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   12692篇
  免费   666篇
  国内免费   3篇
  13361篇
  2022年   47篇
  2021年   132篇
  2020年   85篇
  2019年   111篇
  2018年   160篇
  2017年   147篇
  2016年   248篇
  2015年   395篇
  2014年   454篇
  2013年   909篇
  2012年   821篇
  2011年   828篇
  2010年   552篇
  2009年   509篇
  2008年   842篇
  2007年   882篇
  2006年   786篇
  2005年   826篇
  2004年   806篇
  2003年   749篇
  2002年   707篇
  2001年   123篇
  2000年   118篇
  1999年   195篇
  1998年   187篇
  1997年   133篇
  1996年   121篇
  1995年   100篇
  1994年   95篇
  1993年   101篇
  1992年   111篇
  1991年   81篇
  1990年   80篇
  1989年   74篇
  1988年   53篇
  1987年   73篇
  1986年   62篇
  1985年   55篇
  1984年   64篇
  1983年   50篇
  1982年   65篇
  1981年   54篇
  1980年   57篇
  1979年   25篇
  1978年   24篇
  1977年   30篇
  1976年   36篇
  1975年   36篇
  1973年   22篇
  1970年   21篇
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
71.
Pinus pumila (Pallas) Regel. is a dominant dwarf tree in alpine regions of Japan. The possible factors limiting the net photosynthetic rate (Pn) of the needles of P. pumila were examined in the snow-melting (May) and the summer (August) seasons. In August, in situ maximum Pn was 20 mol kg–1 needle s–1 in the current-year needles and 25 mol kg–1 needle s–1 in the 1-year-old needles. Diurnal trends of Pn in August were positively related to fluctuations in photosynthetic photon flux density (PPFD) and no midday depression of Pn was found, indicating that a decrease in PPFD rather than an increase in needle-to-air vapor pressure deficit (W) might cause the reduction of Pn. Both stomatal conductance (gs) and Pn were lower in May than in August. In May, Pn and gs were almost zero in the morning, but gradually increased with decreasing W, reaching maximum Pn values (4 mol kg–1 needle s–1) and gs (60 mmol kg–1 needle s–1) at 16.00 hours. The daytime Pn in May was positively related to gs. Relative water content in the exposed needles above the snow in May was 83%, which was far above the lethal level. This indicates that the water flow from stems or soils to needles was enough to compensate for a small amount of water loss due to the low gs in May, although the water supplied to needles would be impeded by the low temperatures. Thus, the reduced gs in May would be important for avoiding needle desiccation, and would result in a reduced Pn.  相似文献   
72.
Using structure-based drug design, we identified and optimized a novel series of pyrimidodiazepinone PLK1 inhibitors resulting in the selection of the development candidate TAK-960. TAK-960 is currently undergoing Phase I evaluation in adult patients with advanced solid malignancies.  相似文献   
73.
Sarcoplasmic reticulum Ca2+-ATPase couples the motions and rearrangements of three cytoplasmic domains (A, P, and N) with Ca2+ transport. We explored the role of electrostatic force in the domain dynamics in a rate-limiting phosphoenzyme (EP) transition by a systematic approach combining electrostatic screening with salts, computer analysis of electric fields in crystal structures, and mutations. Low KCl concentration activated and increasing salt above 0.1 m inhibited the EP transition. A plot of the logarithm of the transition rate versus the square of the mean activity coefficient of the protein gave a linear relationship allowing division of the activation energy into an electrostatic component and a non-electrostatic component in which the screenable electrostatic forces are shielded by salt. Results show that the structural change in the transition is sterically restricted, but that strong electrostatic forces, when K+ is specifically bound at the P domain, come into play to accelerate the reaction. Electric field analysis revealed long-range electrostatic interactions between the N and P domains around their hinge. Mutations of the residues directly involved and other charged residues at the hinge disrupted in parallel the electric field and the structural transition. Favorable electrostatics evidently provides a low energy path for the critical N domain motion toward the P domain, overcoming steric restriction. The systematic approach employed here is, in general, a powerful tool for understanding the structural mechanisms of enzymes.  相似文献   
74.
75.
This study was undertaken to investigate whether the concentration of carbonic anhydorase isoenzyme I (CA-I) in canine feces and urine is useful as a temporary marker of occult blood. Concentrations of CA-I were measured by enzyme-linked immunosorbent assay (ELISA). Fecal CA-I concentrations in 113 healthy beagle dogs (50 male and 63 female) of various ages ranged from 4.3 to 16.7 ng/g feces (mean; 7.0 +/- 2.9 ng/g feces). One milliliter of blood from 3 healthy beagle dogs was found to contain 1,047, 1,062 and 1,150 microg CA-I. The fecal CA-I concentrations of dogs receiving intragastric infusions of autologous blood (10 ml) were very low. However, the fecal CA-I concentrations of dogs receiving infusion of autologous blood (5 ml) into the ascending colon were very high. Detection of fecal CA-I would be useful for identifying dogs with hemorrhaging of the large intestine. Of 55 urinary samples collected from healthy beagle dogs by catheter, chemical tests for occult blood were negative in 44, but CA-I concentrations ranged from 1.8 to 12.6 ng/ml (mean; 6.9 +/- 5.4 ng/ml) by ELISA. The CA-I concentrations of the other 11 samples, which tested positive for occult blood on chemical testing, ranged from 41.2 to 525.0 ng/ml by ELISA. Although CA-I is not a specific marker of erythrocytes, CA-I may be used to detect occult blood in canine feces and urine until a specific immunological test kit using antibody for Hb is developed.  相似文献   
76.
Venomous snakes have various types of phospholipase A(2) inhibitory proteins (PLIs) in their circulatory system to protect them from attack by their own phospholipase A(2)s (PLA(2)s). Here we show the first evidence for the existence of circulating PLI against secretory PLA(2)s (sPLA(2)s) in mammals. In mouse serum, we detected specific binding activities of group IB and X sPLA(2)s, which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA(2) receptor (PLA(2)R), a type I transmembrane glycoprotein related to the C-type animal lectin family. Western blot analysis after partial purification with group IB sPLA(2) affinity column confirmed the identity of serum sPLA(2)-binding protein as a soluble form of PLA(2)R (sPLA(2)R) that retained all of the extracellular domains of the membrane-bound receptor. Both purified sPLA(2)R and the recombinant soluble receptor having all of the extracellular portions blocked the biological functions of group X sPLA(2), including its potent enzymatic activity and its binding to the membrane-bound receptor. Protease inhibitor tests with PLA(2)R-overexpressing Chinese hamster ovary cells suggested that sPLA(2)R is produced by cleavage of the membrane-bound receptor by metalloproteinases. Thus, sPLA(2)R is the first example of circulating PLI that acts as an endogenous inhibitor for enzymatic activities and receptor-mediated functions of sPLA(2)s in mice.  相似文献   
77.
The nm23 gene is overexpressed in many hematological malignancies and other neoplasms. Some tumor cell lines that overexpress NM23 secrete this protein into extracellular environment. In this study, we found that the serum concentration of NM23-H1 protein was significantly higher in patients with various hematological malignancies. The serum level of NM23-H1 protein was clinically useful as a prognostic factor in malignant lymphoma and acute myelogeneous leukemia (AML). The level of NM23-H1 protein in all of the normal serum samples examined was lower than 10 ng/mL, while those in the tumors varied from about 0 to 1000 ng/mL. Exogenously added NM23-H1 protein did not affect the growth or survival of various leukemia and lymphoma cell lines. However, NM23-H1 protein inhibited the survival of adherent normal peripheral blood mononuclear cells (PBMNC) at 100–1000 ng/mL, and slightly stimulated the survival of nonadherent PBMNC. These results suggest that the effect of NM23-H1 protein on normal PBMNC may be associated with a poor prognosis in hematological malignancies.  相似文献   
78.
The viability or developmental ability of porcine embryos after slow-freezing and thawing differs depending on the embryonic stage or the batch, which is defined as a group of embryos obtained from one donor at one time. We froze porcine blastocysts in batches and assessed their cryotolerance by using two expanded blastocysts (EBs) as samples to predict the developmental potential of other blastocysts from the same batch at different stages. Two EBs from the same batch that had been separately frozen were thawed and cultured in vitro for 48 h to examine their in vitro ability to develop to the hatched blastocyst stage. Thereafter, each batch was assigned to Grade A, B, or C according to the viability of the two EBs, i.e., 100% viability (2/2: number of hatched blastocysts/number of cultured EBs) was Grade A; 50% (1/2) was Grade B; and 0% (0/2) was Grade C. The viability of EBs after freeze-thawing and in vitro culture varied depending on the batch and was lower (31.0+/-10.2%, mean+/-S.E.M.; P<0.01) than that of unfrozen controls (96.8+/-2.3%). The viability of frozen-thawed hatched blastocysts (HBs) did not differ among the graded batches, but the blastocyst diameter decreased (from 409 to 326 microm) as the batch grade decreased (from A to C). When both EBs and HBs from batches of the same grade were transferred to recipients (average 11.7 EBs and 16.0 HBs per recipient), the rate of pregnancy and farrowing in recipients decreased (from 77.8% to 0%) and the number of piglets obtained decreased (from 15.3 to 0) as the batch grade decreased. However, when not only frozen-thawed EBs from Grade B or C batches, but also four helper embryos at the morula to early blastocyst stage (which were expected to support the pregnancy) were transferred, the number of piglets generated was higher from EBs from Grade B batches (16.0) than from EBs from Grade C batches (0.0). When frozen-thawed HBs and helper embryos were transferred, the number of piglets generated was higher from HBs from Grade B batches (12.7) than that from HBs from Grade C batches (1.9). After slow-freezing of porcine blastocysts, their rate of survival to the piglet stage differs batchwise, and in vitro viability assessment of sample EBs after freezing and thawing may help in assessing the post-freezing and post-thawing developmental potential of other blastocysts at different stages from the same batch.  相似文献   
79.
Wood samples (Picea jezoensis Carr.) were treated with aqueous NaOH solutions (0-0.20 concentration fraction, 12 conditions), and bending tests were performed to measure stress relaxation. The relationship between mechanical properties and NaOH concentration is discussed. The relaxation modulus and relaxation rate were divided into three concentration ranges. Both decreased slightly for NaOH concentrations less than 0.10, decreased drastically for concentrations between 0.11 and 0.14, and decreased slightly for concentrations greater than 0.15. The change in relaxation behavior upon NaOH treatment was due to an increase in molecular chain mobility in non-crystallized regions along the microfibril longitudinal axis in wood as well as lignin swelling. Furthermore, the molecular chain response in this region required time; thus, the dependence of crystallinity on the relaxation rate was apparent in the long time region.  相似文献   
80.
In an attempt to gain insight into the physiological role of phosphatidylinositol turnover enhanced by extracellular stimuli, the physical properties of artificial membranes (egg yolk phosphatidylcholine/bovine brain phosphatidylserine) containing phosphatidylinositol or diacylglycerol were studied by ESR using spin probes and freeze-fracture electron microscopy. Diacylglycerol lost both the ability to form lipid bilayer structures and its susceptibility to calcium ions. Yeast phosphatidylinositol included in dipalmitoylphosphatidylcholine liposomes lowered the phase transition temperature of dipalmitoylphosphatidylcholine and expanded the temperature range of phase transition. However, diacylglycerol at the same concentration did not undergo the effects caused by phosphatidylinositol but the phase transition temperature was slightly raised. Phase separation of phosphatidylserine induced by calcium ions was enhanced when the phosphatidylinositol was replaced by diacylglycerol in phosphatidylcholine/ phosphatidylserine/phosphatidylinositol (3:5:2, by molar ratio) mixtures. The mobility of phosphatidylcholine spin probe was decreased in phosphatidylcholine/ phosphatidylserine/diacylglycerol (3:5:2, by molar ratio) liposomes compared with phosphatidylcholine/phosphatidylserine/phosphatidylinositol (3:5:2, by molar ratio) liposomes. An additional component from protonated stearic acid spin probes was observed in phosphatidylcholine/phosphatidylinositol (8:2, by molar ratio) liposomes at 40°C, whereas the component was not seen in phosphatidylcholine/diacylglycerol (8:2, by molar ratio) liposomes. This may indicate the alteration of surface charge induced by the replacement of phosphatidylinositol by diacylglycerol. Indeed, in the presence of 1 mM Ca2+, the additional component was removed by an electrostatic interaction between Ca2+ and phosphatidylinositol molecules in phosphatidylcholine/phosphatidylinositol liposomes at 40°C. These results support the hypothesis that the enhanced turnover of phosphatidylinositol may play a triggering role for various cellular responses to exogenous stimuli by altering membrane physical states.  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