全文获取类型
收费全文 | 1055篇 |
免费 | 41篇 |
国内免费 | 39篇 |
专业分类
1135篇 |
出版年
2023年 | 14篇 |
2022年 | 14篇 |
2021年 | 17篇 |
2020年 | 30篇 |
2019年 | 25篇 |
2018年 | 19篇 |
2017年 | 27篇 |
2016年 | 28篇 |
2015年 | 16篇 |
2014年 | 41篇 |
2013年 | 46篇 |
2012年 | 36篇 |
2011年 | 48篇 |
2010年 | 31篇 |
2009年 | 52篇 |
2008年 | 54篇 |
2007年 | 48篇 |
2006年 | 58篇 |
2005年 | 65篇 |
2004年 | 39篇 |
2003年 | 29篇 |
2002年 | 25篇 |
2001年 | 31篇 |
2000年 | 27篇 |
1999年 | 18篇 |
1998年 | 16篇 |
1997年 | 12篇 |
1996年 | 25篇 |
1995年 | 24篇 |
1994年 | 16篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1978年 | 5篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1970年 | 3篇 |
1956年 | 1篇 |
1953年 | 1篇 |
排序方式: 共有1135条查询结果,搜索用时 11 毫秒
71.
摘要 目的:探讨与分析慢性阻塞性肺疾病(COPD)患者运动负荷气道反应性与T细胞亚群的关系。方法:2020年1月到2022年4月选择在本院诊治的慢阻肺患者88例作为慢阻肺组,同期选择在本院进行健康体检者88例作为健康组,检测两组T细胞亚群含量,判定两组的运动负荷气道反应性情况并进行相关性分析。结果:慢阻肺组的CD8+T淋巴细胞比例明显高于健康组,CD3+T淋巴细胞、CD4+T淋巴细胞比例明显低于健康组(P<0.05)。慢阻肺组的运动负荷气道反应性发生率为20.9 %,明显高于健康组的1.2 %(P<0.05)。在慢阻肺中,Spearsman分析显示运动负荷气道反应性发生率与CD3+T淋巴细胞、CD4+T淋巴细胞、CD8+T淋巴细胞比例存在相关性(P<0.05)。logistic回归分析显示CD3+T淋巴细胞、CD4+T淋巴细胞、CD8+T淋巴细胞比例都为影响运动负荷气道反应性发生的重要危险因素(P<0.05)。结论:慢阻肺患者多伴随有T细胞亚群异常,也多伴随有运动负荷气道反应性,运动负荷气道反应性与T细胞亚群存在相关性,也表明T细胞亚群紊乱是导致运动负荷气道反应性发生的重要因素。 相似文献
72.
A+T content, phylogenetic relationships, codon usage, evolutionary rates, and ratio of synonymous versus non-synonymous substitutions
have been studied in partial sequences of the atpD and aroQ/pheA genes of primary (Buchnera) and secondary symbionts of aphids and a set of selected non-symbiotic bacteria, belonging to the five subdivisions of the
Proteobacteria. Compared to the homologous genes of the last group, both genes belonging to Buchnera behave in a similar way, showing a higher A+T content, forming a monophyletic group, a loss in codon bias, especially in
third base position, an evolutionary acceleration and an increase in the number of non-synonymous substitutions, confirming
previous results reported elsewhere for other genes. When available, these properties have been partly observed with the secondary
symbionts, but with values that are intermediate between Buchnera and free living Proteobacteria. They show high A+T content, but not as high as Buchnera, a non-solved phylogenetic position between Buchnera, and the other γ-Proteobacteria, a loss in codon bias, again not as high as in Buchnera and a significant evolutionary acceleration in the case of the three atpD genes, but not when considering aroQ/pheA genes. These results give support to the hypothesis that they are symbionts at different stages of the symbiotic accommodation
to the host. 相似文献
73.
ABSTRACT An improved method has been developed for the induction of selfing conjugation in Paramecium. Methyl cellulose induces selfing conjugation simply and efficiently in all species examined. Induction of conjugation by methyl cellulose was characterized in P. caudatum , where it occurred only in sexually mature, mating reactive cells. Conjugation produced sexual offspring and the time course of nuclear processes was substantially the same as that in natural conjugation between cells of complementary mating types. The method is useful for genetic studies in a wide variety of Paramecium species including P. caudatum, P. tetraurelia, P. multimicronucleatum and P. bursaria. 相似文献
74.
M. Kaliva C. Gabriel C.P. Raptopoulou J.-P. Tuchagues 《Inorganica chimica acta》2006,359(14):4535-4548
In a pH-specific fashion, V2O5, citric acid and H2O2 reacted at pH 5.5-6.0 and afforded a red crystalline product at 4 °C. Elemental analysis pointed to the molecular formulation . Complex 1 was further characterized by UV/Vis, FT-IR, NMR, cyclic voltammetry, and X-ray crystallography. The X-ray structure of 1 reveals two dinuclear vanadium-peroxo-citrate subunits, A and B, linked through a hydrogen bond. In both A and B, the citrate ligands have different protonation states, ultimately affording a pentagonal bipyramidal geometry around each V(V) ion. The peroxide ligands bind V(V) in a side-on fashion. pH-Dependent, non-thermal and thermal transformations of 1 unravel its connection with key participants in the vanadium-peroxo-citrate ternary system and project its association with other non-peroxo binary complexes of variable vanadium oxidation state, geometry, citrate binding mode and state of protonation. Overall, the surprising twist in the aqueous synthetic chemistry of the investigated ternary system: (a) projects a new pH structural variant (species A) as a component of the speciation; (b) provides an in-depth look at that speciation under specific pH conditions; and (c) offers significant insight into the aqueous structural speciation of vanadium with peroxide and citrate, and its potential relevance to biological processes. 相似文献
75.
76.
《Chronobiology international》2013,30(1):107-129
The role of endogenous circadian rhythmicity in autonomic cardiac reactivity to different stressors was investigated. A constant routine protocol was used with repeated exposure to a dual task and a cold pressor test. The 29 subjects were randomly divided into two groups in order to manipulate prior wakefulness. Group 1 started at 09:00 h immediately after a monitored sleep period, whereas group 2 started 12 h later. Measures of interbeat intervals (IBI), respiratory sinus arrythmia (RSA, a measure of parasympathetic activity), pre-ejection period (PEP, a measure of sympathetic activity), as well as core body temperature (CBT) were recorded continuously. Multilevel regression analyses (across-subjects) revealed significant (mainly 24 h) sinusoidal circadian variation in the response to both stressors for IBI and RSA, but not for PEP. Individual 24 + 12 h cosine fits demonstrated a relatively large interindividual variation of the phases of the IBI and RSA rhythms, as compared to that of the CBT rhythm. Sinusoidal by group interactions were found for IBI and PEP, but not for RSA. These findings were interpreted as an indication for endogenous circadian and exogenous parasympathetic (vagal) modulation of cardiac reactivity, while sympathetic reactivity is relatively unaffected by the endogenous circadian drive and mainly influenced by exogenous factors. 相似文献
77.
Yasuo Aizono Masaru Funatsu Yukio Fujiki Masayoshi Watanabe 《Bioscience, biotechnology, and biochemistry》2013,77(2):317-324
A species of rice bran lipase (lipase II) was purified by ammonium sulfate precipitation, followed by successive chromatographies on DEAE-cellulose, Sephadex G–75 and CH-Sephadex C–50. Both polyacrylamide disc electrophoresis and ultracentrifugation demonstrated that the enzyme protein is homogeneous. The isoelectric point of the enzyme was 9.10 by ampholine electrophoresis. The sedimentation coefficient of the enzyme was evaluated to be 2.60 S, and the molecular weight to be 33,300 according to Archbald’s method. The enzyme showed the optimum pH between 7.5 and 8.0, and the optimum temperature at about 27°C. It was stable over the pH range from 5 to 9.5 and below 30°C. In substrate specificity, the enzyme exhibited a high specificity toward triglycerides having short-carbon chain fatty acids, although it was capable of hydrolyzing the ester bonds in the rice and olive oil. 相似文献
78.
Cranial integration in Homo: singular warps analysis of the midsagittal plane in ontogeny and evolution 总被引:1,自引:0,他引:1
Bookstein FL Gunz P Mitteroecker P Prossinger H Schaefer K Seidler H 《Journal of human evolution》2003,44(2):167-187
This study addresses some enduring issues of ontogenetic and evolutionary integration in the form of the hominid cranium. Our sample consists of 38 crania: 20 modern adult Homo sapiens, 14 sub-adult H. sapiens, and four archaic Homo. All specimens were CT-scanned except for two infant H. sapiens, who were imaged by MR instead. For each specimen 84 landmarks and semi-landmarks were located on the midsagittal plane and converted to Procrustes shape coordinates. Integration was quantified by the method of singular warps, a new geometric-statistical approach to visualizing correlations among regions. The two classic patterns of integration, evolutionary and ontogenetic, were jointly explored by comparing analyses of overlapping subsamples that span ranges of different hypothetical factors. Evolutionary integration is expressed in the subsample of 24 adult Homo, and ontogenetic integration in the subsample of 34 H. sapiens. In this data set, vault, cranial base, and face show striking and localized patterns of covariation over ontogeny, similar but not identical to the patterns seen over evolution. The principal differences between ontogeny and phylogeny pertain to the cranial base. There is also a component of cranial length to height ratio not reducible to either process. Our methodology allows a separation of these independent processes (and their impact on cranial shape) that conventional methods have not found. 相似文献
79.
Summary The effects of seed size on growth, biomass allocation and competitive ability of Rumex acetosella plants grown either individually or in competition were studied in two populations (6 months and 15 years old respectively) sampled from a postcultivation successional gradient. For plants grown individually there were highly significant effects of seed weight on growth after 43 days, with a higher relative growth rate (RGR) observed for plants raised from heavier seeds. However at the end of the experiment, seedlings developed from lighter seeds had a RGR 2 times greater than those from heavier seeds. Final biomass of the two types was not significantly different after 73 days of growth. When plants were grown individually, there were only slight differences between populations, but when grown in monocultures of 4 plants per pot, plants from the old population had higher root and leaf biomass per pot whereas those from the young population had a higher reproductive effort per pot. This suggests that a trade-off between allocation to sexual and vegetative reproduction occurs over successional time. In mixtures of light and heavy seeds, plants from light seeds were shorter, had fewer leaves and lower biomass than plants from heavy seeds, which were also taller and produced more dry matter than plants grown from heavy seeds in monoculture. The significant effects of seed weight and population on biomass parameters persisted unit the end of the experiment. Seedlings from heavy seeds were strong competitors: those from the young population grew better in the presence of neighbors than in monoculture and those from the late successional population suppressed the more the growth of their partners. Seedlings from light seeds were subordinate competitors. These results suggest that seedlings from seeds of different sizes benefit from contrasting ecological conditions and that selection acts on reproductive output along successional gradients. 相似文献
80.
Probability is closely related to biological organization and adaptation to the environment. Living systems need to maintain their organizational order by producing specific internal events non-randomly, and must cope with the uncertain environments. These processes involve increases in the probability of favorable events for these systems by reducing the degree of uncertainty of events. Systems with this ability will survive and reproduce more than those that have less of this ability. Probabilistic phenomena have been deeply explored using the mathematical theory of probability since Kolmogorov's axiomatization provided mathematical consistency for the theory. However, the interpretation of the concept of probability remains both unresolved and controversial, which creates problems when the mathematical theory is applied to problems in real systems. In this article, recent advances in the study of the foundations of probability from a biological viewpoint are reviewed, and a new perspective is discussed toward a comprehensive theory of probability for understanding the organization and adaptation of living systems. 相似文献