全文获取类型
收费全文 | 21920篇 |
免费 | 1499篇 |
国内免费 | 9篇 |
出版年
2024年 | 19篇 |
2023年 | 126篇 |
2022年 | 311篇 |
2021年 | 580篇 |
2020年 | 351篇 |
2019年 | 470篇 |
2018年 | 593篇 |
2017年 | 478篇 |
2016年 | 809篇 |
2015年 | 1215篇 |
2014年 | 1343篇 |
2013年 | 1659篇 |
2012年 | 1948篇 |
2011年 | 1892篇 |
2010年 | 1174篇 |
2009年 | 1016篇 |
2008年 | 1376篇 |
2007年 | 1287篇 |
2006年 | 1179篇 |
2005年 | 1081篇 |
2004年 | 981篇 |
2003年 | 927篇 |
2002年 | 751篇 |
2001年 | 117篇 |
2000年 | 106篇 |
1999年 | 135篇 |
1998年 | 125篇 |
1997年 | 115篇 |
1996年 | 89篇 |
1995年 | 86篇 |
1994年 | 90篇 |
1993年 | 88篇 |
1992年 | 53篇 |
1991年 | 56篇 |
1990年 | 62篇 |
1989年 | 45篇 |
1988年 | 50篇 |
1987年 | 38篇 |
1986年 | 42篇 |
1985年 | 52篇 |
1984年 | 39篇 |
1983年 | 45篇 |
1982年 | 32篇 |
1981年 | 42篇 |
1980年 | 26篇 |
1979年 | 34篇 |
1978年 | 32篇 |
1977年 | 29篇 |
1976年 | 25篇 |
1975年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Anna Swierzko Lore Brade Elisabeth C. Höffgen Helmut Brade 《FEMS immunology and medical microbiology》1993,7(3):265-270
Abstract Rabbit polyclonal antibodies against the lipopolysaccharide (LPS) of the Rd1 P− mutant strain R7 of Salmonella minnesota were serologically characterized using R7 LPS, dephosphorylated LPS, deacylated LPS, deacylated, dephosphorylated and reduced LPS, and synthetic partial structures. The latter comprised partial structures of the core region of Rd1 P− LPS bound to the β 1 → 6-linked glucosamine disaccharide with two amide-linked 3-hydroxytetradecanoic acid residues or artificial glycoconjugates comprised of the synthetic oligosaccharides coupled to bovine serum albumin. Using a passive hemolysis and an enzyme immunoassay, absorption and inhibition experiments, the antibody specificities present could be determined. One group of antibodies required components of the core region and the phosphorylated glucosamine disaccharide of the lipid A moiety for binding. The majority of phosphate-independent antibodies was directed against the trisaccharide l -glycero-α- d -manno-heptopyranose(1 → 3)- l -glycero-α- d -manno-heptopyranose(1 → 5)3-deoxy- d -manno-octulosonic acid. Antibodies against the 1 → 3- and 1 → 7-linked heptose disaccharides and against a single heptose were also detected, however, with low titers. No antibodies were found which required the presence of fatty acids. 相似文献
152.
153.
Debbie C. Thurmond Anna B. Tang Manabu T. Nakamura Judith S. Stern Stephen D. Phinney 《Obesity (Silver Spring, Md.)》1993,1(2):118-125
Obese Zucker rats (fa/fa) have low levels of arachidonic acid (AA) in liver phospholipids (PL). We have previously shown that a 70% gamma-linolenate concentrate (GLA; an AA intermediate) fed at a fixed dose (0.07 g/day) normalized hepatic PL AA and reduced weight gain selectively in the obese animals. In a follow-up study, 16 obese (fa/fa) and 16 lean (Fa/Fa) 4-week-old male rats were randomized into 4 groups of 8 each and gavaged daily with soybean oil (SOY) containing 55% 18:2ω6 (an AA precursor) or GLA, using a progressive dose (≤ 5% of total calories) based on body weight. A defined diet with 11% of energy as SOY was fed ad libitum for 60 days. GLA obese had lower body weight (p<0.0001) and 60-day cumulative food intake (p<0.05) compared to SOY obese, but neither parameter differed between the lean groups. For the last twenty days cumulative food intake was identical for GLA obese and SOY lean, whereas SOY obese consumed 18% more (p<0.05). Thus the progressive dose of GLA selectively suppressed hyperphagia in obese Zucker rats. Erythrocytes collected at 15-day intervals showed parallel increases in AA in both genotypes over time, suggesting normal AA availability during rapid growth. Thus, the reduced PL AA in the livers from the obese rats probably reflects impaired distribution in selected tissues rather than reduced hepatic production. Due to the potential health risks of enriching tissue lipids with AA, great caution is advised in considering GLA as therapy for human obesity. 相似文献
154.
155.
The possibility of a relation between the expression of root inducing (Ri) T-DNA genes of Agrobacterium rhizogenes and changes in polyamine metabolism has been explored in fast-growing tobacco hairy roots. Transformed root cultures have been established on hormone-fee medium; they came from transgenic plants of Nicotiana tabacum L. cv. Xanthi with different altered phenotypes, designated transformed (T) and supertransformed (T'). T and especially T' roots developed more rapidly both by elongation and lateral branching, and showed a higher growth rate than the untransformed control. After 3 weeks in culture, normal roots showed a very reduced meristematic zone, and flow cytometric analysis indicated that 2C nuclei were predominant in the apical parts in contrast to T and T' roots, in which endopolyploidisation also appeared. Putrescine, spermidine and traces of spermine were present in all the samples, both in free and in conjugated forms. Putrescine was the major polyamine detected in controls and in transformed roots. At the time of excision, the polyamine levels were similar in normal, T and T' roots. Significant differences were found during the progression of growth, particularly in the TCA-insoluble fraction in which polyamines varied differently according to the type of roots, increasing considerably in T roots on day 8, then decreasing. The lower polyamine contents found in growing transformed roots were concomitant to low arginine (EC 4.1.1.19) and ornithine (EC 4.1.1.17) decarboxylase activities. It is suggested that polyamine levels and related enzyme activities are linked to growth kinetics rather than being a consequence of foreign gene expression. 相似文献
156.
Anna Metaxas 《Journal of phycology》1989,25(3):605-608
Two species of marine diatoms [Skeletonema costatum (Greville) Cleve and Thalassiosira pseudonana (Hustedt) Hasle and Heimdal] were grown in glass and polyarbonate containers. S. costatum exhibited a signzJicantly lower exponential growth rate and maximal yield and a signajcantly longer lag phase when grown in polycarbonate. Exponential growth rate and maximal yield of T. pseudonana was significantly reduced (P < 0.05 in all cases). This study suggests that a difference in diatom growth between glass and polyarbonate containers might arise in certain cases. However, such a difference may not be detectable with all biomass measurement techniques or with low within-treatment replication. 相似文献
157.
Anna Nasulewicz Andrzej Mazur Adam Opolski 《Journal of trace elements in medicine and biology》2004,18(1):1-8
The formation of new blood vessels is the initial step in progressive tumour development and metastasis. The first stage in tumour angiogenesis is the activation of endothelial cells. Copper ions stimulate proliferation and migration of endothelial cells. It has been shown that serum copper concentration increases as the cancer disease progresses and correlates with tumour incidence and burden. Copper ions also activate several proangiogenic factors, e.g., vascular endothelial growth factor, basic fibroblast growth factor, tumour necrosis factor alpha and interleukin 1. This review concerns a brief introduction into the basics of tumour blood vessel development as well as the regulatory mechanisms of this process. The role of copper ions in tumour angiogenesis is discussed. The new antiangiogenic therapies based on a reduction of copper levels in tumour microenvironment are reviewed. 相似文献
158.
Sayan Mukherjee Pablo Tamayo Simon Rogers Ryan Rifkin Anna Engle Colin Campbell Todd R Golub Jill P Mesirov 《Journal of computational biology》2003,10(2):119-142
A statistical methodology for estimating dataset size requirements for classifying microarray data using learning curves is introduced. The goal is to use existing classification results to estimate dataset size requirements for future classification experiments and to evaluate the gain in accuracy and significance of classifiers built with additional data. The method is based on fitting inverse power-law models to construct empirical learning curves. It also includes a permutation test procedure to assess the statistical significance of classification performance for a given dataset size. This procedure is applied to several molecular classification problems representing a broad spectrum of levels of complexity. 相似文献
159.
Marina S. Trofimova Inna M. Zhestkova Valentina P. Kholodova Igor M. Andreev Evgeny M. Sorokin Anna G. Kruglova Vladimir V. Kuznetsov 《Physiologia plantarum》2003,118(2):232-239
The osmotic water permeability ( P os ) of cell membranes isolated from leaves of 40-, 50- and 60-day-old Mesembryanthemum crystallinum plants was estimated by measuring light-scattering kinetics using stopped-flow spectrophotometry. The measurements were performed on the plasma membrane (PM), purified tonoplast (TP), and TP-enriched vesicles. The PM and TP-enriched vesicles were obtained by partitioning the microsomal fraction in an aqueous polymer two-phase system, whereas the purified TP vesicles were prepared by microsomal vesicle flotation on a sucrose cushion. The P os of isolated membranes declined with plant age. The kinetic experiments showed that there was no difference between the P os of the PM and TP isolated from plants of all ages. A 24-h exposure of plants to 400 m M NaCl caused a decline in the P os as well. These findings suggest that, during M. crystallinum transition to CAM, which was induced by plant ageing or salinity, plant osmoregulatory responses included changes in the P os of the leaf-cell membranes. These variations in the P os are discussed in the context of adaptive mechanisms responsible for the maintenance of the water balance in the common ice plant. 相似文献
160.
Peter D. Nichols Anna C. Palmisano John K. Volkman Glen A. Smith David C. White 《Journal of phycology》1988,24(1):90-96
The lipid and hydrocarbon composition of natural populations of diatom communities collected during the austral spring bloom of 1985 in the sea-ice at McMurdo Sound, Antartica was analyzed by TLC-FID, GC and GC-MS. Sea-ice diatom communities were dominated by Amphiprora sp., Nitzschia stellata Manguin and Berkeleya sp. at Cape Armitage; N. stellata, Amphiprora, Pleurosigma, N. kerguelensis (O'Meara) Hasle and some small centric diatoms adjacent to the Erebus Ice Tongue; and Porosira pseudodenticulata (Hustedt) Jouse at Wohlschlag Bay. Lipid distributions of the sea ice diatom communities from the Cape Armitage and Ereus sites were characterized by high concentrations of tracylaglecycerol (triacylglycerolplar lipid = 1.0 to 1.5). The hydrocarbon n-C21:6, common in temperate diatoms, and an isoprenoid C25 diunsaturated alkene were the dominant hydrocarons detected at these two sites. Hydrogenation of the C25 diene produced the known alkane 2, 6, 10, 14-tetramethyl-7- (3-methylpentyl)-pentadecane. The C25 diene is one of several structurally related hydrocarbons reported in many estuarine, coastal and ocean ic sediments. We propose that certain species of diatoms are a likely source of these alkenes in sediments. The first reported biological occurrence of the C25 diene in the green seaweed Enteromorpha prolifera may have been due to the presence of epiphytic microalgae in the field sample analysed. 相似文献