全文获取类型
收费全文 | 5785篇 |
免费 | 568篇 |
国内免费 | 1篇 |
出版年
2022年 | 42篇 |
2021年 | 88篇 |
2020年 | 50篇 |
2019年 | 45篇 |
2018年 | 72篇 |
2017年 | 60篇 |
2016年 | 130篇 |
2015年 | 192篇 |
2014年 | 207篇 |
2013年 | 254篇 |
2012年 | 320篇 |
2011年 | 343篇 |
2010年 | 241篇 |
2009年 | 205篇 |
2008年 | 330篇 |
2007年 | 297篇 |
2006年 | 266篇 |
2005年 | 256篇 |
2004年 | 296篇 |
2003年 | 282篇 |
2002年 | 306篇 |
2001年 | 86篇 |
2000年 | 61篇 |
1999年 | 76篇 |
1998年 | 95篇 |
1997年 | 69篇 |
1996年 | 58篇 |
1995年 | 59篇 |
1994年 | 59篇 |
1993年 | 60篇 |
1992年 | 61篇 |
1991年 | 54篇 |
1990年 | 64篇 |
1989年 | 46篇 |
1988年 | 46篇 |
1987年 | 56篇 |
1986年 | 37篇 |
1985年 | 64篇 |
1984年 | 51篇 |
1983年 | 74篇 |
1982年 | 92篇 |
1981年 | 71篇 |
1980年 | 64篇 |
1979年 | 43篇 |
1978年 | 50篇 |
1977年 | 43篇 |
1976年 | 45篇 |
1975年 | 43篇 |
1974年 | 41篇 |
1973年 | 44篇 |
排序方式: 共有6354条查询结果,搜索用时 15 毫秒
141.
Methanogenic Activity and Structural Characteristics of the Microbial Biofilm On a Needle-Punched Polyester Support 总被引:2,自引:0,他引:2
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)
Martin Harvey Cecil W. Forsberg Terry J. Beveridge Jack Pos John R. Ogilvie 《Applied microbiology》1984,48(3):633-638
In a downflow stationary fixed-film anaerobic reactor receiving a swine waste influent, few bacteria were observed to be tightly adherent to the surfaces of the needle-punched polyester support material. However, there was a morphologically complex, dense population of bacteria trapped within the matrix. Frequently large microcolonies of a uniform morphological type of bacteria were observed. These were particularly evident for methanosarcina-like bacteria which grew forming large aggregates of unseparated cells. Leafy deposits of electron-dense, calcium- and phosphorus-enriched material coated the polyester matrix and some cells. As the biofilm matured there was more extensive mineral deposition which completely entrapped cells. The entrapped cells appeared to autolyze, and many were partially degraded. Further impregnation of the matrix with minerals and apparent cell death may eventually have a deleterious effect on the methanogenic activity of the biofilm. 相似文献
142.
Jack T. Moyer 《Journal of Ethology》1984,2(2):85-98
Social organization and reproductive behavior were studied in 4 species of ostraciid fishes.Lactoria fornasini andLactoria diaphana from Japan andAcanthostracion polygonius from the western Atlsnyiv were usually found in single male, haremic social groups, although the latter 2 species usually
included only 2 females per harem, and individuals in all 3 species foraged solitarily. Elements of both “resource defense”
and “female defense” (Emlen & Oring 1977) were evident in the 2Lactoria species, and female defense appeared particularly intense inL. diaphana. Resource defense was apparent inA. polygonius. Lactophrys triquetor occurred in lek-like breeding assemblages at Curacao (Netherlands Antilles) and on the Atlantic coast of Panama. These variations
in social organization are discussed and a relationship with feeding habits is suggested. All 4 species appear to be gonochores
(demonstrated inLactoria by Moyer 1979). The 2Lactoria spp. showed operational sex ratios (OSR: Emlen & Oring 1977) approaching 1∶1 and a high degree of female spawning synchrony.
The occurrence of polygyny in species with 1∶1 OSRs and female spawning synchrony is discussed with reference to the Knowlton
(1979) model which predicts monogamy to occur in such situations. 相似文献
143.
Mohamed Kouider Randal Hauptmann Jack M. Widholm Robert M. Skirvin Safi S. Korban 《Plant cell reports》1984,3(4):142-145
Protoplasts could be successfully isolated and cultured from callus and suspension cultures of Malus xdomestica cv. Jonathan. Protoplast-derived colonies were recovered when the osmoticum (glucose) was gradually reduced in semi-solid 8p medium or by the use of feeder plates. Formation of embryo-like structures was induced from the protoplast-derived callus on media supplemented with IAA and BA. These structures formed roots but plants failed to develop. Protoplasts could be isolated from leaves, but not from stems or petioles. The leaf protoplasts failed to divide.List of abbreviations BA
benzyladenine
- 2,4-D
2,4-dichlorophenoxyacetic acid
- ABA
abscisic acid
- IAA
indole acetic acid 相似文献
144.
Periodic structurally similar oligomers are found on one side of the axes of symmetry in the lac, trp, and gal operators 总被引:2,自引:0,他引:2
Three well-defined E. coli operator regions were examined for recurring conformational deviation from a regular B-DNA helix. All three, the lac, trp, and gal, show repeats of the same set of neighboring helical twist angles. These angles recur with a periodicity equal to the helix periodicity on one side of the operator's axes of symmetry. The probability that their occurrence is random was found to be extremely small. Therefore, we propose that in addition to specific bases, repeating twist angles patterns are likely to be among the local parameters involved in repressor-operator recognition. 相似文献
145.
146.
Fractionation of several type II specific restriction endonucleases was achieved by separation on two novel biospecific matrices. The matrices are pyran, a copolymer of divinyl ether of maleic anhydride, and Cibacron Blue F3GA, a blue dye commonly used for the calibration of molecular sieves. Both compounds are insolubilized by coupling to sepharose through a cyanogen bromide linkage and in their soluble form inhibit the restriction endonucleases which we have tested. These affinity matrices can be used to obtain restriction endonucleases from crude extracts after removal of nucleic acids. They have also proven to have a high capacity when used as subsequent steps in enzyme purification. Their additional advantage is the rapid development time and reusability of columns packed with the two matrices. 相似文献
147.
Rats were injected with combinations of morphine-N-14CH3 and morphine-63H and the isotope content of the brain and liver was measured by combustion in a tissue oxidizer. The liver of intact male rats showed a significant increase in the 3H to 14C isotope ratio relative to the blood reflecting the existence of N-demethylation in this organ. This increase was not observed in the liver of either intact females or castrated males or females. Centrally, the hypothalamus, medial thalamus, and corpus striatum of both intact and castrated male and female rats exhibited increases in 3H to 14C isotope ratios indicating the presence of N-demethylation in these tissues. These results indicate that testicular hormones serve to increase the hepatic N-demethylation of morphine, but apparently reduce the comparable reaction in the CNS. 相似文献
148.
The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone 总被引:5,自引:0,他引:5
Jack H. Y. Li Lawrence G. Palmer Isidore S. Edelman Bernd Lindemann 《The Journal of membrane biology》1982,64(1-2):77-89
Summary Urinary bladders ofBufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly recruited from a pool of electrically silent channels. 相似文献
149.
A photoreactive analog of aminopterin, 2′-azidoaminopterin (VI), was synthesized and evaluated as a potential inhibitor and photoaffinity label of folate-utilizing enzymes. The compound was tightly bound to dihydrofolate reductase (DHFR) from escherichia coli (MB 1428) with K1 equal to 3 × 10?11M and to the enzyme from mouse (S-180) cells with K1 approximately equal to 2 × 10?10M. Dissociation constants measured by equilibrium dialysis using radioactive 2′-azidoaminopterin gave a value of KD = 3.2 × 10?9M for the bacterial enzyme. The presence of NADPH enhanced the affinity by more than an order of magnitude. Azidoaminopterin is also an inhibitor of thymidylate synthetase from Lactobacillus casei, competitive with methylene-tetrahydrofolate (Ki 7 × 10?7M). Photolysis of the radioactive inhibitor in complex with DHFR from E. coli led to approximately 3% covalent incorporation of label into protein. The greater part of this attachment was nonspecific as shown by the lack of protection in the presence of methotrexate. Thymidylate synthetase from L. casei was not significantly inactivated upon photolysis in the presence of the inhibitor and deoxyuridylate. Model studies showed that photoreaction of the inhibitor led to covalent linkages with thiol, lysyl amino groups, and the hydroxyl groups of alcohols. Azidoaminopterin may be useful in labeling other enzymes of folate metabolism, although a minor photoproduct reacts nonspecifically with many proteins. The antifolate can be photoconjugated to polylysine as well as to proteins. The polylysine conjugates inhibit DHFR. Difference spectrum analysis of the photoproducts from the irradiation of the DHFR I complex indicates that water reacts efficiently with the enzyme-bound nitrene and must therefore have access to at least part of the bound p-aminobenzoyl group. This analysis suggests that azide analogs of protein ligands may be useful as reporter groups in probing the hydrophobicity of binding sites. 相似文献
150.
Charles R. Kleeman Jack W. Coburn Arnold S. Brickman David B. N. Lee Robert G. Narins Richard M. Ehrlich 《The Western journal of medicine》1980,132(4):313-332
The prevalence of kidney stones has steadily risen during this century; passage of a calculus and a positive family history increase the probability of recurrence. Findings from recent studies on the cause of renal calculi have stressed crystallization and crystal aggregation of stone minerals from supersaturated urine, rather than excessive organic matrix. Absence of normal urine inhibitors of calcium salts is also stressed. Formation of calcium oxalate stones is the major problem. Therapy with decreased calcium and oxalate intake, thiazides, phosphate salts and allopurinol in various combinations has substantially decreased the prevalence of recurrent stones. The rationale for the use of allopurinol is that uric acid salts enhance the tendency for calcium oxalate to crystallize from supersaturated urine. The hypercalciuria seen in 30 percent to 40 percent of patients with oxalate stones is usually caused by intestinal hyperabsorption of calcium. Although patients with uric acid calculi constitute only a small fraction of those in whom stones form, they represent a group in whom good medical therapy, based on sound physiologic principles, has proved extremely successful. Renal tubular syndromes lead to nephrocalcinosis and lithiasis through hypercalciuria, alkaline urine and hypocitraturia, the latter an inhibitor of calcium salt precipitation. Recent advances in surgical techniques are discussed, including the rationale for removing staghorn calculi. The ileal ureter and coagulum pyelolithotomy deserve special emphasis. 相似文献