全文获取类型
收费全文 | 15090篇 |
免费 | 1341篇 |
国内免费 | 17篇 |
出版年
2022年 | 103篇 |
2021年 | 205篇 |
2020年 | 106篇 |
2019年 | 151篇 |
2018年 | 206篇 |
2017年 | 144篇 |
2016年 | 293篇 |
2015年 | 477篇 |
2014年 | 540篇 |
2013年 | 737篇 |
2012年 | 944篇 |
2011年 | 975篇 |
2010年 | 637篇 |
2009年 | 624篇 |
2008年 | 826篇 |
2007年 | 841篇 |
2006年 | 822篇 |
2005年 | 827篇 |
2004年 | 853篇 |
2003年 | 870篇 |
2002年 | 891篇 |
2001年 | 198篇 |
2000年 | 116篇 |
1999年 | 168篇 |
1998年 | 232篇 |
1997年 | 176篇 |
1996年 | 130篇 |
1995年 | 127篇 |
1994年 | 135篇 |
1993年 | 166篇 |
1992年 | 128篇 |
1991年 | 125篇 |
1990年 | 127篇 |
1989年 | 92篇 |
1988年 | 104篇 |
1987年 | 107篇 |
1986年 | 98篇 |
1985年 | 132篇 |
1984年 | 115篇 |
1983年 | 116篇 |
1982年 | 142篇 |
1981年 | 149篇 |
1980年 | 123篇 |
1979年 | 87篇 |
1978年 | 100篇 |
1977年 | 93篇 |
1976年 | 89篇 |
1975年 | 69篇 |
1974年 | 104篇 |
1973年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
121.
Robert Puskas Natalie Fredd Celia Gazdar Alan Peterkofsky 《Archives of biochemistry and biophysics》1983,223(2):503-513
Under certain growth conditions, some strains of Escherichia coli accumulate toxic levels of methylglyoxal. This report characterizes a strain which synthesizes a mutant cAMP receptor protein in an adenylate cyclase deletion background. When cultured in glucose 6-phosphate minimal medium, this strain (222) was prematurely growth arrested due to methylglyoxal production; growth inhibition did not occur when the strain was grown in glucose minimal medium. A comparison of a variety of enzyme and cofactor levels in the related strains 222 (mutant) and 225 (wild-type) grown on either glucose or glucose 6-phosphate medium was carried out. The only difference found that might explain an increase in methylglyoxal accumulation was an elevated level of phosphofructokinase in strain 222 grown on glucose 6-phosphate. Since this enzyme activity probably limits hexose phosphate metabolism, it is suggested that growth inhibition in strain 222 may be due to increased production of triose phosphate, some of which is converted to methylglyoxal. 相似文献
122.
Photosynthetic membrane fragments separated from whole cells of the green alga Dunaliella parva, were oriented by incorporation into multilayers on thin Mylar films. These partially dehydrated films were then examined by EPR spectroscopy for evidence of orientation of paramagnetic components. Five previously identified paramagnetic components, the reduced states of iron-sulfur clusters A and B, the intermediate acceptor X?, the reduced Rieske iron-sulfur cluster, and oxidized cytochrome b-559, displayed EPR signals showing orientation. In addition, several previously unknown paramagnetic components were also observed to be oriented. Four components, previously characterized in spinach chloroplast preparations, the iron-sulfur clusters A and B, the intermediate acceptor X?, and cytochrome b-559, were shown to be similar in the green alga, D. parva. The orientations of iron-sulfur clusters A and B, however, were determined unambiguously in this preparation; this was not possible in previous work with spinach. The heme plane orientation of cytochrome b-559 was found to be perpendicular to the membrane plane in agreement with the results in spinach preparations. A new photoinduced EPR signal with g values of 1.88, 1.97 and 2.12 was seen only in the oriented preparations and was indicative of a reduced iron-sulfur cluster with an orientation different from that of iron-sulfur cluster A or B. This suggests the existence of a previously unidentified acceptor in Photosystem I of green plants. These studies clearly show that the orientation of these components in bioenergetic membranes are conserved over a large span of evolutionary development and are, therefore, an important aspect of the mechanism of electron transfer. 相似文献
123.
Alterations in the Structure of the Oligosaccharide of Vesicular Stomatitis Virus G Protein by Swainsonine 总被引:8,自引:0,他引:8 下载免费PDF全文
Swainsonine, an inhibitor of glycoprotein processing, inhibits the formation of the normal oligosaccharide chain of the G protein of vesicular stomatitis virus. Thus, when vesicular stomatitis virus was grown in baby hamster kidney cells in the presence of swainsonine (15 to 500 ng/ml) and labeled with [2-(3)H]mannose, the oligosaccharide portion of the G protein was completely susceptible to the action of endoglucosaminidase H. However, the normal viral glycoprotein is not susceptible to this enzyme. Various enzymatic treatments and methylation studies of the mannose-labeled oligosaccharides suggest that swainsonine causes the formation of a hybrid-type oligosaccharide having an oligomannosyl core (Man(5)GlcNAc(2)-Asn) characteristic of neutral oligosaccharides plus the branch structure (NeuNAc-Gal-GlcNAc) characteristic of the complex oligosaccharides. A structure for this hybrid oligosaccharide is proposed. Swainsonine had no effect on the incorporation of [(14)C]leucine into viral proteins, nor did it change the number of PFU produced in these cultures. It did, however, slightly decrease the incorporation of [(3)H]glucosamine and increase the incorporation of [(3)H]mannose. Vesicular stomatitis virus raised in the presence of swainsonine bound much more tightly to columns of concanavalin A-Sepharose than did control virus. Swainsonine had to be added within the first 4 or 5 h of virus infection to be effective. Thus, when 100 ng of the alkaloid per ml was added at any time within the first 3 h of infection, essentially all of the glycoprotein was susceptible to digestion by endoglucosaminidase H. However, when swainsonine was added 4 h after the start of infection, 30% of the glycopeptides became resistant to endoglucosaminidase H; at 5 h, 70% were resistant. The effect of swainsonine was reversible since removal of the alkaloid allowed the cells to form the normal complex glycoproteins. However, the time of removal was critical in terms of oligosaccharide structure. 相似文献
124.
Matthijs G.P. Feenstra Hans Rollema Theo B.A. Mulder Ben H.C. Westerink Alan S. Horn 《Life sciences》1983,32(12):1313-1323
Evidence is presented that a low dose of peripherally administered N, N-dipropylamino-5, 6-dihydroxytetralin (DiPr-5, 6-ADTN) specifically labels dopamine (DA) receptors in rat brain.Concentrations of this potent DA receptor agonist were determined by a highly selective method using reversed phase liquid chromatography and amperometric detection. The binding characteristics satisfy all criteria regarding saturability, stereospecificity, regional distribution and relation with pharmacological effects that are associated with DA receptor interactions. A rough estimation of the density of binding sites in the striatum resulted in values of 60–70 pmol/g. Lesioning the nigrostriatal pathway does not significantly alter the amount of ligand bound, nor do pretreatments with serotonergic, α-adrenergic or β-adrenergic antagonists. DiPr-5, 6-ADTN has thus been shown to be a useful ligand for labeling central DA receptors and a powerful tool in the study of DA-ergic mechanisms. 相似文献
125.
Alan S. Fairhurst Stanley A. Thayer Jack E. Colker David A. Beatty 《Life sciences》1983,32(12):1331-1339
The sarcoplasmic reticulum (S.R.) of rabbit skeletal muscle has been found to contain a single, high affinity binding site for the Ca antagonist drug [3H] -nitrendipine. Two subfractions of the reticulum were studied, the heavy (HSR) and light (LSR) preparations, which exhibited similar nitrendipine equilibrium dissociation constants (KD) of 1nM. Crude cardiac and brain membranes assayed under the same conditions exhibited KD values of 0.2–0.3nM. The concentration of binding sites per mg. protein (Bmax) in HSR was found to be very high, namely 6.7 picomoles/mg, some four times greater than that of LSR. [3H] -nitrendipine binding to HSR was reversible and inhibited by the Ca antagonists flunarizine and verapamil, and by the intracellular Ca release antagonist TMB-8 (8-diethylamino-octyl 3,4,5- trimethylbenzoate hydrochloride). However, unlabelled nitrendipine at 2 × 10?5M had no effect on contraction of isolated electrically stimulated rabbit lumbrical or rat diaphragm muscles, nor did it affect the neuromuscular junction as studied in rat phrenic nerve-diaphragm preparations. Also, little effect of 2 × 10?5M nitrendipine was seen on net 45Ca uptake by HSR. These results suggest that [3H] -nitrendipine binding to skeletal muscle S.R. resembles that of brain membranes, which also contain a high affinity binding site for [3H] -nitrendipine and which similarly are pharmacologically insensitive to this dihydropyridine type of Ca channel blocking agent. Since HSR is also enriched in calsequestrin and terminal cysternae from which Ca is released in vivo, it seems likely that the [3H]- nitrendipine binding sites in S.R. are associated with Ca channels in the S.R. 相似文献
126.
Alan H. Teramura 《Physiologia plantarum》1983,58(3):415-427
This paper reviews growth chamber, greenhouse, and field studies on the effects of ultraviolet-B (UV-B. between 280 and 320 nm) radiation on agricultural crop plants. Our understanding of the physiological effects of UV-B radiation comes primarily from growth chamber studies, where UV-B is artificially supplied via filtered lamps. Both photosystems I and II, as well as carboxylating enzymes, are sensitive to UV-B radiation. Ultraviolet-B radiation also affects stomatal resistance, chlorophyll concentration, soluble leaf proteins, lipids, and carbohydrate pools. In general, the effects of UV-B radiation are accentuated by the low levels of visible radiation typically found inside growth chambers. Ultraviolet-B radiation has also been shown to affect anatomical and morphological plant characteristics. Commonly observed UV-B induced changes include plant stunting, reductions in leaf area and total biomass, and alterations in the pattern of biomass partitioning into various plant organs. In sensitive plants, evidence of cell and tissue damage often appears on the upper leaf epidermis as bronzing, glazing, and chlorosis. Epidermal transmission in the UV region decreases in irradiated leaves. This decrease is primarily associated with a stimulation in flavonoid biosynthesis and is thought to be a protective, screening response to the deleterious effects of UV-B. A considerable degree of variability exists in sensitivity to UV-B radiation between different species. Approximately 30% of the species tested were resistant, another 20% were extremely sensitive, and the remainder were of intermediate sensitivity, in terms of reductions in total dry weight. In addition to this sizable interspecific variability, there appears to be a similarly wide intraspecific variability in UV-B response. The effects of UV-B radiation on crop yield have only been examined in a limited number of field studies, with ambient levels of UV-B radiation being supplemented with fluorescent sun lamps. Due to various deficiencies, all these field experiments to date have only limited utility for assessing the potential impact of enhanced levels of UV-B on crop productivity. 相似文献
127.
Masaji Koshioka Alan Jones Mayumi N. Koshioka Richard P. Pharis 《Phytochemistry》1983,22(7):1585-1590
The native gibberellin A4 (GA4), in radioactive form ([1,2-3H]GA4, 1.06 Ci/mmol), was fed to carrot somatic cell cultures (suspension and immobilized cell systems) and its metabolism over a 48 hr period was investigated. It was found that the [3H]GA4 was metabolized to at least two GAs, [3H]GA1 and [3H]GA8, six GA glucosyl conjugates, [3H]GA1-0(3)-glucoside, [3H]GA1-0(13)-glucoside, [3H]GA1-glucosyl ester, [3H]GA4-glucoside, [3H]GA4-glucosyl ester, a [3H]GA8 glucosyl conjugate(s) and a previously unknown [3H]GA1 glucosyl conjugate ([3H]GA1-0(3,13)-diglucoside-like compound). The GA1-diglucoside-like compound was found only in extracts of cells and was present in significant amounts (33 % of total extractable radioactivity). All other metabolites were present in both cells and medium. For extracts of the medium, no differences between the suspension and immobilized cultures existed in types of [3H]GA4 metabolites although quantitative differences were apparent. 相似文献
128.
Features of tumor and host zinc metabolism are described. Emphasis is placed on tumor-host interactions. Using the model of the Ehrlich ascites tumor in mice, one clear site of modulation of cellular zinc by the amount of nutrient zinc available in the host is a zinc-binding protein with the properties of metallothionein. The selective depletion of zinc from this protein is correlated with the loss of cell proliferation by tumors injected into zinc-deficient animals. The properties of isolated metallothionein are consistent with a role for it as a reactive pool of intracellular zinc which can be donated to apozinc proteins and other structures. The presence of the Ehrlich tumor in mice also perturbs their distribution of zinc: zinc leaves the plasma and is accumulated by liver in the form of newly synthesized zinc metallothionein. During host zinc deficiency, this redistribution is not observed. This may be caused not only by a lack of mobile plasma zinc, but also by an inhibition of the initiation of this host response at the site of the tumor in the peritoneum. 相似文献
129.
Alan Shiels Stephen Jeffery Ian R. Phillips Elizabeth A. Shephard Catherine A. Wilson Nicholas D. Carter 《Biochimica et Biophysica Acta (BBA)/General Subjects》1983,760(3)
Using radioimmunoassay, the concentration of carbonic anhydrase III in the livers of adult male rats was found to be approx. 30-times greater than that observed in mature females. Castration of male rats led to a marked reduction in liver carbonic anhydrase III concentrations which could be partially restored to control levels by testosterone replacement. Administration of testosterone to ovariectomised female rats induced about a 5-fold increase in liver carbonic anhydrase III concentration. Immunoprecipitation analysis of the products of liver mRNA translation in vitro with antiserum specific for carbonic anhydrase III showed that hormonal control of the levels of carbonic anhydrase III in liver is mediated by changes in the amount of translatable carbonic anhydrase III mRNA. Marked changes in liver carbonic anhydrase III concentrations were also observed in developing and ageing male rats. 相似文献
130.
Recent spectroscopic and magnetic susceptibility studies of the iron center in the two-iron ferredoxins provide criteria which any model for the iron-sulfur complex in these proteins must satisfy. These criteria are most stringent for parsley and spinach ferredoxin: the reduced proteins contain a high-spin ferric atom antiferromagnetically exchange-coupled (presumably via sulfide bridging ligands) to a high-spin ferrous atom. In the oxidized proteins the iron atoms are antiferromagnetically spin-coupled, high-spin ferric atoms. Arguments are given to substantiate the claim that the ferrous atom in the reduced protein is ligated by four sulfur atoms in a distorted tetrahedral configuration: two are the bridging sulfides, two are cysteinyl sulfurs. A treatment of proton contact shifts based upon the above model is pertinent to proton magnetic resonance data already available and provides a means to identify directly the ligands at both iron atoms via further PMR experiments. 相似文献