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31.
Diane R. Campbell Nickolas M. Waser Mary V. Price Elizabeth A. Lynch Randall J. Mitchell 《Evolution; international journal of organic evolution》1991,45(6):1458-1467
In the hummingbird-pollinated herb Ipomopsis aggregata, selection through male function during pollination favors wide corolla tubes. We explored the mechanisms behind this selection, using phenotypic selection analysis to compare effects of corolla width on two components of male pollination success, pollinator visit rate and pollen exported per visit. During single visits by captive hummingbirds, flowers with wider corollas exported more pollen, and more dye used as a pollen analogue, to stigmas of recipient flowers. Corolla width was less strongly related to visit rate in the field, and had no direct effect on visit rate after nectar production and corolla length were controlled for. Moreover, the phenotypic selection differential was 80% higher for the effect on pollen exported per visit, suggesting that this is the more important mechanism of selection. 相似文献
32.
A A Salyers A Reeves J D'Elia 《Journal of industrial microbiology & biotechnology》1996,17(5-6):470-476
Polysaccharide digestion by bacteria is an important activity in many ecosystems, and a number of bacterial genera can perform this function. Although many papers have been published about the properties of isolated polysaccharide-degrading enzymes, relatively little is known about how intact bacteria degrade polysaccharides. This review summarizes recent findings suggesting that there are at least three different strategies. the most familiar one is the excretion of extracellular polysaccharidases, which diffuse to and degrade nearby polysaccharides. An example of this type of strategy is provided by the plant pathogen,Erwinia spp. A second strategy is to have the enzyme exposed to the extracellular medium but attached to the surface of the cell. Examples of this strategy are provided by the pullulanase system ofKlebsiella oxytoca and the cellulosomes ofClostridium thermocellum. A strategy that could be seen as a combination of the extracellular enzyme strategy and the surface organelle strategy is provided byVibrio harveyi, which attaches to its substrate, chitin, via proteins that appear to be specialized for attachment and produces extracellular enzymes that attack the chitin. A third strategy is to import the polysaccharide, as appears to be done byBacteroides spp. In this instance, the polysaccharide is bound to an outer membrane receptor, then passes into the periplasm where the degradative enzymes are located. The ecological advantages and disadvantages of these systems are discussed, and areas where further research is needed are defined. 相似文献
33.
Stefan Mihailescu John M. Matsoukas Francis L. Abel Valeriu Nestianu Florin Romanescu Cristina Alecu-Parvu 《Letters in Peptide Science》1996,3(4):181-184
Summary The angiotensin II (ANG II) receptor blocker properties of sarmesin and its influence on the homotropic cooperativity of ANG II receptors were studied in two experimental models: isolated rabbit aorta and isolated rabbit atria. The results show that: (i) sarmesin is a specific competitive antagonist of ANG II receptors, with high affinity (pA2=8.93 in the isolated aorta and 8.66 in the isolated atria); and (ii) the slope of the concentration-response curves to ANG II and the Hill coefficient increased in the presence of sarmesin, the latter suggesting an enhancement of the positive homotropic cooperativity of ANG II receptors. These results may be explained overall by the reciprocal negative modulation of receptor affinity between sarmesin and ANG II, due, possibly, to opposite effects on the binding of G-proteins to ANG II receptors. 相似文献
34.
E. D. Williams 《Mutation research》1995,333(1-2):123-129
The transformation of the normal fully differentiated thyroid follicular cell to the rapidly growing undifferentiated anaplastic thyroid carcinoma cell involves a number of stages which have been defined morphologically and are now being related to various growth pathways and to molecular biological defects. The two main factors involved in this transformation are growth stimulation and mutagenesis. Growth stimulation alone, through elevated TSH, can lead to the development of thyroid tumours, usually benign, and retaining TSH dependency in some cases. Mutagens alone, if growth is suppressed, do not produce tumours, the combination of mutagens and increased growth is a potent carcinogenic regime. Non-genotoxic carcinogenesis in the thyroid involves growth, without mutagenesis the agent often causes this through affecting one component of thyroid hormone synthesis or metabolism, leading to a fall in thyroid hormone levels and a rise in TSH. Growth stimulation increases the rate of cell division, and therefore increases the chance of a mutation. Continued growth increases the change of subsequent events, in particular loss of heterozygosity in a tumour suppressor gene. The main oncogenes involved in human thyroid carcinogens are ras in the follicular tumour pathway, and ret in the papillary carcinoma pathway. p53 is involved in the progression of either papillary or follicular adenoma to an undifferentiated carcinoma. In experimental thyroid carcinogenesis, ras is again involved, with a link between the mutagenic agent used and the type of ras gene showing mutation. Analysis of the involvement of different growth factors and oncogenes in thyroid carcinogenesis suggests that genes related to the two receptors concerned with normal TSH stimulated growth, TSH receptor and the IGF1 recpptor may be involved in the progression of thyroid tumours of follicular pathology. Several tyrosine kinase receptors with unknown ligands or of uncertain physiological function are linked to papillary carcinoma. The recent large increase in papillary carcinoma of the thyroid in children exposed to fallout from the Chernobyl nuclear accident underlines the importance of understanding the pathobiology of thyroid neoplasia. 相似文献
35.
Minimal photosynthetic catalytic F1() core complexes, containing equimolar ratios of the and subunits, were isolated from membrane-bound spinach chloroplast CF1 and Rhodospirillum rubrum chromatophore RrF1. A CF1-33 hexamer and RrF1-11 dimer, which were purified from the respective F1() complexes, exhibit lower rates and different properties from their parent F1-ATPases. Most interesting is their complete resistance to inhibition by the general F1 inhibitor azide and the specific CF1 inhibitor tentoxin. These inhibitors were earlier reported to inhibit multisite, but not unisite, catalysis in all sensitive F1-ATPases and were therefore suggested to block catalytic site cooperativity. The absence of this typical property of all F1-ATPases in the 11 dimer is consistant with the view that the dimer contains only a single catalytic site. The 33 hexamer contains however all F1 catalytic sites. Therefore the observation that CF1-33 can bind tentoxin and is stimulated by it suggests that the F1 subunit, which is required for obtaining inhibition by tentoxin as well as azide, plays an important role in the cooperative interactions between the F1-catalytic sites.Abbreviations CF0F1
chloroplast F0F1
- CF1
chloroplast F1
- CF1
chloroplast F1 subunit
- CF1
chloroplast F1 subunit
- CF1()
a complex containing equal amounts of the CF1 and subunits
- MF1
mitochondrial F1
- RrF0F1
Rhodospirillum rubrum F0F1
- RrF1
R. rubrum F1
- RrF1
R. rubrum F1 subunit
- RrF1
R. rubrum F1 subunit
- RrF1()
a complex containing equal amounts of the RrF1 and subunits
- Rubisco
Ribulose-1,5-bisphosphate carboxylase
- TF1
thermophilic bacterium PS3 F1 相似文献
36.
Claudia M. Caillaud J. S. Pierre B. Chaubet J. P. Di Pietro 《Entomologia Experimentalis et Applicata》1995,75(1):9-18
The behaviour ofSitobion avenae (F.), was compared on resistant wheat lines ofTriticum monococcum (L.) and a susceptible variety ofTriticum aestivum (L.). Firstly, stylet penetration activities were monitored with the Electrical Penetration Graph (EPG) technique and subsequently
analysed using flow charts combined with correspondence analysis. Plant resistance was shown to be associated with repeated
penetrations without access to either the xylem or the phloem, and with numerous failures in starting a sustained sap ingestion
(as represented by pattern E2). Access to sieve elements of the phloem did not seem to be much affected on resistant plants
but it took the aphid three times as long to produce a sap ingestion pattern when maintained on the resistant lineT. monococcum no 44 (Tm44) as compared with aphids maintained on susceptible plants. As a result the total time spent in ingesting from sieve
elements was reduced by 72% on Tm44. Secondly, direct observations of freely-moving apterous adults were performed. Aphids
did not discriminate between resistant and susceptible wheat during the first 30 min of access to test leaves, but only 4
out of 25 aphids were still probing after eight hours on resistant Tm44.
The relevance of these results to possible location of the resistance factor(s) are discussed. Although detection of plant
resistance before sieve elements are reached can not be rigorously excluded, the factors involved inT. monococcum resistance toS. avenae undoubtedly occur within the phloem vessels. 相似文献
37.
L. H. Schmitt D. J. Kitchener R. A. How 《Evolution; international journal of organic evolution》1995,49(3):399-412
This study investigated allozyme and morphometric variability within the genus Cynopterus, with particular emphasis on C. nusatenggara, which is endemic to Wallacea, the area encompassing the Oriental-Australian biogeographic interface. The genetic distances between Cynopterus species are small by mammalian standards and suggest that this genus has undergone a recent series of speciation events. The genetic distance between populations of C. nusatenggara is strongly correlated with both the contemporary sea-crossing distance between islands and the estimated sea crossing at the time of the last Pleistocene glacial maximum, 18,000 b .p . This observation, together with low levels of population substructure within islands as shown by F-statistics, indicates that the sea is a primary and formidable barrier to gene exchange. The genetic distance and the great-circle geographical distance between the populations of C. nusatenggara are not correlated, although a principal-coordinates analysis of genetic distance reveals relationships between the populations that are similar to their geographical arrangement. A strong negative correlation exists between the level of heterozygosity within island populations of C. nusatenggara and the minimum sea-crossing distance to the nearest large source population. This is interpreted as reflecting an isolation effect of the sea, leading to reduced heterozygosity in populations that have larger sea barriers between them and the large source islands. Independently of this, heterozygosity is negatively associated with longitude, which in turn is associated with systematic changes in the environment such as a gradual decline in rainfall from west to east. The association between heterozygosity and longitude is interpreted as reflecting an association between genetic and environmental variance and supports the niche-width theory of genetic variance. Morphometric variability did not show any of the main effects demonstrated in the genetic data. Furthermore, there was no evidence that, at the level of individuals, genetic and morphometric variability were associated. 相似文献
38.
K. A. Koehler M. K. Jain D. A. Gabriel H. -Y. C. Chang O. P. Malhotra 《Journal of Protein Chemistry》1995,14(7):537-548
The interaction of bovine prothrombin with Ca2+ and Mg2+ ions was investigated by following H+ release as a function of metal ion concentration at pH 6 and pH 7.4 at high and low ionic strength. Prothrombin Ca2+ and Mg2+ binding is characterized by high- and low-affinity sites. M2+ binding at these sites is associated with intramolecular conformational changes and also with intermolecular self-association. The pH dependence of H+ release by M2+ is bell shaped and consistent with controlling pKa values of 4.8 and 6.5. At pH 6 and low ionic strength, both Ca2+ and Mg2+ titrations following H+ release clearly show independent low- and high-affinity binding sites. Laser light scattering reveals that at pH 7.4 and low ionic strength, and at pH 6.0 and high ionic strength, the prothrombin molecular weight is between 73 and 98 kD. At pH 7.4 and high ionic strength, prothrombin is monomeric in the absence of metal ions, but appears to dimerize in the presence of M2+. At pH 6.0 and low ionic strength prothrombin exists as a dimer in the absence of metal ions and is tetrameric in the presence of Ca2+ and remains dimeric in the presence of Mg2+. These results and those for metal ion-dependent H+ release indicate that H+ release occurs concomitantly with association processes involving prothrombin.Abbreviations GLA
-carboxyglutamic acid; fragment 1. amino terminal residues 1–156 of bovine prothrombin
- MES
2-(N-morpholino) ethanesulfonic acid
- MOPS
3-(N-morpholino) propanesulfonic acid
- PS/PC
phosphatidylserine/phosphatidylcholine vesicles
-
ionic strength 相似文献
39.
Seven strains of Rhizobium loti were tested for acid tolerance in yeast-extract mannitol (YEM) broth at pH values ranging from 4.0 to 8.0. The strains that grew at pH 4.0 showed the slowest generation time when grown at pH above 7.0 and also produced the most acid. The acid tolerance was related to the composition and structure of the membrane. pH influenced protein expression in acid-tolerant strains growing at pH 4.0 or 7.0. Acid tolerant strains showed one membrane protein of 49.5 kDa and three soluble proteins of 66.0, 58.0 and 44.0kDa; their expression increased when the cells grew at pH 4.0. It is suggested that acid tolerance in Rhizobium loti involves constitutive mechanisms, such as permeability of the outer membrane together with adaptive responses, including the state of bacterial growth and concomitant changes in protein expression. 相似文献
40.
S. W. Englander L. Mayne Y. Bai T. R. Sosnick 《Protein science : a publication of the Protein Society》1997,6(5):1101-1109
This discussion, prepared for the Protein Society's symposium honoring the 100th anniversary of Kaj Linderstrøm-Lang, shows how hydrogen exchange approaches initially conceived and implemented by Lang and his colleagues some 50 years ago are contributing to current progress in structural biology. Examples are chosen from the active protein folding field. Hydrogen exchange methods now make it possible to define the structure of protein folding intermediates in various contexts: as tenuous molten globule forms at equilibrium under destabilizing conditions, in kinetic intermediates that exist for less than one second, and as infinitesimally populated excited state forms under native conditions. More generally, similar methods now find broad application in studies of protein structure, energetics, and interactions. This article considers the rise of these capabilities from their inception at the Carlsberg Labs to their contemporary role as a significant tool of modern structural biology. 相似文献