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   76068篇
  免费   7591篇
  国内免费   50篇
  83709篇
  2022年   469篇
  2021年   896篇
  2020年   570篇
  2019年   701篇
  2018年   910篇
  2017年   844篇
  2016年   1460篇
  2015年   2326篇
  2014年   2748篇
  2013年   3584篇
  2012年   4593篇
  2011年   4549篇
  2010年   3048篇
  2009年   2770篇
  2008年   3979篇
  2007年   4211篇
  2006年   3973篇
  2005年   3785篇
  2004年   3744篇
  2003年   3672篇
  2002年   3420篇
  2001年   1101篇
  2000年   884篇
  1999年   1111篇
  1998年   1058篇
  1997年   807篇
  1996年   756篇
  1995年   719篇
  1994年   721篇
  1993年   738篇
  1992年   857篇
  1991年   756篇
  1990年   744篇
  1989年   726篇
  1988年   744篇
  1987年   715篇
  1986年   683篇
  1985年   755篇
  1984年   841篇
  1983年   746篇
  1982年   817篇
  1981年   869篇
  1980年   820篇
  1979年   607篇
  1978年   658篇
  1977年   592篇
  1976年   546篇
  1975年   516篇
  1974年   610篇
  1973年   543篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Although the central and stable position of female rhesus monkeys in semi-free-ranging bands has been observed and described, the position of the developing male in the social group is not as well delineated. The present study is an attempt to describe interactions of 18 males with members of their families. Quantitative observations were made on one band (E) of rhesus monkeys on Cayo Santiago. Incidences of behavior recorded were joining, grooming, and threatening. The resulting data were analyzed by comparing the behavior of the males studied toward members of their genealogy as well as unrelated members of the social group. Behavior of family and non-relatives toward the males was also revealed. Statistical analysis revealed that the male subjects engaged in more positive social interactions within the matrifocal family than with other members of the genealogy, and in turn, more such behavior was observed within the genealogy than with unrelated monkeys. Threatening behavior within the genealogies was infrequent. Age effects were noted in that family members directed more interactions toward younger males and as males matured, they groomed more but joined family members less frequently. These data suggest mechanisms of socialization within the genealogy that may be different from those employed within the unrelated social group.  相似文献   
82.
83.
84.
85.
1H-Detected 15N-edited NMR in solution was used to study the conformational differences between the GDP- and GTP gamma S-bound forms of human N-p21ras. The amide protons of 15N-labeled glycine and isoleucine were observed. Resonances were assigned to residues of particular interest, glycines-60 and -75 and isoleucines-21 and -36, by incorporating various 13C-labeled amino acids in addition to [15N]glycine and [15N]iosleucine and by replacing Mg2+ by Co2+. When GTP gamma S replaced GDP in the active site of p21ras, only 5 of the 14 glycine amide resonances show major shifts, indicating that the conformational effects are fairly localized. Responsive glycines-10, -12, -13, and -15 are in the active site. Gly-75, located at the far end of a conformationally-active loop and helix, also responds to substitution of GTP gamma S for GDP, while Gly-77 does not, supporting a role for Gly-75 as a swivel point for the conformational change. The amide proton resonances of isoleucines-36 and -21 and a third unidentified isoleucine also undergo major shifts upon replacement of GDP by GTP gamma S. Thus, the effector-binding loop containing Ile-36 is confirmed to be involved in the conformational change, and the alpha-helix containing Ile-21 is also shown to be affected.  相似文献   
86.
Tubulin has been purified from human blood and tonsil lymphocytes. Using gel filtration, the molecular weight of human lymphocyte tubulin was estimated to be 119 000. The proteins was shown to consist of two subunits, with molecular weights of 61 000 and 58 000 comparable to the α and β polypeptides of human brain tubulin. A partial identity reaction was observed between lymphocyte tubulin and human tubulin when tested by double immunodiffusion against a rabbit anti-human brain tubulin antibody. In the presence of GTP, the purified protein polymerized to form microtubules. Tubulin was localized to the cell's juxtacentriolar region by immunofluorescence and electron microscopy. When assayed by a colchicine-binding assay corrected for time decay, the binding affinity was 1.50 ± 0.86 · 106M?1 and a level in normal lymphocytes of 1.21 · 10?2 ± 0.79 g/g of soluble protein was determined. Since chronic lymphocytic leukemia lymphocytes have an anomalous capping behavior as well as an unusual susceptibility to colchicine toxicity, the properties and levels of tubulin were determined in these cells. Similar values were obtained for the level, decay rate, molecular weight, and Ka for colchicine as for normal lymphocytes. Chronic lymphocytic leukemia lymphocyte tubulin polymerized in a normal fashion. It thus appears that a decrease in the quantity or function of tubulin does not account for these anomalies in the chronic lymphocytic leukemia lymphocyte.  相似文献   
87.
88.
Eleven male subjects were investigated to detect a possible circadian rhythm of the polymorphonuclear phagocytosis. Both cell activity and serum opsonins were studied for numerical detection of granulocytes having ingested at least one particle and for the mean number of ingested particles per cell. No significant temporal differences (ANOVA and cosinor) were found.  相似文献   
89.
Presented a choice between conspecific males and 2 congeneric males, virgin females ofMelittobia australica andM. digitata chose conspecific males disproportionately more often, whereasM. femorata females distributed themselves evenly among the choices. Empty tubes, provided as the fourth choice in the test apparatus, were entered much less often than tubes containing live males. Females of all species chose “wrong” males about equally frequently. These observations suggest that even non-conspecific males possess some degree of attractiveness to virgin females. Chemicals in the sex pheromone of the males are presumed to be the source of the males' attractancy. The incomplete species specificity is interpreted in light of the life history of this genus, and it is suggested that specific recognition cues operate primarily after the sexes come together. Supported by a grant from Kagoshima Prefecture in Japan under the exchange program of faculty members between Kagoshima University and the University of Georgia.  相似文献   
90.
Three experiments were performed to study the influence of challenging incentives on feedback-assisted heart rate reduction for coronary-prone (Type A) and non-coronary-prone (Type B) males. In the first experiment, when subjects were given a competitive instructional set, Type As were significantly more successful relative to Type Bs in reducing their heart rate; with a noncompetitive set, Type Bs were significantly more successful than were Type As. In the second experiment, when told that heart rate reduction was a scarce ability, Type As reduced heart rate significantly better than did Type Bs; when told that heart rate reduction was a common ability, Type Bs achieved significantly greater heart rate reduction than did Type As. In the third experiment, when heart rate reduction was described as being instrumental to time-urgency (i.e., getting more done in less time), Type As reduced heart rate significantly bettern than did Type Bs; when heart rate reduction was described as being instrumental to relaxation, Type Bs were significantly better able to reduce heart rate. In all three studies, the incentives had no effect on heart rate when feedback was not provided. The results are discussed as support for the notion that Type A behavioral pattern characteristics can be exploited to reduce Type A symptoms. Implications for how coronary-prone individuals may be challenged to modify symptoms within the clinical setting are discussed.  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