全文获取类型
收费全文 | 136034篇 |
免费 | 9725篇 |
国内免费 | 24篇 |
出版年
2023年 | 714篇 |
2022年 | 825篇 |
2021年 | 1716篇 |
2020年 | 1428篇 |
2019年 | 1618篇 |
2018年 | 3257篇 |
2017年 | 2957篇 |
2016年 | 4080篇 |
2015年 | 5943篇 |
2014年 | 6214篇 |
2013年 | 8215篇 |
2012年 | 9947篇 |
2011年 | 9314篇 |
2010年 | 6102篇 |
2009年 | 4826篇 |
2008年 | 7660篇 |
2007年 | 7592篇 |
2006年 | 6950篇 |
2005年 | 6385篇 |
2004年 | 6068篇 |
2003年 | 5516篇 |
2002年 | 5132篇 |
2001年 | 2982篇 |
2000年 | 2817篇 |
1999年 | 2470篇 |
1998年 | 1187篇 |
1997年 | 919篇 |
1996年 | 819篇 |
1995年 | 829篇 |
1994年 | 782篇 |
1993年 | 639篇 |
1992年 | 1561篇 |
1991年 | 1464篇 |
1990年 | 1298篇 |
1989年 | 1218篇 |
1988年 | 1123篇 |
1987年 | 996篇 |
1986年 | 898篇 |
1985年 | 965篇 |
1984年 | 808篇 |
1983年 | 704篇 |
1982年 | 567篇 |
1981年 | 510篇 |
1979年 | 710篇 |
1978年 | 528篇 |
1977年 | 473篇 |
1976年 | 468篇 |
1975年 | 527篇 |
1974年 | 575篇 |
1973年 | 561篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Summary The gap junction morphology was quantified in freeze-fracture replicas prepared from rat auricles that had been either quickly frozen at 6 K or chemically fixed by glutaraldehyde, in a state of normal cell-to-cell conduction or in a state of electrical uncoupling. The general appearance of the gap junctions was similar after both preparative procedures. A quantitative analysis of three gap junctional dimensions provided the following measurements in the quickly frozen conducting auricles (mean±sd): (a) P-face particles' diameter 8.27±0.74 nm (n =5709), (b) P-face particles' center-to-center distance 10.78±2.12 nm (n=4800), and (c) E-face pits' distance 9.99±2.19 nm (n=1600). Corresponding values obtained from chemically fixed tissues were decreased by about 3% for the particle's diameter and about 5% for the particles' and pits' distances. Electrical uncoupling by the action of either 1 mM 2–4-dinitrophenol (DNP), or 3.5 mMn-Heptan-1-ol (heptanol), induced a decrease of the particle's diameter, which amounted to –0.69±0.01 nm (mean ±se) in the quickly frozen preparations and –0.71±0.01 nm in the chemically fixed ones. The particles' distance was decreased by –0.96±0.04 nm in the quickly frozen samples and by –0.90 ±0.03 nm in the chemically fixed ones and the E-face pits' distance was similarly reduced. All differences were statistically significant (P<0.001 for all dimensions). Electrical recoupling after the heptanol effect promoted a return of these gap junctional dimensions towards normal values, which was about 50% complete within 20 min. It is concluded that very similar morphological alterations of the gap junctional structure are induced in the mammalian heart by different treatments promoting electrical uncoupling and that these conformational changes appear independently of the preparative procedure. The suggestion that the observed decrease of the particles' diameter is genuinely related to the closing mechanism of the unit cell-to-cell channel set in thei centers is thus confirmed. 相似文献
942.
Mini-F E protein: the carboxy-terminal end is essential for E gene repression and mini-F copy number control 总被引:5,自引:0,他引:5
F Bex P Piérard A Desmyter P Drèze M Colet M Couturier 《Journal of molecular biology》1986,189(2):293-303
Mini-F is a segment of the conjugative plasmid F consisting of two origins of replication flanked by regulatory regions, which ensure a normal control of replication and partitioning. Adjacent to the ori-2 origin is a complex coding region that consists of the E gene overlapped by three open reading frames with the coding potential for 9000 Mr polypeptides here designated 9 kd-1, 9 kd-2 and 9 kd-3. In this paper, we show that open reading frame 9 kd-3 is preceded by active promoter and Shine-Dalgarno sequences. The E coding region specifies: an initiator of replication, which acts at the ori-2 site; a function that negatively regulates the expression of the E gene; and a function involved in mini-F copy number control. To assign one of these functions to one of the overlapping coding sequence, we have isolated, characterized and sequenced mutations mapping in the E coding region. In this paper, we analyse two mutations (cop5 and pla25) that abolish the repression of the E gene. As these mutations affect the primary structure of protein E itself but not the 9 kd polypeptides, we conclude that protein E takes part in the negative regulation of its own synthesis. In addition, the localization of the cop5 and pla25 mutations indicates that the carboxy-terminal end of the E protein is involved in the autorepression function. The cop5 mutation causes an eightfold increase of the mini-F copy number. The pla25 mutation leads to the inability of the derived mini-F plasmid to give rise to plasmid-harbouring bacteria. The ways in which the cop5 and pla25 mutations may lead to such phenotypes are discussed in relation to the different functions mapping in the E coding sequence. 相似文献
943.
The toxic effects of sucrose and the conditions of in-straw glycerol removal after freezing and thawing were studied using Day-3 mouse embryos. At 20 degrees C, exposure to less than or equal to 1.0 M-sucrose for periods up to 30 min had no adverse effects on freshly collected embryos. At 25 and 36 degrees C, however, greater than or equal to 1.0 M-sucrose significantly reduced the developmental potential (P less than 0.001). In the freezing experiments the embryos were placed in 0.5 ml straws containing 40 microliters freezing medium separated by an air bubble from 440 microliters sucrose solution. The straws were frozen rapidly in the vapour about 1 cm above the surface of liquid nitrogen. The post-thaw viability was substantially better after sucrose dilution at 20 degrees C than at 36 degrees C. Mixing the freezing medium with the sucrose diluent immediately after thawing further improved the rate of survival relative to mixing just before freezing (P less than 0.001). The best survival was obtained when the freezing medium contained 3.0 M-glycerol + 0.25 M-sucrose; it was mixed with the diluent after thawing and the glycerol was removed at 20 degrees C. Under such conditions the sucrose concentration in the diluent had no significant effect on the rate of development (0.5 M, 69%; 1.0 M, 73%; 1.5 M, 64%). The results show that during sucrose dilution the temperature should be strictly controlled and suggest that intracellular and extracellular concentrations of glycerol are important in the cryoprotection of embryos. 相似文献
944.
Role of equilibration before rapid freezing of mouse embryos 总被引:1,自引:0,他引:1
The time requirements for permeation by glycerol and dehydration by sucrose before rapid freezing of Day-3 mouse embryos by direct transfer to -180 degrees C were studied. When the embryos were equilibrated in 2.0, 3.0, or 4.0 M-glycerol + 0.25 M-sucrose for 2.5 to 40 min, the post-thaw viability increased (P less than 0.001) with the length of equilibration period at 4 degrees C. At 20 degrees C the volume of embryos increased with the duration of equilibration up to 20 min (P less than 0.001), but the post-thaw viability was not affected. The effect of equilibration in glycerol-sucrose was determined at 20 degrees C for embryos which were previously permeated by glycerol, dehydrated by sucrose or left in PBS + 5% FCS. The survival of previously permeated embryos was not affected by equilibration for 1-16 min in glycerol-sucrose. The maximum survival rate was attained after shorter equilibration in glycerol-sucrose for embryos without pretreatment (4 min) than for those previously dehydrated (8 min). It is concluded that increases in the intracellular glycerol level are beneficial for the viability of rapidly frozen mouse embryos and previous or concomitant exposure to sucrose unfavourably affects glycerol permeation. 相似文献
945.
Taurine Levels in Discrete Brain Nuclei of Rats 总被引:7,自引:1,他引:6
Miklós Palkovits István Elekes Tibor Láng rás Patthy 《Journal of neurochemistry》1986,47(5):1333-1335
Concentrations of taurine have been measured in 44 microdissected rat brain nuclei or areas. Taurine is ubiquitously present and distributed unevenly in the rat brain: the ratio of the highest (pyriform cortex) to lowest (midbrain reticular formation) concentrations is 4.7:1. High taurine levels were found in cerebral cortical areas, caudate-putamen, cerebellum, median eminence, and supraoptic nucleus. Acute pain stress reduced taurine levels in the hypothalamus and the lower brainstem nuclei but not in cortical areas. Increased locomotor and behavioral activities following a high dose of amphetamine elevated taurine concentrations significantly in the substantia nigra and locus ceruleus. 相似文献
946.
Enhancive and suppressive effects of cytomegalovirus on human lymphocyte responses in vitro. 总被引:7,自引:3,他引:4
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Virus-specific lymphocyte proliferation in the presence of cytomegalovirus (CMV) without and with monocytes was studied in healthy persons. Three categories of lymphocyte response could be distinguished: seropositive low responders, naturally high responders, and lymphocyte populations responding well to CMV antigen in the presence of added CMV-incubated autologous monocytes. This latter category could be identified by preincubating autologous monocytes with CMV. CMV-seronegative persons were nonresponders. Early CMV antigens were produced in monocytes but not in lymphocytes by all CMV isolates. Infection of monocytes as detected by antibody to early viral protein did not appear to abort the antigen-presenting ability. The virus-specific responding lymphocytes were mainly of the T4+ phenotype. In contrast, addition of CMV to polyclonal mitogens significantly suppressed total lymphocyte DNA synthesis. CMV thus may have an enhanced virus-specific stimulatory effect on lymphocytes together with monocytes but a suppressive effect on the total lymphocyte population. 相似文献
947.
The effects of centrally administered kentsin (H-Thr-Pro-Arg-Lys-OH) on intestinal motility and on pain perception were investigated in rats chronically equipped with lateral ventricle catheters. Intestinal motility was recorded electromyographically from electrodes placed on the duodeno-jejunum; analgesia was evaluated by the hot-plate and tail-flick tests. Kentsin (4.0 ug/kg), injected intracerebroventricularly (ICV) 2 hours after the beginning of a meal, restores the "fasted" i.e. the migrating myoelectric complex of intestinal motility, while a 5 times higher dose administered subcutaneously was inactive. The ICV effect of kentsin was blocked by previous ICV administration of naloxone (400 ug/kg). In contrast, kentsin administered ICV (40 ug/kg) or SC (200 ug/kg) did not affect significantly (P greater than 0.05) the time latency in the two analgesic tests during 90 minutes after its administration and did not significantly modify the analgesic effects of (D5-Ala2, Met5) enkephalinamide. We conclude that kentsin when centrally administered acts on opiate receptors to alter gastrointestinal motility but without effects on pain perception. 相似文献
948.
Flora Sánchez Angeles Touriño Susana Traseira Agustín Pérez-Aranda Víctor Rubio Miguel A. Peñalva 《Molecular & general genetics : MGG》1986,205(2):248-252
Summary We cloned the Penicillium chrysogenum trpC gene from a genomic library by complementation of an Escherichia coli trpC mutant lacking phosphoribosylanthranilate isomerase activity. The gene ecodes a 2.7 kb poly(A)+ RNA. We localized the gene by sequence analysis in a 2.9 kb DNA insert found in the smallest plasmid selected from the library. Sequence data strongly suggest that the organization of the gene is similar to that described in other Ascomycetes. We found that a DNA fragment which codes only for the carboxy-terminal protion of the polypeptide is sufficient for complementation of the E. coli trpC9830 mutation. 相似文献
949.
Membrane lipid changes in root cells of rape (Brassica napus) as a function of water-deficit stress 总被引:1,自引:0,他引:1
Rape seedlings ( Brassica napus L. cv. Brink) were exposed to repeated water-deficit stress. The water-stress program started after 19 days of growth and consisted of three 24 h stress periods interspersed with 24 h rewatering periods. After the third stress period the seedlings were harvested and the membrane lipids of the roots were extracted, isolated and quantified. The stress caused an increased ratio of dry weight roots/shoot. Furthermore, the total amount of acyl lipids as well as phospholipids decreased drastically. However, the relative distribution of individual phospholipids was constant and independent of stress. Free and esterified sterols showed only a small decrease in response to water stress. As a consequence the ratio free sterols/phospholipids increased from 0.07 in the control root cells to 0.15 in the stressed cells. The lipid changes are discussed in relation to membrane activity. 相似文献
950.
Cell Surfaces in Plant-Microorganism Interactions : VI. Elicitors of Ethylene from Colletotrichum lagenarium Trigger Chitinase Activity in Melon Plants 总被引:5,自引:0,他引:5
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Treatment of melon leaves or seedlings with elicitors of Colletotrichum lagenarium, a fungal pathogen of melon, increases chitinase activity. In treated leaves, chitinase is enhanced within the first 6 hours and becomes 2 to 10 times higher than in control leaves after 24 hours. Ethylene is increased simultaneously and is correlated with chitinase elicitation. In the presence of aminoethoxyvinylglycine, an inhibitor of ethylene synthesis, both elicitor-induced ethylene and elicitor-induced chitinase are inhibited. This inhibition is overcome by added exogenous ethylene. On the other hand, 1-aminocyclopropane-1-carboxylic acid the direct precursor of ethylene, triggers chitinase activity. Chitinase elicitation is thought to be a protein synthesis dependent process, as it does not occur in the presence of cycloheximide. 相似文献