9.0 μM and 1.67 mM for 4 h and then dialyzed. The clearance rate of native α2M was unchanged following incubation with cis-DDPt. α2M-trypsin and α2M-CH3NH2 cleared rapidly from the ciruculation; however, reaction with cis-DDPt significantly decreased the plasma elimination rate of both complexes. Non-denaturing gel electrophoresis and α2M activity assays demonstrated relative stability following incubations with cis-DDPt which markedly altered clearance. Evidence for cis-DDPt crosslinking of α2M subunits was obtained: however, whether this crosslinking is involved in altered clearance remains undetermined. Iodoacetamide treatment of α2M did not duplicate the effect of cis-DDPton α2M clearance, nor did it inhibit the effect of cis-DDPt on α2M clearance. Plasma elimination of α2M complex was also unaltered by pretreatment of mice with intravenous free cis-DDPt.  相似文献   
964.
965.
966.
《Journal of morphology》2017,278(8):1075-1090
Musculo‐skeletal morphology is an indispensable source for understanding functional adaptations. Analysis of morphology of the branchial apparatus of Hexanchiform sharks can provide insight into aspects of their respiration that are difficult to observe directly. In this study, I compare the structure of the musculo‐skeletal system of the gill apparatus of Heptranchias perlo and Squalus acanthias in respect to their adaptation for one of two respiratory mechanisms known in sharks, namely, the active two‐pump (oropharyngeal and parabranchial) ventilation and the ram‐jet ventilation. In both species, the oropharyngeal pump possesses two sets of muscles, one for compression and the other for expansion. The parabranchial pump only has constrictors. Expansion of this pump occurs only due to passive elastic recoil of the extrabranchial cartilages. In Squalus acanthias the parabranchial chambers are large and equipped by powerful superficial constrictors. These muscles and the outer walls of the parabranchial chambers are much reduced in Heptranchias perlo , and thus it likely cannot use this pump. However, this reduction allows for vertical elongation of outer gill slits which, along with greater number of gill pouches, likely decreases branchial resistance and, at the same time, increases the gill surface area, and can be regarded as an adaptation for ram ventilation at lower speeds.  相似文献   
967.
The effect of lactoperoxidase (LPO) on dextran sulfate sodium-induced colitis was examined in mice. After 9 d of colitis induction, weight loss, colon shortening, and the histological score were significantly suppressed in mice orally administered LPO (62.5 mg/body/d) as compared to a group administered bovine serum albumin. These results suggest that LPO exhibits anti-inflammatory effects in the gastrointestinal tract.  相似文献   
968.
Ricin A-chain, a protein that inactivates ribosomes by a specific RNA N-glycosidase activity, has been shown to be inactivated by chemical modification of a few arginine residues. When two or fewer arginine residues in the A-chain were modified with [14C]phenylglyoxal, arginines at positions of 193, 196, 213, and 234/235 were found to be modified, from amino acid compositions and radioactivities of the modified peptides that were obtained by cyanogen bromide cleavage followed by tryptic and chymotryptic digestion. All these arginines have side chains outside the active site cleft; the side chain of Arg213 is adjacent to the edge of the cleft, while other modified arginines are located on the opposite side of the cleft. Kinetic analysis showed that the modification of two arginine residues caused a 8-fold loss in kcat with a 3-fold increase in Km, suggesting that this modification mainly decrease the rate of depurination with an additional effect on the affinity for ribosomes. Neither the environment of tryptophan 211 at the bottom of the cleft nor an interaction of adenine with the cleft was changed by this modification, as judged by fluorescence spectroscopy, suggesting that a conformational change of the catalytic site does not occur upon the modification. These results, taken together with other works, suggest that some of the above arginine residues outside the active site cleft may additively contribute to the catalysis of depurination and/or the initial formation of the A-chain/ribosome complex.  相似文献   
969.
The use of peptide mass fingerprinting with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was demonstrated to identify and phenotypically characterize toluene-degrading bacteria via biomarkers of degradation and taxonomical classification. Pseudomonas putida F1, P. mendocina KR1, and Burkholderia sp. JS150 were grown on toluene, extracted, electrophoretically separated, and analyzed by MALDI-TOF MS. Catabolic enzymes were identified and results substantiated using tandem MS.  相似文献   
970.
Summary

In order to get a better understanding in the mechanism by which tryptophan-N-formylated gramicidin (NFG) and gramicidin kill the malaria parasite Plasmodium falciparum in vitro, we studied the capacity of these peptides to change the potassium, as well as the sodium, composition of normal human erythrocytes, and their ability to cause cell lysis. It is shown that both peptides are able to induce potassium leakage from, and sodium flux into, erythrocytes in such a manner that it is most likely that they are able to form cation channels in the membrane of these cells. For both peptides, potassium efflux proceeds at a faster rate than sodium influx, but this difference is greater for NFG than for gramicidin. This explains the observation that gramicidin is more lytic than NFG is, even when comparing concentrations that show the same antimalarial activity. The finding that gramicidin is approximately 10 times more active than NFG in causing potassium efflux from normal erythrocytes, as well as in killing the malaria parasite, supports the hypothesis that peptideinduced parasite death is related to their capacity to induce potassium leakage from infected erythrocytes. Finally, the observation that erythrocytes are able to restore their normal ion contents after losing more than 50% of their potassium content by incubation with NFG or gramicidin, suggests that, in vivo, and upon treatment with drug concentrations that cause full inhibition of parasite growth, these cells would not be irreversibly damaged by action of the drugs.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号