全文获取类型
收费全文 | 6103篇 |
免费 | 718篇 |
国内免费 | 3篇 |
出版年
2021年 | 76篇 |
2020年 | 53篇 |
2019年 | 49篇 |
2018年 | 59篇 |
2016年 | 90篇 |
2015年 | 183篇 |
2014年 | 181篇 |
2013年 | 241篇 |
2012年 | 295篇 |
2011年 | 270篇 |
2010年 | 151篇 |
2009年 | 129篇 |
2008年 | 223篇 |
2007年 | 227篇 |
2006年 | 195篇 |
2005年 | 191篇 |
2004年 | 194篇 |
2003年 | 164篇 |
2002年 | 190篇 |
2001年 | 157篇 |
2000年 | 170篇 |
1999年 | 154篇 |
1998年 | 80篇 |
1997年 | 86篇 |
1996年 | 58篇 |
1995年 | 72篇 |
1994年 | 72篇 |
1993年 | 79篇 |
1992年 | 108篇 |
1991年 | 134篇 |
1990年 | 128篇 |
1989年 | 141篇 |
1988年 | 127篇 |
1987年 | 128篇 |
1986年 | 113篇 |
1985年 | 115篇 |
1984年 | 102篇 |
1983年 | 87篇 |
1982年 | 116篇 |
1981年 | 119篇 |
1980年 | 98篇 |
1979年 | 106篇 |
1978年 | 69篇 |
1977年 | 62篇 |
1976年 | 62篇 |
1975年 | 61篇 |
1974年 | 57篇 |
1973年 | 57篇 |
1972年 | 71篇 |
1970年 | 52篇 |
排序方式: 共有6824条查询结果,搜索用时 203 毫秒
991.
Polymerization mechanism of polypeptide chain aggregation 总被引:3,自引:0,他引:3
The misfolding of polypeptide chains and aggregation into the insoluble inclusion body state is a serious problem for biotechnology and biomedical research. Developing a rational strategy to control aggregation requires understanding the mechanism of polymerization. We investigated the in vitro aggregation of P22 tailspike polypeptide chains by classical light scattering, nondenaturing gel electrophoresis, two-dimensional polyacrylamide gel electrophoresis (PAGE), and computer simulations. The aggregation of polypeptide chains during refolding occurred by multimeric polymerization, in which two multimers of any size could associate to form a larger aggregate and did not require a sequential addition of monomeric subunits. The cluster-cluster polymerization mechanism of aggregation is an important determinant in the kinetic competition between productive folding and inclusion body formation. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 333-343, 1997. 相似文献
992.
Chromosomal Localization of Three Human Dual Specificity Phosphatase Genes (DUSP4, DUSP6, and DUSP7)
Anna Smith Cathy Price Martin Cullen Marco Muda Andrea King Bradford Ozanne Steve Arkinstall Alan Ashworth 《Genomics》1997,42(3):524
Mitogen-activated protein (MAP) kinase phosphatases constitute a growing family of dual specificity phosphatases thought to play a role in the dephosphorylation and inactivation of MAP kinases and are therefore likely to be important in the regulation of diverse cellular processes such as proliferation, differentiation, and apoptosis. For this reason it has been suggested that MAP kinase phosphatases may be tumor suppressors. We have determined the chromosomal locations of three human dual specificity phosphatase genes by fluorescencein situhybridization and radiation hybrid mapping. The genes were localized to three different chromosomes,MKP2(DUSP4) to 8p11–p12,MKP3(DUSP6) to 12q22–q23, andMKPX(DUSP7) to 3p21. This will allow the potential roles of these genes in disease processes to be evaluated. 相似文献
993.
Aquaporins in complex tissues. I.Developmental patterns in respiratory and glandular tissues of rat 总被引:14,自引:0,他引:14
King Landon S.; Nielsen Soren; Agre Peter 《American journal of physiology. Cell physiology》1997,273(5):C1541
Developmentalexpression of aquaporin water transport proteins is not well understoodin respiratory tract or secretory glands; here we define aquaporinprotein ontogeny in rat. Expression of aquaporin-3 (AQP3), AQP4, andAQP5 proteins occurs within 2 wk after birth, whereas AQP1 firstappears before birth. In most tissues, aquaporin protein expressionincreases progressively, although transient high-level expression isnoted in distal lung (AQP4 at postnatal day+2) and trachea (AQP5 at postnatalday +21 and AQP3 at postnatal day+42). In mature animals, AQP5 is abundant in distallung and salivary glands, AQP3 and AQP4 are present in trachea, andAQP1 is present in all of these tissues except salivary glands.Surprisingly, all four aquaporin proteins are highly abundant innasopharynx. Unlike AQP1, corticosteroids did not induce expression ofAQP3, AQP4, or AQP5 in lung. Our results seemingly implicate aquaporinsin proximal airway humidification, glandular secretion, and perinatalclearance of fluid from distal airways. However, the studies underscorea need for detailed immunohistochemical characterizations anddefinitive functional studies. 相似文献
994.
S. Brul A. King J.M. van der Vaart J. Chapman F. Klis C.T. Verrips 《Antonie van Leeuwenhoek》1997,72(3):229-237
In yeast, glucanase extractable cell wall proteins are anchored to the plasma membrane at an intermediate stage in their biogenesis via a glycosylphosphatidylinositol (GPI) moiety before they become anchored to the wall glucan via a 1,6-glucan linkage. The mechanism of the membrane processing step of cell wall proteins is not known. Here, we report that Ascomycete filamentous fungi involved in food spoilage such as Aspergillus, Paecilomyces and Penicillium, also contain GPI membrane-anchored proteins some of which are processed by an endogenous phospholipase C activity. Furthermore, similar to the situation in yeast, their cell walls contain mannoproteins which are linked to the glucan backbone through a 1,6-glucan linkage. Interestingly, one mould which contains a significant amount of non covalently linked 1,6-glucosylated cell wall proteins, is much more sensitive towards 1,3-glucanases and membrane perturbing peptides than the others. 相似文献
995.
Morphology and tissue quality of seedling root systems of Pinus taeda and Pinus ponderosa as affected by varying CO2, temperature,and nitrogen 总被引:2,自引:0,他引:2
Rising atmospheric carbon dioxide, nitrogen deposition and warmer temperatures may alter the quantity and quality of plant-derived organic matter available to soil biota, potentially altering rates of belowground herbivory and decomposition. Our objective was to simulate future growth conditions for an early successional (loblolly) and late successional (ponderosa) species of pine to determine if the physical and chemical properties of the root systems would change. Seedlings were grown for 160 days in greenhouses at the Duke University Phytotron at 35 or 70 Pa CO2 partial pressure, ambient or ambient + 5 °C temperature, and 1 or 5 mMNH4O3. Roots from harvested seedlings were analyzed for changes in surface area, specific root length, mass, total nonstructural carbohydrates (TNC), and concentrations of macro-nutrients. Surface area increased in both species under elevated CO2, due primarily to increases in root length, and this response was greatest (+138%) in loblolly pine at high temperature. Specific root length decreased in loblolly pine at elevated CO2 but increases in mass more than compensated for this, resulting in net increases in total length. TNC was unaffected and nutrient concentrations decreased only slightly at elevated CO2, possibly from anatomical changes to the root tissues. We conclude that future growth conditions will enhance soil exploration by some species of pine, but root carbohydrate levels and nutrient concentrations will not be greatly affected, leaving rates of root herbivory and decomposition unaltered. 相似文献
996.
A metabolic pathway, known as the mannitol cycle in fungi, has been identified as a new entity in the eulittoral mangrove red algaCaloglossa leprieurii (Montagne) J. Agardh. Three specific enzymes, mannitol-1-phosphate dehydrogenase (Mt1PDH; EC 1.1.1.17), mannitol-1-phosphatase (MtlPase; EC 3.1.3.22), mannitol dehydrogenase (MtDH; EC 1.1.1.67) and one nonspecific hexokinase (HK; EC 2.7.1.1) were determined and biochemically characterized in cell-free extracts. Mannitol-1-phosphate dehydrogenase showed activity maxima at pH 7.0 [fructose-6-phosphate (F6P) reduction] and pH 8.5 [oxidation of mannitol-1-phosphate (Mt1P)], and a very high specificity for both carbohydrate substrates. TheK
m values were 1.4 mM for F6P, 0.09 mM for MOP, 0.020 mM for NADH and 0.023 mM for NAD+. For the dephosphorylation of MOP, MtlPase exhibited a pH optimum at 7.2, aK
m value of 1.2 mM and a high requirement of Mg2+ for activation. Mannitol dehydrogenase had activity maxima at pH 7.0 (fructose reduction) and pH 9.8 (mannitol oxidation), and was less substrate-specific than Mt1PDH and MtlPase, i.e. it also catalyzed reactions in the oxidative direction with arabitol (64.9%), sorbitol (31%) and xylitol (24.8%). This enzyme showedK
m values of 39 mM for fructose, 7.9 mM for mannitol, 0.14 mM for NADH and 0.075 mM for NAD+. For the non-specific HK, only theK
m values for fructose (0.19 mM) and glucose (7.5 mM) were determined. The activities of the anabolic enzymes Mt1PDH and MtlPase were always at least two orders of magnitude higher than those of the degradative enzymes, indicating a net carbon flow towards a high intracellular mannitol pool. The function of mannitol metabolism inC. leprieurii as a biochemical adaptation to the environmental extremes in the mangrove habitat is discussed.Abbreviations F6P
fructose-6-phosphate
- HK
hexokinase
- Mt1P
mannitol-1-phosphate
- Mt1PDH
mannitol-1-phosphate dehydrogenase
- Mt1Pase
mannitol-1-phosphatase
- MtDH
mannitol dehydrogenase 相似文献
997.
Returns to service after mating and removal of sows for reproductive reasons from commercial swine farms 总被引:2,自引:0,他引:2
We studied the records of 30 herds with an average inventory of 11,705 sows, 25,719 farrowings and 25,040 daily feed intake logs. Production events were recorded by producers using the PigCHAMP production information system. Of 21,505 matings, 7.2% of sows subsequently returned to estrus after service. The proportionate rates of intervals from service to the subsequent post service event were 0 to 17 d, 2.1%; 18 to 25 d, 27.9%; 26 to 37 d, 13.8%; 38 to 46 d, 15.8%; 47 to 108 d, 30.4%; and >108 d, 10.0%. Sows returned to service after mating were categorized into groups that either regularly or irregularly returned to service after mating. Of a total inventory of 19,076 sows, 10.0% were removed following weaning for reproductive reasons. The reasons for removal included those of anestrus (25.2%), failure to conceive (37.0%), failure to farrow (15.0%), not pregnant (1.4%), negative pregnancy check (14.0%), and abortion (7.4%). The last 5 types of post weaning reproductive failure were grouped into the category of did not farrow. Categorical additive models and comparisons using contrasts were used to analyze the influence of risk factors on reproductive failure. Parity 1 sows had a higher proportion (P < 0.01) of returns to service and a greater proportion of sows remaining anestrous post weaning relative to Parity 3 sows. The proportion of sows that did not farrow was higher (P < 0.01) in Parities 9 and 10 than in Parity 3. More sows were removed for anestrus during the spring (P < 0.01) and summer (P = 0.06) than during the winter. All categories of lactation length had similar rates of reproductive failure except for the lactation length 1 to 7 d, which had a higher (P < 0.05) proportion of reproductive failure. Lower lactational feed intake was associated with an increased risk of occurrence of each reproductive failure category. The odds ratios of lactation feed intake in logistic regression analyses were 0.84, 0.89, 0.82 and 0.88 for regularly and irregularly returned to service, anestrus, and did not farrow groups, respectively. This means, for example, that a sow was 0.88 times less likely to have an occurrence of not farrowing for each 1 kg increase in average daily feed intake during lactation. Our results indicate that lower and higher parities, spring and summer seasons, a lactation length of less than 8 d and lower feed intake during lactation affect the occurrence of return to service after mating and of herd removal for reproductive reasons. 相似文献
998.
The objective of this contribution is the design of optimal feeding strategies for fed-batch bioprocesses, where complex dynamic models with input and state constraints are present. For the solution of this dynamic optimization problem a transformation to a finite dimensional optimization problem is made using piecewise linear control profiles. The optimization of these profiles is performed by a sequential approach, that includes an ODE solver for the solution of the model ODE's. Further an adaptive mesh selection algorithm was investigated for an appropriate discretization of the control profiles. The implementation of the resulting optimal feeding profiles is shown for a process example, namely the production of nikkomycin by Streptomyces tendae. This implementation uses a hierarchical process control framework, that consists of components for process monitoring, state estimation, and trajectory control. 相似文献
999.
1000.
A D Venosa M T Suidan D King B A Wrenn 《Journal of industrial microbiology & biotechnology》1997,18(2-3):131-139
Much of the variability inherent in crude oil bioremediation field studies can be eliminated by normalizing analyte concentrations
to the concentration of a nonbiodegradable biomarker such as hopane. This was demonstrated with data from a field study in
which crude oil was intentionally released onto experimental plots on the Delaware shoreline. Five independent replicates
of three treatments were examined: no nutrient addition, addition of inorganic mineral nutrients alone, and nutrient addition
plus indigenous oil-degrading microorganisms from the site. Samples collected biweekly were analyzed for the Most Probable
Numbers (MPNs) of alkane and aromatic degraders and oil component analysis by GC/MS. The data were normalized to either
the mass of sand that was extracted or to the concentration of hopane that was measured. Hopane normalization enabled detection
of significant treatment differences in hydrocarbon biodegradation that were not detected when the data were normalized to
sand mass. First-order loss rates for the hopane-normalized data were lower than those for the sand-normalized data because
hopane normalization accounts only for loss due to biodegradation whereas sand normalization includes all loss mechanisms.
Plots amended with nutrients alone and nutrients plus the inoculum showed enhanced removal of hydrocarbons compared to unamended
control plots. However, no differences were detected between the nutrient-amended plots and the nutrient/inoculum-amended
plots.
Received 06 November 1995/ Accepted in revised form 26 June 1996 相似文献