全文获取类型
收费全文 | 20041篇 |
免费 | 1658篇 |
国内免费 | 5篇 |
专业分类
21704篇 |
出版年
2023年 | 88篇 |
2022年 | 232篇 |
2021年 | 432篇 |
2020年 | 233篇 |
2019年 | 306篇 |
2018年 | 423篇 |
2017年 | 354篇 |
2016年 | 636篇 |
2015年 | 1032篇 |
2014年 | 1169篇 |
2013年 | 1411篇 |
2012年 | 1747篇 |
2011年 | 1650篇 |
2010年 | 1063篇 |
2009年 | 936篇 |
2008年 | 1266篇 |
2007年 | 1295篇 |
2006年 | 1105篇 |
2005年 | 1065篇 |
2004年 | 1036篇 |
2003年 | 940篇 |
2002年 | 892篇 |
2001年 | 206篇 |
2000年 | 145篇 |
1999年 | 157篇 |
1998年 | 234篇 |
1997年 | 151篇 |
1996年 | 138篇 |
1995年 | 125篇 |
1994年 | 113篇 |
1993年 | 102篇 |
1992年 | 80篇 |
1991年 | 66篇 |
1990年 | 67篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 55篇 |
1986年 | 42篇 |
1985年 | 46篇 |
1984年 | 43篇 |
1983年 | 62篇 |
1982年 | 37篇 |
1981年 | 32篇 |
1980年 | 36篇 |
1979年 | 28篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 32篇 |
1975年 | 25篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Nienow AW Langheinrich C Stevenson NC Emery AN Clayton TM Slater NK 《Cytotechnology》1996,22(1-3):87-94
Because of concern for cell damage, very low agitation energy inputs have been used in industrial animal cell bioreactors, typical values being two orders of magnitude less than those found in bacterial fermentations. Aeration rates are also very small. As a result, such bioreactors might be both poorly mixed and also unable to provide the higher oxygen up-take rates demanded by more intensive operation. This paper reports experimental studies both of K
L
a and of mixing (via pH measurements) in bioreactors up to 8 m3 at Wellcome and of scaled down models of such reactors at Birmingham. Alongside these physical measurements, sensitivity of certain cell lines to continuously controlled dO2 has been studied and the oxygen up-take rates measured in representative growth conditions. An analysis of characteristic times and mixing theory, together with other recent work showing that more vigorous agitation and aeration can be used especially in the presence of Pluronic F-68, indicates ways of improving their performance. pH gradients offer a special challenge. 相似文献
72.
Heidrun Herrmann Christian Müller Ingmar Schmidt Jens Mahnke Lothar Petruschka Karin Hahnke 《Molecular genetics and genomics : MGG》1995,247(2):240-246
The genetic organization of the DNA region encoding the phenol degradation pathway ofPseudomonas putida H has been investigated. This strain can utilize phenol or some of its methylated derivatives as its sole source of carbon and energy. The first step in this process is the conversion of phenol into catechol. Catechol is then further metabolized via themeta-cleavage pathway into TCA cycle intermediates. Genes encoding these enzymes are clustered on the plasmid pPGH1. A region of contiguous DNA spanning about 16 kb contains all of the genetic information necessary for inducible phenol degradation. The analysis of mutants generated by insertion of transposons and cassettes indicates that all of the catabolic genes are contained in a single operon. This codes for a multicomponent phenol hydroxylase andmeta-cleavage pathway enzymes. Catabolic genes are subject to positive control by the gene product(s) of a second locus. 相似文献
73.
Abstract. Altitudinal and latitudinal distribution limits of trees are mainly controlled by temperature. Therefore climate warming is expected to induce upslope or poleward migrations. In the Swiss Central Alps, summers in the period 1982-1991 were on average 0.8 °C warmer than those of the period 30 yr before. We investigated whether populations of conifers at the montane Pinus sylvestris-Pinus cembra ecocline exhibit demographic trends in response to that warming. We found no evidence for this. Young seedlings of Pinus sylvestris, the species which is expected to expand its range upward in a warmer climate, were virtually absent from all sites, whereas large fractions of Pinus cembra populations were observed in the seedling and juvenile categories even below the present lower distribution limit of adult trees. This suggests that there are no major altitudinal shifts in response to the recent sequence of warmer summers. Germination and seedling survival trials with Pinus sylvestris suggest that temperature per se would not exclude this species even from establishing at the current treeline in the Swiss Central Alps. Similar results were found at the polar treeline. Phytotron tests of seedling survival showed much less drought resistance in Pinus sylvestris than in Pinus cembra which is in contrast to their phytogeographic distributions. Thus, the montane pine ecocline in the Swiss Central Alps seems to be stabilized by species interactions and may not be directly responsive to moderate climatic change, which needs to be taken into account in predictive attempts. 相似文献
74.
Indigenous Arbuscular Mycorrhizal Fungi Associated with Acacia albida Del. in Different Areas of Senegal 下载免费PDF全文
Tahir A. Diop Mamadou Gueye Bernard L. Dreyfus Christian Plenchette Desire G. Strullu 《Applied microbiology》1994,60(9):3433-3436
The influences of seasons, plant age, and physicochemical properties of the soil on surface and deep biological arbuscular mycorrhizal fungus parameters associated with Acacia albida were assessed in different areas of Senegal. More indigenous arbuscular mycorrhizal propagules were found in the localities of the Sudano-Guinean zone (Djinaki and Kabrousse) than in those of the Sahelian zone (Louga and Diokoul), and species belonging to the genera Glomus, Gigaspora, Acaulospora, and Sclerocystis prevailed. The numbers of total and viable spores increased more during the rainy season than during the dry season (about 108% more total spores and 262% more viable spores). Similarly, both total and viable spores were more prevalent around young Acacia trees than old trees. However, the intensities of root colonization did not differ in each ecoclimatic zone. 相似文献
75.
Murielle Rinaldi-Carmona Francis Barth Michel Haulme David Shire Bernard Calandra Christian Congy Serge Martinez Jeanne Maruani Gervais Nliat Daniel Caput Pascual Ferrara Philippe Soubri Jean Claude Brelire Grard Le Fur 《FEBS letters》1994,350(2-3):240-244
SR141716A is the first selective and orally active antagonist of the brain cannabinoid receptor. This compound displays nanomolar affinity for the central cannabinoid receptor but is not active on the peripheral cannabinoid receptor. In vitro, SR141716A antagonises the inhibitory effects of cannabinoid receptor agonists on both mouse vas deferens contractions and adenylyl cyclase activity in rat brain membranes. After intraperitoneal or oral administration SR141716A antagonises classical pharmacological and behavioural effects of cannabinoid receptor agonists. This compound should prove to be a powerful tool for investigating the in vivo functions of the anandamide/cannabinoid system. 相似文献
76.
Ann E. Ehrenhofer-Murray Friedrich E. Würgler Christian Sengstag 《Molecular & general genetics : MGG》1994,244(3):287-294
Several pleiotropic drug sensitivities have been described in yeast. Some involve the loss of putative drug efflux pumps analogous to mammalian P-glycoproteins, others are caused by defects in sterol synthesis resulting in higher plasma membrane permeability. We have constructed a Saccharomyces cerevisiae strain that exhibits a strong crystal violet-sensitive phenotype. By selecting cells of the supersensitive strain for normal sensitivity after transformation with a wild-type yeast genomic library, a complementing 10-kb DNA fragment was isolated, a 3.4-kb subfragment of which was sufficient for complementation. DNA sequence analysis revealed that the complementing fragment comprised the recently sequenced SGE1 gene, a partial multicopy suppressor of gal11 mutations. The supersensitive strain was found to be a sge1 null mutant. Overexpression of SGE1 on a high-copy-number plasmid increased the resistance of the supersensitive strain. Disruption of SGE1 in a wild-type strain increased the sensitivity of the strain. These features of the SGE1 phenotype, as well as sequence homologies of SGE1 at the amino acid level, confirm that the Sge1 protein is a member of the drug-resistance protein family within the major facilitator superfamily (MFS). 相似文献
77.
Synopsis Similarities between the freshwater fish faunas of 52 west African rivers have been investigated and three main zoogeographic regions recognized. The Sudanian region includes all rivers from Senegal to the Omo, as well as coastal basins from Ivory Coast to the Cross and the Wouri. The Upper Guinean region comprises the coastal basins from Guinea to Liberia and the Lower Guinean one, the coastal rivers of Cameroon and Gaboon. The Sudanian region can be sub-divided into a Sudanian region sensu stricto, including the Sahelo-Sudanese rivers, and the Eburneo-Ghanean region corresponding to coastal basins from the Cess (or Nipoué, Ivory Coast) to the Pra (Ghana). These delimitations give an highly significant within region faunal homogeneity, even if the effect of geographical proximity between rivers is removed. 21 to 71% of the fish species in each region are endemics. The present patterns of distribution are the result of past climatic and geological events affecting west Africa and, given this framework, the role of alternating wet and dry periods during the early Quarternary is emphasized as well as the importance of mountains as dispersal barriers. Role of recent river connections and links via lagoon is emphasized in explaining river faunal similarities within biogeographical regions. 相似文献
78.
Gerardo D. Castro Christian J. Stamato Jos A. Castro 《Free radical biology & medicine》1994,16(6):693-701
The interaction between free radicals derived from the catalytic decomposition of carbon tetrachloride and tyrosine (the N-acetyl tyrosine ethyl ester, ATEE) under anaerobic and aerobic conditions was studied. The structure of the reaction products formed was desciphered by the GLC/MS analysis of their trimethylsilyl derivatives. Under anaerobic conditions the formation of the following products was found: (1) an unsaturated derivative of the amino acid; (2) the trimethylsilyl derivative of N-acetyl chloro tyrosine ethyl ester; (3) a hydroxyl adduct of ATEE ; (4) an ATEE adduct having a chlorine and a CCl3 group in the molecule (it is suggested that CCl3 is attached to the benzyl carbon and the chlorine located in the benzene ring); (5) an ATEE adduct having only a CCl3 group tentatively assigned to be located on the benzyl carbon; and (6) and (7) were found to be two isomers of an ATEE having one CCl3 on the aromatic ring. Under aerobic conditions the following reaction products were identified: Two products which were similar to those numbered (1) and (2) and formed anaerobically; (8) and (11) two isomeric dichlorinated adducts of ATEE; (9) and (10) two isomeric dichlorinated monohydroxylated derivatives of ATEE. Concerning the potential relevance of these findings, we consider that if similar interactions to those here reported occurred during CCl4 poisoning, the activity of enzymes having tyrosine in their active center might result in impairment. Further, enzymes operating on tyrosine moieties in proteins might be perturbed in their action if tyrosine groups were attacked by the free radicals arising from catalytic decomposition of CCl4 evidenced here. 相似文献
79.
The severity of a root rot disease of cereals, caused by Rhizoctonia solani Kühn AG8, was inversely correlated to the Zn status of plants in field studies in 1989 and 1990. In 1989, a preliminary survey was conducted in a farmer's field in South Australia where Zn deficiency and disease were both widespread. Zn concentration in Spear wheat plants at the 3-leaf to early tillering stage was negatively correlated with severity of the disease. For the elevent elements analysed, a correlation matrix showed that Zn had the highest, and only significant (R2=0.52**) association with disease. The effect of Zn applications and their residual value on disease severity was further studied in a long-term field experiment in 1989 and 1990 to which Zn had been applied in 1986. There was a decrease in the area of Rhizoctonia bare patch as Zn rate was increased, a result consistent with the field survey results; the recommended rate of 2.5 kg Zn ha–1 reduced the area affected by bare patch from 42% to 21% of the total crop area compared with no Zn application, overcame Zn deficiency and increased grain yield from 1.1 to 2.8 t ha–1. In 1990, fresh Zn application treatments were applied to trial plots designed for this purpose, in order to compare the response with the older Zn treatments applied in 1986. The areas of bare patch in the older Zn treatments were approximately 5% greater than those in the fresh Zn treatments. The results are consistent with the hypothesis that Zn deficient plants are more susceptible to root rot caused by R. solani. Testing this hypothesis is the subject of a companion paper. 相似文献
80.
Inci Arisan-Atac Regina Hodits Doris Kristufek Christian P. Kubicek 《Applied microbiology and biotechnology》1993,39(1):58-62
Trichoderma reesei was studied for its ability to produce -mannanase activity on a variety of carbon sources. The highest -mannanase activity was produced on cellulose, whereas -mannan-containing carbon sources (such as kojac powder or locust bean gum) gave lower enzyme titres. The enzyme responsible for the major -mannanolytic activity from T. reesei was purified to physical homogeneity by preparative chromatofocusing and anion exchange fast protein liquid chromatography. This -mannanase is a glycoprotein, with a molecular mass of 46 (±2) kDa and an isoelectric point of 5.2. It has an optimal pH at 5.0 and broad pH stability (2.5–7.0). It is stable for 60 min at 55° C, and has an optimal temperature for activity at 75° C. During incubation with locust bean gum, the enzyme releases mainly tri- and disaccharides.
Correspondence to: C. P. Kubicek 相似文献