全文获取类型
收费全文 | 15922篇 |
免费 | 971篇 |
国内免费 | 5篇 |
专业分类
16898篇 |
出版年
2022年 | 89篇 |
2021年 | 159篇 |
2019年 | 107篇 |
2018年 | 148篇 |
2017年 | 122篇 |
2016年 | 234篇 |
2015年 | 366篇 |
2014年 | 423篇 |
2013年 | 945篇 |
2012年 | 789篇 |
2011年 | 735篇 |
2010年 | 412篇 |
2009年 | 463篇 |
2008年 | 724篇 |
2007年 | 725篇 |
2006年 | 715篇 |
2005年 | 722篇 |
2004年 | 774篇 |
2003年 | 761篇 |
2002年 | 679篇 |
2001年 | 565篇 |
2000年 | 552篇 |
1999年 | 473篇 |
1998年 | 218篇 |
1997年 | 191篇 |
1996年 | 175篇 |
1995年 | 152篇 |
1994年 | 158篇 |
1993年 | 163篇 |
1992年 | 348篇 |
1991年 | 328篇 |
1990年 | 331篇 |
1989年 | 297篇 |
1988年 | 254篇 |
1987年 | 257篇 |
1986年 | 224篇 |
1985年 | 194篇 |
1984年 | 160篇 |
1983年 | 144篇 |
1982年 | 146篇 |
1981年 | 129篇 |
1980年 | 114篇 |
1979年 | 128篇 |
1978年 | 102篇 |
1977年 | 94篇 |
1976年 | 89篇 |
1975年 | 80篇 |
1974年 | 88篇 |
1973年 | 97篇 |
1971年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The effects of defoliation treatments on plant growth in sunflower (Helianthus annuus) were studied in the field. Four defoliation treatments, 0 (control), 37.4, 56.1 and 93.4% of the total leaf dry weight,
were applied to plants that had small third leaves. Decreased leaf weight/whole plant weight (F/W) ratios in defoliated plants
rapidly recovered to almost the same ratio as that observed in the control within 12 to 16 days after defoliation according
to the degree of defoliation. The mechanism involved in the recovery of the F/W ratio in defoliated plants mainly consisted
of three parameters: enhancement of (1) carbon distribution ratios in the leaves, (2) photosynthetic activity in the remaining
leaves, and (3) retranslocation of carbon from the stem and/or roots to leaves.
Inhibitive effects of defoliation on relative growth rate and net assimilation rate were seen at an early stage, but subsequently
both rates became larger in defoliated plants than in controls. Defoliated plants tended to show rapid development and expansion
of new leaves, and to show increased specific leaf area and protein synthesis in individual leaves. The sugar content of leaves
in defoliated plants was higher than that in controls, while the content in both stem and roots was lower. These responses
seem to be advantageous for development of the photosynthetic system. Heights of defoliated plants were clearly depressed
according to the degree of defoliation, and this was attributed largely to differences in the elongation rates of the internodes
resulting from defoliation. 相似文献
42.
Purification of bleomycin hydrolase with a monoclonal antibody and its characterization 总被引:1,自引:0,他引:1
We established a hybridoma clone that produced anti-bleomycin hydrolase antibody. The subclass of the monoclonal antibody was immunoglobulin M. The antibody significantly reacted with bleomycin hydrolase from rabbit tissues, mouse livers, sarcoma 180, and adenocarcinoma 755 but not significantly with that from MH 134 and Ehrlich carcinoma. The enzyme from L5178Y cells showed an intermediate reactivity. Bleomycin hydrolase was purified from rabbit liver by immunoaffinity with the monoclonal antibody and DEAE gel chromatography. Approximately 1300-fold-purified bleomycin hydrolase was obtained. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing on a polyacrylamide slab gel of purified bleomycin hydrolase showed a single band with an apparent Mr of 48K and an isoelectric pH of 5.2. The molecular weight of bleomycin hydrolase determined on gel filtration high-performance liquid chromatography was ca. 300K, suggesting a hexameric enzyme. The enzyme showed an optimum pH of 6.8-7.8 and gave a Vmax value of 6.72 mg min-1 mg-1 for peplomycin and 9.24 mg min-1 mg-1 for bleomycin B2 and a Km value of 0.79 mM for both substrates. The enzyme was inhibited by E-64, leupeptin, p-tosyl-L-lysine chloromethyl ketone, N-ethylmaleimide, Fe2+, Cu2+, and Zn2+ but was enhanced by dithiothreitol. The results suggest that bleomycin hydrolase is a thiol enzyme. 相似文献
43.
Defense mechanisms in invertebrates and fish 总被引:6,自引:0,他引:6
44.
Identification of the cell surface molecule involved in sexual cell fusion of Dictyostelium discoideum 总被引:6,自引:0,他引:6
Kazuyoshi Suzuki Kaichiro Yanagisawa 《Differentiation; research in biological diversity》1989,40(3):159-165
Dictyostelium discoideum was used as a model system for elucidating the molecular mechanism of sexual cell fusion. In heterothallic strains NC4 and HM1 of D. discoideum, complements in mating type, amoeboid cells acquire fusion competence only under certain environmental conditions, such as the presence of excess water and a certain period of darkness, to fuse sexually. The surface of cells which acquired fusion competence was found to possess specific antigens. Monovalent antibodies prepared from rabbit antiserum against fusion-competent NC4 cells inhibit the sexual cell fusion of these cells completely. Two specific antigenic proteins, 39 and 138 k Da in relative molecular mass and specific for fusion-competent cells, were detected. Only one, the 138-k Da protein, was capable of neutralizing the fusion-inhibitory activity of the monovalent antibody. These results show that the 139-k Da protein is the one involved in the sexual cell fusion of NC4 and HM1 strains in D. discoideum. 相似文献
45.
A single point mutation confers tetrodotoxin and saxitoxin insensitivity on the sodium channel II 总被引:22,自引:0,他引:22
A single point mutation of the rat sodium channel II reduces its sensitivity to tetrodotoxin and saxitoxin by more than three orders of magnitude. The mutation replaces glutamic acid 387 with a glutamine and has only slight effects on the macroscopic current properties, as measured under voltage-clamp in Xenopus oocytes injected with the corresponding cDNA-derived mRNA. 相似文献
46.
The expression plasmid containing human prepro-endothelin cDNA was constructed and introduced into COS-7 cells. Mature endthelin, consisting of 21 amino acid residues, was secreted into the culture medium of the transfected cells and was also synthesized by non-transfected COS-7 cells. Normal kidney cells derived from other species also synthesized and secreted endothelin. Partial characterization of endothelins produced by kidney cells suggested that existence of new types of endothelin. This is the first report of the vasoconstrictor peptide endothelin being synthesized in kidney cells. 相似文献
47.
F Suzuki S Miyazaki M Uozumi N Okamoto S Yamashita K Murakami Y Nakamura 《FEBS letters》1989,256(1-2):47-50
The release of prorenin as well as renin from rat renal slices was confirmed by a rat prorenin-prosegment ELISA system and an assay system for determining the renin activity. A significant increase of the prorenin release was found by adding rat submandibular gland extract to the slice medium, indicating the existence of a prorenin-releasing stimulator (PRS) in the extract. The pI and molecular mass of PRS were 8.5-8.7 and 28-30 kDa, respectively. The PRS was completely inactivated by boiling or a proteinase treatment. 相似文献
48.
Endothelin, the most potent vasoconstrictor found in nature, is thought to be important in the regulation of blood pressure and/or local blood distribution. Human placenta cDNA fragment encoding preproendothelin-1 (preproET-1) and its carboxyl terminal mature precursor (C-matured precursor) was expressed in E. coli. These products were characterized by both enzyme immunoassay and Western blot analysis. 相似文献
49.
H Miyata H Hayashi S Suzuki N Noda A Kobayashi H Fujiwake M Hirano N Yamazaki 《Biochemical and biophysical research communications》1989,163(1):500-505
Isolated rat heart myocytes were loaded with both the Ca2+ sensitive fluorescent probe fura-2/AM and the fluorescent pH indicator 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF/AM). Changes in [Ca2+]i and pHi were measured simultaneously using digitized video fluorescence microscopy. In measurement of [Ca2+]i and pHi, the ratios of dual-loaded cells were not different from single-loaded cells. Using this method, [Ca2+]i and pHi in myocytes were 48 +/- 7 nM and 7.17 +/- 0.05. It is concluded that [Ca2+]i and pHi could be measured simultaneously in isolated myocyte using dual-loading of fura-2 and BCECF. 相似文献
50.
H Wada S Ohno K Kubo C Taya S Tsuji S Yonehara K Suzuki 《Biochemical and biophysical research communications》1989,165(1):533-538
By the use of cloned cDNAs for protein kinase C isozymes alpha, beta I, beta II, gamma, and those for novel protein kinase C, epsilon and zeta, the expression of the corresponding mRNA species was examined in various mouse tissues, human lymphoid cell lines, and mouse cell lines of neuronal origin. In adult brain, mRNAs for all the isozymes of PKC family are expressed. However, the expression of these mRNA species in brain is low at birth. A similar pattern of expression was also observed for beta I/beta II mRNAs in spleen. These expression patterns are in clear contrast to that for beta I/beta II mRNAs in thymus where the mRNAs are expressed at birth and the levels of expression decrease with age. Human lymphoid cell lines express large amounts of PKC beta mRNAs in addition to PKC alpha. Further, nPKC epsilon mRNA is expressed in some of these cell lines. On the other hand, all the mouse cell lines of neuronal origin tested express nPKC epsilon and zeta in addition to PKC alpha. In a mouse neuroblast cell line, Neuro 2a, down modulation of mRNAs for both PKC alpha and nPKC epsilon was observed in association with in vitro differentiation. 相似文献