全文获取类型
收费全文 | 23435篇 |
免费 | 1225篇 |
国内免费 | 786篇 |
出版年
2023年 | 212篇 |
2022年 | 304篇 |
2021年 | 422篇 |
2020年 | 486篇 |
2019年 | 651篇 |
2018年 | 769篇 |
2017年 | 562篇 |
2016年 | 444篇 |
2015年 | 543篇 |
2014年 | 925篇 |
2013年 | 1134篇 |
2012年 | 650篇 |
2011年 | 816篇 |
2010年 | 595篇 |
2009年 | 704篇 |
2008年 | 791篇 |
2007年 | 881篇 |
2006年 | 810篇 |
2005年 | 690篇 |
2004年 | 611篇 |
2003年 | 558篇 |
2002年 | 529篇 |
2001年 | 417篇 |
2000年 | 418篇 |
1999年 | 413篇 |
1998年 | 319篇 |
1997年 | 340篇 |
1996年 | 350篇 |
1995年 | 303篇 |
1994年 | 311篇 |
1993年 | 305篇 |
1992年 | 289篇 |
1991年 | 302篇 |
1990年 | 273篇 |
1989年 | 279篇 |
1988年 | 275篇 |
1987年 | 263篇 |
1986年 | 253篇 |
1985年 | 747篇 |
1984年 | 907篇 |
1983年 | 511篇 |
1982年 | 822篇 |
1981年 | 624篇 |
1980年 | 589篇 |
1979年 | 455篇 |
1978年 | 345篇 |
1977年 | 311篇 |
1976年 | 247篇 |
1975年 | 232篇 |
1974年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
The redistribution of surface membrane immunoglobulin molecules (sIg) was studied in two functionally distinct populations of mouse splenic B lymphocytes, namely, those bearing membrane IgM(IgG?) and those bearing IgG. Brief exposure to mitogenic doses of bacterial lipopolysaccharide (LPS) produced direct but differential effects on the subsequent ability of specific antibodies to induce this redistribution on each cell type. Studied as a function of temperature, antibody-induced redistribution of sIgM on cells previously exposed to LPS was observed to occur at temperatures lower than the temperatures required for similar sIgM redistribution on lymphocytes not exposed to LPS. In contrast, mitogen-treated sIgG+ cells demonstrated an opposite and long-lasting effect (at least 40 hr), requiring higher temperatures to allow sIgG movement comparable to that seen on untreated sIgG-bearing lymphocytes. Thus, we conclude that LPS interacts with both IgM+(IgG?) and IgG+ lymphocytes, but that such interactions produced different membrane effects on each B-cell subset. This membrane change can therefore be useful as a quasi-functional differentiation marker. Furthermore, differences in sensitivity to cellular activation by LPS seen between sIgM-bearing (sIgG?) and sIgG-bearing B cells may be a reflection of such direct, although different, membrane effects. 相似文献
62.
The transport of carnitine by rat kidney cortex slices against a concentration gradient has been demonstrated. Similarities to other transport systems included a linear period of uptake, as well as indications of saturability of the system with increasing concentrations of substrate. The transport of carnitine was inhibited by anoxia, and carbonyl cyanide-m-chloro-phenylhydroxazone (CCC1P), an uncoupler of oxidative phosphorylation. Carnitine uptake was stimulated approximately 50% when kidney slices were treated with dibutyryl cAMP. 相似文献
63.
Automated determinations of 5-hydroxytryptamine and its main metabolite, 5-hydroxyindoleacetic acid, have been described (Technicon autoanalyzer). The determinations are based on an extraction procedure from deproteinized tissue extracts or cerebrospinal fluid by means of butanolheptane mixtures. The indoles are transferred from the organic phase to a water phase and determined fluormetrically with the cysteine-o-phthaldialdehyde method. The method is highly sensitive: solutions containing 1 ng/ml can be reproducibly determined. Twenty samples per hour can be passed through the system. The determination of both 5-hydroxyindoles is performed with the same manifold. 相似文献
64.
G E Brown S A Fischkoff J V Ordonez 《Biochemical and biophysical research communications》1984,123(3):937-943
The ability of a myeloid leukemia cell line (HL-60) to undergo membrane electrical potential changes was followed during neutrophilic differentiation induced by 2 compounds. Membrane-potential changes were induced with 12-O-tetradecanoylphorbol 13-acetate (TPA) or formyl-methionyl-leucyl-phenylalanine (FMLP) and were monitored by flow cytometry. The magnitude of the membrane-potential response to TPA increased in a more uniform manner as the population of cells matured than did acquisition of mature morphology or ability to undergo the respiratory burst in response to TPA. The response to TPA and FMLP of HL-60 cells, maximally induced to differentiate by dimethylsulfoxide, closely resembled that of neutrophils. Thus, HL-60 cells may be a useful tool in the study of the relation between membrane depolarization and subsequent cellular activation. 相似文献
65.
Analysis of the effects of cesium ions on potassium channel currents in biological membranes 总被引:3,自引:0,他引:3
Cesium ions block potassium channels in biological membranes in a voltage dependent manner. For example, external cesium blocks inward current with little or no effect on outward current. Consequently, it produces a characteristic N-shaped current-voltage relationship. We have modeled this result by single file diffusion of ions in a narrow channel spanning the membrane with a special blocking site in the channel for cesium ions. The model enables us to make detailed comparisons of the effects of cesium on potassium channels in different types of biological membranes. 相似文献
66.
Proteins undergoing protease reactions, heat denaturation, or interactions with sodium dodecyl sulfate (SDS) were used to demonstrate the effectiveness of a near-infrared method for the quantitative study of changes in hydration or water binding during such processes. The spectra of different proteins showed that the liberation of groups during a protease reaction is associated with a large increase in hydration and excluded volume. On the basis of experiments with model compounds, other spectral changes, including development of continuum absorbance between 1.55 and 1.85 μm and a band with a peak near 2.1 μm, were also attributed to the liberation of these groups. After heat denaturation or in the presence of SDS, the rate of proteolytic hydrolysis was markedly increased, consistent with the view that some preliminary denaturation is necessary for protease activity. The validity of the hydration changes calculated for protease reactions was supported by model studies with l-lysine, and with poly-l-lysine before and after hydrolysis. The near-infrared spectrum of the protein substrate with no added protease was largely unaffected by heat treatment alone, indicating that the hydration as such was not changed to a large extent by the structural modifications of denaturation. In contrast to the protease reaction, the interactions between SDS and the proteins resulted in a decrease in hydration. Results of this paper are compared with those obtained from other methods. Some unique advantages of the near-infrared method for the study of hydration changes during reactions in aqueous solution are described. 相似文献
67.
Summary The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted, solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation. 相似文献
68.
69.
R L Melnick L G Monti S M Motzkin 《Biochemical and biophysical research communications》1976,69(1):68-73
The mechanism of integration of λll, which is deleted of all the known λ recombination genes, was studied using deleted hosts as recipients. The presence of BC DNase and I in the recipient cells affected the fate of λll DNA. In nine of ten transductants, insertion of the λll genome took place somewhere between J and N and the remaining one had abnormally permuted prophage λ. In this lysogen (#42), the sequence of prophage genes was similar to that of vegetative phage λ. The properties of lysogen #42 were compared with those of other lysogens. 相似文献
70.
A. R. Marques G. W. Fernandes I. A. Reis R. M. Assunç;ão 《Plant biology (Stuttgart, Germany)》2002,4(1):94-103
Abstract: This study focuses on the sex ratio and spatial distribution of males and females in three populations of the endemic and restricted tropical dioecious shrub, Baccharis concinna (Asteraceae) in the mountainous region of Serra do Cipó, southeastern Brazil. The proportion of female plants in the population at lower elevation (1000 m a.s.l.) was significantly greater than of male plants. At this elevation of P/N and Ca/Al ratios in the soil were also greater indicating better nutritional status of the soils. The concentration of aluminium increased significantly with the elevation ( p < 0.001), perhaps rendering soils less conducive to female plants at higher elevations. Female plants are possibly adversely affected to a greater extent by soil quality than male plants. The spatial distribution of the populations within habitat was tested by the K(t) function, where the neighbourhood of a given individual was defined by a circle with a radius (t) up to 3 m. Despite the strong tendency for aggregation, the distribution of the sexes within habitats was random and the hypothesis was not supported. The independent distribution of the sexes within habitats may be explained by nutrient homogeneity of the soils, as well as by an absence of antagonism between the sexes. Nevertheless, we found a trend for males and females to be aggregated according to their gender. 相似文献