全文获取类型
收费全文 | 4675篇 |
免费 | 544篇 |
国内免费 | 12篇 |
出版年
2022年 | 59篇 |
2021年 | 100篇 |
2020年 | 52篇 |
2019年 | 75篇 |
2018年 | 105篇 |
2017年 | 101篇 |
2016年 | 123篇 |
2015年 | 266篇 |
2014年 | 258篇 |
2013年 | 299篇 |
2012年 | 333篇 |
2011年 | 342篇 |
2010年 | 245篇 |
2009年 | 174篇 |
2008年 | 227篇 |
2007年 | 234篇 |
2006年 | 215篇 |
2005年 | 212篇 |
2004年 | 210篇 |
2003年 | 183篇 |
2002年 | 181篇 |
2001年 | 131篇 |
2000年 | 120篇 |
1999年 | 82篇 |
1998年 | 47篇 |
1997年 | 37篇 |
1996年 | 25篇 |
1995年 | 22篇 |
1994年 | 33篇 |
1993年 | 23篇 |
1992年 | 43篇 |
1991年 | 58篇 |
1990年 | 39篇 |
1989年 | 52篇 |
1988年 | 24篇 |
1987年 | 40篇 |
1986年 | 44篇 |
1985年 | 32篇 |
1984年 | 41篇 |
1983年 | 33篇 |
1982年 | 36篇 |
1981年 | 29篇 |
1980年 | 25篇 |
1979年 | 23篇 |
1978年 | 28篇 |
1977年 | 19篇 |
1976年 | 22篇 |
1975年 | 14篇 |
1974年 | 17篇 |
1973年 | 17篇 |
排序方式: 共有5231条查询结果,搜索用时 15 毫秒
71.
72.
The major nucleoside transporter of the human T leukemia cell line CEM has been identified by photoaffinity labeling with the transport inhibitor nitrobenzylmercaptopurine riboside (NBMPR). The photolabeled protein migrates on SDS-PAGE gels as a broad band with a mean apparent molecular weight (75,000 +/- 3000) significantly higher than that reported for the nucleoside transporter in human erythrocytes (55,000) (Young et al. (1983) J. Biol. Chem. 258, 2202-2208). However, after treatment with endoglycosidase F to remove carbohydrate, the NBMPR-binding protein in CEM cells migrates as a sharp peak with an apparent molecular weight (47,000 +/- 3000) identical to that reported for the deglycosylated protein in human erythrocytes (Kwong et al. (1986) Biochem. J. 240, 349-356). It therefore appears that the difference in the apparent molecular weight of the NBMPR-sensitive nucleoside transporter between the CEM cell line and human erythrocytes is a result of differences in glycosylation. The NBMPR-binding protein from CEM cells has been solubilized with 1% octyl glucoside and reconstituted into phospholipid vesicles by a freeze-thaw sonication technique. Optimal reconstitution of uridine transport activity was achieved using a sonication interval of 5 to 10 s and lipid to protein ratios of 60:1 or greater. Under these conditions transport activity in the reconstituted vesicles was proportional to the protein concentration and was inhibited by NBMPR. Omission of lipid or protein, or substitution of a protein extract prepared from a nucleoside transport deficient mutant of the CEM cell line resulted in vesicles with no uridine transport activity. The initial rate of uridine transport, in the vesicles prepared with CEM protein, was saturable with a Km of 103 +/- 11 microM and was inhibited by adenosine, thymidine and cytidine. The Km for uridine and the potency of the other nucleosides as inhibitors of uridine transport (adenosine greater than thymidine greater than cytidine) were similar to intact cells. Thus, although the nucleoside transporter of CEM cells has a higher molecular weight than the human erythrocyte transporter, it exhibits typical NBMPR-sensitive nucleoside transport activity both in the intact cell and when reconstituted into phospholipid vesicles. 相似文献
73.
Comparison of Extracellular Cellulase Activities of Clostridium thermocellum LQRI and Trichoderma reesei QM9414 总被引:16,自引:9,他引:7
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The crude extracellular cellulase of Clostridium thermocellum LQRI (virgin strain) was very active and solubilized microcrystalline cellulose at one-half the rate observed for the extracellular cellulase of Trichoderma reesei QM9414 (mutant strain). C. thermocellum cellulase activity differed considerably from that of T. reesei as follows: higher endoglucanase/exoglucanase activity ratio; absence of extracellular cellobiase or β-xylosidase activity; long-chain oligosaccharides instead of short-chain oligosaccharides as initial (15-min) hydrolytic products on microcrystalline cellulose; mainly cellobiose or xylobiose as long-term (24-h) hydrolysis products of Avicel and MN300 or xylan; and high activity and stability at 60 to 70°C. Under optimized reaction conditions, the kinetic properties (Vmax, 0.4 μmol/min per mg of protein; energy of activation, 33 kJ; temperature coefficient, 1.8) of C. thermocellum cellulose-solubilizing activity were comparable to those reported for T. reesei, except that the dyed Avicel concentration at half-maximal velocity was twofold higher (182 μM). The cellulose-solubilizing activity of the two crude cellulases differed considerably in response to various enzyme inhibitors. Most notably, Ag2+ and Hg2+ effectively inhibited C. thermocellum but not T. reesei cellulase at <20 μM, whereas Ca2+, Mg2+, and Mn2+ inhibited T. reesei but not C. thermocellum cellulase at >10 mM. Both enzymes were inhibited by Cu2+ (>20 mM), Zn2+ (>1.0 mM), and ethylene glycol-bis(β-aminoethyl ether)- N,N-tetraacetic acid (>10 mM). T. reesei but not C. thermocellum cellulose-solubilizing activity was 20% inhibited by glucose (73 mM) and cellobiose (29 mM). Both cellulases preferentially cleaved the internal glycosidic bonds of cellooligosaccharides. The overall rates of cellooligosaccharide degradation were higher for T. reesei than for C. thermocellum cellulase, except that the rates of conversion of cellohexaose to cellotriose were equivalent. 相似文献
74.
Ethanol Production by Thermophilic Bacteria: Fermentation of Cellulosic Substrates by Cocultures of Clostridium thermocellum and Clostridium thermohydrosulfuricum 总被引:13,自引:6,他引:7
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The fermentation of various saccharides derived from cellulosic biomass to ethanol was examined in mono- and cocultures of Clostridium thermocellum strain LQRI and C. thermohydrosulfuricum strain 39E. C. thermohydrosulfuricum fermented glucose, cellobiose, and xylose, but not cellulose or xylan, and yielded ethanol/acetate ratios of >7.0. C. thermocellum fermented a variety of cellulosic substrates, glucose, and cellobiose, but not xylan or xylose, and yielded ethanol/acetate ratios of ~1.0. At nonlimiting cellulosic substrate concentrations (~1%), C. thermocellum cellulase hydrolysis products accumulated during monoculture fermentation of Solka Floc cellulose and included glucose, cellobiose, xylose, and xylobiose. A stable coculture that contained nearly equal numbers of C. thermocellum and C. thermohydrosulfuricum was established that fermented a variety of cellulosic substrates, and the ethanol yield observed was twofold higher than in C. thermocellum monoculture fermentations. The metabolic basis for the enhanced fermentation effectiveness of the coculture on Solka Floc cellulose included: the ability of C. thermocellum cellulase to hydrolyze α-cellulose and hemicellulose; the enhanced utilization of mono- and disaccharides by C. thermohydrosulfuricum; increased cellulose consumption; threefold increase in the ethanol production rate; and twofold decrease in the acetate production rate. The coculture actively fermented MN300 cellulose, Avicel, Solka Floc, SO2-treated wood, and steam-exploded wood. The highest ethanol yield obtained was 1.8 mol of ethanol per mol of anhydroglucose unit in MN300 cellulose. 相似文献
75.
Ira S. Kass David A. Larsen Ching C. Wang Antony O.W. Stretton 《Experimental parasitology》1982,54(2):166-174
Avermectin B1a, a novel antiparasitic agent, paralyzes Ascaris suum without causing either flaccid paralysis or a hypercontraction. It reduces the lengthening of the acetylcholine-preconditioned A. suum muscle strip caused by γ-aminobutyric acid. It does not affect the contraction of the isolated muscle strip preparation caused by applying acetylcholine. However, preinjection with Avermectin B1a does significantly reduce the shortening caused by acetylcholine injection without affecting the paralysis of an intact ascarid worm. These results suggest that Avermectin B1a may act on the central nervous system of Ascaris sp. nematodes. 相似文献
76.
Extracellular siderophores of rapidly growing Aspergillus nidulans and Penicillium chrysogenum
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
G Charlang R M Horowitz P H Lowy B Ng S M Poling N H Horowitz 《Journal of bacteriology》1982,150(2):785-787
The highly active extracellular siderophores previously detected in young cultures of Aspergillus nidulans and Penicillium chrysogenum have been identified as the cyclic ester fusigen (fusarinine C), and its open-chain form, fusigen B (fusarinine B). 相似文献
77.
Uwe G. Goehlert N. M. K. Ng Ying Kin Leonhard S. Wolfe 《Journal of neurochemistry》1981,36(3):1192-1201
Abstract: Microvessels, predominantly capillaries, were isolated from rat cerebrum by a modification of published procedures. The morphology and purity of the preparations were monitored by light and electron microscopy and by enrichment in alkaline phosphatase, γ-glutamyl transpeptidase, and prostacyclin synthetase. A reversed-phase high-pressure liquid chromatographic method was used in the purification of prostaglandins after extraction from aqueous incubation solutions. Prostacyclin synthesis in brain is localized in cerebral blood vessels and capillaries. The endogenous biosynthetic capacity of the isolated cerebral capillary fractions for prostacyclin, measured as its chemically stable breakdown product, 6-keto-prostaglandin F1α , was 11 ng/mg protein/10 min. Choroid plexus and intact surface vessels synthesized 6-keto-prostaglandin F1α at 37 and 35 ng/mg protein/10 min, respectively. The prostacyclin-synthesizing enzyme of the cerebral capillaries also converted the exogenously added prostaglandin endoperoxides to 6-keto-prostaglandin F1α . Comparison of the synthesis of prostaglandins 6-keto-F1α , E2 , and F2α showed that 6-keto-prostaglandin F1α was the major prostaglandin formed in the microvessels, in the larger surface vessels, and in the choroid plexus. Prostaglandin D2 was not detected. Prostacyclin synthesis by the cerebral vasculature is similar to that in other blood vessels and cultured human endothelial cells. Possible physiological roles of prostacyclin in the cerebral microvasculature are discussed with special regard to the autoregulation of cerebral blood flow. 相似文献
78.
Effects of actinomycin-D (ACT), cycloheximide (CH), rifampicin (RIF) and chloramphenicol (CAP) on senescence of soybean leaf discs were investigated. All inhibitors tested are effective in retarding senescence of soybean leaf discs. However, CH is more effective than ACT, RIF and CAP, suggesting that activation of preexisting, latent metabolic systems present in the cytoplasm piays predominant role in the initiating of leaf senescence. However, the possibility that events taking place in the nucleus or chloroplast are essential for the initiation of leaf senescence cannot be excluded. 相似文献
79.
80.
The effects of propranolol (DL-propranolol) and D-propranolol on thyroid hormone metabolism were studied in six euthyroid volunteers receiving L-thyroxine (T4) and six hypothyroid patients receiving T4 replacement. D-propranolol as well as propranolol decreased L-triiodothyronine (T3) concentrations and the ratio of T3 to T4 in the euthyroid subjects, and D-propranolol decreased these variables in the subjects with hypothyroidism (propranolol was not given to this group). It is concluded from this study and from parallel invitro investigations that the effect of propranolol on the conversion of T4 to T3 is unrelated to its beta-adrenergic blocking activity, and that at low therapeutic doses propranolol may exert appreciable "membrane-stabilising" effects in vivo. 相似文献