全文获取类型
收费全文 | 4057篇 |
免费 | 324篇 |
出版年
2022年 | 23篇 |
2021年 | 48篇 |
2020年 | 34篇 |
2019年 | 36篇 |
2018年 | 62篇 |
2017年 | 47篇 |
2016年 | 97篇 |
2015年 | 164篇 |
2014年 | 189篇 |
2013年 | 208篇 |
2012年 | 300篇 |
2011年 | 273篇 |
2010年 | 180篇 |
2009年 | 163篇 |
2008年 | 216篇 |
2007年 | 201篇 |
2006年 | 208篇 |
2005年 | 213篇 |
2004年 | 192篇 |
2003年 | 191篇 |
2002年 | 207篇 |
2001年 | 30篇 |
2000年 | 35篇 |
1999年 | 51篇 |
1998年 | 58篇 |
1997年 | 32篇 |
1996年 | 38篇 |
1995年 | 28篇 |
1994年 | 42篇 |
1993年 | 40篇 |
1992年 | 32篇 |
1991年 | 26篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 27篇 |
1987年 | 27篇 |
1986年 | 22篇 |
1985年 | 31篇 |
1984年 | 47篇 |
1983年 | 19篇 |
1982年 | 40篇 |
1981年 | 29篇 |
1980年 | 32篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 30篇 |
1976年 | 28篇 |
1975年 | 22篇 |
1974年 | 21篇 |
1973年 | 27篇 |
排序方式: 共有4381条查询结果,搜索用时 62 毫秒
21.
22.
23.
The major phosphoglycerides in grey and white matter from the brain of the koala have been separated and examined. The major polyunsaturated fatty acids present in both the diacyl- and alk-1-enyl acylglycerophosphorylethanolamines from grey matter were 22:6 omega 3, 20:4 omega 6, and 22:4 omega 6. In both grey and white matter, 22:6 omega 3 and 20:4 omega 6 were concentrated in the 2-position of diacylglycerophosphorylethanolamines and 22:4 omega 6 in the 2-position of alk-1-enylacylglycerophosphorylethanolamines; polyunsaturated fatty acid levels were higher in diacylglycerophosphorylethanolamines. Ethanolamine phosphoglyceride fractions from grey matter were enriched in polyunsaturated fatty acids compared with those from white matter. The acyl groups 18:0, 18:1, and 16:0 and their alk-1-enyl analogues were prominent in grey and white matter ethanolamine phosphoglycerides; 18:1 was dominant in white matter alk-1-enylacylglycerophosphorylethanolamines. The plasmalogen composition of ethanolamine phosphoglycerides was 55% in grey matter and 76% in white matter. Choline phosphoglycerides contained negligible plasmalogen and low polyunsaturated fatty acid levels. Diacylglycerophosphorylcholine was characterized by high levels of 16:0 and 18:1. Similar acyl group distributions were estimated in the 1-position in both grey and white matter, 16:0 being present at greater than 50%. The presence of the molecular species 18:0/22:6 omega 3 was indicated in grey matter diacylglycerophosphorylethanolamine, 18:1/18:1 in white matter alk-1-enylcylglycerophosphorylethanolamine, and 16:0/18:1 in white matter diacylglycerophosphorylcholine. 相似文献
24.
Analyses for phytate by an indirect precipitation method and for the minerals calcium (Ca), zinc (Zn), iron (Fe), copper (Cu),
and manganese (Mn) by atomic absorption spectrophotometry were carried out on 100 foods available in New Zealand. Foods with
1% phytate (dry weight basis) included untoasted muesli, rolled oats, wheat germ, wheat bran, soybean, and some soy products.
Most breads contained between 0.35 and 0.60% phytate; legumes on average had 0.62% phytate, as did snack bars. There was a
wide variation in Ca and Zn contents: There was a tenfold variation in Ca content among the legume products, whereas there
was a seventyfold variation in Zn content among the cereals. The phytate: Zn molar ratio, which is presumed to indicate the
biovailability of Zn, was above 20∶1 for two-thirds of the cereals and almost all of the snack bars; it was above 15∶1 for
one-third of the breads, almost all of the legumes, and half of the legume products. These high phytate: Zn molar ratios,
as well as some Ca: phytate molar ratios above 6∶1, indicate that there might be a reduced biovailability of Zn in many of
the foods analyzed in this study. 相似文献
25.
Hooper JE 《The EMBO journal》1986,5(9):2321-2329
The segmental musculature of Drosophila melanogaster larvae consists of 24-30 muscles per segment. Unique patterns of muscles are found in the three thoracic segments and the first and last abdominal segments; the remaining abdominal segments share the same pattern. Mutations in Ultrabithorax (Ubx) cause partial transformation of the muscle pattern of larval abdominal segments towards metathorax. The muscles of the thorax are not affected. In the first two abdominal segments the changes include the loss of at least 11 `abdominal' muscles and the gain of 11 `thoracic' muscles. Less extensive transformations are seen in more posterior abdominal segments. Anterobithorax, bithorax, postbithorax and bithoraxoid mutations also induce transformations of the larval musculature. Each allelic group affects a domain that is a subset of the entire Ubx domain but these domains are not restricted to compartments or segments and may extend through as many as five segments. In the muscles the segmental distribution of Ubx antigen correlates with the segments affected by Ubx mutations. The different domains of Ubx in mesoderm and ectoderm argue that the segmental diversity of the muscle pattern is not simply induced by the overlying epidermis and that Ubx function in the mesoderm is required for the correct development of abdominal segments. 相似文献
26.
Dr. E. Joan Blanchette-Mackie Thomas Briggs Sidney S. Chernick Robert O. Scow 《Cell and tissue research》1986,244(1):95-105
Summary Cultured mouse (J774.1) macrophages accumulated triacylglycerol, but no cholesteryl ester or cholesterol, when incubated in albumin-poor medium with serum-activated lipid particles containing 84 mol% trioleoylglycerol and 9 mol% cholesteryl oleate. Accumulation of triacylglycerol by cells was associated with hydrolysis of particulate triacylglycerol to fatty acid and glycerol. Both acyl and glyceryl moieties of particulate triacylglycerol were recovered in cellular triacylglycerol with a molar ratio of 3.6. The cells also accumulated fatty acid and monoacylglycerol. Whether acylglycerol was taken up as a single molecular species, such as monoacylglycerol, or as several species can not be determined by the present findings. Macrophages incubated with lipid particles for 24 h had many lipid particles attached to cell surfaces and numerous intracellular lipid droplets. The surface film of attached particles was continuous with the outer leaflet of plasma membrane of the cells. Particles partially depleted of core triacylglycerol and collapsed surface films were found attached to surfaces of macrophages. There was no morphological evidence that lipid particles were taken up intact by cells, through endocytosis or phagocytosis. Macrophages incubated with lipid particles also contained intracellular lamellar structures. They varied in size and shape, and were located in the periphery of cells, sometimes near lipid droplets and endoplasmic reticulum. Only 3% of the lamellar structures were associated with lysosomes, indicating they probably were not of lysosomal origin. Lipid particles attached to cells decreased in size and number, and lamellar structures developed at the surface of particles, or replaced the particles, when glutaraldehyde-fixed specimens were incubated at 25° C, demonstrating lipolytic activity at the surface of macrophages. Our findings suggest that particulate triacylglycerol was hydrolyzed by lipoprotein lipase at the surface of macrophages, and that fatty acid and monoacylglycerol formed by lipolysis were transported directly into the cells to be reesterified. When lipolytic products were taken up faster than they could be utilized, they accumulated as lamellar structures in the cells.Abbreviations MEM
Eagle's alpha modification of minimum essential medium 相似文献
27.
Synopsis Developmental patterns as seen in cyprinodontiforms fishes with different reproductive styles are compared, and discussed
in relation to ecology and evolutionary significance. The discussion centres around Adinia xenica (its detailed ontogeny presented in two previous sequels to this paper), and, from the existing literature, Fundulus heteroclitus (closely related), Austrofundulus myersi (an annual) and Platypoecilus maculatus (a livebearer). The embryonic resting interval is present in various forms in the first three species, and differences in
it and the overall patterns of development are shown to be consistent with ecological conditions. Termination of the resting
interval leads immediately to hatching, a process in A. xenica, as in F. heteroclitus, apparently initiated by the appropriate summation of internal and external factors. These factors include any or all of:
metabolic changes and increased oxygen requirements, response to light, reduced environmental oxygen, agitation, and increased
hydrostatic pressure. They all can cause increased movement by the embryo which is credited with rupturing hatching gland
cells and releasing the enzyme(s). Annual fishes experience 3 pronounced resting intervals, termed diapauses. These are discussed
in the context of apparent steps and thresholds, and evolutionary ecology. A possible evolutionary sequence, from a simple
fractional spawning pattern to diapause, is presented. Morphological differences in primary embryonic respiratory surfaces,
as seen in the four species, are related to environmental conditions. The above illustrate ways in which the same basic structures
and events are modified to cope with different habitats. 相似文献
28.
The structural transitions occurring during the assembly and disassembly of pig brain microtubule protein were investigated by time-resolved X-ray scattering using synchrotron radiation. The reactions were introduced by a slow temperature scan (2 deg.C/min) from 0 °C to 37 °C and back. Several structurally distinct states could be resolved during one cycle of assembly/disassembly. During the temperature rise, one observes four main phases: prenucleation events, microtubule nucleation, growth, and postassembly events.Heating from 0 °C to 22 °C results in a biphasic breakdown of rings and other aggregates, while the apparent mean diameter increases from 38 to 41 nm. Parallel time-resolved electron microscopic observations suggest that the initial solution contains several types of aggregates, mostly double concentric and single rings, but also rod-like particles, clusters of rings and other aggregates. All of these tend to break down with increasing temperature. Double concentric rings seem to dissociate into large and small single rings before both types of rings break down into protofilament fragments and tubulin subunits. From the breakdown products, associations of several protofilament fragments are formed, which are important for initiating microtubule nucleation. Assembly of nuclei begins around 22 °C. Microtubule elongation takes place between 25 and 30 °C. They grow mainly by addition of tubulin subunits but not via rings.During the reverse temperature scan, microtubules shorten by the release of subunits and/or small protofilament fragments from their ends. The degree of disassembly is strongly increased below 22 °C. Below about 10 °C rings are reformed, probably from the fragments, but their final number is much less than initially.Conditions that prevent microtubule nucleation such as GDP or Ca2+ also stabilize rings, even at 37 °C. Thus, rings are viewed as storage aggregates of tubulin and microtubule associated proteins, whose breakdown is a prerequisite for microtubule formation, and whose reformation is independent of microtubule breakdown.The midpoints of microtubule growth and breakdown differ by about 12 deg.C so that the system shows hysteresis-like behavior. It is dependent on microtubule formation and is not seen when the temperature is cycled below that required for nucleation. Thus, even during a slow temperature scan, microtubule assembly is kinetically limited by nucleation. By contrast, depolymerization proceeds close to equilibrium.The radius of gyration of the tubulin heterodimers is 3.1 nm. The weight average diameter of rings in cold solutions is 38 nm, that of microtubules is 24.5 nm.At radiation dose rates of about 100 rad/s. radiation damage is of minor importance, as judged by the criterion of polymerizability. Total doses of up to 500,000 rad can be applied.Some concepts of analyzing time-resolved X-ray scattering data are presented. They make use of the fact that the scattering intensities vary continuously both with scattering angle and time. Cross-correlation of different regions of the pattern, and comparison of their temperature derivatives, reveals structural transitions not seen by other techniques. 相似文献
29.
30.
Bacteriophages can be stained with a flagella stain, making them visible by light microscopy. 相似文献