全文获取类型
收费全文 | 9499篇 |
免费 | 745篇 |
国内免费 | 478篇 |
专业分类
10722篇 |
出版年
2024年 | 24篇 |
2023年 | 93篇 |
2022年 | 253篇 |
2021年 | 408篇 |
2020年 | 286篇 |
2019年 | 296篇 |
2018年 | 328篇 |
2017年 | 263篇 |
2016年 | 383篇 |
2015年 | 573篇 |
2014年 | 626篇 |
2013年 | 728篇 |
2012年 | 818篇 |
2011年 | 745篇 |
2010年 | 480篇 |
2009年 | 347篇 |
2008年 | 484篇 |
2007年 | 398篇 |
2006年 | 383篇 |
2005年 | 317篇 |
2004年 | 275篇 |
2003年 | 231篇 |
2002年 | 230篇 |
2001年 | 170篇 |
2000年 | 190篇 |
1999年 | 143篇 |
1998年 | 84篇 |
1997年 | 81篇 |
1996年 | 78篇 |
1995年 | 61篇 |
1994年 | 66篇 |
1993年 | 48篇 |
1992年 | 92篇 |
1991年 | 77篇 |
1990年 | 77篇 |
1989年 | 61篇 |
1988年 | 47篇 |
1987年 | 68篇 |
1986年 | 45篇 |
1985年 | 37篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 25篇 |
1980年 | 22篇 |
1979年 | 26篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1975年 | 18篇 |
1973年 | 21篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Nagilactone E, a norditerpene dilactone isolated from a gymnosperm, Podocarpus nagi (Podocarpaceae), was able to stimulate the growth of cultured cells of Lactuca sativa cv. Grand Rapids at 0.1 g ml-1 but not at 0.01 or 1 g ml-1. Cell wet weight/unit time and cell number/unit wet weight were increased when they were recorded at the end of the 14-day incubation period. Also, the population of cells treated with that concentration of nagilactone E had a higher percentage of cells with shorter cell length compared to the control (untreated). 相似文献
22.
The present study describes substance P-like immunoreactivity in the ciliary ganglia of monkey (Macaca fascicularis) and cat. About 60% of neurons in the monkey ciliary ganglion and 40% in the cat ciliary ganglion were substance P-like immunoreactive, ranging from faint to moderate staining. Substance P-like immunoreactivity was located in cell bodies, dendritic profiles and axons. In the monkey, substance P-like immunoreactive pericellular arborisations were associated with about 0.5%–3% of the ganglion cells, which were either negatively, faintly or moderately stained. An electron-microscopic study demonstrated the presence of either substance P-like immunoreactive positive or negative axon terminals synapsing or closely associated with positive dendritic profiles in both the monkey and cat ciliary ganglia. The results suggest that substance P plays an important role in the ciliary ganglion, perhaps as a modulator or transmitter. 相似文献
23.
研究了天然杀伤(NK)细胞对受致死剂量γ线照射的同系小鼠的造血调控作用。AMS/5小鼠经9Gyγ线全身照射后立即经尾静脉注射NK细胞(5×105),可明显提高受照小鼠30d活存率,照后8d小鼠骨髓中CFU-GM数量明显高于对照和脾细胞注射组,照射后30d,NK细胞注射组活存小鼠的骨髓有核细胞数和CFU-GM数已恢复到正常的76%─96%。病理组织学观察显示,输注NK细胞可使小鼠骨髓、脾脏的组织损伤程度减轻,造血功能增强,表现为造血灶数增多,造血细胞功能活跃,核分裂相增多,且涉及红系、粒系、巨核细胞系造血。NK细胞可能通过直接与造血干细胞相互作用或改善造血微环境等促进“内源性”造血功能,从而发挥对造血的正调控作用。提示NK细胞在小鼠造血功能的平衡维持中起重要作用。 相似文献
24.
In an initial study of anti-nuclear antibodies in the chronic inflammatory bladder disease interstitial cystitis, we reported that 7% of interstitial cystitis patients studied had autoantibodies to the nucleolus. We now report that, using an autoimmune serum from a patient with interstitial cystitis, we have identified and partially characterized a novel protein with an M(r) of approximately 55 kDa (hereafter referred to as No55) localized to the granular component of the nucleolus. No55 was initially characterized by diffuse nucleolar immunofluorescence staining in interphase cells and by Western blotting as a 55-kDa doublet on whole-cell extracts. During mitosis, No55 was associated with chromosomes and appeared in prenucleolar bodies during telophase, but it did not colocalize with p80-coilin in coiled bodies. Immunoelectron microscopy revealed that No55 was localized uniformly throughout the granular component of the nucleolus compared with a more peripheral localization of nucleolar granular component protein B23. On segregation of the nucleolus with actinomycin D, No55 remained with the granular component of the segregated nucleolus, whereas protein B23 was found predominantly in the nucleoplasm. Finally, a cDNA expression library was screened with the human autoantibody against No55, and a 2.4-kb insert was isolated, subcloned to homogeneity, and then sequenced. Analysis of this sequence showed an open reading frame of approximately 1.3 kb coding for 437 amino acids with a predicted molecular weight of 50 kDa. A search of the gene sequence database indicated homology with SC65, a rat synaptonemal complex protein. Therefore, on the basis of molecular weight, nucleolar sublocalization, response to actinomycin D, and cDNA sequence determination, No55 is a novel protein of the interphase nucleolus. 相似文献
25.
Efficiency of reinitiation of translation on human immunodeficiency virus type 1 mRNAs is determined by the length of the upstream open reading frame and by intercistronic distance. 总被引:16,自引:4,他引:12 下载免费PDF全文
In this study, we examined the mechanism of translation of the human immunodeficiency virus type 1 tat mRNA in eucaryotic cells. This mRNA contains the tat open reading frame (ORF), followed by rev and nef ORFs, but only the first ORF, encoding tat, is efficiently translated. Introduction of premature stop codons in the tat ORF resulted in efficient translation of the downstream rev ORF. We show that the degree of inhibition of translation of rev is proportional to the length of the upstream tat ORF. An upstream ORF spanning 84 nucleotides was predicted to inhibit 50% of the ribosomes from initiating translation at downstream AUGs. Interestingly, the distance between the upstream ORF and the start codon of the second ORF also played a role in efficiency of downstream translation initiation. It remains to be investigated if these conclusions relate to translation of mRNAs other than human immunodeficiency virus type 1 mRNAs. The strong inhibition of rev translation exerted by the presence of the tat ORF may reflect the different roles of Tat and Rev in the viral life cycle. Tat acts early to induce high production of all viral mRNAs. Rev induces a switch from the early to the late phase of the viral life cycle, resulting in production of viral structural proteins and virions. Premature Rev production may result in entrance into the late phase in the presence of suboptimal levels of viral mRNAs coding for structural proteins, resulting in inefficient virus production. 相似文献
26.
肌肽对绵羊精子无氧酵解的影响 总被引:3,自引:0,他引:3
在无氧条件下,绵羊精子通过酵解途径获得能量,代谢结果产生大量乳酸,本实验通过测定精子悬液中果糖摄取以及乳酸生成量,研究肌肽、棉酚对绵羊精子酵解途径的影响,结果表明:4mM肌肽对绵羊精子酵解有显著增强作用,并能刺激精子对果糖的摄取。12μM棉酚对绵羊精子无明显抑制,棉酚能部分抑制肌肽对精子的酵解作用。 相似文献
27.
28.
催产素在脊髓水平对电针镇痛的影响 总被引:3,自引:0,他引:3
采用玻璃微电极胞外记录和脊髓表面给药的方法观察了催产素(OT)、抗催产素血清(AOTS)以及电针穴位对背角神经元伤害性诱发放电的影响。结果表明:电针穴位或脊髓表面施加OT可部分抑制脊髓背角神经元的伤害性诱发放电;在电针的基础上施加OT则明显加强电针的抑制效应;相反,用AOTS预处理后,电针的抑制作用放取消。提示OT在脊髓水平参与了对痛觉信息的调制,并与一定频率的针刺镇痛有关。 相似文献
29.
Summary Batch cultures of Saccharomycopsis lipolytica were grown in minimal medium with olive oil as carbon source. Inocula of glucose-grown cells commenced growth with little lag at rates largely unaffected by variations in the stirring rate or oil concentration. However, growth rates declined when the medium pH was below 7.0. In all cultures, media pH declined with increasing cell concentration. Cell composition during exponential growth was 42% protein and 2% fat. Carbon-limited cells maintained this composition after oil exhaustion but during nitrogen- and oxygen-limited growth, protein content decreased and fat content increased although the protein decrease was only transient with oxygen limitation. Yield coefficients for triglyceride were near unity for all cultures. Free acid concentrations rose rapidly after inoculation. As fermentations progressed, free glycerol appeared and concentrations of di- and monoglycerides passed through maximal values although peak concentrations of di- and monoglycerides persisted for extended times in oxygen- and nitrogen-limited cultures respectively. The fraction of free glycerol consumed was greater in oxygen-limited than in carbon- or nitrogen-limited culture. The basic requirements for growth of yeasts on fatty wastes are discussed with reference to these observations. 相似文献
30.
Desired family size and contraceptive behavior of 986 ever-married women aged 15-54 from Northern Mindanao, the Philippines, are described. Using the Dow and Werner typology of demographic and contraceptive patterns, it was found that a majority of the women were traditional in their demographic attitude and contraceptive behavior. A substantial proportion of the women desired medium to large families. In consistency in attitude and behavior was also manifested by women who desired small families but have never used efficient family planning methods. Sociodemographic characteristics of these women are described. A majority of the women who have never used efficient contraception also never practiced family planning. Rhythm was the primary method of birth control among demographic innovators and traditionals who have ever used inefficient methods; the pill was popular among contraceptive innovators and moderns. A considerable proportion of the demographic innovators and the traditionals were not currently using any family planning method; a lesser proportion of contraceptive innovators and moderns were also not using birth control methods. Relevance of the findings to family planning programs is pointed out. Recommendations are given. Motivation for a small family is a prerequisite for a successful family planning program. Without motivation, what family planning programs do is reduce the number of unwanted births. Family planning programs should reach out to those who are sufficiently motivated to have small families but have never used efficient family planning methods. A major finding of this research is that a majority of the women who never used efficient contraception also never used inefficient or traditional methods. Efforts should be made to change this. Additionally, efforts should be made to bring those modern and contraceptive innovators who had stopped using any family planning method to practice contraception again. 相似文献