首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3645篇
  免费   339篇
  国内免费   2篇
  2021年   40篇
  2020年   23篇
  2019年   41篇
  2018年   48篇
  2017年   34篇
  2016年   58篇
  2015年   102篇
  2014年   110篇
  2013年   157篇
  2012年   185篇
  2011年   164篇
  2010年   105篇
  2009年   106篇
  2008年   164篇
  2007年   160篇
  2006年   160篇
  2005年   178篇
  2004年   170篇
  2003年   179篇
  2002年   176篇
  2001年   100篇
  2000年   73篇
  1999年   61篇
  1998年   63篇
  1997年   50篇
  1996年   47篇
  1995年   35篇
  1994年   33篇
  1993年   43篇
  1992年   63篇
  1991年   44篇
  1990年   48篇
  1989年   28篇
  1988年   47篇
  1987年   50篇
  1986年   43篇
  1985年   44篇
  1984年   47篇
  1983年   50篇
  1982年   51篇
  1981年   47篇
  1980年   36篇
  1978年   32篇
  1977年   31篇
  1976年   22篇
  1975年   30篇
  1974年   36篇
  1973年   32篇
  1972年   31篇
  1968年   24篇
排序方式: 共有3986条查询结果,搜索用时 484 毫秒
21.
Indole compounds secreted byFrankia sp. HFPArI3 in defined culture medium were identified with gas chromatography-mass spectrometry (GC-MS). WhenFrankia was grown in the presence of13C(ring-labelled)-L-tryptophan,13C-labelled indole-3-acetic acid (IAA), indole-3-ethanol (IEtOH), indole-3-lactic acid (ILA), and indole-3-methanol (IMeOH) were identified.High performance liquid chromatography (HPLC) and GC-MS with selected ion monitoring were used to quantify levels of IAA and IEtOH inFrankia culture medium. IEtOH was present in greater abundance than IAA in every experiment. When no exogenous trp was supplied, no or only low levels of indole compounds were detected.Seedling roots ofAlnus rubra incubated in axenic conditions in the presence of indole-3-ethanol formed more lateral roots than untreated plants, indicating that IEtOH is utilized by the host plant, with physiological effects that modify patterns of root primordium initiation.  相似文献   
22.
Macvicaria obovata (Molin) n. comb. is redescribed from Sparus aurata off the Mediterranean coast of France and a neotype is designated. Specimens from Oblada melanura off Israel may belong to the same species. It is mainly characterized by the uterine extension between the ovary and the anterior testis and the lack of a vitelline confluence in the forebody. M. maillardi n. sp. is also described from Sparus aurata off the southern coast of France. Its uterus does not pass between ovary and testes and the vitelline fields are confluent in the forebody. M. crassigula (Linton) n. comb. is redescribed from Diplodus annularis, D. sargus, D. vulgaris, Pagellus erythrinus and Sparus pagrus off Corsica, Calamus bajonado off Bermuda, Spicara smaris, D. annularis off Yugoslavia, D. sargus off Israel, and D. cervinus, Sparodon durbanensis and Cheilodactylus fasciatus in the SW Indian Ocean. It is similar to M. maillardi, but differs in being smaller, having a greater sucker ratio and a larger pharynx. It may well be a species-complex. M. dubia (Stossich) n. comb. is redescribed from Oblada melanura off Corsica and Yugoslavia. It is similar to M. maillardi and M. crassigula, but has a more anteriorly situated genital pore.  相似文献   
23.
A new species, Neolepidapedon smithi, is described from the fish Mora moro in the north-eastern Atlantic Ocean and is distinguished from other members of the genus. It differs from its congeners in the extension of the vitellarium into the forebody and/or the body shape. The genus Neolepidapedon is reviewed and a key to species given. The genus has previously contained up to 20 species, but is here restricted to eight species, which possess a very thick male duct wall within the cirrus-sac, a narrow internal seminal vesicle, an excretory vesicle restricted to the hindbody and no eye-spots. The remaining nominal species with a relatively thin-walled internal male duct, a globular internal seminal vesicle, an excretory system which reaches into the forebody and eye-spots are considered to belong to a separate group which is related to Opechona. Some of these species were included by Yamaguti (1971) in his new subgenus Neolepidapedon (Neolepidapedoides), which is raised to full generic status. Eight new combinations are made: Neolepidapedoides belizensis (Fischthal, 1977), N. dollfusi (Durio & Manter, 1968), N. equilatum (Siddiqi & Cable, 1960), N. hypoplectri (Nahhas & Cable, 1964), N. israelense (Fischthal, 1980), N. macrum (Overstreet, 1969), N. medialunae (Montgomery, 1957) and N. mycteropercae (Siddiqi & Cable, 1960).  相似文献   
24.
Pulmonary surfactant contains phospholipids including dipalmitoyl-phosphatidylcholine and three surfactant-associated proteins designated SP-A, SP-B and SP-C. A cDNA for rabbit SP-B has been isolated from a fetal (30 days gestation) rabbit lung cDNA library constructed in lambda gt11. The cDNA and deduced amino acid sequences show strong homology with the cDNAs and predicted 40 kDa proproteins for human and canine SP-B. Strong homology is also observed with the amino acid sequences directly determined for the mature 8 kDa bovine and porcine SP-B isolated from lung lavage. SP-B is remarkable for its high cysteine and proline content and for the hydrophobic nature of the organic solvent-soluble, mature protein. In vitro translation of sense but not antisense RNA transcribed from the cDNA led to the production of 40 kDa and 32 kDa proteins. These proteins were immunoprecipitated by an antibody raised against bovine SP-B. Northern blot analysis revealed the mRNA for rabbit SP-B appears in fetal rabbit lung late in gestation and falls slightly in the neonate.  相似文献   
25.
26.
27.
The role of the hypothalamic tripeptide L-prolyl-L-leucyl-glycinamide (PLG) in modulating the agonist binding to bovine striatal dopamine D2 receptor was investigated using a selective high-affinity agonist, n-propylnorapomorphine (NPA). PLG caused an enhancement in [3H]NPA binding in striatal membranes in a dose-dependent manner, the maximum effect being observed at 10(-7)-10(-6) M concentration of the tripeptide. The Scatchard analysis of [3H]NPA binding to membranes preincubated with 10(-6) M PLG revealed a significant increase in the affinity of the agonist binding sites. In contrast, there was no effect of PLG on the binding pattern of the antagonist [3H]spiroperidol. The antagonist versus agonist competition curves analyzed for agonist high- and low-affinity states of the receptor displayed an increase in the population and affinity of the high-affinity form of the receptor with PLG treatment. The low-affinity sites concomitantly decreased with relatively small change in the affinity for the agonists. Almost similar results were obtained when either NPA or apomorphine was used in the competition experiments. A partial antagonistic effect of PLG on 5'-guanylylimidodiphosphate [Gpp(NH)p]-induced inhibition of high-affinity agonist binding was also observed, as the ratio of high- to low-affinity forms of the receptor was significantly higher in the PLG-treated membranes compared to the controls. Direct [3H]NPA binding experiments demonstrated that PLG attenuated the Gpp(NH)p-induced inhibition of agonist binding by increasing the EC50 of the nucleotide (concentration that inhibits 50% of the specific binding). No effect of PLG on high-affinity [3H]NPA binding, however, could be observed when the striatal membranes were preincubated with Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
28.
Embryonic cell number in miniature pigs inbred for specific SLA haplotypes (a, c, and d) was determined on Day 6 by nuclear staining and, on Days 9 and 11, by DNA analyses (first day of oestrus = Day 0). Pigs exhibiting first behavioural oestrus at 08:00 h were hand-mated to an SLA homozygous boar 12 and 24 h later. Numbers of embryos flushed from uteri at 08:00-10:00 h on Days 6, 9 and 11 were greater (P less than 0.05) for SLAd females than for SLAa or SLAc females, which did not differ (8.2 vs 6.8 and 6.2, respectively). Recovery rates (embryos recovered/CL number) were similar, averaging 75.8% for all three SLA haplotypes. Embryos from SLAd dams contained fewer blastomeres (23 cells) on Day 6 than did embryos from SLAa (89 cells) or SLAc (79 cells) females. The reduced cell numbers of SLAd vs SLAa or SLAc embryos continued to Day 9 (28 vs 107 and 67 ng DNA/embryo) and Day 11 (167 vs 674 and 586 ng DNA/embryo). These results suggest an effect of the SLA complex on preimplantation embryonic development.  相似文献   
29.
An enzyme with FAD pyrophosphatase activity was extracted from human placental syncytiotrophoblast microvilli and purified to near-homogeneity. The enzyme has been identified as 5'-nucleotidase by several criteria. Throughout purification, parallel increases in the specific activities of FAD pyrophosphatase and AMP phosphatase were observed. The enzyme was a glycoprotein with a subunit molecular weight of 74,000. EDTA treatment resulted in a marked decline in both activities, and restoration of FAD pyrophosphatase activity but not 5'-nucleotidase activity was accomplished by the addition of Co2+ or, to a lesser extent, Mn2+. The substrate specificity of the 5'-nucleotidase activity that we observed agreed closely with the results of others. The pyrophosphatase activity was relatively specific for FAD. ADP, ATP, NAD(H), and FMN were not hydrolyzed, and ADP strongly inhibited both activities. For FAD pyrophosphatase activity, a Km of 1.2 x 10(-5) M and a Vmax of 1.1 mumol/min/mg protein were determined in assays performed in the presence of Co2+. In the absence of added Co2+, the Vmax declined but the Km was unchanged. For 5'-nucleotidase (AMP as substrate) the Km was 4.1 x 10(-5) M and the Vmax 109 mumol/min/mg protein. Hydrolysis of FMN to riboflavin was observed in partially purified detergent extracts of microvilli that contained alkaline phosphatase activity and lacked FAD pyrophosphatase and 5'-nucleotidase activity. The presence of both FAD pyrophosphatase and FMN phosphatase activities in syncytiotrophoblast microvilli supports the view that the placental uptake of vitamin B2 involves the hydrolysis of FAD and FMN to riboflavin which is then absorbed, a sequence postulated for intestinal absorption and liver uptake.  相似文献   
30.
The developmental expression of adrenocortical steroid hydroxylases was studied in bovine fetuses from 40 to 280 days gestational age. The expression of P-450(17 alpha) is first detected at a gestational age of 50 days and reaches a maximum at 60-70 days. The expression of P-450(17 alpha) then declines and is nondetectable at a gestational age of 100 days. P-450(17 alpha) is not expressed again until about 240 days, i.e. shortly before birth (approximately 280 days). P-450scc, P-450c21, P-450(11 beta) and adrenodoxin were present in fetal adrenals throughout gestation. This "on-off-on" pattern of P-450(17 alpha) expression during fetal development was associated with a corresponding episodic production of cortisol. Immunoreactive corticotropin (ACTH) levels in fetal plasma were elevated in small fetuses (corresponding to less than or equal to 100 days) and in near-term fetuses (corresponding to greater than 250 days) compared with those in mid-gestation fetuses. In primary culture, adrenal cells from mid-gestation fetuses contained no detectable P-450(17 alpha) but rapidly responded to ACTH with an increase in P-450(17 alpha) protein and mRNA. The tissue specificity of the developmental patterns is emphasized by the fact that both P-450(17 alpha) and P-450scc were detectable throughout the development of the fetal testes, whereas only P-450scc was detectable in fetal bovine ovary prior to 200 days. Thus, in fetal bovine adrenal it appears that ACTH is the major regulatory factor effecting the intermittent presence of P-450(17 alpha), whereas the presence of the other steroid hydroxylases is either regulated by additional factors or shows a much different sensitivity to ACTH.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号