全文获取类型
收费全文 | 88737篇 |
免费 | 7933篇 |
国内免费 | 47篇 |
专业分类
96717篇 |
出版年
2023年 | 291篇 |
2022年 | 739篇 |
2021年 | 1594篇 |
2020年 | 907篇 |
2019年 | 1144篇 |
2018年 | 1471篇 |
2017年 | 1288篇 |
2016年 | 2186篇 |
2015年 | 3709篇 |
2014年 | 4142篇 |
2013年 | 4938篇 |
2012年 | 6664篇 |
2011年 | 6622篇 |
2010年 | 4260篇 |
2009年 | 3869篇 |
2008年 | 5552篇 |
2007年 | 5565篇 |
2006年 | 5354篇 |
2005年 | 5151篇 |
2004年 | 5011篇 |
2003年 | 4774篇 |
2002年 | 4493篇 |
2001年 | 919篇 |
2000年 | 697篇 |
1999年 | 1025篇 |
1998年 | 1258篇 |
1997年 | 848篇 |
1996年 | 782篇 |
1995年 | 685篇 |
1994年 | 643篇 |
1993年 | 693篇 |
1992年 | 599篇 |
1991年 | 558篇 |
1990年 | 506篇 |
1989年 | 434篇 |
1988年 | 451篇 |
1987年 | 376篇 |
1986年 | 354篇 |
1985年 | 447篇 |
1984年 | 543篇 |
1983年 | 424篇 |
1982年 | 526篇 |
1981年 | 516篇 |
1980年 | 439篇 |
1979年 | 325篇 |
1978年 | 351篇 |
1977年 | 302篇 |
1976年 | 286篇 |
1975年 | 219篇 |
1974年 | 277篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Rodman Wilson William J. Mills Jr. Donald R. Rogers Michael T. Propst 《The Western journal of medicine》1978,128(6):471-476
Between 1903 and 1975 about 1 percent of climbers on Mount McKinley (Denali) and Mount Foraker in Alaska died. In 1976 a total of ten (1.7 percent) of 587 mountaineers died, but this rate of death was not significantly higher than previously.Nineteen percent of climbers in 1976 suffered major or minor injuries, illness or death. Acute mountain sickness (AMS), frostbite and fractures were common. Thirty-three rescues or retrievals of bodies were mounted at a cost of more than $82,000.Inexperience (particularly with arctic mountaineering), poor leadership, faulty equipment and undue reliance on rescue by helicopter contributed to the alarming incidence of accident, illness and death on big peaks in Mount McKinley National Park in 1976. 相似文献
992.
The heavy and light chains of pooled antibodies of the hybodont shark,Heterodontus francisci (horned shark), were subjected to amino acid sequence analysis. Yield determinations showed that more than 90% of the available polypeptides in the respective pools were sequenced. The heavy chains were homogeneous in the initial framework segment and showed a sequence homology of approximately 70% with the corresponding region of the more recently evolved nurse shark and a 45% homology with a human myeloma heavy chain. The light chains were less homogeneous and not identifiable as either kappa or lambda chains as known in higher species. The first half-cystine characteristics of the variable domain intrachain disulfide bridge of immunoglobulins was present in the same position (22 for heavy chains; 23 for light chains) in the horned shark as in mammalian species. The sequence analysis also suggested the presence of a hypervariable region in the horned shark light chains. The combined data imply that the antigen-binding function of immunoglobulins is mediated in much the same manner in this primitive shark as in more recently evolved species, including mammals. 相似文献
993.
Elongation factor G (EF-G) is rapidly inactivated when irradiated at 253.7 nm. The inactivation follows first-order single-hit kinetics with a quantum efficiency of 3.15 × 10?5 μmol/μE. Inclusion of either GTP or GDP in the irradiation mixture does not alter the kinetics of inactivation, but does result in the covalent attachment of nucleotide to between 10 and 20% of the EF-G. This relatively low percentage of cross-linking is due to the rapid rate of photoinactivation as compared to the slower rate of covalent attachment. If EF-G is reacted before irradiation with N-ethylmaleimide, a modification known to block the nucleotide binding site [Rohrbach and Bodley (1976) J. Biol. Chem.251, 930], essentially no nucleotide can be photo-cross-linked to EF-G. Treatment of the photo-cross-linked GTP-EF-G with Raney nickel led to the liberation of the nucleotide moiety, indicating that the photo-cross-link to EF-G occurred through a sulfur atom. Although the formation of the EF-G nucleotide complex has been shown to be an obligatory first step in the formation of the EF-G nucleotide ribosome complex [Rohrbach and Bodley (1976) Biochemistry15, 4565], the covalent EF-G-nucleotide adduct cannot form a ternary complex with the ribosome. The presence of both nucleotide and ribosomes during irradiation drastically alters the kinetics of inactivation. The inactivation under these conditions follows multiple-hit kinetics with an initial period during which no EF-G activity is lost. Following this lag period, EF-G is inactivated at the same rate at which ribosomes lose their ability to bind EF-G. No nucleotide is cross-linked to EF-G or the ribosome under these conditions. 相似文献
994.
The effects of high oxygen pressure on pyruvate dehydrogenase (pyruvate: lipoate oxidoreductase (decarboxylating and acceptor-acylating), EC 1.2.4.1) activity, tissue concentration of ATP, and CO2 production from glucose were studied in rat brain cortical slices. The increase in pyruvate dehydrogenase activity and the lowering of cellular ATP, occurring during potassium-induced depolarization at 1 atm of oxygen, were reversed by increasing the oxygen pressure to 5 atm. When brain slices were incubated at 1 atm oxygen with [U-14C]glucose, a high potassium medium approximately doubled the production of 14CO2. Oxygen at 5 atm abolished this potassium-dependent increase in 14CO2 production with no significant effect on glucose oxidation in normal Krebs-Ringer phosphate medium. Adding 4 atm helium to 1 atm oxygen did not interfere with the ability of potassium ions to activate pyruvate dehydrogenase, lower ATP, or increase glucose oxidation. The results show that toxic effects of hyperbaric oxygen, not manifest in “resting” tissue, may be revealed during stress such as potassium depolarization. The site of the toxic effects of oxygen is probably the cell membrane where excess oxygen appears to interfere with the action of the sodium pump, calcium transport or other processes stimulated by increased concentrations of extracellular potassium. 相似文献
995.
Enzymic analyses of aqueous extracts of barley obtained at 25–100° demonstrated an exponential relationship between β-d-glucan content and temperature. Purified β-d-glucans, in particular those from extracts made at 40° and 100°, were compared by the Smith-degradation method followed by g.l.c. analysis of the methyl and the trimethylsilyl ethers of the products. All extracts yielded qualitatively the same products, including oligosaccharides which were characteristic of sequences of adjacent (1→3)-linkages. The material extracted at 100° contained relatively more of these sequences, and also showed a much higher specific viscosity, than did the material extracted at low temperatures. The structural implications of these findings are discussed. 相似文献
996.
997.
A method has been developed for the measurement of DNA synthesis in vivo using the incorporation of multilabeled, non-radioactive thymidine. Simultaneous intraperitoneal injection of hexalabeled thymidine and tritiated thymidine into a normal adult rat resulted in the incorporation of both labeled nucleosides into the DNA of cells undergoing replication. The DNA of several tissues and organs was analysed, including liver, thymus, spleen, bone marrow, and small intestine. Following extraction with hot trichloroacetic acid, acid hydrolysis, and thin-layer chromatography of the hydrolysates, the isotopic compositions of the thymine products were determined by field ionization mass spectrometry and by scintillation counting. The relative incorporation of radioactive and stable isotope-labeled thymidine was similar in all tissues, and corresponded to the ratio of the two labeled nucleosides in the injected material. These results indicate the feasibility of utilizing thymidine multilabeled with stable isotopes for measurement of cellular proliferation rates in conjunction with cancer therapy. 相似文献
998.
Michael A. Kirschenbaum 《Prostaglandins & other lipid mediators》1977,13(6):1103-1112
It has been suggested that increased prostaglandin release may mediate the natriuresis seen during the administration of renal vasodilators. To further investigate this possibility, either acetylcholine (40 μg/min) or bradykinin (5 μg/min) was infused into the left renal artery of anesthetized dogs previously given an inhibitor of prostaglandin synthesis. During the infusion of either vasodilator drug in the prostaglandin-inhibited dogs, urinary sodium excretion increased to a similar degree as in a group of normal dogs with intact prostaglandin synthesis. These studies therefore do not confirm that prostaglandins play a significant role in the natriuresis of drug-induced vasodilatation. 相似文献
999.
1000.