全文获取类型
收费全文 | 4346篇 |
免费 | 305篇 |
国内免费 | 542篇 |
出版年
2024年 | 11篇 |
2023年 | 122篇 |
2022年 | 119篇 |
2021年 | 162篇 |
2020年 | 181篇 |
2019年 | 157篇 |
2018年 | 145篇 |
2017年 | 160篇 |
2016年 | 156篇 |
2015年 | 174篇 |
2014年 | 209篇 |
2013年 | 378篇 |
2012年 | 150篇 |
2011年 | 202篇 |
2010年 | 161篇 |
2009年 | 208篇 |
2008年 | 229篇 |
2007年 | 212篇 |
2006年 | 190篇 |
2005年 | 190篇 |
2004年 | 146篇 |
2003年 | 151篇 |
2002年 | 118篇 |
2001年 | 91篇 |
2000年 | 87篇 |
1999年 | 101篇 |
1998年 | 80篇 |
1997年 | 84篇 |
1996年 | 63篇 |
1995年 | 80篇 |
1994年 | 62篇 |
1993年 | 63篇 |
1992年 | 60篇 |
1991年 | 41篇 |
1990年 | 55篇 |
1989年 | 30篇 |
1988年 | 35篇 |
1987年 | 32篇 |
1986年 | 39篇 |
1985年 | 43篇 |
1984年 | 51篇 |
1983年 | 36篇 |
1982年 | 43篇 |
1981年 | 21篇 |
1980年 | 20篇 |
1979年 | 14篇 |
1978年 | 4篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1974年 | 5篇 |
排序方式: 共有5193条查询结果,搜索用时 15 毫秒
991.
Our knowledge about fossil algae has been biased for a long time towards groups with calcified tissues such as dasyclads and crustose rhodophytes. This picture has recently begun to change owing to the growing number of discoveries of silicified microbial fossils and fossil microbial endoliths. There is good evidence that both groups contain a large proportion of photosynthetic organisms, particularly algae. Silicified microbial fossils are best known from the Precambrian strata, while microbial endoliths, which apparently evolved during middle to late Precambrian times are common throughout the Phanerozoic including the present. Because of the small size of these fossils, it is possible to study and morphometrically evaluate entire populations rather than individual specimens or body fragments. Thus, we are able to approach fossil taxa using largely biological rather than strictly paleontological criteria and to apply interpretations and reconstructions of biological species. Phycologists entering this field can contribute to the interpretation of such fossils by applying comparisons with modern algae, their life cycles, cellular organization, and cell division patterns. However, they have to learn to deal with post-mortem degradation and diagenetic changes, and to recognize algal remains from mineral features that surround and often obliterate the fossils. This work is often similar to that of a detective or coroner where the use of intuition and imagination is just as dangerous as it is necessary. Detection of microfossils between and within single carbonate grains in otherwise unfossiliferous rocks can provide valuable information for applied geology including petroleum exploration. 相似文献
992.
Enzymatic degradation of alginate by marine fungi 总被引:4,自引:0,他引:4
A total of 72 pre-selected strains of 19 species of marine fungi were tested for their ability to decompose sodium alginate, calcium alginate or freshly prepared calcium alginate gel. Active alginate decomposition was evident in 18 strains (25% of total tested). These belong to only three different species: Asteromyces cruciatus, Corollospora intermedia, and Dendryphiella salina. In broth culture, decomposition of sodium alginate by the two deuteromycetes was followed by gravimetric, electrometric, viscometric, photometric and chromatographic methods in order to characterize the alginase enzyme system and its degradation products. The alginase enzyme complex consisted of at least two different enzyme components: the already known alginate lyase (eliminase) and a new endo-alginate hydrolase. In summary, a model is presented on the alginase-mediated structural and molecular decomposition of sodium alginate by marine fungi. 相似文献
993.
Ripening of pericarp tissue from mature green, early breaker and late breaker stages of tomato ( Lycopersicon esculentum Mill. cv. Dombito) fruit development was inhibitied by tunicamycin. Ripening was evaluated by lycopene accumulation, chlorophyll degradation, rate of ethylene production and cell wall-bound polygalacturonase (EC 3.2.1.15) activity. Maximum inhibition of these ripening parameters occurred at a treatment of 240 μ M tunicamycin for 2 h except for cell wall-bound polygalacturonase activity, which was greatly inhibited by concentrations of 12 μ tunicamycin or higher. Tunicamycin treatment at 120 μ M for 2 h inhibited the incorporation of [3 H]-mannose into macromolecules (about 70%) and pronase-sensitive material (about 65%) and the incorporation of [3 H]-leucine into proteins (about 20%). Our results indicate that protein glycosylation plays an important role in the ripening of tomato pericarp tissue. 相似文献
994.
Cell-wall-degrading enzymes of aquatic hyphomycetes: a review 总被引:7,自引:0,他引:7
ANNE-CAROLE CHAMIER 《Botanical journal of the Linnean Society. Linnean Society of London》1985,91(1-2):67-81
Reports of cell-wall-degrading enzymes of aquatic hyphomycetes are reviewed, including pectinases, cellulases, hemicellulases, laminarinases and chitinases and the ability of these fungi to degrade lignin and straw. New evidence of enzymic activity is presented for 14 species. 相似文献
995.
Raphael Lamed Rina Kenig Eva Setter Edward A. Bayer 《Enzyme and microbial technology》1985,7(1):37-41
The properties of the cellulosome (a cellulose-binding, multiple cellulase-containing protein complex isolated from Clostridium thermocellum) have been compared with the previously reported characteristics for crude cellulase [see 1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4] preparations. Similar to the crude enzyme system, true cellulolytic activity was demonstrated for the purified cellulosome on the basis of extensive solubilization of microcrystalline cellulose. The cellulolytic activity of the purified cellulosome was enhanced both by calcium ions and by thiols, and was inhibited by cellobiose (the major end product of the cellulosome-mediated cellulose degradation). In addition, at low ionic strength, cellulose-adsorbed cellulosome was detached intact from the cellulose matrix. Using controlled conditions, maximum enzymatic activity was shown to correspond to suboptimal conditions of cellulosome adsorption to cellulose. The results suggest that previous data accumulated for the crude cellulase system in C. thermocellum essentially reflect the contribution of the cellulosome. 相似文献
996.
During somatic embryogenesis in Papaver orientale tissue cultures a permanent starch accumulation and a transient triacylglycerol accumulation were observed. The degradation of the lipids during plantlet development from embryoids was paralleled by an activity increase of the glyoxylate-cycle enzymes malate synthase (EC 4.1.3.2) and isocitrate lyase (EC 4.1.3.1). Fat accumulation and breakdown was interpreted as a reflection of seed formation and germination during normal development. 相似文献
997.
998.
Summary The mammalian urinary bladder contains in its apical membrane and cytoplasmic vesicles, a cation-selective channel or activating fragment which seems to partition between the apical membrane and the luminal (or vesicular space). To determine whether it is an activating fragment or whole channel, we first demonstrate that solution known to contain this moiety can be concentrated and when added back to the bladder causes a conductance increase, with a percent recovery of 139±25%. Next, we show that using tip-dip bilayer techniques (at 21°C) and a patch-clamp recorder, the addition of concentrated solution resulted in the appearance of discrete current shots, consistent with the incorporation of a channel (as opposed to an activating fragment) into the bilayer. The residency time of the channel in the bilayer was best described by the sum of two exponentials, suggesting that the appearance of the channel involves an association of the channel with the membrane before insertion. The channel is cation selective and more conductive to K+ than Na+ (by a factor of 1.6). It has a linearI–V relationship, but a singlechannel conductance that saturates as KCl concentration is raised. This saturation is best described by the Michaelis-Menten equation with aK
m of 160mm KCl and aG
max of 20 pS. The kinetics of the channel are complex, showing at least two open and two closed states.Since the characteristics of this channel are similar to a channel produced by the degradation of amiloride-sensitive Na+ channels by the proteolytic enzyme kallikrein (which is released by the cortical collecting duct of the kidney), we suggest that this channel then is not synthesized by the cell but is rather a degraded form of the epithelial Na+ channel. 相似文献
999.
[Carbonyl-14C] methabenzthiazuron (MBT) was applied to an arid region soil at a rate of 5mg kg−1 soil to give a14C content of 2400 KB kg−1 soil. After 15 weeks of incubation at 22°C and 50% of the maximum water holding capacity of the soil, 7.2% of the applied14C was mineralized to14CO2. Where the soil was amended with wheat straw, total mineralization increased to 17.3%. Soil disturbance caused a significant
increase while chloroform fumigation caused a significant decrease in the rate of14CO2 production, both from amended and unamended soils. These results suggest that MBT is degraded mainly through microbial co-metabolism.
Wheat straw amendment resulted in increased transformation of MBT into soil humus. In unamended soil, a major portion of14C was recovered in fulvic acid and in fractions extracted with organic solvents. Recovery of14C in non-extractable bound residues (humins) increased as incubation progressed and seemed to be derived from the fulvic acid
fraction, which showed a concomitant decrease.
More than 99% of the residual14C in unamended soil consisted of unaltered MBT; the remainder occurred as 1-methyl-1 (benzthiazolyl) urea. In amended soil,
a relatively higher percentage of the extractable14C was found in the metabolite. Small amounts of three unidentified14C-labelled compounds were also observed.
In amended soil, disturbance caused a decrease in extractable-14C whereas fumigation caused a significant increase, as compared to the untreated control. The effects were more pronounced
when the soils were reated at an early stage of incubation. In general, soil disturbance increased the availability of MBT
for further transformations while chloroform fumigation decreased the process. 相似文献
1000.
Biodegradation of polycyclic aromatic hydrocarbons 总被引:67,自引:0,他引:67
Carl E. Cerniglia 《Biodegradation》1992,3(2-3):351-368
The intent of this review is to provide an outline of the microbial degradation of polycyclic aromatic hydrocarbons. A catabolically diverse microbial community, consisting of bacteria, fungi and algae, metabolizes aromatic compounds. Molecular oxygen is essential for the initial hydroxylation of polycyclic aromatic hydrocarbons by microorganisms. In contrast to bacteria, filamentous fungi use hydroxylation as a prelude to detoxification rather than to catabolism and assimilation. The biochemical principles underlying the degradation of polycyclic aromatic hydrocarbons are examined in some detail. The pathways of polycyclic aromatic hydrocarbon catabolism are discussed. Studies are presented on the relationship between the chemical structure of the polycyclic aromatic hydrocarbon and the rate of polycyclic aromatic hydrocarbon biodegradation in aquatic and terrestrial ecosystems. 相似文献