全文获取类型
收费全文 | 96855篇 |
免费 | 8704篇 |
国内免费 | 46篇 |
出版年
2023年 | 363篇 |
2022年 | 818篇 |
2021年 | 1685篇 |
2020年 | 960篇 |
2019年 | 1266篇 |
2018年 | 1559篇 |
2017年 | 1431篇 |
2016年 | 2300篇 |
2015年 | 3939篇 |
2014年 | 4454篇 |
2013年 | 5340篇 |
2012年 | 7175篇 |
2011年 | 6935篇 |
2010年 | 4442篇 |
2009年 | 4191篇 |
2008年 | 5938篇 |
2007年 | 5958篇 |
2006年 | 5549篇 |
2005年 | 5398篇 |
2004年 | 5182篇 |
2003年 | 4994篇 |
2002年 | 4691篇 |
2001年 | 1013篇 |
2000年 | 774篇 |
1999年 | 1042篇 |
1998年 | 1329篇 |
1997年 | 907篇 |
1996年 | 829篇 |
1995年 | 783篇 |
1994年 | 661篇 |
1993年 | 744篇 |
1992年 | 685篇 |
1991年 | 582篇 |
1990年 | 577篇 |
1989年 | 554篇 |
1988年 | 522篇 |
1987年 | 479篇 |
1986年 | 457篇 |
1985年 | 620篇 |
1984年 | 694篇 |
1983年 | 628篇 |
1982年 | 732篇 |
1981年 | 666篇 |
1980年 | 665篇 |
1979年 | 392篇 |
1978年 | 468篇 |
1977年 | 403篇 |
1976年 | 403篇 |
1974年 | 361篇 |
1973年 | 356篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
David G. Griffiths Michael D. Partis Perry Churchill Stephen C. Brenner Sidney Fleischer Roger J. Moore R. Brian Beechey 《Journal of bioenergetics and biomembranes》1990,22(5):691-707
A series of amphiphilic polymethylenecarboxymaleimides has been synthesized for use as sulfhydryl reagents applicable to membrane proteins. Physical properties of the compounds which are relevant to their proposed mode of action have been determined. By comparing rates of reaction in aqueous and aprotic solvents, the compounds have been shown to react exclusively with the thiolate ion. The effects of the reagents on three membrane-associated proteins are reported, and in two cases a comparative study has been made of the effects on the proteins in the absence of membranes. A mechanism is proposed whereby the reagents are anchored at the lipid/water interface by the negatively charged carboxyl group, thus siting the reactive maleimide in a plane whose depth is defined by the length of the reagent. Supporting evidence for this model is provided by the inability of the reagents to traverse membranes, and variation of their inhibitory potency with chain length when the proteins are embedded in the membrane, but not when extracted into solution. As examples of general use of the reagents to probe sulfhydryl groups in membrane proteins, the reagents have been used to (a) determine the depths in the membrane at which two populations of sulfhydryl groups occur in the mitochondrial phosphate transporter; (b) locate a single sulfhydryl associated with the active site ofD--hydroxybutyrate dehydrogenase in the inner mitochondrial membrane; (c) examine sulfhydryl groups in theD-3-glyceraldehyde phosphate dehydrogenase associated with the human red blood cell membrane. 相似文献
932.
Elizabeth E. Hood David H. Clapham Inger Ekberg Thomas Johannson 《Plant molecular biology》1990,14(2):111-117
The hypervirulent Agrobacterium tumefaciens strain A281 formed frequent tumors (31%) on Picea abies (Norway spruce), an economically important tree species in Swedish forests. Three-month-old seedlings were inoculated and tumors were established that grew hormone-independently in culture. Tumors contained agropine and mannopine/mannopinic acid as determined by acid pH paper electrophoresis. In addition, DNA hybridization studies showed that the DNA from these tumor lines contained sequences homologous to Ti plasmid T-DNA, whereas wild-type spruce seedling DNA did not. These results suggest that Agrobacterium vectors can be used for gene transfer into this important forest species. 相似文献
933.
Véronique Guellec Chantal David Michel Branchard Jacques Tempé 《Plant Cell, Tissue and Organ Culture》1990,20(3):211-215
Genetically transformed grapevine (Vitis vinifera L.) roots were obtained after inocultation of in vitro grown whole plants (cv. Grenache) with Agrobacterium rhizogenes. The strain used contains two plasmids: the wild-type Ri plasmid pRi 15834 and a Ti-derived plasmid which carries a chimaeric neomycin phosphotrans-ferase gene (NPT II) and the nopaline synthase gene. Expression of the NPT II gene can confer kanamycin resistance to transformed plant cells. Slowly growing axenic root cultures derived from single root tips were obtained. Opine analysis indicated the presence of agropine and/or nopaline in established root cultures. For one culture, the presence of T-DNA was confirmed by dot-blot hybridization with pRi 15834 TL-DNA. Callogenesis was induced by subculturing root fragments on medium supplemented with benzylaminopurine and indoleacetic acid.Transformation of in vitro cultured grapevine cells has recently been reported (baribault T.J. et al., Plant Cell Rep (1989) 8: 137–140). In contrast with the results presented here, expession of the NPT II gene Conferred kanamycin resistance to Vitis vinifera calli that was sufficient for selection of trasformed cells.Abbreviations BAP
benzylaminopurine
- IAA
indoleacetic acid
- NAA
naphtaleneacetic acid
- NPT II
neomycin phosphostransferase II
- EDTA
ethylenediaminetetraacetic acid 相似文献
934.
Ivan Valiela Joseph Costa Kenneth Foreman John M. Teal Brian Howes David Aubrey 《Biogeochemistry》1990,10(3):177-197
Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers. The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn, increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the characteristically high respiration rates found in enriched waters. 相似文献
935.
A newly-developed field-portable multi-flash kinetic fluorimeter for measuring the kinetics of the microsecond to millisecond reactions of the oxidizing and reducing sides of photosystem 2 in leaves of intact plants is described and demonstrated. The instrumental technique is a refinement of that employed in the double-flash kinetic fluorimeter (Joliot 1974 Biochim Biophys Acta 357: 439–448) where a low-intensity short-duration light pulse is used to measure the fluorescence yield changes following saturating single-turnover light pulses. The present instrument uses a rapid series of short-duration (2 s) pulses to resolve a complete microsecond to millisecond time-scale kinetic trace of fluorescence yield changes after each actinic flash. Differential optics, using a matrix of optical fibers, allow very high sensitivity (noise levels about 0.05% Fmax) thus eliminating the need for signal averaging, and greatly reducing the intensity of light required to make a measurement. Consequently, the measuring pulses have much less actinic effect and an entire multi-point trace (seven points) excites less than 1% of the reaction centers in a leaf. In addition, bu combining the actinic and measuring pulse light in the optical fiber network, the tail of the actinic flash can be compensated for, allowing measurements of events as rapidly as 20 s after the actinic flash. This resolution makes practical the routine measurement of the microsecond turnover kinetics of the oxygen evolving complex in leaves of intact plants in the field. The instrument is demonstrated by observing flash number dependency and inhibitor sensitivity of the induction and decay kinetics of flash-induced fluorescence transients in leaves of intact plants. From these traces the period-two oscillations associated with the turnover of the two-electron gate and the period-four oscillations associated with the turnover of the oxygen evolving complex can be observed. Applications of the instrument to extending our knowledge of chloroplast function to the whole plant, the effects on plants of environmental stress, herbicides, etc, and possible applications to screening of mutants are discussed.Abbreviations DCMU
3-(3,4-Dichlorophenol)-1,1-dimethylurea
- PS 2
photosystem 2
- PS 1
photosystem 1
- P680
primary electron donor of the PS 2 reaction center
- QA
primary acceptor quinone of PS 2
- QB
secondary acceptor quinone of PS 2
- CCCP
carbonyl cyanide-m-chlorophenylhydrazone
- Yz
donor to P680
+
- F0
level of fluorescence with all PS 2 centers open
- Fmax
maximum level of fluorescence with all PS 2 centers closed
- P680QA
Open reaction centers with P680 reduced and QA oxidized (low fluorescence)
- P680QA
-
Closed reaction centers, in which P680 is reduced (high fluorescence)
- P680
+QA
-
Closed reaction centers, in which P680 is oxidized (low fluorescence) 相似文献
936.
The effect of dicyclohexylcarbodiimide (DCCD) on electron transfer in the acceptor quinone complex of reaction centers (RC) from Rhodobacter sphaeroides is reported. DCCD covalently labelled the RC over a wide concentration range. At low concentrations (<10 M) the binding was specific for the L subunit. At relatively high concentrations (>100 M) DCCD accelerated the rate of charge recombination of the P+QB
- state, consistent with a decrease in the equilibrium constant between QA
-QB and QAQB
-. At similar concentrations, in the presence of cytochrome c as exogenous donor, turnover of the RC was inhibited such that only three cytochromes were oxidized in a train of flashes. Both these inhibitory effects were fully reversed by dialysis, indicating that stable covalent binding was not involved. Possible mechanisms of action are discussed in terms of the putative role of specific residues in proton transfer and protonation and release of quinol from the RC. 相似文献
937.
David P. Watts 《International journal of primatology》1990,11(1):21-45
Understanding the principles that underly primate social evolution depends on integrated analysis of data on behavioral ecology, demography, life history tactics, and social organization. In this paper, data on the behavioral ecology of gorillas are reviewed and comparisons made among the three subspecies. Gorillas are selective feeders; and, their patterns of food choice are consistent with models of feeding by large generalist herbivores. They rely heavily on terrestrial herbaceous vegetation, which provides an abundant supply of densely distributed food. Availability of this food varies little in space and time; and, gorilla foraging activity can maintain its productivity. The level of frugivory and the extent of seasonal variation in diet and habitat use vary among and within populations. Low variability in food distribution patterns makes cooperative defense of foraging areas not worthwhile; but, it also means that ecological costs associated with gregariousness are low. However, demographic and life history data on mountain gorillas show that these costs may be sufficient to reduce female reproductive success as group size increases. Advantages to being with high quality males apparently can outweigh these costs. The implications of these data for the evolution of the mountain gorilla social system, and the possible roles of male protection, predation, and female/female competition in this regard, are discussed. 相似文献
938.
Paramyxovirus P genes are transcribed into two mRNAs which differ from each other by either one (measles and Sendai virus) or two (SV5 and mumps virus) G insertions, and which code for either the P or V proteins. The G insertions always occur within a short run of Gs, and a stuttering mechanism for the insertions has been suggested in which the viral polymerase reiteratively copies a template C residue during mRNA synthesis. Support for this mechanism was obtained by varying the reaction conditions during Sendai virus mRNA synthesis in vitro. A stuttering model is proposed which accounts for how the ratio of inserted to uninserted mRNAs is controlled, and why some paramyxoviruses insert one G and others two Gs when insertions occur. 相似文献
939.
Distinct sequence elements control the specificity of G protein activation by muscarinic acetylcholine receptor subtypes. 总被引:25,自引:5,他引:20
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J Lechleiter R Hellmiss K Duerson D Ennulat N David D Clapham E Peralta 《The EMBO journal》1990,9(13):4381-4390
Relatively little is understood concerning the mechanisms by which subtypes of receptors, G proteins and effector enzymes interact to transduce specific signals. Through expression of normal, hybrid and deletion mutant receptors in Xenopus oocytes, we determined the G protein coupling characteristics of the functionally distinct m2 and m3 muscarinic acetylcholine receptor (mAChR) subtypes and identified the critical receptor sequences responsible for G protein specificity. Activation of a pertussis toxin insensitive G protein pathway, leading to a rapid and transient release of intracellular Ca2+ characteristic of the m3 receptor, could be specified by the transfer of as few as nine amino acids from the m3 to the m2 receptor. In a reciprocal manner, transfer of no more than 21 residues from the m2 to the m3 receptor was sufficient to specify activation of a pertussis toxin sensitive G protein coupled to a slow and oscillatory Ca2+ release pathway typical of the m2 subtype. Notably, these critical residues occur within the same region of the third cytoplasmic domain of functionally distinct mAChR subtypes. 相似文献
940.
Giving between generations in American families 总被引:4,自引:0,他引:4
This paper documents the types and amounts of aid exchanged between adults and their non-coresidential parents. Data for the
study are drawn from a representative national sample survey of Americans age 19 and older conducted in 1987–1988. Exchanges
of monetary and material resources, childcare, household assistance, and companionship and advice are considered.
Patterns of intergenerational exchange are found to differ by gender, family structure, age, ethnicity, and socioeconomic
situation. Differences in exchange between males and females and between whites and Mexican-Americans are related to other
life-course characteristics, and to the availability and proximity of kin. Blacks and persons living in poverty are shown
to be less involved than other groups in intergenerational exchanges. Finally, patterns of prior assistance and the available
needs and resources of the respondents and their parents are found to influence current patterns of exchange.
Support for this research was provided by NICHD Grant No. 1 R01 HD26070-01, “Intergenerational Exchanges in Families with
Children,” Dennis P. Hogan, Principal Investigator. Funds for the computer analysis were provided by the Pennsylvania State
University Intercollege Research Programs.
David Eggebeen is an Assistant Professor of Human Development in the Department of Human Development and Family Studies and
a research associate at the Population Issues Research Center at Pennsylvania State University. He trained in sociology and
demography at the University of North Carolina. His current research interests, besides those related to intergenerational
relations, are the recent changes in the demographic structure of childhood in America and their implications for children’s
social and economic well-being.
Dennis P. Hogan is a professor of sociology and the director of the Population Issues Research Center at Pennsylvania State
University. His current research interests, besides those related to intergenerational relations, are in the interrelation
of social structures and the demographic life course. He is coauthor with David I. Kertzer ofFamily, Political Economy, and Demographic Change: The Transformation of Life in Casalecchio, Italy, 1861–1921, University of Wisconsin Press, 1989. 相似文献