首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1764篇
  免费   175篇
  国内免费   195篇
  2024年   9篇
  2023年   39篇
  2022年   37篇
  2021年   74篇
  2020年   67篇
  2019年   83篇
  2018年   63篇
  2017年   75篇
  2016年   83篇
  2015年   74篇
  2014年   87篇
  2013年   181篇
  2012年   78篇
  2011年   84篇
  2010年   60篇
  2009年   82篇
  2008年   117篇
  2007年   98篇
  2006年   90篇
  2005年   76篇
  2004年   67篇
  2003年   46篇
  2002年   55篇
  2001年   35篇
  2000年   32篇
  1999年   29篇
  1998年   25篇
  1997年   30篇
  1996年   25篇
  1995年   21篇
  1994年   22篇
  1993年   23篇
  1992年   21篇
  1991年   16篇
  1990年   14篇
  1989年   14篇
  1988年   13篇
  1987年   11篇
  1986年   11篇
  1985年   9篇
  1984年   7篇
  1983年   11篇
  1982年   4篇
  1981年   10篇
  1980年   3篇
  1979年   5篇
  1978年   4篇
  1977年   5篇
  1973年   2篇
  1972年   3篇
排序方式: 共有2134条查询结果,搜索用时 31 毫秒
61.
Measurements made in fresh water wetland have shown that temperature, pH, dissolved oxygen and bicarbonate alkalinity in the hyacinth infested area were lower, but dissolved free carbon dioxide concentration was excitingly greater than in the open water area. This extreme hypoxic and hypercarbic condition of hyacinth covered areas was caused by the thick coverage of Eichhornia crassipes which was crucial for the composition of the characteristic flora and fauna.  相似文献   
62.
Summary Three human melanoma cell lines derived from one primary and two metastatic tumors from three different patients were characterized for growth properties usually associated with malignant transformation; these include cell morphology, growth rate, saturation density, growth in semisolid media, colony-forming ability on contact-inhibited monolayers of normal fibroblasts and epithelial cells, and tumorigenicity in immunosuppressed mice. Variations in expression of aberrant properties were evident among the lines. One of the metastatic lines satisfied all the parameters of malignancy tested and the other showed a number of these properties, whereas the primary essentially fulfilled only one. These results suggest that cultured melanoma cells reflect the clinical variability often observed among melanoma patients and the metastatic melanoma seems to display a higher degree of malignant transformation than the primary. THis work was supported in part by USPHS Grant No. 5 T01 AI00332-06 from the National Institutes of Health, Contract E73-2001-N01-CP-3-3237 from the Virus Cancer Program of the National Cancer Institute, and USPHS Grant No. 0H00714-02 from the National Institute for Occupational Safety and Health.  相似文献   
63.
The blood of two fresh water cobitids — Botia lohachata, an exclusive water breathing form and Lepidocephalus guntea a dual breather, — showed a comparatively higher range of Hb (16.0–19.0 g%), Hct (50.0–61.1%) and number of RBC (2.71–6.7 millions/mm3) than many other water and air breathing fishes. Significant sexual difference exists in these characteristics (P > 0.05).The impact of life in oxygen depleted water, also inhabited by L. guntea as a result of its air breathing habit, is well reflected in its greater RBC size (11.86 × 8.66 µm) and their larger surface area (83.96 µm2) compared to that of Botia (53.16 µm2). The smaller size (9.92 × 6.45 µm) and consequently greater number of erythrocytes (4.67 millions/mm3) in Botia, are related to its active mode of life in the swiftly flowing water of hilly rivers. Though the higher nucleo-cytoplasmic ratio of 1 : 5.2 in Botia and 1 : 6.9 in Lepidocephalus suggest a slower red cell metabolism, the greater number of erythrocytes seems to have compensated for their active mode of life.  相似文献   
64.
SPATIAL FLOWER PARAMETERS AND INSECT SPATIAL VISION   总被引:6,自引:0,他引:6  
The present article reviews recent and older literature on the spatial parameters that flowers display, as well as on the capacities of anthophilous insects to perceive and use these parameters for optimizing their foraging success. Although co-evolution of plants and pollinators has frequently been discussed with respect to floral colours and insect colour vision, it has rarely been assessed with respect to insect spatial vision and spatial floral cues, such as shape, pattern, size, contrast, symmetry, spatial frequency, contour density and orientation of contours. This review is an attempt to fill this gap. From experimental findings and observations on both flowers and insects, we arrive at the conclusion that all of the spatial and spatio-temporal parameters that flowers offer are relevant to the foraging task and are tuned to the insect's visual capacities and visually guided behaviour. We try, in addition, to indicate that temporal cues are closely related to spatial cues, and must therefore be included when flower–pollinator interactions are examined. We include results that show that colour vision and spatial vision have diverged over the course of evolution, particularly regarding the processing of spatio-temporal information, but that colour vision plays a role in the processing of spatial cues that are independent of temporal parameters. By presenting this review we hope to contribute to closer collaboration among scientists working in the vast fields of botany, ecology, evolution, ethology and sensory physiology.  相似文献   
65.
BackgroundResearch to date suggests that nickel affects not only the metabolism of vitamin B12 but also folates and thus may affect hematopoiesis processes.ObjectiveThe aim of the study was to examine the relationship of nickel (Ni) status to red blood cell (RBC) parameters and serum vitamin B12, folate and homocysteine concentrations in the course of normal pregnancy and in pregnant women with anemia.MethodsThe study included fifty-three pregnant women recruited to the study from the Lower Silesia region of Poland, 17 % of whom developed anemia. Nickel concentration was determined in urine, whole blood and food samples by atomic absorption spectrometry. At the same time as the food and urine samples were taken, blood was also collected for the determination of RBC parameters and serum vitamin B12, homocysteine and folate concentrations.ResultsThe median reported Ni intake, and the urinary and whole blood nickel contents for the studied pregnant women for the first trimester were respectively – 162.46 μg/day, 3.98 μg/L and 3.32 μg/L; for the second trimester – 110.48 μg/day, 6.86 μg/L and 1.04 μg/L; and for the third trimester – 132.20 μg/day, 3.41 μg/L and 0.70 μg/L. With regard to Ni concentration in whole blood (p = 0.0204) and in urine (p = 0.0003), the differences in the values for individual trimesters were statistically significant. The whole blood Ni level was significantly higher (9.28 vs 3.62 μg/L, p = 0.0114), while the concentration of homosysteine was significantly lower (4.09 vs 5.04 μmol/L, p = 0.0165) in pregnant women with anemia compared to those without anemia. The whole blood Ni concentration was negatively correlated with almost all RBC parameters in non-anemic pregnant women.ConclusionsNi status changes with the development of normal pregnancy, and in the case of anemia, an increase in Ni concentration in whole blood is observed. The demonstrated correlations between the Ni status in pregnant women and RBC parameters as well as serum vitamin B12 and folate concentrations suggest that nickel is associated with the methionine–folate cycle, iron homeostasis and bacterial synthesis of vitamin B12 in humans.  相似文献   
66.
67.
韩志萍  程慧莹  王飞 《菌物学报》2020,39(8):1520-1529
桔黄赛多孢菌Scedosporium aurantiacum是慢性肺病患者的常见呼吸道定植菌,在免疫缺陷人群中可引起侵袭性感染,致死率高,但由于致病机理不明,目前仍然缺乏有效的防控手段。我们在前期研究中,通过差异蛋白组学及酶工程技术发现分泌胰蛋白酶是桔黄赛多孢菌的潜在毒力因子,目前对这种蛋白酶的遗传信息、结构及致病机制并不清楚。本研究用Superdex S-200分子筛和DEAE-Sepharose离子交换两种填料将这种蛋白酶分离纯化出来,通过酶谱验证了纯化效果。进一步研究发现,这种胰蛋白酶对bFSR、bLSTR和bEKK 3种底物的水解性能最佳,对zFR和bzLR的水解性能最差。酶解最快的反应所对应的Km为6.09μmol/L,Vmax为13.01μmol/L/s,Kcat为23.65/s;酶解最慢的反应所对应的Km为29.94μmol/L,Vmax为11.35μmol/L/s,Kcat为20.63/s。研究结果对于填补赛多孢菌毒力因子研究的空白、针对毒力因子开发新型的抗真菌药物和治疗方法都具有重要意义。  相似文献   
68.
Grape seed procyanidins (GSPs), widely known for their beneficial health properties, fail to bring about the expected improvement in piglets’ growth performance. The effects of dietary supplementation with GSPs on nutrient utilisation may be a critical influencing factor. Hence, the purpose of this study was to investigate the effects of dietary supplementation with GSPs on nutrient utilisation and gut function in weaned piglets. One hundred and twenty crossbred piglets were allocated randomly to four treatment groups, with three replicate pens per treatment and 10 piglets per pen. Each group was given one of the four dietary treatments: the basal diet (control group) or the basal diet with the addition of 50-, 100- or 150-mg/kg GSPs. The trial lasted 28 days. Faeces were collected from d 12 to 14 and from d 26 to 28 for measuring the coefficient of total tract apparent digestibility (CTTAD) of the nutrients. Blood samples were collected on d 14 and 28 for detecting the blood biochemical parameters. Two piglets per pen were slaughtered to collect the pancreas and intestinal digesta for evaluating the digestive enzyme activity and the coefficient of ileal apparent digestibility (CIAD) of the nutrients. On d 14 and 28, supplementation with 150-mg/kg GSPs significantly decreased the CTTAD of DM and CP in piglets. On d 14, GSPs supplementation at a concentration of 150 mg/kg led to a remarkable decrease in the CIAD of CP and gross energy (GE). On d 28, GSPs supplementation at a dose of 150 mg/kg generated a marked decline in the CIAD of DM, GE, CP and ether extract. Grape seed procyanidins supplementation at concentrations of 100 or 150 mg/kg inhibited the activities of lipase and amylase. In contrast, the jejunum mucosa maltase and sucrase activities increased due to the inclusion of GSPs at a concentration of 100 mg/kg in the piglet diet. Compared with the levels of the control group, the serum glucose and total protein levels were enhanced significantly by supplementation with GSPs at 100 mg/kg and reduced dramatically at 150 mg/kg. The serum diamine oxidase activity and endotoxin levels were decreased by GSPs supplementation in piglet diets. In conclusion, higher concentrations of GSPs in weaned piglet diets attenuated nutrient digestion and inhibited digestive enzyme activity; however, suitable concentrations of GSPs could promote brush-border enzyme activity, enhance serum glucose and total protein concentrations and decrease epithelial permeability.  相似文献   
69.
In the mink industry, feed costs are the largest variable expense and breeding for feed efficient animals is warranted. Implementation of selection for feed efficiency must consider the relationships between feed efficiency and the current selection traits BW and litter size. Often, feed intake (FI) is recorded on a cage with a male and a female and there is sexual dimorphism that needs to be accounted for. Study aims were to (1) model group recorded FI accounting for sexual dimorphism, (2) derive genetic residual feed intake (RFI) as a measure of feed efficiency, (3) examine the relationship between feed efficiency and BW in males (BWM) and females (BWF) and litter size at day 21 after whelping (LS21) in Danish brown mink and (4) investigate direct and correlated response to selection on each trait of interest. Feed intake records from 9574 cages, BW records on 16 782 males and 16 875 females and LS21 records on 6446 yearling females were used for analysis. Genetic parameters for FI, BWM, BWF and LS21 were obtained using a multivariate animal model, yielding sex-specific additive genetic variances for FI and BW to account for sexual dimorphism. The analysis was performed in a Bayesian setting using Gibbs sampling, and genetic RFI was obtained from the conditional distribution of FI given BW using genetic regression coefficients. Responses to single trait selection were defined as the posterior distribution of genetic superiority of the top 10% of animals after conditioning on the genetic trends. The heritabilities ranged from 0.13 for RFI in females and LS21 to 0.59 for BWF. Genetic correlations between BW in both sexes and LS21 and FI in both sexes were unfavorable, and single trait selection on BW in either sex showed increased FI in both sexes and reduced litter size. Due to the definition of RFI and high genetic correlation between BWM and BWF, selection on RFI did not significantly alter BW. In addition, selection on RFI in either sex did not affect LS21. Genetic correlation between sexes for FI and BW was high but significantly lower than unity. The high correlations across sex allowed for selection on standardized averages of animals’ breeding values (BVs) for RFI, FI and BW, which yielded selection responses approximately equal to the responses obtained using the sex-specific BVs. The results illustrate the possibility of selecting against RFI in mink with no negative effects on BW and litter size.  相似文献   
70.
BackgroundSome observational studies indicate an association of 25-hydroxy vitamin D (25(OH)D) insufficiency and atherogenic cholesterol concentrations. The aim of this study was to investigate relationship between 25(OH)D concentrations and lipid parameters in end stage renal disease (ESRD) patients, separately for predialysis, hemodialysis and peritoneal dialysis patients.MethodsWe have adjusted 25(OH)D concentrations for seasonal variability with cosinor analysis, and performed all further analysis using these corrected 25(OH)D concentrations. Concentrations of 25(OH)D and the lipid parameters were determined in 214 ESRD patients and 50 control group participants. The analysis included the measurement of 25(OH)D by HPLC, apolipoprotein (Apo) AI, ApoB and Lp(a) by nephelometry, total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and triglyceride (TG) by spectrophotometry and manually calculated ApoB/ApoAI and LDL-C/HDL-C ratio.ResultsESRD patients with adjusted 25(OH)D concentrations of 50 nmol/L had significantly higher TC (P = 0.005) and ApoAI (P = 0.049). Significantly higher HDLC (P = 0.011) and ApoAI (P = 0.020) were found in hemodialysis patients with the 25(OH)D concentrations of 50 nmol/L. The other analyzed lipid parameters differed significantly between predialysis, hemodialysis and peritoneal dialysis patients with 25(OH)D concentrations of < 50 nmol/L.ConclusionsOur study indicate the significant relationship between 25(OH)D repletion and optimal concentrations of lipid parameters in ESRD patients. Further research is necessary to explain whether joint evaluation of vitamin D status and lipid abnormalities could improve cardiovascular outcome in ESRD patients.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号