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101.
A hypothesis describing the mechanism of photoactive protochlorophyllide (P) photoreduction in vivo, relating mainly to the molecular nature of the intermediates, is proposed. The hypothesis is compatible with currently published
experimental data. After illumination of etiolated barley leaves at 143 to 153 K, the absorption of P remains essentially
unchanged, but a new absorption band at 690 nm is observed. Appearance of this new intermediate enables to distinguish between
light and dark stages of the photoconversion reaction. When returned to the higher temperature in the dark, the treated leaves
begin accumulating chlorophyllide (Chlide), concomitant with the disappearance of the 690-nm band. The decay time of the excited
P (P*) is estimated at 300 ps, which approximates the time constant of photoinduced electron transfer (ET). It is suggested that
the charge-transfer complex (CTC) in its ground state (GS) (ground state of CTC formed by the partial (δ) electron transfer),
i.e. (Pδ−•••H–Dδ+), between P and NADPH – the electron and proton donor (H–D) – accumulates in the following sequence: P* + H–D → (P*•••H–D)→[(P*•••H–D)←(P−•••H–D+)] → 1(P−•••H–D+)] → 3(P−•••H–D+) → (Pδ−•••H–D δ+), where an equilibrium state (ES) – [(P*•••H–D)←(P−•••H–D+)] – with a lifetime of about 1 to 2 ns, exists between the local excited (LE) and ET states. The existence of a triplet ET
state – 3(P−•••H–D+) – is proposed because the time interval between recording of the ES and appearance of the CTC GS (35–250 ns) does not fit
the lifetime of the singlet excited complex (exciplex). It is feasible that apart from NADPH, other intermediate proton carriers
are contemporaneously involved in the dark reaction (Pδ−•••H–Dδ+) → Chlide, because proton binding to the C7–C8 bond in vivo takes place in the trans-configuration. The hydride ion may approach the C7–C8 bond from one side by heterolytic fission and an additional proton, donated by the protein group, may be simultaneously added
to this bond from the opposite side of the porphyrin nucleus surface.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
102.
103.
十种濒危植物的种群生态学特征及致危因素分析 总被引:51,自引:11,他引:51
从保护生态学的观点出发,对10个濒危植物的地理分布、生境条件、种群数量动态、空间分布格局、种间关系、种群的生命表、生殖力表、有性生殖、无性繁殖等种群生态学特征进行了综合分析。论述了濒危植物以光合、蒸腾、呼吸为主的生理生态学规律;预测种群数量动态的Leslie矩阵、刻画种群空间分布格局的模型、刻画种群生长和数量增长的Logistic方程和多元回归模型所反映出的生态学特征。分析了濒危植物种群动态、发展趋势以及在内外因素作用下的生态学特征和过程。针对濒危植物保护研究存在的问题,从生态学角度对我国未来濒危植物保护研究应注意的问题提出了建议。 相似文献
104.
小型核酶的结构和催化机理 总被引:4,自引:1,他引:4
自然界存在的小型核酶主要有锤头型核酶、发夹型核酶、肝炎δ病毒(HDV)核酶和VS核酶.锤头型核酶由3个短螺旋和1个广义保守的连接序列组成;发夹型核酶的催化中心由两个肩并肩挨着的区域构成;HDV核酶折叠成包含五个螺旋臂(P1~P4)的双结结构;VS核酶由五个螺旋结构组成,这些螺旋结构通过两个连接域连接起来.小型核酶的催化机理与其分子结构密切相关.金属离子或特定碱基都可作为催化反应的关键成分.锤头型核酶的催化必须有金属离子(尤其是二价金属离子)参与,而发夹型核酶则完全不需要金属离子.基因组HDV核酶进行催化时要有金属离子和特定碱基互相配合. 相似文献
105.
Hans‐Peter Piepho Hanspeter Thni Hans‐Martin Müller 《Biometrical journal. Biometrische Zeitschrift》2002,44(6):657-670
Bivariate samples may be subject to censoring of both random variables. For example, for two toxins measured in batches of wheat grain, there may be specific detection limits. Alternatively, censoring may be incomplete over a certain domain, with the probability of detection depending on the toxin level. In either case, data are not missing at random, and the missing data pattern bears some information on the parameters of the underlying model (informative missingness), which can be exploited for a fully efficient analysis. Estimation (after suitable data transformation) of the correlation in such samples is the subject of the present paper. We consider several estimators. The first is based on the tetrachoric correlation. It is simple to compute, but does not exploit the full information. The other two estimators exploit all information and use full maximum likelihood, but involve heavier computations. The one assumes fixed detection limits, while the other involves a logistic model for the probability of detection. For a real data set, a logistic model for the probability of detection fitted markedly better than a model with fixed detection limits, suggesting that censoring is not complete. 相似文献
106.
E. van Hunnik H. van den Ende K. R. Timmermans P. Laan J. W. de Leeuw 《Plant biology (Stuttgart, Germany)》2000,2(6):624-627
Abstract: The ability of the green alga Tetraedron minimum to acquire inorganic carbon from its environment was investigated and compared with that of Chlamydomonas monoica. T. minimum showed a higher affinity for bicarbonate ions than C. monoica, regardless of whether it was grown at high or low CO2 concentrations. Furthermore, T. minimum was distinguished by the fact that it maintained a large intracellular pool of inorganic carbon. These features may explain why this alga is able to proliferate in alkaline conditions. 相似文献
107.
Chloroplasts of peridinin-containing dinoflagellates have recently been shown to contain Form II Rubisco, which consists of large subunits only and is coded by nuclear genes. We have used immunoelectron microscopy to determine the distribution of Form II and Form I Rubisco in dinoflagellates. In sections of Amphidinium carterae Hulburt, the pyrenoid was intensely labeled and the rest of the chloroplast moderately labeled by antisera to Form II Rubisco from the purple non-sulfur bacterium Rhodospirillum rubrum and the symbiotic dinoflagellate Symbiodinium sp. No labeling was observed when sections were exposed to antiserum against Form I Rubisco of the haptophyte alga Isochrysis galbana. In contrast, cell sections of the dinoflagellate Peridinium foliaceum (Stein) Biecheler, whose chloroplasts belong to a diatom endosymbiont, showed no labeling with the two antisera against Form II Rubisco, but heavy pyrenoid labeling was present after treatment with antiserum against Form I Rubisco of I. galbana. The same immunolabeling results were obtained with the free-living diatom Phaeodactylum tricornutum Bohlin. Volumetric analysis of the distribution of Form II Rubisco in the chloroplast of A. carterae showed that, in cells grown under moderate photon irradiance, 72.9% of the plastid's Rubisco was localized in the pyrenoid, whereas in cells grown under low irradiance only 37.0% of the Rubisco was found in the pyrenoid. This light-induced concentration of Rubisco in the pyrenoid suggests that a CO2 –concentrating mechanism may elevate CO2 within the pyrenoid, favoring the efficient fixation of CO2 by pyrenoid Rubisco. 相似文献
108.
Barbara R. Evans Amanda K. Gilman Kimberley Cordray Jonathan Woodward 《Biotechnology letters》2000,22(9):735-740
A cellulase from the thermophile, Thermotoga maritima, hydrolyzed oligosaccharide substrates by an exoglucanase mode of action but acted as an endoglucanase to rapidly reduce the viscosity of the soluble polysaccharides carboxymethylcellulose and barley -glucan. The V
max for hydrolysis of the substrate, p-nitrophenyl -d-cellobioside, was 42 mol min–1 (mg protein)–1, while that for barley -glucan was 637. The enzyme had little activity on crystalline cellulose. 相似文献
109.
110.
介绍了近年来转基因植物中RNA介导的抗性特征和解释这种抗性机制的两种模型,一种是阈值模型,一种是质量模型,并对这两种模型的优缺点进行了讨论. 相似文献