全文获取类型
收费全文 | 38848篇 |
免费 | 3501篇 |
国内免费 | 19篇 |
专业分类
42368篇 |
出版年
2023年 | 236篇 |
2022年 | 494篇 |
2021年 | 1062篇 |
2020年 | 610篇 |
2019年 | 772篇 |
2018年 | 921篇 |
2017年 | 770篇 |
2016年 | 1196篇 |
2015年 | 1992篇 |
2014年 | 2210篇 |
2013年 | 2347篇 |
2012年 | 3319篇 |
2011年 | 3024篇 |
2010年 | 1941篇 |
2009年 | 1667篇 |
2008年 | 2400篇 |
2007年 | 2349篇 |
2006年 | 2124篇 |
2005年 | 2006篇 |
2004年 | 1843篇 |
2003年 | 1736篇 |
2002年 | 1614篇 |
2001年 | 339篇 |
2000年 | 238篇 |
1999年 | 341篇 |
1998年 | 413篇 |
1997年 | 273篇 |
1996年 | 245篇 |
1995年 | 209篇 |
1994年 | 215篇 |
1993年 | 219篇 |
1992年 | 216篇 |
1991年 | 203篇 |
1990年 | 189篇 |
1989年 | 165篇 |
1988年 | 173篇 |
1987年 | 162篇 |
1986年 | 128篇 |
1985年 | 129篇 |
1984年 | 165篇 |
1983年 | 124篇 |
1982年 | 153篇 |
1981年 | 145篇 |
1980年 | 135篇 |
1979年 | 103篇 |
1978年 | 107篇 |
1977年 | 99篇 |
1976年 | 82篇 |
1974年 | 71篇 |
1973年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
391.
Culture fluids of 4 type A, 6 type B, and 1 type EClostridium botulinum strains were compared for their toxicities in 9–11-day-old (infant) and adult mice. In intraperitoneal challenges, most toxins were about equally potent for infants and adults; the exceptions were 4 type B preparations, which were 500 to 5,000 times more lethal for infants. In orogastric challenges, samples were 10 to >15,000 times more toxic for infants, and the previously mentioned type B toxins constituted a group having the highest relative toxicity for infants. 相似文献
392.
Daniel P. Rossignol Aimee J. Roschelle William J. Lennarz 《Journal of cellular biochemistry》1981,15(4):347-358
We have attempted to identify a surface component of echinoderm eggs that is involved in the species-specific binding of sperm. Cell surface membranes from eggs of the sea urchins Strongylocentrotus purpuratus or Arbacia punctulata were radioiodinated, detergent-treated, and subjected to density-gradient centrifugation. In the presence of bindin, the complementary binding protein isolated from sperm, one component of the membranes sedimented to a different density. This membrane component bound-species specifically to sperm that had undergone the acrosome reaction. This binding led to an inhibition of the ability of treated sperm to fertilize eggs. Exhaustive proteolytic digestion of this receptor fraction yields a high molecular weight glycopeptide that can also bind to bindin. It therefore appears that this egg surface membrane fraction contains a functionally intact, species-specific receptor for sperm. 相似文献
393.
Daniel Johnston 《Cellular and molecular neurobiology》1981,1(1):41-55
The passive electrical cable properties of CA3 pyramidal neurons from guinea pig hippocampal slices were investigated by applying current steps and recording the voltage transients from 25 CA3 neurons, using a single intracellular microelectrode and a 3-kHz time-share system. Two independent methods were used for estimating the equivalent electrotonic length of the dendrites, L, and the dendritic to somatic conductance ratio, . The first method is similar to that used by Gorman and Mirolli (1972) and gave an average L of 0.96; the average was 2.44. The second method is derived here for the first time and assumes a finite-length cable with lumped soma. It is an exact solution for L and , using the slopes and intercepts of the first two peeled exponentials. The average L was 0.94; the average was 1.51. The results, using both methods, are in close agreement. The average membrane time constant for all 25 CA3 neurons was 23.6 ms, suggesting a large (23,600 cm2) average membrane resistivity. It is concluded that CA3 neurons are electronically short.This work was supported by Grants NS 11535 and NS 15772 from the National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health, U.S. Public Health Service. 相似文献
394.
395.
396.
Androgen receptors have been found in duck preen glands by using dextran-coated charcoal adsorption. They bound DHT with high affinity (KD = 0.2 nM), limited capacity (45 fmoles/mgP) and good specificity. They sedimented at 8 S in a sucrose gradient and were destroyed by pronase digestion and heating. An estrogen receptor having different binding specificity was also demonstrated. On the basis of a marked annual cycle of gonadal activity in ducks, this system appears appropriate for studying the regulation of sex steroid hormone receptors. 相似文献
397.
Hendrik C. Potgieter Nicolaas M.J. Vermeulen Daniel J.J. Potgieter Hein F. Strauss 《Phytochemistry》1977,16(11):1757-1759
An amino acid, lethal to New Hampshire chickens (LD50, 150 mg/kg) was isolated from dried sclerotia of the fungus Sclerotium rolfsii (Sacc.). Purification of the rather unstable compound was effected on a cation exchange column by means of displacement chromatography and the amino acid was crystallised from 80% methanol. A structure was assigned to the compound on the basis of available chemical and physical data, namely 2(S),3(R)-2- amino-3-hydroxypent-4-ynoic acid. Confirmation of this structure was gained by direct and indirect synthetic procedures. 相似文献
398.
The role of citrate and the citrate cleavage enzyme in lipidsynthesis in developing soybean cotyledons (Glycine max L. Merr.var. Harosoy 63) was investigated. The activity of the enzymewas inhibited by () hydroxycitrate, which is a specificinhibitor of citrate cleavage by this enzyme. Incorporationof label from citrate-1-14C and -5-14C indicated that the citratemolecule is cleaved between carbons 3 and 4. Acetyl CoA-14Cand oxaloacetate-14C phenylhydrazone were isolated as productsof the citrate cleavage reaction. The production of oxaloacetate-14C-phenylhdrazonefrom citrate-6-14C was carried out using a nucleotide free enzymepreparation and did not require the addition of ATP or CoA.Therefore it would appear that the citrate cleavage reactionis not CoA dependent in developing soybean seeds. Incorporationof pyruvate-2-14C into the crude lipid fraction was shown torequire both the particulate and soluble fractions. Apparentlyin soybeans, as in animal systems, pyruvate is oxidized by thepyruvate dehydrogenase complex and the acetyl CoA formed condenseswith oxaloacetate to produce citrate in the mitochondria. Citrateis then transported out of the mitochondria to the cytosol whereit is cleaved to form acetyl CoA for lipid synthesis.
1 Cooperative investigations of the Agricultural Research Service,U.S. Department of Agriculture, and Illinois Agricultural ExperimentStation.
2 This research represents partial fulfillment of the Ph. D.requirements of Daniel R. Nelson. Presently at Monsanto AgriculturalProducts Co., St. Louis, MO 63141, U.S.A. (Received January 12, 1977; ) 相似文献
399.
Robert L. Stolfi Leslie M. Stolfi Ruth A. Fugmann Daniel S. Martin 《Cancer immunology, immunotherapy : CII》1977,3(2):137-143
Summary Because it had been reported that splenectomy produces a tumor-inhibitory effect in several transplantable tumor systems when the surgery is performed before tumor challenge, we attempted to examine this putative immunological manipulation in a therapeutic situation.A spontaneous, autochthonous, murine breast tumor system was utilized in the present studies, and treatment was initiated in animals bearing large tumors (averaging 0.5 g). To amplify any immunological benefit ensuing from splenectomy, the tumor burden in the host was reduced by ancillary treatment with enucleative tumor surgery or with enucleative tumor surgery plus cytoreductive combination chemotherapy.Splenectomy performed in conjunction with enucleative tumor surgery was associated with an increment of cure in each of four separate experiments in comparison to treatment with enucleative tumor surgery alone. In four of five experiments utilizing different combinations or schedules of chemotherapeutic agents following enucleative tumor surgery, the addition of splenectomy resulted in a decrease in the rate of tumor recurrence as well as an increment in the cure rate. In the fifth experiment, splenectomy resulted in a decrease in the rate of tumor recurrence, but did not effect the ultimate cure rate.Although the nature of the immunological changes resulting from splenectomy are incompletely defined at present, these results provide encouragement in the search for immunological treatments for solid tumors.This work was supported in part by Contract No. N01-CM-73703 and Grant IR01CA-14768-01A1, both from the National Cancer Institute, National Institutes of Health, Department of Health, Education and Welfare, USA, and in part by a grant from the Chemotherapy Foundation of New York, Inc. 相似文献
400.
Daniel J. Crawford 《Phytochemistry》1978,17(9):1680-1681
A new chalcone glycoside, okanin 4′-O-diglucoside, is identified from the ray florets of Coreopsis petrophiloides. The known distribution of anthochlors in Coreopsis indicates that two of the most primitive sections produce in their floral parts complex glucosides of butein (and sulfuretin). The more advanced sections synthesize the monoglucosides of okanin (marein), butein (coreospin) and lanceoletin (lanceolin) and their corresponding aurones. The co-occurrence of marein and lanceolin is thus far restricted to members of sect. Coreopsis. 相似文献