全文获取类型
收费全文 | 4534篇 |
免费 | 541篇 |
出版年
2021年 | 50篇 |
2020年 | 37篇 |
2019年 | 47篇 |
2018年 | 57篇 |
2017年 | 49篇 |
2016年 | 76篇 |
2015年 | 128篇 |
2014年 | 140篇 |
2013年 | 173篇 |
2012年 | 216篇 |
2011年 | 203篇 |
2010年 | 149篇 |
2009年 | 114篇 |
2008年 | 202篇 |
2007年 | 203篇 |
2006年 | 167篇 |
2005年 | 178篇 |
2004年 | 157篇 |
2003年 | 138篇 |
2002年 | 136篇 |
2001年 | 128篇 |
2000年 | 158篇 |
1999年 | 130篇 |
1998年 | 61篇 |
1997年 | 61篇 |
1996年 | 59篇 |
1995年 | 53篇 |
1994年 | 42篇 |
1993年 | 48篇 |
1992年 | 100篇 |
1991年 | 99篇 |
1990年 | 88篇 |
1989年 | 78篇 |
1988年 | 84篇 |
1987年 | 88篇 |
1986年 | 65篇 |
1985年 | 72篇 |
1984年 | 75篇 |
1983年 | 56篇 |
1982年 | 47篇 |
1981年 | 47篇 |
1980年 | 56篇 |
1979年 | 81篇 |
1978年 | 57篇 |
1977年 | 36篇 |
1976年 | 48篇 |
1975年 | 37篇 |
1974年 | 42篇 |
1973年 | 45篇 |
1969年 | 35篇 |
排序方式: 共有5075条查询结果,搜索用时 234 毫秒
61.
62.
Evidence is presented that rat serosal mast cells and cells of the rat basophilic leukemia line, RBL-1, each contain a calcium-activated, phospholipid-dependent protein kinase. The enzymes are very similar in their activation requirements to the calcium-dependent enzymes termed protein kinase Cs in brain. The enzyme is selectively stimulated by diolein and phosphatidylserine and is inhibited by several local anesthetics. The Ka for Ca2+ is 1.0 X 10(-3) M and 1.5 X 10(-4) M in mast cells and RBL-1 cells, respectively. The enzyme in mast cells is rapidly activated and apparently changed in its intracellular distribution when intact mast cells are stimulated with 48/80, A-23187, and anti-IgE and 12-O-tetradecanoyl-13-acetate in combination. 相似文献
63.
Factors Affecting High-Oxygen Survival of Heterotrophic Microorganisms from an Antarctic Lake 总被引:2,自引:2,他引:0 下载免费PDF全文
We sought to determine factors relating to the survival of heterotrophic microorganisms from the high-dissolved-oxygen (HDO) waters of Lake Hoare, Antarctica. This lake contains perpetual HDO about three times that of normal saturation (40 to 50 mg liter−1). Five isolates, one yeast and four bacteria, were selected from Lake Hoare waters by growth with the membrane filter technique with oxygen added to yield dissolved concentrations 14 times that in situ, 175 mg liter−1. One bacterial isolate was obtained from the microbial mat beneath the HDO waters. This organism was isolated at normal atmospheric oxygen saturation. The bacteria were gram-negative rods, motile, oxidase positive, catalase positive, and superoxide dismutase positive; they contained carotenoids. The planktonic isolates grew in media containing 10 mg of Trypticase soy (BBL Microbiology Systems)-peptone (2:1) liter−1 but not at 10 g liter−1. Under low-nutrient levels simulating Lake Hoare waters (10 mg liter−1), two of the planktonic isolates tested were not inhibited by HDO. Growth inhibition by HDO increased as nutrient concentration was increased. A carotenoid-negative mutant of one isolate demonstrated a decreased growth rate, maximal cell density, and increased cell lysis in the death phase under HDO compared with the parent strain. The specific activity of superoxide dismutase was increased by HDO in four of the five bacterial isolates. The superoxide dismutase was of the manganese type on the basis of inhibition and electrophoretic studies. The bacterial isolates from Lake Hoare possess several adaptations which may aid their survival in the HDO waters, as well as protection due to the oligotrophic nature of the lake. 相似文献
64.
Expression and purification of glutamine synthetase cloned from Bacteroides fragilis 总被引:7,自引:0,他引:7
A glutamine synthetase (GS) gene, glnA, from Bacteroides fragilis was cloned on a recombinant plasmid pJS139 which enabled Escherichia coli glnA deletion mutants to utilize (NH4)2SO4 as a sole source of nitrogen. DNA homology was not detected between the B. fragilis glnA gene and the E. coli glnA gene. The cloned B fragilis glnA gene was expressed from its own promoter and was subject to nitrogen repression in E. coli, but it was not able to activate histidase activity in an E. coli glnA ntrB ntrC deletion mutant containing the Klebsiella aerogenes hut operon. The GS produced by pJS139 in E. coli was purified; it had an apparent subunit Mr of approximately 75,000, which is larger than that of any other known bacterial GS. There was very slight antigenic cross-reactivity between antibodies to the purified cloned B. fragilis GS and the GS subunit of wild-type E. coli. 相似文献
65.
Wolterbeek, H. Th. and De Bruin, M. 1986. Xylem and phloem importof Na+, K+ , Rb+, Cs+ and in tomato fruits: differential contributions from stem and leaf.J.exp. Bot. 37: 928939. The transport of Na+, K+, Rb+, Cs+ and into developing fruits of tomato (an inbred lineof Lycopersicon esculentum Mill. cv. Tiny Tim) was measured.Element solutions were introduced into the transpiration streamthrough the cut stem bases of plant parts consisting of a stempart with single green fruit, both with and without attachedfully expanded leaf. Measurements were carried out of the accumulationin the fruit of the gamma-ray emitting radiotracers 24Na+, 42K+,86Rb+, 134Cs+ and The transport into the fruit was expressed by a single parameter taking intoaccount volume flows varying with time and experiments. Xylemto phloem transfer in the stem as a source of fruit elementsupply was shown to be inversely related with the velocity offlow of the stem xylem. The results also indicated that thetransfer system in the stem was more rapidly equilibrated thanit was in the leaf. Stem loading of the phloem is suggested as a possible mechanismregulating the solute influx in fruits under varying flow velocitiesof the stem xylem, while fruit influx of phloem solutes, whichwere loaded in the leaf, may play a major role in influx regulationunder conditions of varying solute concentrations. Key words: Alkali ions, tomato fruits, stem and leaf phloem loading 相似文献
66.
Effect of serotonin on murine macrophages: suppression of Ia expression by serotonin and its reversal by 5-HT2 serotonergic receptor antagonists 总被引:2,自引:0,他引:2
Serotonin (5-HT), a mediator released from platelets at sites of inflammation, suppressed IFN-gamma-induced Ia expression in mouse bone marrow macrophages maintained in vitro. (Mean percent suppression = 63.9% +/- 9.2, n = 40.) This suppression was not toxic or endotoxin-related, was concentration-dependent, and occurred at the physiologic concentrations of 5-HT present at inflammatory sites. The concentration of 5-HT producing the half-maximal effect was 2.5 to 5.5 X 10(-8) M. Related compounds, dopamine, histamine, and tryptamine, were much less potent in suppressing IFN-gamma-induced Ia, with maximally suppressing concentrations more than 100-fold higher than the maximally suppressing 5-HT concentration. L-5-hydroxytryptophan (5-HTP), the most potent analog tested, was 10-fold less potent than 5-HT in suppressing Ia expression. The concentration of 5-HTP producing the half-maximal effect = 4 X 10(-7) M. 5-HT suppression of IFN-gamma-induced Ia expression was antagonized by the 5-HT2 type receptor antagonists spiperone, ketanserin, and LY53857. Concentrations of these agents resulting in 50% inhibition of the serotonin effect were 1.5 X 10(-8) M, 7.5 X 10(-8) M, and 4.5 X 10(-12) M, respectively. 5-HT was most effective in suppressing IFN-gamma-induced Ia when added early in culture simultaneously with IFN-gamma. These data provide functional evidence that 5-HT suppression of IFN-gamma-induced Ia expression is mediated through a 5-HT receptor with some characteristics of the 5-HT2 type. 5-HT may play a physiologic role at sites of inflammation as a modulator of the effects of IFN-gamma on macrophage function. 相似文献
67.
Loyd J. E.; Nolop K. B.; Parker R. E.; Roselli R. J.; Brigham K. L. 《Journal of applied physiology》1986,60(1):198-203
Because pulmonary edema has been associated clinically with airway obstruction, we sought to determine whether decreased intrathoracic pressure, created by selective inspiratory obstruction, would affect lung fluid balance. We reasoned that if decreased intrathoracic pressure caused an increase in the transvascular hydrostatic pressure gradient, then lung lymph flow would increase and the lymph-to-plasma protein concentration ratio (L/P) would decrease. We performed experiments in six awake sheep with chronic lung lymph cannulas. After a base-line period, we added an inspiratory load (20 cmH2O) and allowed normal expiration at atmospheric pressure. Inspiratory loading was associated with a 12-cmH2O decrease in mean central airway pressure. Mean left atrial pressure fell 11 cmH2O, and mean pulmonary arterial pressure was unchanged; calculated microvascular pressure decreased 8 cmH2O. The changes that occurred in lung lymph were characteristic of those seen after other causes of increased transvascular hydrostatic gradient, such as increased intravascular pressure. Lung lymph flow increased twice base line, and L/P decreased. We conclude that inspiratory loading is associated with an increase in the pulmonary transvascular hydrostatic gradient, possibly by causing a greater fall in interstitial perimicrovascular pressure than in microvascular pressure. 相似文献
68.
Subunit interactions of class I histocompatibility antigens 总被引:5,自引:0,他引:5
The kinetics of dissociation of iodinated beta 2-microglobulin (beta 2m) from the papain-solubilized class I histocompatibility antigen HLA-B7 have been investigated. In the presence of unlabeled beta 2m, most of the HLA dissociates according to a single rate constant, whereas in the absence of unlabeled beta 2m, the system approaches an equilibrium dependent upon the initial HLA concentration. When iodinated beta 2m is incubated with unlabeled HLA-B7, the rate of incorporation of beta 2m into the complex is much less dependent on the concentration than is expected for a simple association/dissociation system; instead, the system behaves as if the "activity" (in a thermodynamic sense) of the HLA heavy-chain intermediate cannot surpass a critical concentration. The dissociation rate for each class I specificity is a function of temperature, ionic strength, pH, and the status of the heavy chain (papain solubilized vs. detergent solubilized). High temperature, high ionic strength, and extremes of pH promote dissociation. The intact molecule dissociates about 10 times more slowly than the papain-solubilized molecule. In contrast, the rate of dissociation of all papain-solubilized class I antigens tested falls within the range of about a factor of 2. The presence of the carbohydrate has no effect on the rate of dissociation. The possibility that HLA class I antigen dissociation may occur in vivo within acidic internal vesicles is discussed. 相似文献
69.
Prevost, I. and Le PageDegivry, M. Th. 1985. Changesin absicisic acid content in axis and cotyledons of developingPhaseolus vulgaris embryos and their physiological consequences.J.exp. Bot. 36: 19001905.Changes in abscisic acid (ABA)content with time were measured in embryonic axes and in cotyledonsof Phaseolus vulgaris embryos using a radioimmunoassay.During embryogenesis, a similar pattern was observed in bothtissues: ABA increased to a maximum 29 d after an thesis, followedby a decrease as the seed matured. The level of ABA in the cotyledonswas always much higher than that in the axes. In in vitro cultures,the duration of the lag phase before germination of isolatedembryonic axes increased with ABA content. The presence of cotyledonsalways lengthened the lag phase; longer lag phases were associatedwith greater concentrations of ABA in the cotyledons. Moreoverthe presence of cotyledons stimulated the growth of seedlings. Key words: ABA distribution, embryo maturation, axis and embryo germinability 相似文献
70.
The developmental success of interspecific Lepomis hybrids is used as an index of gene regulatory divergence between the green sunfish, L. cyanellus, and each of three other parental species, longear sunfish, L. megalotis, warmouth, L. gulosus, and bluegill, L. macrochirus. This gene regulatory divergence is compared to the degree of structural gene divergence among these four species (genetic distance [Nei, '78], D, ranged from 0.206 to 0.586). The developmental success of the hybrid embryos at the level of morphogenesis was higher than expected from the genetic distance between the parental species. The rates of morphogenesis of the hybrid embryos were the same as that for the green sunfish embryos. The percentage of embryos that hatched was relatively high in all crosses. However, two of the hybrid crosses resulted in enhanced percentages of hatched embryos. Slight increases in the extent of morphological abnormalities were observed in hybrids from crosses between more distantly related parental species. The schedules and levels of enzyme locus expression of the hybrids, assessed spectrophotometrically and electrophoretically for nine enzyme systems (encoded in a total of 14 loci), were different from each other and from those of the green sunfish embryos. Alterations in the time of first enzyme appearance and in the time of first increase in enzyme activity in the developing hybrid embryos were not correlated with genetic distance between parental species. However, the extents of alteration of enzyme activities over the entire period of hybrid embryogenesis were correlated with the genetic distance. We attribute the morphological and molecular anomalies observed in the hybrids to gene regulatory incompatibilities between species. Although the exact number of mutational differences and their relative developmental impacts are not known, some inferences can be drawn about the degree of divergence in gene regulation between species. It appears that an uncoupling of the rates of structural and regulatory gene evolution can occur between species of some taxa, an observation that has implications for the roles of gene regulatory differences in organismic evolution. 相似文献