全文获取类型
收费全文 | 30019篇 |
免费 | 3782篇 |
国内免费 | 29篇 |
出版年
2016年 | 270篇 |
2015年 | 344篇 |
2014年 | 430篇 |
2013年 | 716篇 |
2012年 | 696篇 |
2011年 | 731篇 |
2010年 | 519篇 |
2009年 | 439篇 |
2008年 | 658篇 |
2007年 | 700篇 |
2006年 | 672篇 |
2005年 | 660篇 |
2004年 | 666篇 |
2003年 | 666篇 |
2002年 | 685篇 |
2001年 | 1890篇 |
2000年 | 1861篇 |
1999年 | 1460篇 |
1998年 | 487篇 |
1997年 | 490篇 |
1996年 | 486篇 |
1995年 | 436篇 |
1994年 | 430篇 |
1993年 | 425篇 |
1992年 | 1083篇 |
1991年 | 1096篇 |
1990年 | 1075篇 |
1989年 | 1042篇 |
1988年 | 969篇 |
1987年 | 908篇 |
1986年 | 776篇 |
1985年 | 756篇 |
1984年 | 595篇 |
1983年 | 518篇 |
1982年 | 375篇 |
1981年 | 346篇 |
1980年 | 334篇 |
1979年 | 570篇 |
1978年 | 457篇 |
1977年 | 416篇 |
1976年 | 342篇 |
1975年 | 459篇 |
1974年 | 479篇 |
1973年 | 422篇 |
1972年 | 403篇 |
1971年 | 318篇 |
1970年 | 262篇 |
1969年 | 241篇 |
1968年 | 228篇 |
1967年 | 207篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
During the last decade new anthracycline-type structures with potential usefulness in cancer treatment have been supplied both by new microbial strains and by bioconversions of precursor molecules employing cells or enzymes. We highlight recent advances in bioconversion of anthracycline structures with the main focus on late transformations such as are carried out by oxidoreductases. 相似文献
982.
Chitosan-elicited synthesis of callose and of coumarin derivatives in parsley cell suspension cultures 总被引:8,自引:0,他引:8
In suspension cultured cells of parsley (Petroselinum crispum), chitosan elicited a rapid deposition of the 1,3-ß-glucan callose on the cell wall and a slower formation of coumarins. With cells remaining in conditioned growth medium, fully N-deacetylated chitosans and partially N-acetylated chitosans were about equally active, the potency increased with the degree of polymerization up to several thousand and addition of reduced glutathione increased the sensitivity of the cells. These results indicate common initial events in the induction of callose and coumarin synthesis although two fully independent metabolic pathways are involved. When the cells were suspended in fresh growth medium, less chitosan was required, and fully N-deacetylated chitosan became the best callose elicitor.Abbreviations DP
average degree of polymerization
- GSH
reduced glutathione
- PE
pachyman equivalents
- Pmg
Phytophthora megasperma f. sp.glycinea 相似文献
983.
Summary The lung of the deep diving Weddell Seal is characterized by an unusually well developed periacinar dense collagenous connective tissue, and a thick coat of smooth musculature particularly in the distal bronchioli. Both, collagen and smooth musculature appear to be functionally interrelated, the first serving presumably as site of origin or attachment for the latter. The orientation of the bronchiolar smooth muscle cells is complex: there exists a basic pattern of two crisscrossing helical bundles that wind in opposite direction. In addition, longitudinal bundles are frequent both at the inside and the outside of the muscular coat. Furthermore, more or less complete ringshaped bundles occur as well as groups of muscle fibres running radially into the collagenous tissue of the surroundings of a bronchiolus. This complex architecture presumably allows active adjustment to various physiological needs of the Weddell Seal including as extremes both closing and widening of the bronchiolar lumen. Isometric contractions of the smooth musculature may stiffen the wall of the distal airways while diving. In the Crabeater Seal which dives for shorter durations and by far less deeply than the Weddell Seal, both periacinar collagen and bronchiolar smooth musculature are of similar arrangement, however, occur in considerably reduced amounts. A rich supply of autonomie nerve fibres with abundant varicosities controls the smooth muscle cells, which are interconnected by gap junctions and receive their innervation par distance (visceral type of smooth musculature). The majority of varicosities contains small clear vesicles, as is typical for cholinergic nerves, suggesting a strong parasympathetic influence. Other varicosities are presumably of peptidergic type. Mast cells and epithelial endocrine cells may exert additional influence on the musculature. 相似文献
984.
Nuclear Overhauser effect and computational characterization of the beta-spiral of the polypentapeptide of elastin 总被引:1,自引:0,他引:1
D K Chang C M Venkatachalam K U Prasad D W Urry 《Journal of biomolecular structure & dynamics》1989,6(5):851-858
The structure of the elastin polypentapeptide, poly(VPGVG), was studied by nuclear Overhauser effect experiments using perdeuterated Val1 and Val4 samples under the condition where intermolecular interactions are absent. More extensive interaction was found between the Val1 gamma CH and Pro2 beta CH protons than between the Val4 gamma CH and Pro2 beta CH protons. The Val1 gamma CH3-Pro2 beta CH interaction does not occur within the same pentamer as previously shown experimentally and as expected from steric considerations. The results are incompatible with the presence of a random chain network in poly(VPGVG) at room temperature but are readily explicable in terms of interturn interactions in a beta-spiral structure. More specifically, the results indicate that the beta-spiral conformation with 2.9 pentamers/turn is more prevalent than that with 2.7 pentamers/turn. Using conformations developed by molecular mechanics calculations, molecular dynamics simulations were carried out to compare the relative energies of these two variants of this class of beta-spiral structures. It was found in vacuo that the structure with 2.9 pentamers/turn is indeed more stable than that of 2.7 pentamers/turn by approximately 1 kcal/mole-pentamer. 相似文献
985.
A. Vincent N. Dahl I. Oberl A. Hanauer J. L. Mandel H. Malmgren U. Pettersson 《Genomics》1989,5(4):797-801
The fragile X syndrome, which is the most common cause of inherited mental retardation, poses important diagnostic problems for genetic counseling. The development of diagnostic strategies based on DNA analysis has been impaired by the lack of polymorphic markers very close to the disease locus. Here we report that the polymorphic probe U6.2 (locus DXS304) is much closer to the fragile X locus than all the previously reported markers. A recombination fraction of 0.02 between DXS304 and the fragile X locus was estimated by multipoint linkage analysis (confidence interval 0.002 to 0.05). Our data suggest that DXS304 is distal to the fragile X locus. This marker thus represents a major improvement for carrier detection and prenatal diagnosis in fragile X families. 相似文献
986.
Genetic and physical mapping of a novel region close to the fragile X site on the human X chromosome 总被引:11,自引:0,他引:11
M. N. Patterson M. V. Bell J. Bloomfield T. Flint H. Dorkins S. N. Thibodeau D. Schaid G. Bren C. E. Schwartz b. Wieringa H. -H. Ropers D. F. Callen G. Sutherland U. Froster-Iskenius H. Vissing K. E. Davies 《Genomics》1989,4(4):570-578
We report the isolation and characterization of a novel DNA marker (1A1) in Xqter in the region of the fragile X. Genetic studies in families segregating for the fragile X syndrome suggest that 1A1 lies between the disease mutation and the distal locus, DXS52. Studies in normal and fragile X families show that 1A1 is tightly linked to DXS52 (Zmax = 17.20; theta max = 0.03) and F8 (Zmax = 7.01; theta max = 0.08). Multipoint mapping of families supports the order Xcen-DXS105-FRAXA-1A1-DXS52-(F8, DXS115)-Xqter. Pulsed-field gel electrophoresis (PFGE) studies demonstrate that 1A1 defines a new region of at least 2 Mb of DNA not physically linked to DXS52 or F8, thus extending the physical map of Xq27-qter to over 4 Mb. Complex partial digestion PFGE patterns, probably due to differing degrees of methylation, are observed with 1A1 in unrelated normal and fragile-X-positive individuals, whereas other distal markers give uniform digestion profiles. Physical data suggest that 1A1 lies in a region less CpG rich than other distal markers in Xq27-qter. 相似文献
987.
L Zychlinski P Raska-Emery J U Balis M R Montgomery 《Journal of biochemical toxicology》1989,4(4):251-254
Bioenergetics of isolated lung and heart mitochondria from adult and aged rats were examined in the presence of glutamate (NAD-linked substrate) or succinate + rotenone (FAD-linked substrate) following ozone exposure (3.0 ppm, 8 hr). In controls, several differences were observed between adults and aged in both organ preparations. Following exposure, all bioenergetic parameters were decreased significantly in lung preparations from both adult and aged rats. In heart mitochondria, the respiration rates in state 3 and in uncoupled state, and the ADP/O ratio were decreased significantly in both exposed age groups. The respiratory control ratio (RCR) was decreased significantly only in the aged exposed rats. These results suggest that acute exposure to high levels of ozone alters energy production in both lung and heart mitochondria of adult and aged rats. 相似文献
988.
T A Voyno-Yasenetskaya V A Tkachuk E G Cheknyova M P Panchenko G Y Grigorian R J Vavrek J M Stewart U S Ryan 《FASEB journal》1989,3(1):44-51
In this paper we examine the effect of the vasodilator peptide bradykinin on endothelial cell regulation of phosphoinositide (PI) turnover. The data show that the activation of PI turnover by bradykinin in bovine pulmonary artery endothelial cells is insensitive to pertussis toxin, which ADP ribosylates a membrane protein of mol wt 40,000. However, this effect of bradykinin can be potentiated by guanosine 5'-O-(3-thio)triphosphate (GTP gamma S), an activator of G proteins, and depressed by guanosine 5'-O-(2-thio)diphosphate (GDP beta S), an inhibitor of G proteins. After endothelial cells were preincubated for 1 h with GTP gamma S, there was a three- to fourfold increase in PI turnover. Preincubation of cells with GDP beta S did not affect the basal level of PI turnover, but completely prevented activation of PI turnover by bradykinin. 4 beta-Phorbol-12 beta-myristate-13 alpha-acetate can block the bradykinin-stimulated inositol monophosphate formation in cultured endothelial cells. The effects of bradykinin on PI turnover were blocked by B2 antagonists but not by B1 antagonists. Taken together, these results indicate that in endothelial cells the bradykinin B2 receptor is coupled to phospholipase C via a G protein (or proteins) that is not a substrate for pertussis toxin (neither Gi nor Go). 相似文献
989.
Free radical reduction in the human epidermis 总被引:1,自引:0,他引:1
The human epidermis presents the first line of defense against invading free radicals. Therefore, the surface of the skin must be equipped to deal with both the penetration of ultra-violet light as well as the neutralization of reactive photochemical products such as superoxide anion radical, hydrogen peroxide and especially hydroxyl radicals. Consequently, the human epidermis contains a variety of anti-oxidants to reduce oxygen radicals and hydrogen peroxide. The photochemical production of hydroxyl radicals, from both extracellular and intracellular hydrogen peroxide, is of special significance to the integrity of cells in the human epidermis. Recently, both biochemical and clinical studies on the healthy human population, and on patients with pigmentation disorders, suggested a connection between free radical defense by plasma membrane associated thioredoxin reductase and melanin biosynthesis. This research provided the first evidence for a direct relationship between free radical concentration and pigmentation. Furthermore, this system has been shown to be regulated by both extracellular and intracellular calcium concentrations. Clinical studies show depigmentation disorders vitiligo and tyrosinase positive albinism (Hermansky-Pudlak syndrome) appear to have defective calcium uptake systems influencing both free radical defense and melanin biosynthesis. 相似文献
990.
Johan U. Grobbelaar 《Journal of applied phycology》1989,1(4):333-340
It has been suggested that turbulence with the resultant light/dark cycle and light gradient through which phytoplankton move,
enhances their productivity. The stationary bottle incubation technique for estimating rates of primary productivity has mainly
been criticized because of bottle effects, the elimination of natural turbulence and the presence of photo-inhibition. In
a series of experiments where productivity was measured over static profiles and compared to the productivity in a mixed system,
no definite conclusion could be reached regarding the effect of varying light/dark cycles of medium frequency (seconds to
minutes). It appeared as though the ratio of the euphotic depth to mixing depth (Z
eu/Z
m) influenced productivity more than the duration of the light/dark cycle. The static bottle incubation method gave higher
integral productivities than the mixed samples at low ratio's ofZ
eu/Z
m. It is suggested that mixing has two separate, but synergistic effects i.e. it not only moves the phytoplankton cells through
a light/dark cycle, but also decreases the boundary layer, which increases the rate of exchange through the cell wall of nutrients
and metabolites. In doing so more nutrients are available and light could be utilized more efficiently and therefore, productivity
is increased. 相似文献