全文获取类型
收费全文 | 1678篇 |
免费 | 139篇 |
国内免费 | 1篇 |
出版年
2023年 | 37篇 |
2022年 | 17篇 |
2021年 | 30篇 |
2020年 | 29篇 |
2019年 | 38篇 |
2018年 | 59篇 |
2017年 | 54篇 |
2016年 | 95篇 |
2015年 | 84篇 |
2014年 | 102篇 |
2013年 | 231篇 |
2012年 | 104篇 |
2011年 | 141篇 |
2010年 | 157篇 |
2009年 | 86篇 |
2008年 | 59篇 |
2007年 | 23篇 |
2006年 | 38篇 |
2005年 | 17篇 |
2004年 | 29篇 |
2003年 | 17篇 |
2002年 | 16篇 |
2001年 | 17篇 |
2000年 | 10篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1996年 | 20篇 |
1995年 | 16篇 |
1994年 | 16篇 |
1993年 | 24篇 |
1992年 | 27篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 15篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1972年 | 12篇 |
1971年 | 8篇 |
1962年 | 4篇 |
1961年 | 4篇 |
排序方式: 共有1818条查询结果,搜索用时 31 毫秒
11.
Some observations on coleoptile cell ultrastructure in ungerminated grains of rice (Oryza sativa L) 总被引:1,自引:0,他引:1
Helgi Öpik 《Planta》1971,102(1):61-71
Summary The ultrastructure of coleoptile cells of ungerminated rice grains has been examined following fixation in glutaraldehyde and osmium tetroxide. In many respects the cell structure resembles that reported for other dormant seed tissues: the cells contain protein bodies and lipid droplets, mitochondria and plastids show little internal structure but cytoplasm invaginates into many plastids; golgi cisternae cannot be discerned. Rough ER is present as cisternae surrounding protein bodies, as occasional regions of parallel layers, and in concentric whorls where it alternates with smooth paired membranes of an unknown nature. The ribosomes on the ER are at least partly arranged into regular rows. Various crystalline, presumably proteinaceous, inclusions lie in the groundplasm, plastids and nuclei. 相似文献
12.
In a variety of tumour systems, individuals carrying progressively growing neoplasms have lymphoid cells with a specific cytotoxic effect on cultured tumour cells from the same individual1–4. Since the sera of tumour-bearing individuals have been shown to prevent tumour cell destruction by immune lymphocytes in vitro2,5–8 and since this serum blocking activity appears early in primary and transplant tumour development5,7, it has been suggested that the appearance of this serum blocking activity might be responsible for the progressive growth of tumours in individuals having cytotoxic lymphocytes. Counteraction of this blocking activity would thus be of primary importance in facilitating the function of an already existing or bolstered cell-mediated immunity. The serum blocking activity might be inhibited in various ways, by preventing the formation of blocking antibody or by interfering with its action (“unblocking”), as demonstrated in Moloney sarcoma regressor sera9. This type of serum also has a therapeutic effect on Moloney sarcomas in vivo10,11, which has been tentatively attributed to its unblocking activity8,9 or, possibly, to a complement-dependent cytotoxicity10. Tumour growth in the Moloney sarcoma system, however, might be due in part to continuous recruitment of neoplastic cells by virus-induced transformation and so the therapeutic effect could be due to a virus-neutralizing serum activity9,10. 相似文献
13.
Association of Viral Reverse Transcriptase with an Enzyme degrading the RNA Moiety of RNA-DNA Hybrids 总被引:44,自引:0,他引:44
K. MÖLLING D. P. BOLOGNESI H. BAUER W. BÜSEN H. W. PLASSMANN P. HAUSEN 《Nature: New biology》1971,234(51):240-243
DURING replication of RNA tumour viruses, the genetic information contained in the viral RNA seems to be transferred to DNA1,2. Studies on the enzymatic activities present in the virus particles suggest that this transfer is mediated by an RNA dependent DNA polymerase3,4. RNA-DNA hybrids have been demonstrated to occur as intermediates in this reaction5 and single stranded DNA is generated as an early reaction product6, which is then replicated to give a double stranded DNA product6–8. The mechanism by which the single stranded DNA is displaced from the RNA template is, however, not known. 相似文献
14.
DURING each step of peptide chain elongation the ribosome shifts up one triplet along the messenger RNA with concomitant movement of the peptidyl-transfer RNA from the donor to the acceptor site. This process, commonly known as translocation, is triggered by a supernatant protein, factor G, which in association with the ribosome cleaves GTP into GDP and inorganic phosphate1,2 and it has been argued that the energy liberated in this reaction is used “to carry the complex one triplet forward”3. 相似文献
15.
Ohne Zusammenfassung 相似文献
16.
Jan-Erik Edstrm 《The Journal of cell biology》1960,8(1):47-51
Microphoretic purine-pyrimidine analyses of the ribonucleic acid (RNA) in nucleoli, nucleoplasm, cytoplasm, and yolk nuclei of spider oocytes have been carried out. The material necessary for the analyses was isolated by micromanipulation. Determinations of the amounts of RNA in the different parts of the cell were also performed. No differences between the composition of RNA in the nucleolus and the cytoplasm could be disclosed. Nucleoplasmic RNA was, on the other hand, distinctly different from that in the nucleolus and in the cytoplasm. The difference lies in the content of adenine, which is highest in nucleoplasmic RNA. The few analyses carried out on yolk nuclei showed their RNA to be variable in composition with a tendency to high purine values. The cytoplasm contains about 99 per cent of the total RNA in these cells, the nucleoplasm about 1 per cent, and the nucleolus not more than 0.3 per cent, although the highest concentrations are found in these latter structures. When considered in the light of other recent findings the results are compatible with the view that nucleolar RNA is the precursor of cytoplasmic RNA. 相似文献
17.
Summary A dichromatic contact microradiographic method is presented for quantitative microanalysis of lead in mineralized tissues utilizing the L absorption edges of lead. Sections of mineralized tissue with a thickness of about 100 are exposed to filtered molybdenum K and copper K radiation. Quantitative localization of lead is possible within small areas of the order of 50
2 with an accuracy of 25 · 10–12g lead. The method has been applied to studies of long term accumulation of lead in bone tissue. 相似文献
18.
Estimation of the effect of photoinhibition on the carbon gain in leaves of a willow canopy 总被引:21,自引:0,他引:21
The occurrence of photoinhibition of photosynthesis in leaves of a willow canopy was examined by measuring the chlorophyll-a fluorescence ratio of F
V/F
M (FM is the maximum fluorescence level of the induction curve, and FV is the variable fluorescence, F
V=F
M–F
0, where F0 is the minimal fluorescence). The majority of the leaves situated on the upper parts of peripheral shoots showed an afternoon inhibition of this ratio on clear days. This was the consequence of both a decrease in F
M and a rise in F
O. In the same leaves the diurnal variation in intercepted photosynthetic photon flux density (PPFD) was monitored using leaf-mounted sensors. Using the multivariate method, partial least squares in latent variables, it is shown that the dose of PPFD, integrated and linearly weighted over the last 6-h period, best predicts photoinhibition. Photoinhibition occurred even among leaves that did not intercept PPFDs above 1000 mol·m–2·s–1. Exposure of leaves to a standard photoinhibitory treatment demonstrated that the depression in the F
V/F
M ratio was paralleled by an equal depression in the maximal quantum yield of CO2 uptake and a nearly equal depression in the rate of bending (convexity) of the light-response curve of CO2 uptake. As a result, the rate of net photosynthesis is depressed over the whole natural range of PPFD. By simulating the daily course in the rate of net photosynthesis, it is estimated that in the order of one-tenth of the potential carbon gain of peripheral willow shoots is lost on clear days as a result of photoinhibition. This applies to conditions of optimal temperatures. Photoinhibition is even more pronounced at air temperatures below 23° C, as judged from measurements of the FV/FM ratio on clear days: the afternoon inhibition of this ratio increased in a curvilinear manner from 15% to 25% with a temperature decrease from 23° to 14° C.Abbreviations and Symbols FO
minimum fluorescence
- FV
variable fluorescence
- FM
maximum fluorescence
- PLS
partial least squares in latent variables
- PPFD
photosynthetic photon flux density
- VPD
water vapour-pressure deficit
This study was supported by the Swedish Natural Science Research Council. We are indebted to Dr. Jerry Leverenz (Department of Plant Physiology, University of Umeå, Sweden) for guidance with the modelling of the photosynthesis data. 相似文献
19.
In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385–398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field. 相似文献
20.
Humoral anti-idiotypic and anti-anti-idiotypic immune response in cancer patients treated with monoclonal antibody 17-1A 总被引:4,自引:0,他引:4
Peter Ragnhammar Maria Liljefors Anna-Lena Hjelm Håkan Mellstedt Jan-Erik Frödin J. Fagersberg 《Cancer immunology, immunotherapy : CII》1996,42(2):81-87
A group of 96 patients with advanced colorectal carcinoma were treated with the mouse (m) or chimeric (c) (mouse variable
regions × human IgG1 constant regions) monoclonal antibody (mAb) 17-1A recognizing the tumour-associated antigen GA733-2.
Eighty-two of the 83 patients treated with mmAb17-1A and 69% of the patients given cmAb17-1A (n = 13) developed anti-idiotypic antibodies (ab2). Auto-antibodies binding to tumour cells expressing GA733-2 were found in 7% of the patients. In a further 38 patients (40%)
antitumour-cell antibodies, i.e. anti-anti-idiotypic antibodies (ab3), were induced by the mAb17-1A therapy. Patients with detectable ab3 after treatment had significantly higher ab2 levels than those not developing ab3. Addition of granulocyte/macrophage-colony-stimulating factor (GM-CSF) to mmAb17-1A significantly enhanced the induction
of ab2 as well as induction of anti-anti-idiotypic antibodies (ab3), compared to mmAb17-1A alone. Patients with a high increase in antitumour-cell antibodies (ab3) induced by the therapy lived significantly longer than patients with no or a low level of induction of ab3 (P = 0.016). The results indicate that induction of an idiotypic network response might be an important effector mechanism in
mAb therapy.
Received: 20 October 1995 / Accepted: 18 December 1995 相似文献