全文获取类型
收费全文 | 3241篇 |
免费 | 270篇 |
专业分类
3511篇 |
出版年
2021年 | 40篇 |
2020年 | 21篇 |
2019年 | 28篇 |
2018年 | 34篇 |
2017年 | 29篇 |
2016年 | 58篇 |
2015年 | 118篇 |
2014年 | 124篇 |
2013年 | 173篇 |
2012年 | 196篇 |
2011年 | 171篇 |
2010年 | 102篇 |
2009年 | 83篇 |
2008年 | 145篇 |
2007年 | 139篇 |
2006年 | 132篇 |
2005年 | 140篇 |
2004年 | 121篇 |
2003年 | 108篇 |
2002年 | 121篇 |
2001年 | 94篇 |
2000年 | 85篇 |
1999年 | 88篇 |
1998年 | 51篇 |
1997年 | 42篇 |
1996年 | 31篇 |
1995年 | 42篇 |
1994年 | 33篇 |
1993年 | 35篇 |
1992年 | 64篇 |
1991年 | 61篇 |
1990年 | 64篇 |
1989年 | 63篇 |
1988年 | 66篇 |
1987年 | 44篇 |
1986年 | 39篇 |
1985年 | 54篇 |
1984年 | 35篇 |
1983年 | 46篇 |
1982年 | 32篇 |
1981年 | 35篇 |
1980年 | 22篇 |
1979年 | 24篇 |
1978年 | 23篇 |
1977年 | 30篇 |
1976年 | 19篇 |
1975年 | 21篇 |
1974年 | 20篇 |
1973年 | 23篇 |
1972年 | 20篇 |
排序方式: 共有3511条查询结果,搜索用时 15 毫秒
71.
Dieter Kabelitz Thomas H. Tötterman Magnus Gidlund Kenneth Nilsson Hans Wigzell 《Cellular immunology》1982,70(2):277-286
Stimulatory effects of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) on human T lymphocytes have been investigated. TPA was found to stimulate highly purified T cells (obtained by a three-step isolation procedure involving plastic adherence, nylon wool passage and Ig-anti-Ig column passage) in the absence of accessory cells (stimulation index of 5 to 10), whereas phytohemaglutinin (PHA) and concanavalin A (Con A) did not. This response was, however, increased by the addition of autologous adherent cells. Addition of TPA, but not adherent cells, induced T-cell proliferation in response to the nonmitogenic lectin, wheat germ agglutinin (WGA), while both adherent cells and TPA restored T-cell proliferation to mitogenic lectins such as PHA and Con A. Furthermore, TPA greatly increased the mixed-lymphocyte response of purified T cells to otherwise nonstimulating allogeneic cells such as T lymphocytes or tumor cells from some patients with chronic lymphocytic leukemia. These results suggest that TPA can directly act on human T cells to render them reactive to a variety of stimuli. 相似文献
72.
The Ca2(+)-and calmodulin-dependent protein kinase III, which specifically phosphorylates the eukaryotic elongation factor 2 (eEF-2), has been purified to apparent homogeneity from the post-ribosomal fraction of rabbit reticulocytes by an efficient four-step method. The method results in a more than 4000-fold purification of the enzyme. SDS-gel electrophoresis showed that the purified kinase contained only one polypeptide with the apparent molecular mass of 90 kDa. The kinase activity was associated with the 90-kDa protein as shown by analyzing the phosphorylating activity of SDS gel electrophoretically purified protein electroblotted to nitrocellulose membranes. The purified kinase was dependent on Ca2+, Mg2+ and calmodulin for activity. Kinetic analysis of the phosphorylation reaction indicates that the turnover number of the kinase was approximately 1 s-1. The Km for the two substrates ATP and eEF-2 was calculated to be approximately 100 microM and 10 microM, respectively. The activity of the kinase was competitively inhibited by cAMP. The inhibition constant Ki (0.5 mM) was found to be in the same order of magnitude as that calculated for the competitive product inhibition caused by ADP. GTP was ten-times less efficient as competitor, indicating that the kinase had a preference for adenosine nucleotides. Phosphorylation of eEF-2 did not interfere with the diphtheria-toxin-catalysed ADP-ribosylation of the factor nor did ADP-ribosylation inhibit phosphorylation. 相似文献
73.
74.
Treatment of reticulocyte polysomes with elongation factor eEF-2 and GTP led to an increased sensitivity of peptidyl-tRNA for puromycin as a result of the translocation from the ribosomal A-site to the P-site. Upon addition of an excess of the non-hydrolysable GTP analogue, GuoPP[CH2]P, the puromycin sensitivity decreased rapidly. The decrease in sensitivity required high concentrations of eEF-2 with half maximal effect at an eEF-2 concentration of around 1 microM. The data suggest either that peptidyl-tRNA had re-translocated back to the A-site due to the higher affinity of eEF-2 for the pre-translocation than for the post-translocation ribosome, or that the eEF-2-GuoPP[CH2]P complex blocks the peptidyl-transferase activity. 相似文献
75.
A preferential incorporation of dietary arachidonic acid (20:4, n-6) into chyle lipoprotein phospholipids, a relative resistance of 20:4 esters of chyle triacylglycerol (TG) to hydrolysis by lipoprotein lipase, a preferential utilization of 20:4 for phospholipid acylation, and a low rate of oxidation of 20:4 are factors that may contribute to the differences seen in the incorporation into tissue lipids between absorbed 20:4 and the predominant dietary 16-18 carbon fatty acids. In this study we fed [14C]20:4 and [3H]eicosapentaenoic acid (20:5, n-3) as free fatty acids in a fish oil emulsion to rats and analyzed the radioactivity in different tissue lipids after 1, 2, and 4 h. The purpose was to examine the degree of similarity in the fate of the two major eicosanoid precursors during the absorption of a fish oil meal. The recovery after 2 and 4 h of 14C exceeded that of 3H in lipids of small intestine, serum, liver, heart, kidneys, and spleen. The differences increased with time, e.g., the liver contained 9.7 (+/- 0.7)% 3H and 17.9 (+/- 1.4)% of the 14C (P less than 0.001), and the upper half of the small intestine 10.0 (+/- 0.8)% of the 3H and 22.8 (+/- 1.1)% of the 14C (P less than 0.001) after 4 h. The 14C and 3H radioactivity per g tissue after 4 h ranked as follows: liver and brown adipose tissue greater than kidneys greater than heart, lungs, spleen, and serum greater than colon greater than white adipose tissue and testes, the differences between tissues being up to 50-fold. There were up to fourfold variations in the 14C/3H ratios between tissues after 4 h, the highest value being observed in the heart and the lowest in white adipose tissue. Of the radioactivity retained in liver and intestine, more 14C and 3H was in phospholipids and less in triacylglycerol (TG), the differences being largest in the liver, e.g., after 4 h 57.6 (+/- 0.8)% of the 14C and 29.9 (+/- 0.9)% of the 3H (P less than 0.001) in the liver was in phosphatidylcholine (PC). In both intestine and liver the highest 14C/3H ratios were found in phosphatidylinositiol (PI). Also phosphatidylethanolamine (PE) contained more 14C than 3H but the quantitative differences were relatively small after 4 h. In heart the proportions of 3H and 14C found in PE and PI did not differ, whereas more of the 14C was in PC and more of the 3H was in cardiolipin and phosphatidylserine.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
76.
Plasma and tissue alterations of peptide YY and enteroglucagon in rats after colectomy. 总被引:4,自引:0,他引:4
A P Vukasin G H Ballantyne O Nilsson A J Bilchik T E Adrian I M Modlin 《The Yale journal of biology and medicine》1992,65(1):1-15
Peptide YY (PYY) and enteroglucagon are produced by endocrine cells of the colonic mucosa. PYY inhibits upper gastrointestinal motility, and enteroglucagon is trophic for small bowel mucosa. Adaptive increase in the production and release of these peptides may improve functional results after colorectal resections. We hypothesized that if segments of the colon were resected, then production and release of PYY and enteroglucagon would increase in the remaining segments of bowel. Animals which underwent colonic transections and partial resections had transient elevations of PYY up to 250 +/- 80 pmol/L, which dropped to control group levels in the second week following surgery. Rats with an abdominal colectomy had significantly greater PYY levels than all other groups from the third (208 +/- 30 pmol/L) to the thirty-eighth (100 +/- 16 pmol/L) week of the study. Circulating levels of enteroglucagon were elevated to 156 +/- 35 pmol/L in rats with a right hemicolectomy during the first week following surgery. Enteroglucagon levels did not significantly vary in the other groups studied. Both tissue PYY (413 +/- 33 pmol/gram) and tissue enteroglucagon (171 +/- 17 pmol/gram) were significantly elevated in the rectums of the rats with an abdominal colectomy, as compared to all other groups. The elevated tissue levels may thus account for the ability to maintain elevated plasma PYY. Double immunogold labeling of endocrine cells in the colorectal tissue for PYY and enteroglucagon revealed both peptides within the same endocrine cells and secretory granules. These studies support the hypothesis that circulating levels of PYY are elevated after major colonic resections and suggest that L-type endocrine cells may participate in adaptive responses which improve intestinal function following colonic surgery. 相似文献
77.
Orchid pollination biology 总被引:1,自引:0,他引:1
Anders Nilsson L 《Trends in ecology & evolution》1992,7(8):255-259
Orchids display many unsurpassed floral specializations, as both rewarders and frauds in their interaction with animal pollinators. Accumulating evidence indicates that their floral evolution is driven by pollinator traits and that expenditure for maximized sexual reproduction is parcelled out over their lifetimes in strategies for coping with pollinator and resource limitations. Recent advances in orchid pollination biology center mainly on floral evolutionary processes, pseudocopulation and other deceptive pollination systems, and flower and fruit production in relation to costs of sexual reproduction. 相似文献
78.
The cytologic features of fine needle aspiration smears from 28 hepatocellular carcinomas (HCC) were reviewed. All aspirations except one were guided. There were 14 well-, 11 moderately and 3 poorly differentiated HCC. The better-differentiated HCC were characterized by similarity of the tumor cells to hepatocytes (83%), cohesive cell clusters with a trabecular arrangement (72%) and presence of sinusoidal endothelial cells (66%). Other features included bile production (38%), atypical hepatocytic naked nuclei (52%), acinar formation (31%), intracytoplasmic vacuoles (14%) and abnormal vascular patterns (14%). Poorly differentiated HCC showed dyshesive pleomorphic cells. Unusual cytologic features from a well-differentiated HCC with fatty change and an HCC with a prominent acinar component are described. The identification of fatty change in dissociated well-differentiated hepatocytes or cytologic features suggestive of an adenocarcinoma do not preclude the diagnosis of HCC. The usefulness of cell blocks is emphasized. 相似文献
79.
80.
A Skogl?f I Nilsson S Gustafsson J Deinum P O G?the 《Biochimica et biophysica acta》1990,1041(1):22-30
The angiotensin I-converting enzyme (peptidyl-dipeptide hydrolase, EC 3.4.15.1) inhibitor, ramiprilat (2-[N-[(S)-1-ethoxycarbonyl-3-phenylpropyl]-L-Ala]-(1S,3S,5S)-2- azabicyclo[3.3.0]octane-3-carboxylic acid), is shown to exist in tow conformational isomers, cis and trans, which interconvert around the amide bond. The two conformers were separated by reversed-phase high-performance liquid chromatography. The conformers were identified by nuclear Overhauser effect measurements. From line shape analysis the isomerization rate constants were determined to be kcis----trans = 15 s-1 and ktrans----cis = 5 s-1 at 368 K in [2H]phosphate buffer (p2H 7.5). By enzyme kinetic studies using 3-(2-furylacryloyl)-L-Phe-Gly-Gly as substrate, the trans conformer was found to be the most potent enzyme inhibitor, whereas the cis conformer had a very low inhibitory effect. A new inhibition mechanism is presented for this type of slow, tight-binding inhibitors that contain an amide bond. This mechanism involves an equilibrium between the two conformers and the enzyme-bound inhibitor complex. 相似文献