全文获取类型
收费全文 | 7570篇 |
免费 | 489篇 |
国内免费 | 11篇 |
专业分类
8070篇 |
出版年
2024年 | 20篇 |
2023年 | 84篇 |
2022年 | 285篇 |
2021年 | 414篇 |
2020年 | 165篇 |
2019年 | 199篇 |
2018年 | 297篇 |
2017年 | 207篇 |
2016年 | 294篇 |
2015年 | 443篇 |
2014年 | 436篇 |
2013年 | 567篇 |
2012年 | 577篇 |
2011年 | 545篇 |
2010年 | 348篇 |
2009年 | 283篇 |
2008年 | 330篇 |
2007年 | 333篇 |
2006年 | 269篇 |
2005年 | 213篇 |
2004年 | 210篇 |
2003年 | 161篇 |
2002年 | 160篇 |
2001年 | 101篇 |
2000年 | 77篇 |
1999年 | 63篇 |
1998年 | 42篇 |
1997年 | 38篇 |
1996年 | 31篇 |
1995年 | 32篇 |
1994年 | 29篇 |
1993年 | 30篇 |
1992年 | 60篇 |
1991年 | 50篇 |
1990年 | 53篇 |
1989年 | 52篇 |
1988年 | 39篇 |
1987年 | 35篇 |
1986年 | 38篇 |
1985年 | 34篇 |
1984年 | 46篇 |
1983年 | 36篇 |
1982年 | 37篇 |
1981年 | 34篇 |
1980年 | 41篇 |
1979年 | 19篇 |
1978年 | 28篇 |
1977年 | 19篇 |
1974年 | 23篇 |
1973年 | 22篇 |
排序方式: 共有8070条查询结果,搜索用时 0 毫秒
101.
Habituation to acid in Escherichia coli: conditions for habituation and its effects on plasmid transfer 总被引:3,自引:3,他引:0
N. Raja M. Goodson W.C.M. Chui D.G. Smith R.J. Rowbury 《Journal of applied microbiology》1991,70(1):59-65
Induction of acid resistance (habituation) in Escherichia coli at pH 5·0 took ca 5 min in broth at 37°C and 30–60 min in minimal medium. Induction occurred at a range of pH values from 4·0 to 6·0; it was dependent on continuing protein and RNA synthesis but substantial acid resistance appeared in the presence of nalidixic acid. Acid resistance was long-lasting; organisms grown at pH 5·0 retained most of their resistance after 2 h growth at pH 7·0. Organisms grown at pH 5·0 showed increased synthesis of a number of cytoplasmic proteins compared with the level in cells grown at pH 7·0. DNA repair-deficient strains carrying recA, uvrA or polAl mutations were more acid-sensitive than the repair-proficient parents but were able to habituate at pH 5·0. Organisms grown at pH 5·0 transferred the ColV plasmid much more effectively at acid pH than did those grown at pH 7·0 and habituated recipients appeared better able to repair incoming acid-damaged plasmid DNA than did those that were non-habituated. Induction of acid resistance at pH 5·0 may be significant for the survival of organisms exposed to periodic discharges of acid effluent in the aquatic environment and habituation may also allow plasmid transfer and repair of acid-damaged plasmid DNA during or after such exposure. 相似文献
102.
Summary The uterus and vagina of the guinea pig have been examined, region by region, for acetylcholinesterase, tyrosine hydroxylase, dopamine -hydroxylase and aromatic amino acid decarboxylase activity, as well as for the neuropeptides, neuropeptide Y, vasoactive intestinal peptide, substance P, enkephalin and somatostatin. No acetylcholinesterase activity was localized in the uterus, though it was present in associated paracervical ganglion tissues. Of the catecholamine-synthesizing enzymes, tyrosine hydroxylase and dopamine -hydroxylase activity was found virtually throughout the reproductive tract, whereas aromatic amino acid decarboxylase activity was restricted in its distribution. Neuropeptide distribution was quite varied. Neuropeptide Y was found throughout the endometrium/submucosa but only in the muscularis of the vagina and not in the myometrium. Substance P was localized in the vagina and uterine horn, though not the body of the uterus. Vasoactive intestinal peptide was present in all regions of the endometrium/submucosa, but not in the myometrium of the uterine horn. Enkephalin and somatostatin were not localized in any part of the reproductive tract examined, apart from paracervical ganglion tissues. The types and significance of the nerves supplying the reproductive tract are discussed. 相似文献
103.
To explain our finding that the dimeric beta subunit of tryptophan synthase is only 50% inactivated by beta-chloro-L-alanine (Ahmed, S. A., Ruvinov, S. B., Kayastha, A. M., and Miles, E. W. (1991) J. Biol. Chem. 266, 21548-21557), we have extended our investigation using spectroscopic, steady-state kinetic, and electrophoretic methods. The spectroscopic properties of the half-active beta 2 dimer and the reactivation after alkali treatment show that the inactivation proceeds by an "enamine" mechanism. Although the fully active beta 2 dimer associates with the tryptophan synthase alpha subunit to form alpha 2 beta 2 complex, the inactive beta subunits in the half-active enzyme associate weakly or not at all with the alpha subunit. Our results provide evidence for two conformers of the beta subunit in solution: one is rapidly inactivated by beta-chloro-L-alanine and the other is not inactivated. Thermal inactivation studies and non-denaturing polyacrylamide gel electrophoresis of the half-active enzyme show that the beta 2 dimer exists in both homologous and heterologous combinations of these two forms. After removal of the reaction products and unreacted beta-chloro-L-alanine from the half-active beta 2 dimer by gel filtration, further incubation with beta-chloro-L-alanine results in the loss of 50% of the remaining activity. This result suggests that the subunits undergo rearrangement via an intermediate monomer form to regenerate the two conformers of the active beta subunit. This mechanism of rearrangement is supported by our finding that the extent of inactivation increases at lower concentrations of the beta 2 dimer. 相似文献
104.
Salem MH Mekkawy MY Ahmed NA Abdel-Aziz IY Mohamed AA El-Oksh HA Pursel VG 《Theriogenology》1992,37(5):1061-1074
Ejaculated washed ram spermatozoa showed consistent increases in the intracellular concentration of cyclic 3', 5'-adenosine monophosphate (cAMP) after incubation for 15 minutes with the phosphodiesterase (PDE)-inhibitors, theophylline and caffeine. In vitro addition of cAMP or PDE-inhibitors to ram semen also stimulated and maintained sperm motility and enhanced the rate of fructose utilization. The same doses of cAMP or theophylline significantly stimulated the rate of protein synthesis by the washed spermatozoa, while the PDE-stimulator, imidazole, inhibited protein synthesis significantly. The stimulatory effect of cAMP on sperm protein synthesis was not affected by cycloheximide, but was abolished by the mitochondrial inhibitor, chloramphenicol. The present results indicate a positive correlation between the intracellular concentration of cAMP and the rates of progressive motility, fructose utilization, and protein synthesis by ram spermatozoa. The results suggest that the effect of cAMP is associated with the synthesis of mitochndrial proteins which may be involved with the observed enhancement of sperm motility and metabolism. The data also indicate that cAMP map act either as a first or a second messenger in mature spermatoza. 相似文献
105.
Sylvain Biquand Véronique Biquand-Guyot Ahmed Boug Jean-Pierre Gautier 《International journal of primatology》1992,13(5):533-543
Papio hamadryas was surveyed throughout its range in Saudi Arabia and was observed at altitudes ranging from 0 to 2300 m.
Wild populations occur along the whole range of altitude, while commensal populations are only found above 850 m altitude.
No variation in group size was found with altitude. Comparison of wild and commensal populations showed the following. (1)
Their composition in terms of age and sex classes, overall adult sex ratios, and group size does not significantly differ.
(2) Groups of both populations include, in similar proportions, three types of parties: one-male units (>70%), two-male units
(>13%), and a few other units of variable composition. (3) The mean size of commensal parties is significantly larger than
in the wild population; specifically one-male units are larger in the commensal population due to a larger number of females
per male. Thus, female distribution in commensal groups is more inequitable than that in wild groups. (4) Finally, the number
of females included in two-male units increases with altitude. These differences are discussed in terms of food availability
and predator pressure and are compared with results obtained on other Arabian and Ethiopian populations. 相似文献
106.
The distribution ofPapio hamadryas in Saudi Arabia: Ecological correlates and human influence 总被引:1,自引:0,他引:1
Sylvain Biquand Véronique Biquand-Guyot Ahmed Boug Jean-Pierre Gautier 《International journal of primatology》1992,13(3):223-243
We describe the distribution ofPapio hamadryas in Saudi Arabia in relation to ecological constraints and recent changes in human activities and land use. Baboons are present
only in the Sarawat and southern Hijaz mountains, which border the Red Sea in the southwestern region of the country. Our
distribution data confirms that their range, described as of 1981, should be extended 200 km northward, from 21°40N (Taif)
to 23°20N (Al Akhal). This new limit of range is not due to recent deployment of the baboons, but instead corrects inaccurate
surveys by previous reporters. Ecological factors that affect baboon distribution include low annual rainfall (less than 100
mm per yr), which limits baboon range in the north and east, and the absence of suitable sleeping cliffs in the coastal plain,
which limits their range in the west. Relative abundance is discussed in relation to vegetation and predator distribution.
Human influence is also significant: commensal troops persist near urban centers, while agricultural development reduces baboon
numbers in some regions. Habitat characteristics are discussed in comparison with similar data on the range of hamadryas in
Ethiopia. 相似文献
107.
The haloacetonitrile, dibromoacetonitrile (DBAN), is a direct-acting genotoxic agent that has been detected in drinking water. In a time course study, male Sprague-Dawley rats were treated with DBAN (75 mg/kg PO), and killed at 0.5, 1, 2, and 4 hr after treatment. In a dose response study, animals were treated orally with various doses of DBAN (25, 50, 75, and 100 mg/kg) and killed at one-half hour after treatment. Control animals received 1 ml/kg PO of the vehicle dimethyl sulfoxide (DMSO). In both experiments blood and organs were collected and stored at -80 degrees C until the time of analysis. At 0.5 hr after treatment, a single oral dose of DBAN caused a significant decrease of glutathione (GSH) concentrations in liver (54% of control) and stomach (6% of control). Hepatic GSH depletion was maximal at 0.5 hr and rebound to the control levels by 4 hr. In contrast, gastric GSH concentrations remained low at all time points. DBAN caused an insignificant change in both kidney and blood GSH levels. DBAN significantly inhibited glutathione-S-transferase (GST) activity in liver and stomach. Hepatic GST inhibition was maximal (34% of control) at 2 hr and minimal (80% of control) at 4 hr. Meanwhile, in the stomach GST activity was inhibited at 1 hr (60% of control) and remained low at all times after treatment. Both GSH depletion and GST inhibition were dose-dependent. This study indicates that GSH and GST play an important role in the metabolism and detoxification of DBAN in rats. The prolonged depletion of GSH and inhibition of GST in the gastrointestinal (GI) tissues suggest that the GI tract is a major target for DBAN toxicity. 相似文献
108.
H.H. Ahmed D.A. Van Vugt C.M.Ruiz de Galarreta L.F. Fanjul J. Meites 《Life sciences》1981,29(26):2757-2762
The effects of naltrexone, a specific opiate antagonist, on stimulation by estradiol benzoate (EB) of prolactin (PRL) release and anterior pituitary (AP) weight, were studied in gonadectomized female and male Sprague-Dawley rats. One week after castration, rats were injected for 10 days once daily with 2 μg EB alone, or together with twice daily injections of 2 mg naltrexone/kg body weight (BW). Blood was collected for radioimmunoassay of PRL by orbital sinus puncture on days 0 and 6, and by decapitation on day 11, at which time the AP was quickly removed, weighed and assayed for PRL.Serum PRL concentrations and AP weights were significantly increased by EB administration. These effects of EB were partially but significantly inhibited by naltrexone. These results suggest that endegenous opiates may be involved in the estrogen-induced rise in serum PRL and increase in pituatary weight. 相似文献
109.
Summary The effect of soil moisture stress on growth, nodulation and nitrogenase activity of two tropical forage legumes,Macroptilium atropurpureum cv. Siratro andDesmodium intortum cv. Greenleaf was studied in a pot experiment. After ten weeks growth, the highest moisture stress (20 per cent water holding capacity) significantly reduced only the top weight of both plants. Moisture stress progressively retarded top growth in the two legumes. Similar trends were also observed in defoliated plants. Moisture stress had little or no effect on the nodulation or nitrogenase activity of the plants. 相似文献
110.