全文获取类型
收费全文 | 3192篇 |
免费 | 218篇 |
国内免费 | 2篇 |
专业分类
3412篇 |
出版年
2023年 | 15篇 |
2022年 | 37篇 |
2021年 | 68篇 |
2020年 | 57篇 |
2019年 | 66篇 |
2018年 | 71篇 |
2017年 | 68篇 |
2016年 | 107篇 |
2015年 | 137篇 |
2014年 | 175篇 |
2013年 | 203篇 |
2012年 | 239篇 |
2011年 | 226篇 |
2010年 | 130篇 |
2009年 | 149篇 |
2008年 | 194篇 |
2007年 | 187篇 |
2006年 | 194篇 |
2005年 | 151篇 |
2004年 | 167篇 |
2003年 | 149篇 |
2002年 | 145篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 29篇 |
1998年 | 35篇 |
1997年 | 24篇 |
1996年 | 22篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 26篇 |
1989年 | 22篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 10篇 |
1982年 | 21篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有3412条查询结果,搜索用时 15 毫秒
101.
González R Collado JA Nell S Briceño J Tamayo MJ Fraga E Bernardos A López-Cillero P Pascussi JM Rufián S Vilarem MJ De la Mata M Brigelius-Flohe R Maurel P Muntané J 《Free radical biology & medicine》2007,43(10):1439-1452
Vitamin E (alpha-tocopherol) has demonstrated antioxidant activity and gene-regulatory properties. d-Galactosamine (D-GalN)-induced cell death is mediated by nitric oxide in hepatocytes, and it is associated with hepatic steatosis. The beneficial properties of alpha-tocopherol and their relation to oxidative stress and gene regulation were assessed in D-GalN-induced cell death. Hepatocytes were isolated from human liver resections by a collagenase perfusion technique. alpha-Tocopherol (50 microM) was administered at the advanced stages (10 h) of D-GalN-induced cell death in cultured hepatocytes. Cell death, oxidative stress, alpha-tocopherol metabolism, and NF-kappaB-, pregnane X receptor (PXR)-, and peroxisome proliferator-activated receptor (PPAR-alpha)-associated gene regulation were estimated in the hepatocytes. D-GalN increased cell death and alpha-tocopherol metabolism. alpha-Tocopherol exerted a moderate beneficial effect against apoptosis and necrosis induced by D-GalN. Induction (rifampicin) or inhibition (ketoconazole) of alpha-tocopherol metabolism and overexpression of PXR showed that the increase in PXR-related CYP3A4 expression caused by alpha-tocopherol enhanced cell death in hepatocytes. Nevertheless, the reduction in NF-kappaB activation and inducible nitric oxide synthase expression and the enhancement of PPAR-alpha and carnitine palmitoyl transferase gene expression by alpha-tocopherol may be relevant for cell survival. In conclusion, the cytoprotective properties of alpha-tocopherol are mostly related to gene regulation rather than to antioxidant activity in toxin-induced cell death in hepatocytes. 相似文献
102.
Population size and distribution of Otaria flavescens in central and southern Chubut, Patagonia were determined by aerial censuses conducted during the breeding season in 1989 and 1995 and outside the season. Pup numbers were corrected by means of a linear regression between pup and adult male (AM) numbers obtained from terrestrial censuses carried out in 1994 and 1996. During the 1989 breeding season, 10,557 sea lions occurred at 29 locations; 2,800 (26.3%) were pups. During the 1995 breeding season, 14,887 animals were censused at 32 sites; 3,311 (22.2%) were pups. The corrected pup number increased to 4,852, indicating that around 32% is lost when counting from photographs. The corrected total number increased to 16,483; this fell to less than one half between breeding seasons (November 1990 and 1995). The 1995 breeding season census represents the highest number since 1972 (8,800) and the widest historical dispersion (37 sites year round contrasting with 13 in the late 1940s and 1972). However, the present number is about one half of the 33,000 censused about 50 yr ago. Using a correction factor developed for the north Paragonian population (1.8 × censused number) the present population size in the area can be estimated at around 29,669 individuals. 相似文献
103.
Enrique A. Gonzlez Froiln Vzquez Jos Ignacio Garabal Jorge Blanco 《Microbiology and immunology》1995,39(12):937-942
Fimbriae isolation by means of thermal shock was applied to fifteen K88-positive (three K88ab, nine K88ac and three K88ad) Escherichia coli reference strains belonging to serotypes O8:K87, O32, O45, O138:K81, O141:K85, O147:K89, O149:K91, and O157, as well as to ten K88-positive enterotoxigenic strains isolated from porcine diarrhea in Spain, all of them belonging to the O149 serogroup. Fimbriae were removed from the bacterial cells by thermal shock at 60 C and then precipitated using ammonium sulfate. The final amount of K88 antigen and the purification degree were not related to the serogroup of the bacteria or to the antigen variant but were related to the buffer used for isolation. Phosphate buffer containing urea was shown to be more effective than Tris-HCl for isolation of K88 antigen. The molecular weights by SDS-PAGE for K88ab, K88ac, and K88ad were 28.5, 29.2, and 31.0 kDa, respectively. All enterotoxigenic E. coli strains isolated in Spain showed the K88ac variant. 相似文献
104.
Tracey C. Bourner Enrique Vargas‐Osuna Trevor Williams Candido Santiago‐Alvarez Jenny S. Cory 《Biocontrol Science and Technology》1992,2(4):315-326
Agrotis segetum nuclear polyhedrosis virus (AsNPV) and granulosis virus (AsGV), propagated in laboratory cultures of A. segetum in England and A. ipsilon in Spain, respectively, were applied to plots of maize plants at the one‐ to four‐leaf stage of growth. Plots were arranged in a 6 x 6 Latin square design and infested with second‐instar A. segetum larvae (the common cutworm). Each virus was applied in separate treatments by two application methods; as an aqueous spray containing 0.1% Agral as a wetting agent, and as a bran bait. The NPV was applied at a rate of 4 X 1012 polyhedra/ha, and the GV at 4 X 1013 granules/ha. Soil and plants were sampled for larvae on three occasions following virus treatment: 24 h, 4 days and 11 days. The larvae were reared on diet in the laboratory, until death or pupation, to examine the rate and level of viral infection. Infection data showed 87.5% and 91% NPV infection and 12.5% and 55% GV infection in spray and bait treatments, respectively, in larvae sampled 24 h after treatment. In larvae sampled 4 days after treatment, the results were 78% and 100% NPV infection, and 13% and 6% GV infection. A total of only six larvae were retrieved on day 11. In both treatments larvae infected with AsNPV died significantly more rapidly and at an earlier instar than those infected with AsGV, indicating that AsNPV appears to have better potential as a control agent for A. segetum. 相似文献
105.
David J. Walker Pascual Romero Aránzazu de Hoyos Enrique Correal 《Environmental and Experimental Botany》2008,64(3):217-224
We determined the cold (freezing) tolerance for field-grown plants of Atriplex halimus L. (Chenopodiaceae) in relation to plant ploidy level, leaf water relations and accumulation of osmolytes. Plants were grown at two sites in Murcia (Spain), having average minimum temperatures in the coldest month of 0.6 and 12.1 °C, respectively. LT50 values derived from laboratory freezing tests, using leaves taken from the plants in early winter and in spring, showed greater tolerance for winter-harvested leaves; the acclimation was more pronounced at the cold-winter site. Cold tolerance was related positively with leaf K and/or Na accumulation. Analysis of compatible organic solutes (soluble sugars, total amino acids and quaternary ammonium compounds) showed that cold tolerance (measured both as LT50 and as winter freezing damage in situ) was related most closely with leaf concentrations of soluble sugars. The leaf percentage dry matter content was related to both in vitro and in vivo tolerance, while tolerance in vitro was correlated also with the osmotic (potential ψs) and the relative water content. The two diploid (2n = 2x = 18) populations, from Spain, showed greater cold tolerance than the three tetraploid (2n = 4x = 36) populations, from North Africa and Syria, which may be related to the latter's greater cell size and consequent dilution of osmolytes. In this halophytic species, cold tolerance, like salinity and drought tolerance, seems to depend on osmotic adjustment, driven by vacuolar accumulation of K and Na and cytoplasmic accumulation of compatible solutes. 相似文献
106.
María Nicolas Mercedes Alvarez Enrique Anel Félix Martínez Santiago Borragán Felipe Martínez-Pastor Paulino de Paz Luis Anel 《European Journal of Wildlife Research》2012,58(1):77-84
Sperm cryopreservation protocols for brown bear (Ursus arctos) require the centrifugation of semen samples to increase sperm concentration and to clean urine in contaminated samples.
We evaluated the effect of centrifugation regimes (time and relative centrifugal force—RCF) on the quantity of sperm recovered
and the quality of post-thawed sperm. Thirteen brown bears were electroejaculated. The ejaculates were diluted 1:1 in Tris–citric
acid–glucose (TCG) extender and centrifuged with different RCF/time combinations: 600×g, 1,200×g and 2,400×g, for 3, 6 or 12 min. After centrifugation, spermatozoa were diluted in TES–Tris–fructose extender with egg yolk and glycerol
(final glycerol concentration of 8%) and frozen in 0.25-mL straws. In the post-thawed semen, motility was assessed by CASA,
and acrosomal status (PNA-FITC), viability (SYBR-14 with propidium iodide) and chromatin status (SCSA) were determined by
flow cytometry. The longest centrifugation time (12 min) significantly decreased some motility parameters. Sperm recovery
significantly decreased in brown bear at 600×g. Our results suggest that brown bear spermatozoa are more sensitive to long centrifugation times than to high RCF. Centrifugation
regimes showed no effects on the post-thawing chromatin status. We recommend preparing the brown bear semen for freezing by
centrifugation 1,200×g or 2,400×g for 6 min, after electroejaculation and dilution 1:1 in TCG extender, since these procedures increase the spermatozoa recovery
without harmful effects on the post-thawed quality of brown bear spermatozoa. 相似文献
107.
José Ángel Sánchez Agudo Enrique Rico José Sánchez Sánchez Maria Montserrat Martínez-Ortega 《Grana》2013,52(4):239-257
Pollen grains of 30, mainly annual, species from 134 populations of the genus Veronica (Plantaginaceae, formerly Scrophulariaceae) from the Mediterranean Region, have been studied with light and scanning electron microscopy. Three pollen types are defined based on pollen size and exine surface sculpture. In some cases within the study group pollen size can be useful in species determination. Hypothetical relationships of pollen grain size and aperture number with ploidy level, style length and corolla diameter are discussed. The ornamentation of the exine in Veronica, although generally a conservative character within the genus, gives some additional support to the most recent infrageneric classification of Veronica. 相似文献
108.
Angel López Piñeiro Enrique Saiz 《International journal of biological macromolecules》1983,5(1):37-41
The Rotational Isomeric States model is applied to calculate dipole moments of polypeptides of the twenty natural α-amino acids in the random coil state. Dipole moments of each repeat unit (μi), are evaluated using a quantum mechanics procedure. Dipole moment ratios () of homopolypeptides are calculated and extrapolated to x →?. With a few exceptions, D? = 0.36 ± 0.1. Ten actual proteins and three enzymes are also studied; their dipole ratios (Dx′ =〈μ〉/x) range from 7.34 to 10.57 in 10?59 C2 m2 (6.6–9.5 D2). Diffferences in the values of Dx′ are due mainly to the different contributions, μi, of the amino acid residues contained in each polymer, whereas the sequence of amino acids has a very minor effect. 相似文献
109.
110.
David Gutirrez‐Larruscain Santiago Andrs‐Snchez Enrique Rico María Montserrat Martínez‐Ortega 《植物分类学报:英文版》2019,57(1):42-54
Forty-five populations of Pentanema corresponding to seven species included in the Pentanema conyzae clade have been studied using AFLP fingerprinting. The results show that allopolyploidization could have been involved in the diversification of this group, specifically in species P. langeanum and P. maletii. Molecular data confirm the presence of P. britannicum in the Iberian Peninsula and key steps are provided to identify the species that are morphologically the most challenging. 相似文献