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   2438篇
  免费   62篇
  国内免费   22篇
  2023年   11篇
  2022年   13篇
  2021年   27篇
  2020年   35篇
  2019年   37篇
  2018年   31篇
  2017年   32篇
  2016年   37篇
  2015年   33篇
  2014年   91篇
  2013年   133篇
  2012年   66篇
  2011年   97篇
  2010年   127篇
  2009年   96篇
  2008年   93篇
  2007年   120篇
  2006年   98篇
  2005年   96篇
  2004年   94篇
  2003年   69篇
  2002年   54篇
  2001年   56篇
  2000年   36篇
  1999年   56篇
  1998年   65篇
  1997年   48篇
  1996年   41篇
  1995年   64篇
  1994年   48篇
  1993年   47篇
  1992年   55篇
  1991年   36篇
  1990年   35篇
  1989年   36篇
  1988年   35篇
  1987年   26篇
  1986年   23篇
  1985年   38篇
  1984年   52篇
  1983年   32篇
  1982年   43篇
  1981年   32篇
  1980年   30篇
  1979年   36篇
  1978年   14篇
  1976年   9篇
  1975年   7篇
  1974年   7篇
  1973年   9篇
排序方式: 共有2522条查询结果,搜索用时 15 毫秒
1.
The plasma membrane of the hepatoma cell line, HTC cells, has been characterized and purified by cell fractionation techniques. In the absence of true 5′-nucleotidase in HTC cells, alkaline phosphodiesterase I has been used as a marker enzyme, following conclusions gained from differential and isopycnic centrifugation studies (Lopez Saura, P., Trouet A. and Tulkens P. (1978) Biochim. Biophys. Acta 543, 430–449). To confirm this localization, HTC cells were exposed to anti-plasma membrane IgG at 4°C and fractionated. Alkaline phosphodiesterase I and IgG showed super imposable distribution patterns in linear sucrose gradients. Alkaline phosphodiesterase I is, however, only poorly resolved from enzyme markers of other organelles, especially NADPH-cytochrome c reductase (endoplasmic reticulum) and galactosyltransferase (Golgi complex). Maximal purification from the homogenate is only 13-fold, on a protein basis, even when using a microsomal fraction (67 and 13% of alkaline phosphodiesterase I and protein, respectively) as the starting material. Improved resolution can be obtained after the addition of small quantities of digitonin (equimolar with respect to the cholesterol content). Digitonin increases the buoyant density of alkaline phosphodiesterase I by approx. 0.05 g/cm3, whereas the buoyant densities of galactosyltransferase and NADPH-cytochrome c reductase are increased only by 0.03 and 0.015 g/cm3, respectively. Accordingly, a procedure has been designed which yields a fraction containing 22.8% of alkaline phosphodiesterase I with a purification of 21-fold on a protein basis. The content of NADPH-cytochrome c reductase and galactosyltransferase is 1.2 and 2.1%, respectively. Electron microscopy shows smooth surface membrane elements and vesicles, with only occasional other recognizable elements.  相似文献   
2.
In this study, we examined the effects that antifreeze proteins have on the supercooling and ice-nucleating abilities of aqueous solutions. Very little information on such nucleation currently exists. Using an automated lag time apparatus and a new analysis, we show several dilution series of Type I antifreeze proteins. Our results indicate that, above a concentration of ∼8 mg/ml, ice nucleation is enhanced rather than hindered. We discuss this unexpected result and present a new hypothesis outlining three components of polar fish blood that we believe affect its solution properties in certain situations.  相似文献   
3.
We have studied the effects of diethyldithiocarbamate (DDTC) on the biotransformations of toxic doses of tetrachloro (d,l-trans)1,2-diaminocyclohexaneplatinum(IV) (tetraplatin) in Fischer 344 rats. In animals not treated with DDTC, tetraplatin was rapidly converted to dichloro(d,I-trans)1,2-diaminocyclohexaneplatinum(II) [PtCl2(dach]. Subsequent biotransformations included the transient formation of the (d,I-trans)1,2-diaminocyclohexane-aquachloroplatinum(II) [Pt(H2O)(Cl)(dach)]+ complex, followed by formation of the platinum (Pt)-methionine and either Pt-cysteine or Pt-ornithine complexes. Significant amounts of free (d,I-trans) 1,2-diaminocyclohexane (dach) were observed in plasma as a result of intracellular trans-labilization reactions. DDTC caused a marked decrease in both total and protein-bound platinum in the circulation. A significant increase in the plasma concentration of free dach was also observed as a result of formation of the Pt(DDTC)2 complex. Some of the free dach could have arisen from intracellular reactions with DDTC, but the displacement of platinum from plasma proteins was more than sufficient to account for the increase in free dach in the circulation. DDTC treatment also decreased plasma concentrations of tetraplatin, PtCl2(dach), [Pt(H2O)(Cl)(dach)]+, the Pt-methionine complex, and one unidentified biotransformation product, but had no effect on the Pt-cysteine (or Pt-ornithine) complex. These effects of DDTC on protein-bound platinum and low-molecular-weight biotransformation products in plasma may contribute to the decrease in tetraplatin toxicity seen in DDTC-treated rats.  相似文献   
4.
Summary Dark grown coleoptile segments were floated on solutions of IAA alone and of IAA and the secretion inhibitors cytochalasin and monensin. The secretion inhibitors prevented normal elongation of the tissue segments, the monensin inhibition being virtually complete while cytochalasin gave a 40% reduction over the first six hours with little further further elongation in the following 18 hours. Vesicle production was assessed in outer epidermal cells after 6 hours of IAA-stimulated elongation using the vesicle accumulation method following a cytochalasin-block of vesicle transport. The results were compared with the area of plasma membrane required to enable cell elongation to proceed at the observed rate. The area of vesicle membrane delivered to the cell surface exceeded this requirement to such an extent that at least 65% of the delivered membrane must be recycled back into the cytoplasm. Expressed in terms of the whole cell, the plasma membrane turnover rate was found to be once every 200 minutes. It is concluded that limitation of elongation by secretion inhibitors is more likely to reflect a requirement for the vesicle contents than the vesicle membrane. These results are compared with those obtained from other secretory systems using a similar approach.Abbreviations IAA indole acetic acid - DMSO dimethyl sulphoxide - D dictyosome - ER endoplasmic reticulum - V vesicle  相似文献   
5.
Rat livers were fractionated to obtain intracellular membrane preparations and a highly purified preparation of bile canaliculi. The fraction containing bile canaliculi was homogenized and subfractionated to give fractions representing fragments of contiguous membrane and of canalicular microvilli. The relative purity and extent of contamination of each preparation was determined. When the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene was incorporated into aliquots of each fraction at the same probe: lipid ratio and the steady-state anisotropy of its fluorescence measured, it was found that the plasma membrane preparations were much more ordered than the intracellular membrane preparations. Of the plasma membrane preparations, that containing the canalicular microvilli was the most ordered, even allowing for any contribution of contaminants. Thus the microvillus membrane of the bile canaliculus appears to be the most ordered domain of the plasma membrane of the hepatocyte. The high order in this domain may be a factor in reducing the susceptibility to bile salt damage during bile secretion, since it is this region which is exposed to high concentrations of bile salts in vivo.  相似文献   
6.
Ammineruthenium(III) complexes have been found to act as electron acceptors for the transplasmalemma electron transport system of animal cells. The active complexes hexaammineruthenium(III), pyridine pentaammineruthenium(III), and chloropentaammineruthenium(III) range in redox potential (E 0) from 305 to –42 mV. These compounds also act as electron acceptors for the NADH dehydrogenase of isolated plasma membranes. Stimulation of HeLa cell growth, in the absence of calf serum, by these compounds provides evidence that growth stimulation by the transplasma membrane electron transport system is not entirely based on reduction and uptake of iron.  相似文献   
7.
The measurement of cholinesterase activities in either plasma or cerebrospinal fluid (CSF) may ultimately prove to be relevant in the diagnosis of neurological and neuropsychiatric disorders. However, studies to date have examined only total enzyme activities. Therefore in the present study we have examined the distribution of the individual molecular forms of both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in plasma and CSF using sucrose density gradient centrifugation. Although the total activities of AChE were of the same order of magnitude in plasma and CSF, there was a considerable difference (120-500-fold) between total BChE activity in the CSF and the BChE-rich plasma. The analysis of the individual molecular forms revealed that the predominant molecular species of AChE and BChE in the CSF--both lumbar and ventricular--was the G4 form. The G4 form also constituted the majority of the plasma BChE activity and, on average, over half (56%) of the plasma AChE activity. The significance of the AChE and BChE molecular form compositions of both plasma and CSF and their possible relationship to pathological states are discussed.  相似文献   
8.
Monoclonal antibodies generated by immunization with a plasma-membrane preparation from suspension-cultured cells of Nicotiana glutinosa L. were used in combination with fluoresceinor rhodamine-labeled goat anti-mouse immunoglobulins to identify heterokaryons in protoplast fusion procedures. Antibody labeling did not inhibit callus formation nor plantlet regeneration. The antibodies are non-invasive and surface labeling provides clear optical discrimination of true heterokaryons from unfused aggregates as well as from parental protoplasts and homokaryons. Labeling is stable throughout fusion and hence by pre-labeling parental protoplast populations the strategy is both versatile and of general applicability.  相似文献   
9.
Summary Fluorescein or rhodamine conjugates of seventeen different lectins were tested for their ability to label the plasma membrane of live plant protoplasts. During the investigation, a strong effect of calcium was observed on the binding of several lectins to protoplasts derived from suspension cultured rose cells (Rosa sp. Paul's Scarlet). The binding of these lectins was increased by elevating the calcium concentration from 1 to 10 mM in the buffer. Other divalent cations had variable, but similar, effects on lectin binding. The mechanism of this effect appeared to involve the protoplast surface rather than the lectins. Although the cell wall-degrading enzymes used to isolate protoplasts had generally no effect on lectin binding, one clear exception was observed. Binding ofArachis hypogaea agglutinin was markedly reduced on protoplasts isolated with Driselase as compared to protoplasts isolated with a combination of Cellulysin and Pectolyase Y-23. Although most of the lectins that labeled protoplasts derived from cultured rose cells or from corn root cortex (Zea mays L. WF9 × Mo17) had specificities for galactose or N-acetylgalactosamine, some differences in protoplast labeling between lectins of the same saccharide specificity were observed. Two different analyses of the interaction betweenRicinus communis agglutinin and rose protoplasts showed that binding was cooperative with an apparent association constant of 7.2 × 105M–1 or 9.8 × 105M–1 with a maximum of approximately 108 lectin molecules bound per protoplast. Treatment of protoplasts with glycosidases which hydrolyze either N- or O-glycosidic linkages of glycoproteins slightly enhanced labeling of protoplasts byRicinus communis agglutinin. Interpretation of these results are discussed.Abbreviations MPR medium, minimal organic medium (Nothnagel andLyon 1986) - APA Abrus precatorius agglutinin - CSA Cytisus sessilifolius agglutinin - ECA Erythrina cristagalli agglutinin - GS-I Griffonia simplicifolia agglutinin - LcH Lens culinarus agglutinin - PNA Arachis hypogaea agglutinin - SBA Glycine max agglutinin - VAA Viscum album agglutinin - VFA Vicia faba agglutinin - WGA Triticum vulgaris agglutinin - Con A Canavalia ensiformis agglutinin - HPA Helix pomatia agglutinin - TPA Tetragonolobus purpureas agglutinin - RCA Ricinus communis agglutinin - DBA Dolichos biflorus agglutinin - SJA Sophora japonica agglutinin - BPA Bauhinia purpurea agglutinin - FITC fluorescein isothiocyanate - Ga1NAc N-acetylgalactosamine - FDA fluorescein diacetate - 2-O-Me-D-Fuc 2-O-methyl-D-fucose Parts of the work presented here are also submitted in partial fulfillment of requirements for the Ph.D. degree.  相似文献   
10.
Summary Transmembrane linear terminal complexes considered to be involved in the synthesis of cellulose microfibrils have been described in the plasma membrane ofBoergesenia forbesii. Evidence for the existence of these structures has been obtained almost exlusively using the freeze etching technique. In the present study an attempt has been made to complete these studies using conventional fixation, staining, and sectioning procedures. In developing cells ofBoergesenia forbesii, strongly stained structures traversing the plasma membrane and averaging 598.9 nm ± 171.3 nm in length, 28.7 nm ± 4.2 nm in width, and 35.2 nm ± 6.6 nm in depth have been demonstrated. These structures are considered to be linear terminal complexes. At their distal (cell wall) surface, they appear to be closely associated with cellulose microfibrils. At the proximal (cytoplasmic) surface, they are associated with microtubules and polysomes. A model of the possible interrelation of the terminal complexes and microtubules leading to the generation of cell wall microfibrils is proposed.  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