全文获取类型
收费全文 | 245篇 |
免费 | 21篇 |
国内免费 | 1篇 |
出版年
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 4篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 3篇 |
2013年 | 10篇 |
2012年 | 19篇 |
2011年 | 9篇 |
2010年 | 12篇 |
2009年 | 4篇 |
2008年 | 7篇 |
2007年 | 9篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 5篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 12篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1989年 | 4篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 3篇 |
1966年 | 2篇 |
1948年 | 2篇 |
1928年 | 2篇 |
1918年 | 1篇 |
1915年 | 1篇 |
排序方式: 共有267条查询结果,搜索用时 31 毫秒
1.
The human cholesteryl ester (CE) and triglyceride (TG) exchange protein (denoted LTC or lipid transfer complex) was isolated in a single step from plasma using immunoaffinity batch extraction. Antibodies were raised against two preparations of conventionally purified LTC. LTC-I and LTC-II (purified 20,000-fold and 3500-fold, respectively) were used as immunogens. The antiLTC antibodies were isolated by anion-exchange chromatography and coupled to Affi-Gel 10. Chromatography of plasma on antiLTC Affi-Gel removed all of the CE and TG transfer activity. Moreover, LTC prepared from both antiLTC-I and antiLTC-II-Affi-Gel matrices were identical when analyzed by sodium dodecyl sulfate-polyacrylamide gel LTC electrophoresis. LTC exhibited two protein bands of Mr (apparent) 67,000 and 58,000 and a broad, faintly staining region at greater than 150,000. Analysis of LTC by immunoblotting indicated that both antiLTC-I and antiLTC-II antibodies recognized the same LTC proteins. Isoelectric focussing of LTC gave two pI values, 5.2 and 8.7. These data suggest that LTC is a complex of specific proteins and perhaps lipid. Specific CE and TG exchange activities of immunoaffinity-purified LTC were comparable, although the activities were low with respect to that of the antigen used to generate antiLTC-I. This is not due to contamination of LTC by albumin, lecithin:cholesterol acyltransferase, or apolipoproteins AI, AII, B, CIII, D, or E. 相似文献
2.
Rubisco activity can be measured using gas exchange (in vivo) or using in vitro methods. Commonly in vitro methods yield activities that are less than those obtained in vivo. Rubisco activity was measured both in vivo and in vitro using a spectrophotometric technique in mature Pinus taeda L. (loblolly pine) trees grown using free-air CO2 enrichment in elevated (56 Pa) and current (36 Pa) pCO2. In addition, for studies where both in vivo and in vitro values of Rubisco activity were reported net CO2 uptake rate (A) was modelled based on the in vivo and in vitro values of Rubisco activity reported in the literature. Both the modelling exercise and the experimental data showed that the in vitro values of Rubisco activity were insufficient to account for the observed values of A. A trichloroacetic acid (TCA) precipitation of the protein from samples taken in parallel with those used for activity analysis was co-electrophoresed with the extract used for determining in vitro Rubisco activity. There was significantly more Rubisco present in the TCA precipitated samples, suggesting that the underestimation of Rubisco activity in vitro was attributable to an insufficient extraction of Rubisco protein prior to activity analysis. Correction of in vitro values to account for the under-represented Rubisco yielded mechanistically valid values for Rubisco activity. However, despite the low absolute values for Rubisco activity determined in vitro, the trends reported with CO2 treatment concurred with, and were of equal magnitude to, those observed in Rubisco activity measured in vivo. 相似文献
3.
4.
Leaf lifespan as a determinant of leaf structure and function among 23 amazonian tree species 总被引:26,自引:0,他引:26
Summary The relationships between resource availability, plant succession, and species' life history traits are often considered key to understanding variation among species and communities. Leaf lifespan is one trait important in this regard. We observed that leaf lifespan varies 30-fold among 23 species from natural and disturbed communities within a 1-km radius in the northern Amazon basin, near San Carlos de Rio Negro, Venezuela. Moreover, leaf lifespan was highly correlated with a number of important leaf structural and functional characterisues. Stomatal conductance to water vapor (g) and both mass and area-based net photosynthesis decreased with increasing leaf lifespan (r2=0.74, 0.91 and 0.75, respectively). Specific leaf area (SLA) also decreased with increasing leaf lifespan (r2=0.78), while leaf toughness increased (r2=0.62). Correlations between leaf lifespan and leaf nitrogen and phosphorus concentrations were moderate on a weight basis and not significant on an area basis. On an absolute basis, changes in SLA, net photosynthesis and leaf chemistry were large as leaf lifespan varied from 1.5 to 12 months, but such changes were small as leaf lifespan increased from 1 to 5 years. Mass-based net photosynthesis (A/mass) was highly correlated with SLA (r2=0.90) and mass-based leaf nitrogen (N/mass) (r2=0.85), but area-based net photosynthesis (A/area) was not well correlated with any index of leaf structure or chemistry including N/area. Overall, these results indicate that species allocate resources towards a high photosynthetic assimilation rate for a brief time, or provide resistant physical structure that results in a lower rate of carbon assimilation over a longer time, but not both. 相似文献
5.
Very low and low density lipoprotein synthesis and secretion by the human hepatoma cell line Hep-G2: effects of free fatty acid 总被引:4,自引:0,他引:4
The liver is a major source of the plasma lipoproteins; however, direct studies of the regulation of lipoprotein synthesis and secretion by human liver are lacking. Dense monolayers of Hep-G2 cells incorporated radiolabeled precursors into protein ([35S]methionine), cholesterol ([3H]mevalonate and [14C]acetate), triacylglycerol, and phospholipid ([3H]glycerol), and secreted them as lipoproteins. In the absence of free fatty acid in the media, the principal lipoprotein secretory product that accumulated had a density maximum of 1.039 g/ml, similar to serum low density lipoprotein (LDL). ApoB-100 represented greater than 95% of the radiolabeled apoprotein of these particles, with only traces of apoproteins A and E present. Inclusion of 0.8 mM oleic acid in the media resulted in a 54% reduction in radiolabeled triacylglycerol in the LDL fraction and a 324% increase in triacylglycerol in the very low density lipoprotein (VLDL) fraction. Similar changes occurred in the secretion of newly synthesized apoB-100. The VLDL contained apoB-100 as well as apoE. In the absence of exogenous free fatty acid, the radiolabeled cholesterol was recovered in both the LDL and the high density lipoprotein (HDL) regions. Oleic acid caused a 50% decrease in HDL radiolabeled cholesterol and increases of radiolabeled cholesterol in VLDL and LDL. In general, less than 15% of the radiolabeled cholesterol was esterified, despite the presence of cholesteryl ester in the cell. Incubation with oleic acid did not cause an increase in the total amount of radiolabeled lipid or protein secreted. We conclude that human liver-derived cells can secrete distinct VLDL and LDL-like particles, and the relative amounts of these lipoproteins are determined, at least in part, by the availability of free fatty acid. 相似文献
6.
Ellsworth C. Alvord Jr. Sarka Hruby Russell E. Martenson Gladys E. Deibler Mona J. Law 《Journal of neurochemistry》1986,47(3):764-771
The specificities of two monoclonal IgM antibodies (18.25 and 21.14.2) evoked in mice with guinea pig myelin basic protein were examined and interpreted in terms of a specific folding of the protein's polypeptide chain. Studies with guinea pig and rabbit myelin basic protein fragments showed that a region encompassing the central Phe-Phe (87-88) sequence is obligatory, but not sufficient, for reactivity with antibody 18.25. Appreciable reactivity was observed for rabbit peptides 22-95 and 45-151, and lower, but significant, reactivity was shown by peptide 32-95. Only very weak reactivity was seen with peptide 44-95. No reactivity was observed with peptide 1-95 after its lysine residues were acetylated, acetamidinated, or guanidinated. These results have been interpreted in terms of a polypeptide chain folding that creates an epitope within sequence Val-Val-His-Phe-Phe-Lys-Asn-Ile-Val (84-92). The specific conformation of this epitope, which includes probably the Lys-89 and possibly the Asn-90 and Val-92 side chains, could be formed by the association of sequence 84-92 with either sequence Ile-Leu-Asp-Ser-Ile-Gly-Arg-Phe-Phe (37-45) or with sequence Val-Leu-Ser-Arg-Phe (108-112) to form beta-sheet structures essentially identical with those that appear to be present in the intact BP [Martenson R.E.J. Neurochem. 46, 1612-1622 (1986)]. The second monoclonal antibody, no. 21.14.2, reacts only with guinea pig myelin basic protein and fragments containing the species-restricted sequence Arg-Ala-Asp-Tyr-Lys-Ser-Lys (129-135).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
7.
8.
Darrell L. Ellsworth Rodney L. Honeycutt Nova J. Silvy John W. Bickham W. D. Klimstra 《Evolution; international journal of organic evolution》1994,48(1):122-136
Mitochondrial DNA (mtDNA) was used to characterize patterns of geographic variation among white-tailed deer (Odocoileus virginianus) populations in the southeastern United States. Fifteen restriction enzymes were employed to survey and map 99 restriction sites in 142 deer from 18 localities in five southeastern states. Phylogenetic analysis revealed three primary groups of haplotypes: (1) southern Florida and the Florida Keys, (2) the remainder of peninsular Florida northward to South Carolina, and (3) the Florida panhandle westward to Mississippi. Geographical heterogeneity in haplotype frequencies suggests that stochastic lineage sorting or isolation by distance are not important determinates of mtDNA differentiation among deer populations. The pattern of mtDNA variation in white-tailed deer is concordant spatially with those observed in unrelated taxa suggesting the common influence of historical biogeographic events. The data (1) support previous hypotheses that relate contemporary patterns of intraspecific phylogeography in northern Florida to the physiogeographic history of the region; and (2) suggest that genetic differentiation in southern Florida may be attributable to episodes of Pleistocene dispersal. Despite potentially high vagility and human intervention, ecological and demographic characteristics of deer have effectively preserved the historical pattern of intraspecific mtDNA differentiation. 相似文献
9.
BS Sabna Thankappan Bency Mahendran Ramasamy Muthusamy Gayathri Femil selta Daniel Raja Angayarkanni Jayaraman 《Probiotics and antimicrobial proteins》2021,13(4):993-1004
Probiotics and Antimicrobial Proteins - Gamma-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in the central nervous system and is produced by irreversible decarboxylation of... 相似文献
10.
Kristine Y. Crous Audrey G. Quentin Yan‐Shih Lin Belinda E. Medlyn David G. Williams Craig V. M. Barton David S. Ellsworth 《Global Change Biology》2013,19(12):3790-3807
Eucalyptus species are grown widely outside of their native ranges in plantations on all vegetated continents of the world. We predicted that such a plantation species would show high potential for acclimation of photosynthetic traits across a wide range of growth conditions, including elevated [CO2] and climate warming. To test this prediction, we planted temperate Eucalyptus globulus Labill. seedlings in climate‐controlled chambers in the field located >700 km closer to the equator than the nearest natural occurrence of this species. Trees were grown in a complete factorial combination of elevated CO2 concentration (eC; ambient [CO2] +240 ppm) and air warming treatments (eT; ambient +3 °C) for 15 months until they reached ca. 10 m height. There was little acclimation of photosynthetic capacity to eC and hence the CO2‐induced photosynthetic enhancement was large (ca. 50%) in this treatment during summer. The warming treatment significantly increased rates of both carboxylation capacity (Vcmax) and electron transport (Jmax) (measured at a common temperature of 25 °C) during winter, but decreased them significantly by 20–30% in summer. The photosynthetic CO2 compensation point in the absence of dark respiration (Γ*) was relatively less sensitive to temperature in this temperate eucalypt species than for warm‐season tobacco. The temperature optima for photosynthesis and Jmax significantly changed by about 6 °C between winter and summer, but without further adjustment from early to late summer. These results suggest that there is an upper limit for the photosynthetic capacity of E. globulus ssp. globulus outside its native range to acclimate to growth temperatures above 25 °C. Limitations to temperature acclimation of photosynthesis in summer may be one factor that defines climate zones where E. globulus plantation productivity can be sustained under anticipated global environmental change. 相似文献