全文获取类型
收费全文 | 18877篇 |
免费 | 1695篇 |
国内免费 | 1433篇 |
专业分类
22005篇 |
出版年
2023年 | 246篇 |
2022年 | 531篇 |
2021年 | 899篇 |
2020年 | 607篇 |
2019年 | 689篇 |
2018年 | 730篇 |
2017年 | 540篇 |
2016年 | 699篇 |
2015年 | 1150篇 |
2014年 | 1266篇 |
2013年 | 1369篇 |
2012年 | 1569篇 |
2011年 | 1501篇 |
2010年 | 980篇 |
2009年 | 834篇 |
2008年 | 906篇 |
2007年 | 876篇 |
2006年 | 770篇 |
2005年 | 658篇 |
2004年 | 617篇 |
2003年 | 543篇 |
2002年 | 516篇 |
2001年 | 393篇 |
2000年 | 390篇 |
1999年 | 360篇 |
1998年 | 171篇 |
1997年 | 162篇 |
1996年 | 160篇 |
1995年 | 119篇 |
1994年 | 147篇 |
1993年 | 90篇 |
1992年 | 155篇 |
1991年 | 147篇 |
1990年 | 122篇 |
1989年 | 98篇 |
1988年 | 82篇 |
1987年 | 96篇 |
1986年 | 83篇 |
1985年 | 92篇 |
1984年 | 48篇 |
1983年 | 48篇 |
1982年 | 48篇 |
1981年 | 36篇 |
1980年 | 37篇 |
1979年 | 52篇 |
1978年 | 41篇 |
1977年 | 45篇 |
1976年 | 33篇 |
1975年 | 35篇 |
1974年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Na-, K-, Ba-, and Ca-Y were employed for the separation of fructose and glucose in an adsorption column. Effects of temperature, solvent flow rate, amount of mixture injection, and exchangeable cations on the separation were investigated. Efficiency of separation was used as a criterion to characterize the effectiveness of the separation. The transport and kinetic parameters for the column separation were also presented. From simple pulse experiments and moment analysis, the obtained process information of equilibrium and dynamic parameters might be used to design, operate, and control the separation column. (c) 1992 John Wiley & Sons, Inc. 相似文献
62.
To examine the existence of pressure equilibrium between tributary veins and the central vena cava during the mean circulatory filling pressure manoeuvre, pressures in the hepatic portal vein, renal vein, and inferior vena cava were determined at 4-s intervals over a 20-s period of circulatory arrest induced by inflating a right atrial balloon in normal blood volume, 10% volume depletion, and 10% volume expansion states in urethane-anaesthetized rats. Portal vein pressure determined 8 s after arrest during volume depletion and expansion was significantly higher than vena caval pressure (6.2 +/- 0.8 vs. 3.4 +/- 0.2 and 7.7 +/- 0.5 vs. 6.2 +/- 0.4 mmHg (1 mmHg = 133.32 Pa), respectively; p less than 0.01); this pressure disequilibrium continued for 16 s during volume expansion and for the entire 20 s during volume depletion. Renal vein pressure was equal to vena caval pressure during this manoeuvre. Portal vein pressure at normal blood volume was not significantly different from vena caval pressure following circulatory arrest (4.6 +/- 0.3 vs. 3.8 +/- 0.4 mmHg, respectively). Following ganglionic blockade, portal vein pressure was still significantly higher than vena caval pressure for 12 s during volume alterations. At the 8th s of the arrest the portal pressure determined in volume depletion was 3.6 +/- 0.3 mmHg and the inferior vena caval pressure was 2.6 +/- 0.4 mmHg (p less than 0.05). Under the volume expansion condition, the respective values were 6.5 +/- 0.3 and 5.3 +/- 0.4 mmHg (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
63.
Zn+2 is required for specific binding of c-erbA proteins to the hormone response elements of target genes. It is unclear whether Zn+2 is important for the binding of ligand to c-erbA proteins. The present study evaluated the effect of Zn+2 and other divalent cations on the binding of 3,3',5-triiodo-L-thyronine(T3) to the purified human placental c-erbA protein (h-TR beta 1). Zn+2 induced cross-linking of h-TR beta 1 to form aggregates in a dose-dependent manner with an apparent half-maximal concentration of approximately 200 microM at 22 degrees C. Cross-linking was reversible by the addition of 5 microM EDTA or 10 mM dithiothreitol. The cross-linked h-TR beta 1 bound T3. These results indicated Zn+2 had no effect on T3 binding and suggested that the cysteines and histidines involved in cross-linking are not essential for T3 binding. 相似文献
64.
The cytoplasmic and cell wall components of the Gram-positive bacterium Bacillus megaterium and the cytoplasmic and cell envelope components of the Gram-negative bacterium Escherichia coli were assayed for chemotactic activity for the hemocytes of Crassostrea virginica. The cellular components were separated by differential centrifugation and gel filtration was used to determine the approximate molecular weights of the chemoattractant molecules. Active fractions were assayed for glycoproteins and lipoproteins. As a result, it is known that hemocytes are chemotactically attracted to proteins of approximately 10,000 daltons which are associated with the cell wall of B. megaterium and the cell envelope of E. coli. 相似文献
65.
Seiji Suzuki Hiroshi Oka Hiroko Yasuda Masahiro Ikeda Po Yuan Cheng Toshitsugu Oda 《Biochemical and biophysical research communications》1981,99(3):987-993
Isolated rat pancreatic islets, incubated in the presence of extracellular 32Pi to steady state 32P incorporation into cellular phosphopeptides, were exposed to glucose for 10 min. Glucose (16.7 mM) significantly stimulated the phosphorylation of six phosphoproteins with molecular weights of 15,000, 35,000, 49,000, 64,000, 93,000 and 138,000. Mannoheptulose (16.7 mM) markedly inhibited glucose-stimulated phosphorylation of these six phosphoproteins. This protein phosphorylation might be important in mediating glucose-stimulated insulin release. 相似文献
66.
Two basic cell types occur in the hemolymph of Bulinus truncatus rohlfsi: granulocytes and hyalinocytes. Granulocytes are divided into three subtypes: (1) Granulocytes I, which account for 19% of the hemocytes, are small, young amoebocytes with 1–20 filopodia and small numbers of cytoplasmic granules, including some lysosomes; (2) granulocytes II, which account for 78% of the cells, are large, fully developed amoebocytes that possess 1–20 filopodia and many granules, both acidophilic and basophilic, including numerous lysosomes, phagosomes, and mitochondria; and (3) spent granulocytes, which are rare, have few filopodia, large accumulations of glycogen granules and prominent vacuoles in addition to lysosomes in the cytoplasm. These three subtypes of granulocytes probably represent ontogenetic stages within a single cell line. In addition, granulocytes with 40 or more filopodia and little ectoplasm, found in only 1 of 45 snails examined, probably reflect a pathologic condition. Hyalinocytes, which account for 3% of all hemocytes, are similar in size to mature granulocytes, but have few or no cytoplasmic granules and lack filopodia and glycogen granules. Total hemocyte concentration in hemolymph is 328,000 ± 188,000 cells/ml. 相似文献
67.
Initiation of adventitious bud formation in vitro from Douglas fir cotyledons required both cytokinin and auxin at concentrations of 5 μM BAP and 5 nM NAA. Histological observations showed that these adventitious buds arose de novo from cells residing in hypodermal layers. Development of adventitious buds in culture was characterized by the sequential appearance of four anatomically distinguishable structures: 1) meristemoid, 2) bud primordium, 3) shoot apex with needle primordia, and 4) adventitious bud. The anatomical structure of tissue culture-produced buds was similar to that of vegetative buds produced on intact plants. Cultured cotyledons capable of producing adventitious buds (bud culture) were compared with bud-callus and callus cultures initiated by 5 μM BAP plus 5 μM NAA and 5μM NAA alone without BAP, respectively. Results showed that, during early stages of the culture period (i.e., prior to the appearance of meristemoid structure), cell division of bud culture was mainly located in hypodermal layers, whereas for the other culture types, bud-callus and callus cultures, cell division occurred randomly in all tissues. 相似文献
68.
Adhesion of bacteria to epithelial cell surfaces within the reticulo-rumen of cattle. 总被引:13,自引:9,他引:4 下载免费PDF全文
Blocks of tissue were removed from various locations in the bovine digestive tract and fixed and processed for transmission and scanning electron microscopy by techniques that retained adherent bacteria. The distribution of bacteria on the surface of epithelial cells was examined by scanning electron microscopy. This showed intermittent colonization of the epithelia with the formation of occasional microcolonies of morphologically similar bacterial cells. Transmission electron microscopy of ruthenium red-stained material showed the presence of both the glycocalyx of the bovine epithelial cells and fibrous carbohydrate coats surrounding adherent bacteria. The carbohydrate coats appeared to mediate the attachment of bacteria to the epithelium, to food particles, and to each other so that microcolonies were formed. Careful examination of the bacterial colonization of keratinized cells in the process of being sloughed from the surface of the stratified squamous epithelium of the rumen showed that these dead cells were digested by adherent bacteria of a limited number of morphological types. The spatial relationship of this mixed, adherent, microbial population to living and dead epithelial cells and to food particles indicates that digestive processes of some importance may be accomplished by this stationary component of the microbial flora of the digestive tract. 相似文献
69.
The sarcolemmal fraction prepared from rat skeletal muscle consists of osmotically active vesicles that accumulate D-glucose in preference to L-glucose, apparently by facilitated diffusion into intravesicular space. Stereospecific D-glucose uptake by these vesicles is a saturable rpocess, inhibited by phloridzin, by cytochalasin B, and by certain sugars, and enhanced by counterflow. An additional leak pathway permits entry of both D- and L-glucose into the vesicles. Stereospecific D-glucose transport by sarcolemmal vesicles is enhanced to a small extent by insulin, provided the hormone is administered prior to cell disruption. In membranes prepared from insulin-pretreated muscle, Ca2+ produces a small further enhancement. Local anesthetics preferentially inhibit stereospecific D-glucose transport. Apparent uptake of both D- and L-glucose is greater when vesicles are suspended in salt solutions rather than sucrose, an effect attributed to increased functional vesicular volume. 相似文献
70.
Colchicine inhibits glucose oxidation and the uptake of 2-deoxy-D-glucose in fat cell ghosts but has no effect on glucose oxidation by fat cell homogenates. This inhibition is rapid, reversible, and temperature-independent. Insulin-stimulated glucose oxidation and 2-deoxy-D-glucose transport are also inhibited by colchicine to an extent comparable to the basal processes. 相似文献