全文获取类型
收费全文 | 8038篇 |
免费 | 741篇 |
国内免费 | 697篇 |
专业分类
9476篇 |
出版年
2024年 | 28篇 |
2023年 | 187篇 |
2022年 | 200篇 |
2021年 | 284篇 |
2020年 | 249篇 |
2019年 | 301篇 |
2018年 | 273篇 |
2017年 | 271篇 |
2016年 | 272篇 |
2015年 | 261篇 |
2014年 | 451篇 |
2013年 | 577篇 |
2012年 | 359篇 |
2011年 | 339篇 |
2010年 | 248篇 |
2009年 | 383篇 |
2008年 | 414篇 |
2007年 | 433篇 |
2006年 | 423篇 |
2005年 | 369篇 |
2004年 | 296篇 |
2003年 | 271篇 |
2002年 | 221篇 |
2001年 | 190篇 |
2000年 | 184篇 |
1999年 | 187篇 |
1998年 | 157篇 |
1997年 | 122篇 |
1996年 | 143篇 |
1995年 | 128篇 |
1994年 | 116篇 |
1993年 | 143篇 |
1992年 | 115篇 |
1991年 | 99篇 |
1990年 | 85篇 |
1989年 | 82篇 |
1988年 | 60篇 |
1987年 | 46篇 |
1986年 | 54篇 |
1985年 | 71篇 |
1984年 | 69篇 |
1983年 | 30篇 |
1982年 | 50篇 |
1981年 | 34篇 |
1980年 | 49篇 |
1979年 | 42篇 |
1978年 | 36篇 |
1977年 | 13篇 |
1976年 | 15篇 |
1973年 | 18篇 |
排序方式: 共有9476条查询结果,搜索用时 15 毫秒
991.
To reveal the mechanisms of sedimental H2S accumulation, annual investigations on sedimental environments were conducted in two temperate estuarine lagoons. The lagoons, Gamo and Idoura (Japan), have similar shapes, locations, and topographical properties but different degrees of H2S accumulation. Water stagnation causes a high phytoplankton biomass (Chl. a; 26–52 g l–1) in the inner Gamo Lagoon. Gamo Lagoon sediment was characterized by high bounded sulfides (bounded Smainly FeS) and H2S contents, and low C/N ratios (mean = 10.4) and iron (reactive Fe2+ and total Fe) contents. H2S was not detected in Idoura Lagoon where phytoplankton biomass was much lower (Chl. a; 0.6–4 g l–1). Idoura Lagoon sediment had high C/N ratios (mean=17.9) and high iron contents. The C/N ratio difference implies that organic matter in Gamo Lagoon originates mainly from more decomposable phytoplankton, while organic matter in Idoura Lagoon derives mainly from terrestrial vascular plants with lower decomposability. The excess loading of phytoplanktonic detritus in Gamo accelerates sedimentary microbial activity, including sulfate reduction (i.e., H2S production). High Fe2+and low bounded S contents in Idoura sediment indicate a high chemical buffering capacity toward H2S. In contrast, almost all Fe2+ in Gamo Lagoon had already reacted with H2S as FeS. H2S accumulation in Gamo Lagoon is caused by low sedimentary chemical buffering capacity toward H2S, as well as higher microbial H2S production, caused by the excess loading of phytoplanktonic detritus. 相似文献
992.
Gill PS 《Biometrics》2004,60(2):525-527
We propose a likelihood-based test for comparing the means of two or more log-normal distributions, with possibly unequal variances. A modification to the likelihood ratio test is needed when sample sizes are small. The performance of the proposed procedures is compared with the F-ratio test using Monte Carlo simulations. 相似文献
993.
Intracellular glutathione status regulates mouse bone marrow monocyte-derived macrophage differentiation and phagocytic activity 总被引:2,自引:0,他引:2
Kim JM Kim H Kwon SB Lee SY Chung SC Jeong DW Min BM 《Biochemical and biophysical research communications》2004,325(1):101-108
Although a redox shift can regulate the development of cells, including proliferation, differentiation, and survival, the role of the glutathione (GSH) redox status in macrophage differentiation remains unclear. In order to elucidate the role of a redox shift, macrophage-like cells were differentiated from the bone marrow-derived monocytes that were treated with a macrophage colony stimulating factor (M-CSF or CSF-1) for 3 days. The macrophagic cells were characterized by a time-dependent increase in three major symptoms: the number of phagocytic cells, the number of adherent cells, and the mRNA expression of c-fms, a M-CSF receptor that is one of the macrophage-specific markers and mediates development signals. Upon M-CSF-driven macrophage differentiation, the GSH/GSSG ratio was significantly lower on day 1 than that observed on day 0 but was constant on days 1-3. To assess the effect of the GSH-depleted and -repleted status on the differentiation and phagocytosis of the macrophages, GSH depletion by BSO, a specific inhibitor of the de novo GSH synthesis, inhibited the formation of the adherent macrophagic cells by the down-regulation of c-fms, but did not affect the phagocytic activity of the macrophages. To the contrary, GSH repletion by the addition of NAC, which is a GSH precursor, or reduced GSH in media had no effect on macrophage differentiation, and led to a decrease in the phagocytic activity. Furthermore, we observed that there is checkpoint that is capable of releasing from the inhibition of the formation of the adherent macrophagic cells according to GSH depletion by BSO. Summarizing, these results indicate that the intracellular GSH status plays an important role in the differentiation and phagocytosis of macrophages. 相似文献
994.
A model for environmental sex reversal in fish 总被引:3,自引:0,他引:3
A mathematical model is presented which combines genetic XX-female/XY-male sex determination with environmental pressure for phenotypic sex reversal. This may occur when fishes are exposed to endocrine disrupters, specifically masculinization by exposure to androgens and feminization by exposure to estrogens. A generic model is derived for the sex ratio in successive generations and three special cases, with chronic and constant pressure to sex reverse, are discussed in detail. These show that, with extreme environmental pressure to masculinize, the male genotype is at risk of dying out but with less extreme pressure, masculinization will not be detectable since the proportion of phenotypic males becomes one-half. With feminization at any pressure to sex reverse, the male and female genotypes will be maintained in a stable sex ratio in which the proportion of genotypic males exceeds one-half and is close to one-half if YY offspring (eggs) are not viable. In converse, the model is also applicable to the genetic ZZ-male/ZW-female system of sex determination in fish. At present suitable data are not available with which to validate the model, but proposals are made for relevant experimental studies. 相似文献
995.
Bartos JD Stoler DL Matsui S Swede H Willmott LJ Sait SN Petrelli NJ Anderson GR 《Mutation research》2004,568(2):283-292
Genomic instability in cancer is frequently described as being either chromosomal instability or microsatellite instability, although when events within chromosomes are monitored, extensive intrachromosomal instability is also found. Spectral karyotyping was used to visualize how extensively genomic instability gives rise to intratumor genomic heterogeneity in sporadic colorectal carcinomas. Two factors were then examined which might relate to intrachromosomal instability in colorectal cancers: the presence of the glutathione transferase-Ml gene to detoxify potential carcinogens, and the presence of activated ras which has been associated with chromosomal instability when first expressed. Intrachromosomal genomic instability was previously determined by inter-(simple sequence repeat) PCR (inter-SSR PCR) and by fractional allelic loss rate for 348 markers. GSTM1 status was determined for each of 49 tumors through use of specific PCR, and 28 of the tumors showed the GSTM1 null genotype. A significant association was found between GSTMl-null status and elevated inter-(simple sequence repeat) PCR instability. In contrast, no association was found with fractional allelic loss rate. The first exons of the K-ras and H-ras oncogenes were sequenced in 72 colorectal cancers; 19 of the tumors had a mutation in codon 12 of the K-ras gene (24.5%), but no H-ras mutations were found. A weak correlation (p=0.10) was observed between mutant K-ras and inter-(simple sequence repeat) PCR genomic instability, and no association existed with fractional allelic loss rate. 相似文献
996.
Burke LE Styn MA Steenkiste AR Music E Warziski M Choo J 《Obesity (Silver Spring, Md.)》2006,14(11):2007-2017
Objective: The PREFER study objectives were to examine potential differences in weight loss during a standard behavioral intervention between subjects assigned to one of two calorie‐ and fat‐restricted diets [standard behavior treatment (SBT) and lacto‐ovo‐vegetarian ([SBT+LOV)], with or without regard to their preferred dietary treatment. This article reports the differences in outcomes between diet groups after the first 6 months of the intervention. Research Methods and Procedures: The study used a four‐group design. Subjects (n = 182) were randomized to a treatment preference group and then to a dietary treatment group. For this report, preference groups were combined to permit comparisons by dietary treatment only (SBT, n = 98; SBT+LOV, n = 84). Additional analyses compared SBT+LOV subjects who were 100% adherent (did not consume any meat, fish, or poultry, n = 47) to those who were <100% adherent (n = 24). Results: Significant differences were seen in the baseline to 6‐month change scores between the two groups for carbohydrate consumption (p = 0.013), protein consumption (p < 0.001), polyunsaturated‐to‐saturated fat ratio (p = 0.009), and low‐density lipoprotein‐cholesterol (LDL‐C) level (p = 0.013). Among SBT+LOV subjects, those who were 100% adherent experienced greater reductions in weight (p < 0.001), total cholesterol (p = 0.026), LDL‐C (p = 0.034), and glucose (p = 0.002) and consumed less fat (p = 0.030) compared with those who were <100% adherent. Discussion: Differences between dietary treatment groups at 6 months were minimal, most likely because one‐third of the SBT+LOV group did not follow the vegetarian diet and because both groups had the same calorie and fat restrictions. SBT+LOV subjects who were 100% adherent were more successful at both weight loss and cholesterol reduction than those who were <100% adherent, suggesting that vegetarian diets are efficacious for weight and cholesterol control. 相似文献
997.
Objective: To conduct a clinical and economic evaluation of outpatient weight loss strategies in overweight and obese adult U.S. women. Research Methods and Procedures: This study was a lifetime cost‐use analysis from a societal perspective, using a first‐order Monte Carlo simulation. Strategies included routine primary care and varying combinations of diet, exercise, behavior modification, and/or pharmacotherapy. Primary data were collected to assess program costs and obesity‐related quality of life. Other data were obtained from clinical trials, population‐based surveys, and other published literature. This was a simulated cohort of healthy 35‐year‐old overweight and obese women in the United States. Results: For overweight and obese women, a three‐component intervention of diet, exercise, and behavior modification cost $12,600 per quality‐adjusted life year gained compared with routine care. All other strategies were either less effective and more costly or less effective and less cost‐effective compared with the next best alternative. Results were most influenced by obesity‐related effects on quality of life and the probabilities of weight loss maintenance. Discussion: A multidisciplinary weight loss program consisting of diet, exercise, and behavior modification provides good value for money, but more research is required to confirm the impacts of such programs on quality of life and the likelihood of long‐term weight loss maintenance. 相似文献
998.
The purpose of this research was to investigate the effects of different concentrations of polymer and sucrose stearate, aluminum
tristearate as dispersing agents on microsphere properties and performance. The yield values of microspheres were over the
78%, and the encapsulation efficiencies were found to be ∼735. Particle sizes of microspheres prepared with aluminum tristearate
were between 76 and 448 μm, while that of the microspheres containing sucrose stearate were between 521 and 2000 μm. Morphological
and physicochemical properties of microspheres were investigated by scanning electron micrography and differential scanning
calorimetry (DSC). DSC analysis indicated that verapamil hydrochloride formed a solid solution with acrylic polymers. In vitro
release studies were performed using the flow-through cell method. While ∼80% of drug was released from the microspheres containing
aluminum tristearate in 480 minutes, the same amount of drug was released from microspheres containing sucrose stearate in
only 60 minutes. Chemical structures and concentrations of the dispersing agents were clearly effective on the physical properties
of microspheres and their drug-release characteristics.
Published: February 24, 2006 相似文献
999.
Hae Koo Kim Delphine Luquet Erik van Oosterom Michael Dingkuhn Graeme Hammer 《Annals of botany》2010,106(1):69-78
MethodsFive sorghum hybrids, derived from inbred lines with a common genetic background and with similar phenology and plant height but contrasting tillering, were grown in five experiments. The experiments covered a wide range in radiation and temperature conditions, so that number of tillers produced varied significantly. Data on leaf area, tiller number, and biomass accumulation and partitioning were collected at regular intervals. To quantify internal plant competition for carbohydrates, a carbohydrate supply–demand index (S/Dindex) was developed and related to variation in tillering.ConclusionsThe results support the hypothesis that genotypic differences in tillering were associated with differences in plant carbon S/D balance, associated with differences in leaf size and in the threshold at which tillers grow out. The results provide avenues for phenotyping of mapping populations to identify genomic regions regulating tillering. Incorporating the results in crop growth simulation models could provide insight into the complex genotype-by-management-by-environment interactions associated with drought adaptation. 相似文献
1000.
We investigate the dynamics of a cytoplasmic parasitic element with feminizing effect in a two-population model. We assume that the host species has a ZZ/ZW sex determination system. Our analysis reveals that the feminizer and the W chromosome can stably coexist by dominating different populations if the transmission rate differs significantly between the populations and migration is sufficiently weak. In the equilibrium of coexistence, genetic influx at any host autosomal locus is strongly enhanced in the population where infection is prevalent but not modified in the other population. We further explore conditions for the spread of autosomal suppressor genes that reduce transmission of feminizing elements to the cost of host viability, and compute their equilibrium frequencies. Our results confirm the hypothesis that selfish genetic elements convert infected host populations into genetic sinks, thereby restricting the spread of transmission suppressors. 相似文献