全文获取类型
收费全文 | 4572篇 |
免费 | 220篇 |
国内免费 | 266篇 |
出版年
2024年 | 4篇 |
2023年 | 65篇 |
2022年 | 57篇 |
2021年 | 100篇 |
2020年 | 127篇 |
2019年 | 115篇 |
2018年 | 105篇 |
2017年 | 91篇 |
2016年 | 116篇 |
2015年 | 129篇 |
2014年 | 165篇 |
2013年 | 277篇 |
2012年 | 113篇 |
2011年 | 176篇 |
2010年 | 150篇 |
2009年 | 236篇 |
2008年 | 225篇 |
2007年 | 247篇 |
2006年 | 232篇 |
2005年 | 166篇 |
2004年 | 182篇 |
2003年 | 182篇 |
2002年 | 137篇 |
2001年 | 100篇 |
2000年 | 80篇 |
1999年 | 107篇 |
1998年 | 95篇 |
1997年 | 102篇 |
1996年 | 103篇 |
1995年 | 127篇 |
1994年 | 92篇 |
1993年 | 99篇 |
1992年 | 95篇 |
1991年 | 71篇 |
1990年 | 57篇 |
1989年 | 100篇 |
1988年 | 54篇 |
1987年 | 66篇 |
1986年 | 33篇 |
1985年 | 54篇 |
1984年 | 39篇 |
1983年 | 23篇 |
1982年 | 40篇 |
1981年 | 25篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 21篇 |
1977年 | 14篇 |
1976年 | 8篇 |
1973年 | 4篇 |
排序方式: 共有5058条查询结果,搜索用时 28 毫秒
91.
Plasmid DNA carrying either the nitrate reductase (NR) gene or the argininosuccinate lyase gene as selectable markers and the correspondingChlamydomonas reinhardtii mutants as recipient strains have been used to isolate regulatory mutants for nitrate assimilation by insertional mutagenesis. Identification of putative regulatory mutants was based on their chlorate sensitivity in the presence of ammonium. Among 8975 transformants, two mutants, N1 and T1, were obtained. Genetic characterization of these mutants indicated that they carry recessive mutations at two different loci, namedNrg1 andNrg2. The mutation in N1 was shown to be linked to the plasmid insertion. Two copies of the nitrate reductase plasmid, one of them truncated, were inserted in the N1 genome in inverse orientation. In addition to the chlorate sensitivity phenotype in the presence of ammonium, these mutants expressed NR, nitrite reductase and nitrate transport activities in ammonium-nitrate media. Kinetic constants for ammonium (14C-methylammonium) transport, as well as enzymatic activities related to the ammonium-regulated metabolic pathway for xanthine utilization, were not affected in these strains. The data strongly suggest thatNrg1 andNrg2 are regulatory genes which specifically mediate the negative control exerted by ammonium on the nitrate assimilation pathway inC. reinhardtii. 相似文献
92.
Carlos Hermenegildo Goizane Marcaida Carmina Montoliu Santiago Grisolía María-Dolores Miñana Vicente Felipo 《Neurochemical research》1996,21(10):1237-1244
We proposed that acute ammonia toxicity is mediated by activation of NMDA receptors. To confirm this hypothesis we have tested
whether different NMDA receptor antagonists, acting on different sites of NMDA receptors, prevent death of mice induced by
injection of 14 mmol/Kg of ammonium acetate, a dose that induces death of 95% of mice. MK-801, phencyclidine and ketamine,
which block the ion channel of NMDA receptors, prevent death of at least 75% of mice. CPP, AP-5, CGS 19755, and CGP 40116,
competitive antagonists acting on the binding site for NMDA, also prevent death of at least 75% of mice. Butanol, ethanol
and methanol which block NMDA receptors, also prevent death of mice. There is an excellent correlation between the EC50 for preventing ammonia-induced death and the IC50 for inhibiting NMDA-induced currents. Acute ammonia toxicity is not prevented by antagonists of kainate/AMPA receptors, of
muscarinic or nicotinic acetylcholine receptors or of GABA receptors. Inhibitors of nitric oxide synthase afford partial protection
against ammonia toxicity while inhibitors of calcineurin, of glutamine synthetase or antioxidants did not prevent ammonia-induced
death of mice. These results strongly support the idea that acute ammonia toxicity is mediated by activation of NMDA receptors. 相似文献
93.
Arsenic toxicity is not due to a direct effect on the oxidation of reduced inorganic sulfur compounds by Thiobacillus caldus 总被引:1,自引:0,他引:1
Abstract Thiobacillus caldus is a moderately thermophilic acidophile which has been implicated in the biooxidation of arsenic containing mineral Sulfides. The toxic effects of arsenic on this bacterium are presented here. Addition of arsenite to a growing culture of T. caldus caused a transient increase in the optical density of the culture while causing a simultaneous decrease in cell viability. The increase in optical density was shown to be due to the formation of extracellular sulfur. The oxidation rates of tetrathionate and thiosulfate were decreased by increasing concentrations of arsenite, while in a culture induced to arsenic resistance the rates were not as adversely effected. Sulfur oxidation was also inhibited to the same extent as tetrathionate oxidation, with the oxidation of solid sulfur being slightly more effected than the oxidation of sulfur dissolved in acetone. Thus, bactericidal arsenite causes a transient formation of extracellular sulfur in the culture supernatant of T. caldus yet the toxicity of arsenite is not due to direct inhibitory effects on reduced inorganic sulfur compound oxidation by these bacteria. 相似文献
94.
An exhaustive analysis of registers of births, deaths, and marriages is used to reconstitute genealogies, both ascending and
descending, for all individuals who lived in two mountain valleys: the Valserine valley (in the French Jura), which is studied
from the end of the seventeenth century to the present, and the Vallouise valley (in the French Alps) studied from the seventeenth
to the nieteenth centuries.
This genealogical approach makes it possible to trace the reproductive process in the populations whose members lived in the
valley for several generations. The Vallouise valley forms an important demographic isolate; nearly 98 per cent of those born
between 1820 and 1849 were descendants of ancestors who had lived in the valley two centuries earlier. By contrast, there
has been continuous immigration into the Valserine valley, which has resulted in a constant renewal of the gene pool. Only
18 per cent of the gene pool of individuals living in this valley two centuries later was contributed by those who lived in
the valley and who were born before 1750. 相似文献
95.
Colette M. St. Mary 《Environmental Biology of Fishes》1996,45(2):177-190
Synopsis The sexual pattern of the zebra goby Lythrypnus zebra, is an apparent exception to sex allocation theory. Most L. zebra are simultaneous hermaphrodites (i.e., have active female and male gonadal tissue), yet it appears they do not reproduce as males and females simultaneously. Understanding the maintenance of simultaneous hermaphroditism in L. zebra could expand sex allocation theory. In this study, I used a comparison with the blue-banded goby, Lythrypnus dalli, a sympatric congener with a qualitatively similar sexual pattern, to investigate the role of male spawning rate, body size and sexual flexibility in determining the sexual pattern of L. zebra and to isolate differences between the species that might explain their differing sexual patterns. Using field measurements of male nesting success, I found no differences between the species in the body size of nesting males suggesting that large size is associated with successful male reproduction in both species. In addition, nesting males spawned at approximately three times the rate of females in both species; thus, reproduction via male function can be equally advantageous relative to adopting the female role. However, the nest longevity of L. zebra males was shorter than that of L. dalli males, suggesting reproduction via male function may be less reliable in L. zebra. Finally, under laboratory conditions, L. zebra females tended to prefer large mates, and L. zebra were able to re-allocate in both directions, exhibiting a greater capacity to switch than L. dalli. Given these results, I suggest that switching between the sexes plays a greater role in maintaining simultaneous hermaphroditism in L. zebra than L. dalli, perhaps because male reproduction is not as consistent in L. zebra. Sexual flexibility may be an important factor affecting patterns of sex allocation, generally. 相似文献
96.
Aspects of the reproductive biology of the concha wrasse, Nelabrichthys ornatus, at Tristan da Cunha 总被引:1,自引:0,他引:1
Synopsis The reproductive biology of the concha wrasse, Nelabrichthys ornatus, at Tristan da Cunha in the South Atlantic Ocean is described. Histological evidence shows that the species is a monandric protogynous hermaphrodite. N. ornatus displays full permanent sexual dichromatism with all males occurring in the upper size classes of the population. Size specific habitat preference is described. Aspects of the mating system of the species resemble a lek although the possibility of mating site selection by females is not discounted. Spawning groups have a sex ratio of between 4 and 7 females per male. 相似文献
97.
To examine whether zinc deficiency would increase the toxicity of dietary aluminum, weanling, male Sprague-Dawley rats were
fed purified diets containing either 2 or 30 mg Zn/kg diet, with or without 500 mg Al/kg diet for 28 d. Individually pair-fed
rats were fed the 30 mg Zn/kg diet with or without added aluminum to control for inanition secondary to zinc deficiency. Rats
fed the 2 μg Zn/kg diet showed evidence of zinc deficiency, including anorexia, growth retardation, and depressed concentrations
of zinc in tibias and livers. Zinc deficiency did not significantly increase the concentrations of aluminum in the tibias,
livers, kidneys, or regions of the brain examined (cerebrum, cerebellum, midbrain, and hippocampus). Inclusion of aluminum
in the diet did not alter aluminum concentrations in the various tissues. Under the conditions of this study, zinc deficiency
did not result in greater sensitivity to dietary aluminum exposure. 相似文献
98.
We assessed the reproductive responses of adult female Iphiseius degenerans and Neoseiulus teke to increasing density of three stages of their prey, Mononychellus tanajoa, on cassava leaf discs under laboratory conditions. The oviposition rates increased with number of prey consumed per predator per day with a maximum of approximately two eggs per day for I. degenerans and four eggs per day for N. teke. The oviposition rate of N. teke was higher when consuming eggs than other prey stages. Neoseiulus teke was more efficient than I. degenerans in converting consumed prey into egg production. The data were adequately described by simple mathematical models. 相似文献
99.
To clarify the mechanism of aluminum (Al) toxicity and Al tolerance, we isolated a new clone (pAL201) from a tobacco cDNA library. Northern blot hybridization analysis indicated that the expression of pAL201 is induced by Al treatment and phosphate (P1 ) starvation. The complete cDNA sequence suggested that this clone encodes a moderately anionic peroxidase (EC 1.11.1.7). Analysis by isoelectric focussing indicated that a moderately anionic peroxidase (approximately pI 6.7) and two cationic peroxidases (pI 9.2 and 9.7) in the soluble fraction are activated by Al treatment and P1 starvation, while two moderately anionic isozymes are repressed by these stresses. We suppose that Al ion stress can control the activity of some peroxidase isozymes, one of which is probably induced by enhanced gene expression of pAL201. There is a possibility that some of these isozymes have some functions in Al ion stress. 相似文献
100.
Mike H. Jurke 《Primates; journal of primatology》1996,37(1):109-119
The fate of most nonhuman primate species is intimately related to man. The increasing encroachment on the natural habitat
has resulted in the decimation and even near extinction of some species. Along with this development, the basic concept in
many modern zoos has changed from one of merely display to self-perpetuating units. Primate research facilities are orienting
their research programs towards reproductive physiology and behavior in an effort to provide basic knowledge of reproduction
in these species. This increased emphasis in the area of reproductive biology and the various efforts to improve breeding
of these mostly endangered primates in captivity has stimulated the author to write this review. It represents an attempt
to provide the reader with basic background information relating to the endocrinology and behavior of reproduction in the
clawed New World monkeys as it exists at the time of publication.
The intermediate evolutionary position ofCallimico goeldii between the clawed New World monkeys and the ‘true New World monkeys’ and our relatively poor knowledge about reproduction
and behavior in this particular species fully justifies the focus on Goeldi’s monkey in this essay. This review is an attempt
to provide a brief history of previous studies but also the basis for research in the future. The current status of knowledge
of the small-bodied clawed monkeys is also discussed in an evolutionary context, with an emphasis on the different reproductive
strategies in this dynamic group of primates.
The outcome, not surprisingly, confirms the unique position ofCallimico goeldii in its social, ecological, and evolutionary environment. 相似文献