全文获取类型
收费全文 | 722篇 |
免费 | 51篇 |
出版年
2021年 | 2篇 |
2019年 | 4篇 |
2018年 | 10篇 |
2017年 | 10篇 |
2016年 | 18篇 |
2015年 | 39篇 |
2014年 | 40篇 |
2013年 | 44篇 |
2012年 | 42篇 |
2011年 | 40篇 |
2010年 | 45篇 |
2009年 | 46篇 |
2008年 | 29篇 |
2007年 | 45篇 |
2006年 | 34篇 |
2005年 | 32篇 |
2004年 | 27篇 |
2003年 | 25篇 |
2002年 | 13篇 |
2001年 | 18篇 |
2000年 | 17篇 |
1999年 | 7篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 5篇 |
1995年 | 10篇 |
1994年 | 5篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 13篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 18篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有773条查询结果,搜索用时 15 毫秒
1.
Movement analysis of the 'volume suction' feeding type in Astarotilapia elegans suggests the existence of an inhibiting peripheral feedback control on the fast movements of the head parts, apparently triggered by the food items entering through the mouth aperture. As soon as the prey passes the mouth, rostral expansion of the buccopharyngeal cavity stops. On the basis of a mathematical model and physiological evidence, respectively, visual and chemical perception must probably be excluded as the initial stimulus of the feedback control. The simulation of the hydrodynamic characteristics of the suction flow at the level of the gape reveals sudden changes in the pressure and acceleration waves coupled to the moment of prey uptake. These fluctuations are premised to generate the triggering signal. The possibility of modulation entails re-evaluation of the neuro-motoric preprogamming concept. 相似文献
2.
J M Aerts J Heikoop S van Weely W E Donker-Koopman J A Barranger J M Tager A W Schram 《Experimental cell research》1988,177(2):391-398
We have characterized glucocerebrosidase in various cell types of peripheral blood of control subjects and in cultured human blastoid cells. The intracellular level of glucocerebrosidase in cultured blastoid cells (10-30 nmol substrate hydrolyzed/h.mg protein) resembles closely values observed for leukocyte cell types and various tissues and is significantly lower than that observed in cultured fibroblasts (150-500 nmol substrate hydrolyzed/h.mg protein). Glucocerebrosidase is extracted from leukocyte cell types and cultured blastoid cells almost exclusively in a monomeric, nonactivated form with enzymatic properties identical to those of the tissue enzyme. In contrast, extracts of platelets are rich in an aggregated, activated form of the enzyme. Glucocerebrosidase in blood cells and cultured blastoid cells is heterogeneous with respect to Mr and pI due to a heterogeneous oligosaccharide composition of the enzyme. The different forms seen represent intermediates in the biosynthesis and maturation of the enzyme. Blastoid cells should thus be an attractive model system for studying the natural history of glucocerebrosidase in a cell type related to those cells involved in the pathology of Gaucher disease. 相似文献
3.
J M Aerts W E Donker-Koopman M Koot G J Murray J A Barranger J M Tager A W Schram 《Biochimica et biophysica acta》1986,863(1):63-70
Human urine contains a soluble form of glucocerebrosidase, an enzyme associated with the lysosomal membrane in cells and tissues. Urinary glucocerebrosidase is identical to the enzyme extracted from tissues with respect to the following parameters: Km for natural and artificial substrates, inhibition by conduritol B-epoxide, and stimulation by taurocholate. The enzyme is greater than 90% precipitable by polyclonal anti-(placental glucocerebrosidase) antiserum. Upon isoelectric focussing of urinary glucocerebrosidase multiple peaks of activity were observed. Partial deglycosylation (removal of sialic acid, N-acetylglucosamine and galactose) of the urinary enzyme increased the isoelectric point to a value identical to that of the main form found after partial deglycosylation of the placental enzyme. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate followed by immunoblotting, the immunopurified urinary enzyme shows the same molecular mass forms as the enzyme immunopurified from brain and kidney. In placenta the apparent molecular mass is somewhat higher but upon removal of sialic acid, N-acetylglucosamine and galactose the urinary and the placental enzyme show identical molecular masses of 57 kDa. We conclude that the enzymes extracted from urine and tissue are identical and that differences in apparent molecular mass and isoelectric point are probably due to heterogeneity in the oligosaccharide moieties of the molecules. 相似文献
4.
5.
6.
7.
The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
相似文献
8.
We investigated the effect of increased N-supply on productivity and potential litter decay rates of Carex species, which are the dominant vascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-supply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) under conditions of P-limited plant growth, increased N-supply will not affect productivity but it will lead to lower C:N ratios in plant litter and thereby to a higher potential decay rate of that litter. These hypotheses were tested by fertilization experiments (addition of 10 g N m-2 year-1) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry and potential leaf litter decay. In a P-limited peatland, dominated by Carex lasiocarpa, there was no significant increase of net C-fixation by plant biomass upon enhanced N-supply, although N-uptake had increased significantly compared with the unfertilized control. Due to the N-fertilization the C:N ratio in the plant biomass decreased significantly. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The potential decay rate of that litter, measured as CO2-evolution from the litter under aerobic conditions, was significantly increase upon enhanced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply, whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in response to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. This implies that atmospheric N-deposition may affect the CO2-sink function of peatlands, but the effect is dependent on the nature of nutrient limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under conditions of P-limited plant growth, however, the net C-accumulation will decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is increased. As plant growth in most terrestrial ecosystems is N-limited, increased N-supply will in most peatlands lead to an increase of net C-accumulation. 相似文献
9.
10.