全文获取类型
收费全文 | 33394篇 |
免费 | 2727篇 |
国内免费 | 122篇 |
出版年
2022年 | 292篇 |
2021年 | 558篇 |
2020年 | 398篇 |
2019年 | 539篇 |
2018年 | 656篇 |
2017年 | 533篇 |
2016年 | 805篇 |
2015年 | 1202篇 |
2014年 | 1314篇 |
2013年 | 1796篇 |
2012年 | 1998篇 |
2011年 | 1822篇 |
2010年 | 1283篇 |
2009年 | 1021篇 |
2008年 | 1506篇 |
2007年 | 1415篇 |
2006年 | 1345篇 |
2005年 | 1180篇 |
2004年 | 1204篇 |
2003年 | 1123篇 |
2002年 | 1196篇 |
2001年 | 1055篇 |
2000年 | 943篇 |
1999年 | 851篇 |
1998年 | 397篇 |
1997年 | 400篇 |
1996年 | 309篇 |
1995年 | 312篇 |
1994年 | 235篇 |
1993年 | 284篇 |
1992年 | 557篇 |
1991年 | 531篇 |
1990年 | 485篇 |
1989年 | 442篇 |
1988年 | 361篇 |
1987年 | 350篇 |
1986年 | 341篇 |
1985年 | 380篇 |
1984年 | 361篇 |
1983年 | 320篇 |
1982年 | 232篇 |
1981年 | 236篇 |
1980年 | 216篇 |
1979年 | 279篇 |
1978年 | 251篇 |
1977年 | 270篇 |
1976年 | 256篇 |
1975年 | 251篇 |
1974年 | 250篇 |
1973年 | 236篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Gianna Evelina De Medio Gianfranco Trovarelli Anders Hamberger Giuseppe Porcellati 《Neurochemical research》1980,5(2):171-179
Rabbit synaptosomes have been used to study the effect of the base-exchange reaction in membrane phospholipids on -aminobutyric acid (GABA) transport in vitro. The uptake of GABA was measured after a base-exchange reaction with ethanolamine, choline, orl-serine and after subsequent displacement of these exchanged moieties from lipid by bases of similar or different structures which were added to the synaptosomal medium. Serine incorporation stimulated GABA transport, but its displacement from membrane lipid by choline or ethanolamine induced an inhibition of GABA transport. Ethanolamine incorporation inhibited GABA transport, but its displacement by serine or choline resulted in stimulation of GABA uptake. Choline incorporation also inhibited GABA transport, although less than ethanolamine. The pool size of synaptosomal phospholipids, presumably involved in GABA uptake, accounted for 0.2 to 10% of the total content of membrane phospholipid. Thus, alteration of phospholipid compositior by exchange of the lipid hydrophilic head-groups influences the extent GABA uptake into rabbit synaptosomes. 相似文献
992.
993.
994.
A. De Antoni C. Costa G. Allegri F. Baccichetti S. Vanzan 《Chemico-biological interactions》1981,34(1):11-18
Tryptophan metabolism ‘via kynurenine’ has been studied in rats before and after induction of experimental light-conditioned dermatitis with psoralen. Tryptophan load in animals during the acute phase of dermatitis (one day after induction) causes a markedly increased urinary excretion of total metabolites in comparison with that obtained before dermatitis. During this phase of the skin disease tryptophan pyrrolase activity is significantly increased and kynureninase activity significantly decreased in liver in respect to the control animals. Kynurenine aminotransferase activity shows no significant variations in both liver and kidneys. After 6 days of dermatitis, when the skin damage is in repair, both the excretory values of the urinary metabolites after L-tryptophan load and the enzymic activities are similar to those before dermatitis. 相似文献
995.
Longevity, egg production and sensitivity towards induction of diapause of two laboratory cultures of Leptinotarsa decemlineata Say, the Colorado potato beetle, were compared. One culture was kept ab ovo under long-day conditions (LD: 16 hr photophase; 25°=LD culture); the other was raised ab ovo in short-day condition (SD: 10 hr photophase; 25°), kept in diapause during 9 months at 25° in dry sand and after termination of diapause brought to LD (=DP-LD culture). During 97 days, there were no significant differences in the longevities of the cultures (50% mortality: day 52 in DP-LD; day 58 in LD). The survival curve of each was almost linear, which is rather unusual in animals. If the regression line, representing each survival curve was linearly extrapolated beyond day 97 (end of experiment), a theoretical maximal life span of 121 days was found for the LD culture, and of 110 days for the DP-LD culture. Beetles did not enter a second diapause as easily as the first. The haemolymph protein pattern of old animals that had ceased reproduction was almost similar to that of young reproducing animals. In both cultures, the last egg was laid at day 86. Mated females produced 30–60 eggs per day as compared with virgin females which produced less than ten per day. Application of JH 1, 20-hydroxyecdysone, prostaglandin E2, haemolymph or brain extracts of females did not increase egg production in virgins.
This research was supported by the National Foundation of Scientific Research (N.F.W.O.) of Belgium. 相似文献
Résumé La longévité, la fécondité et la sensibilité à l'induction de la diapause ont été comparées chez deux souches de laboratoire du doryphore, Leptinotarsa decemlineata. Une souche a été conservée ab ovo en jours longs (LD: 16 h de photophase; 25°=LD culture). L'autre souche a été élevée ab ovo en jours courts (SD: 10 h de photophase; 25°); elle est restée en diapause pendant 9 mois à 25° dans du sable sec et a été placée en LD après la fin de diapause (= DP-LD culture). Jusqu'au 97ième jour de l'expérience, il n'y avait pas de différence significative entre les longévités des deux souches. Les courbes de survie des deux souches étaient presque rectilignes, ce qui est plutôt rare chez des animaux.En extrapolant linéairement après le 97ième jour (fin de l'expérience) les lignes de régression linéaire correspondant aux deux courbes, une durée de vie maximale théorique de 121 jours a pu être déduite dans la souche LD et de 110 jours pour la souche DP-LD. Aucune différence significative n'a été observée entre les fécondités des deux souches. Le dernier uf a été pondu le 86ième jour. Les doryphores qui avaient déjà été en diapause, n'y retournaient pas aussi aisément que ceux qui n'avaient jamais été en diapause avant leur vie reproductive. Le protéinogramme de l'hémolymphe des animaux âgés ne différait pas de celui des jeunes animaux reproducteurs. Tandis que les femelles accouplées ont pondu 30 à 60 ufs par jour, les femelles vierges n'en ont pas émis généralement plus de 10. L'application d'hormone juvénile 1, de 20-hydroxyecdysone, de prostaglatine E2, d'hémolymphe ou d'extraits céphaliques de femelles n'a pas accru la production d'ufs.
This research was supported by the National Foundation of Scientific Research (N.F.W.O.) of Belgium. 相似文献
996.
Johannes Boonstra Paul T. Van Der Saag Wouter H. Moolenaar Siegfried W. De Laat 《Experimental cell research》1981,131(2):452-455
Nerve growth factor (NGF) induces neuronal differentiation of rat pheochromocytoma cells (PC12). Here we show that NGF causes a stimulation of Na+,K+-pump mediated K+ influx, with a maximum at 30 min after addition of NGF. The stimulation of the Na+,K+-pump is completely blocked by the Na+-flux inhibitor amiloride (0.2 mM) and can be mimicked by the Na+ ionophore monensin. These results suggest that NGF causes a rapid enhancement of Na+ influx leading to an activation of the Na+,K+-pump, a mechanism similar to the action of other growth factors. 相似文献
997.
998.
999.
Summary The possibility of using propidium iodide, a phenanthridinic fluorochrome specific for double-stranded nucleic acids, for the study of chromatin thermal denaturationin situ has been examined. Smears of lymphocytes and hepatocyte nuclei from 15-day-old rats were fixed in acetic acid-ethanol (13 v/v), treated with RNAse and submitted to different protein extraction procedures, namely, incubation with pepsin, trypsin and sodium chloride.Denaturation experiments were performed in Sörensen buffer at pH 7.4 containing 10% formamide at temperatures between 27 and 95°C. The samples were stained with propidium iodide and mounted in buffer or glycerol. Measurements were performed with a microfluorometer at a wavelength of 546 nm.The results indicate a higher thermostability of lymphocytes as compared to hepatocytes. The denaturation pattern suggests a certain organization complexity of chromatin, better emphasized by the derivative curves which show the presence of at least three fractions with different melting points. After protein extraction, the denaturation curves exhibit a somewhat simplified pattern, with the disappearance of the most stable peak in the derivative curves. The samples mounted in glycerine exhibit a better stability of staining with time, and an increased quantum efficiency of the fluorochrome with regard to those mounted in buffer.These data confirm the importance of protein-DNA interactions in the organization of chromatin and point to some differences, depending on the cell type and on functional activity. 相似文献
1000.
Patrick De Wever 《Geobios》1981,14(1):5-15
Studies of skeleton elements of several jurassic species of Saitoum allow to compare them with Poulpus from Trias. The sub-family Poulpinae is introduced, caracterised by three cephalic arcs and the collar position of the cephalic structure. Among the jurassic forms, 4 species are newly described: S. corniculum, S. elegans, S. levium and S. trichylum. 相似文献