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   6501篇
  免费   392篇
  国内免费   7篇
  6900篇
  2022年   40篇
  2021年   61篇
  2020年   36篇
  2019年   53篇
  2018年   84篇
  2017年   62篇
  2016年   100篇
  2015年   174篇
  2014年   203篇
  2013年   337篇
  2012年   300篇
  2011年   355篇
  2010年   217篇
  2009年   223篇
  2008年   361篇
  2007年   382篇
  2006年   392篇
  2005年   374篇
  2004年   370篇
  2003年   341篇
  2002年   342篇
  2001年   166篇
  2000年   157篇
  1999年   131篇
  1998年   101篇
  1997年   65篇
  1996年   67篇
  1995年   65篇
  1994年   55篇
  1993年   77篇
  1992年   103篇
  1991年   94篇
  1990年   91篇
  1989年   88篇
  1988年   95篇
  1987年   75篇
  1986年   80篇
  1985年   49篇
  1984年   57篇
  1983年   51篇
  1982年   39篇
  1981年   35篇
  1980年   36篇
  1979年   44篇
  1978年   34篇
  1977年   28篇
  1976年   23篇
  1974年   25篇
  1973年   24篇
  1968年   20篇
排序方式: 共有6900条查询结果,搜索用时 0 毫秒
71.
A supernumerary muscle bundle “M. tubero-femoralis Fick” of the biceps femoris was found in a female crab-eating monkey in both hindlimbs.  相似文献   
72.
A gas chromatographic—mass spectrometric analysis was used to separate and identify abnormal compounds in the nail of psoriatic patients. The nail was extracted with heated ethanol, and the extract was analyzed with and without trimethylsilylation. Tetradecanoic acid octadecyl ester, hexadecanoic acid octadecyl ester and octadecanoic acid octadecyl ester were first identified in the psoriatic nail, but were not detected in normal nail.  相似文献   
73.
As a model compound for the growing chain in the activated-NCA type of polymerization of α-amino acid N-carboxyanhydride (NCA), 3-[ω-acetylglycyl-poly(α-amino acid) acyl]-α-amino acid NCA (called the prepolymer) having various degrees of polymerization (DPs) was synthesized by the polymerization of Phe, Val, Glu(OEt), and Asp(OBzl) NCA in the presence of AcGly NCA by the tertiary amine. Activated (S)-Phe, Val, Glu(OEt), and Asp(OBzl) NCA were added to the terminal cyclic group of the corresponding (S)- or (R)- prepolymer, and the enantiomer selectivity in the reaction was investigated. With prepolymers having DPs ranging from 1 to 15, the addition reaction always took place preferentially between species having the same configuration, and the degree of the enantiomer selection increased with increasing DP of the prepolymer. With prepolymers having DP = 1 and 2, we found contributions from the chiral terminal unit and the chiral penultimate unit to the enantiomer selection, respectively. Prepolymer having DP = 5 was shown to take a β-type conformation, which led to higher enantiomer selection; and prepolymers having DP = 10 and 15 were shown to take an α-helix conformation, which led to much higher enantiomer selection than did the β-type conformation. In the present investigation the mechanisms of terminal-unit control, penultimate-unit control and conformational control of the enantiomer selection in the activated-NCA type of polymerization were clearly observed.  相似文献   
74.
75.
The physiochemical properties of pyocin F1 were studied. Pyocin F1 consists of flexuous rod-like particles homogenous in size. Each particle was composed of rod and fiber parts. The rod part was 105.5 +/- 9.5 nm long and 10.0 +/- 1.4 nm wide, and showed regular striations amounting to 23 layers. The fiber part was composed of several filaments; the length of the longest filament was 43.0 +/- 12.0 nm. The amino acid composition, the partial specific volume (0.720 ml/g), the sedimentation coefficient (S020,W = 35.1S), and the translational diffusion constant (0.94 +/- 0.01 x 10(-7) cm2/s) were determined. The particle weight was calculated to be 3.23 x 10(6) daltons.  相似文献   
76.
Protoplats were enzymatically prepared from protonemata of the mossPlagiomnium vesicatum, which was cultured on a Murashige-Skoog's medium supplemented with 0.2% yeast extract and solidified with agar. The protoplasts incubated in soft agar medium containing sucrose began to divide after 15 days of incubation, and after 50 days 10- to 15-cell aggregates were present. When transferred to a sugar-free medium, the cells in the aggregates grew into filaments.  相似文献   
77.
The nature of the interaction between Sendai virus and Sil mutant cells was examined by measuring a change in ESR spectrum of spin-labeled phosphatidylcholine molecules on the viral envelope. When spin-labeled virus was incubated with the Sil cells that had a reduced ability to respond to virus-induced cell fusion, interchange of the phospholipid molecules between viral envelope and cell surface membrane occurred to a smaller extent than that observed with parental cells. Moreover, the degree of the interchanging correlated with the degree of the fusion capacity of the mutant lines. The results show that the mutant cells carry such a lesion(s) on their surface membranes that the viral envelopes can hardly fuse into them.  相似文献   
78.
Summary A large metacentric marker chromosome, m20, in a line of human D98/AH-2 cells was identified by Q bands as being a translocation (1;17)(p36;q21). This was confirmed by means of somatic cell hybridization between D98/AH-2 and thymidine kinase (TK) deficient mouse cells. The hybrid clones by HAT selective system retained m20, indicating the presence of TK locus on this chromosome. The results also provide evidence that TK gene is located on the distal region of the breakpoint in 17q21 but not on 17q21 17pter.  相似文献   
79.
Effect of temperature on motility and chemotaxis of Escherichia coli.   总被引:14,自引:10,他引:4       下载免费PDF全文
K Maeda  Y Imae  J I Shioi    F Oosawa 《Journal of bacteriology》1976,127(3):1039-1046
The swimming velocity of Escherichia coli at various constant temperatures was found to increase with increasing temperature. The frequency of tumbling had a peak at 34 degrees C and was very low both at 20 and at 39 degrees C. The swimming tracks near the surface of a slide glass showed curves, and the curvature increased the temperature. When the temperature of a bacterial suspension was suddenly changed, a transient change of the tumbling frequency was observed. A temperature drop induced a temporary increase in the tumbling frequency, and a quick rise of temperature, on the other hand, resulted in a temporary suppression of the tumbling. These dynamic responses to sudden changes of temperature was not observed in the smoothly swimming nonchemotactic strains bearing the mutations cheA and cheC and also in a mutant with the metF mutation under a smooth swimming condition.  相似文献   
80.
Two enantiomeric cuparene-type sesquiterpenoids, (R)-(−)-cuparene (1) and (R)-(−)-δ-cuparenol (2), have been isolated from the liverwort, Bazzania pompeana. The structures and absolute configurations of the two compounds have been determined.  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