全文获取类型
收费全文 | 1804篇 |
免费 | 134篇 |
国内免费 | 94篇 |
出版年
2023年 | 29篇 |
2022年 | 38篇 |
2021年 | 62篇 |
2020年 | 36篇 |
2019年 | 86篇 |
2018年 | 107篇 |
2017年 | 86篇 |
2016年 | 41篇 |
2015年 | 66篇 |
2014年 | 69篇 |
2013年 | 105篇 |
2012年 | 58篇 |
2011年 | 75篇 |
2010年 | 47篇 |
2009年 | 44篇 |
2008年 | 53篇 |
2007年 | 58篇 |
2006年 | 56篇 |
2005年 | 32篇 |
2004年 | 29篇 |
2003年 | 28篇 |
2002年 | 36篇 |
2001年 | 20篇 |
2000年 | 19篇 |
1999年 | 17篇 |
1998年 | 24篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 19篇 |
1992年 | 5篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1986年 | 8篇 |
1985年 | 28篇 |
1984年 | 81篇 |
1983年 | 91篇 |
1982年 | 76篇 |
1981年 | 57篇 |
1980年 | 45篇 |
1979年 | 51篇 |
1978年 | 25篇 |
1977年 | 45篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1974年 | 21篇 |
1973年 | 18篇 |
排序方式: 共有2032条查询结果,搜索用时 15 毫秒
21.
John W. Brown 《Journal of experimental marine biology and ecology》1984,80(2):197-206
The classification of lysine biosynthetic pathways in various organisms have been used to investigate their descent in evolution. We have attempted these determinations in the diatoms Amphora coffeaeformis var:perpusilla (Grunow Cleve.) and Phaeodactylum tricornutum (Bohlin). Additionally, we have verified earlier results of Vogel in a green alga, Chlorella pyrenoidosa strain Tx 71105 (Texas Culture Collection). Our research indicates that the diaminopimelic acid route is involved in all three organisms. While these studies do not exclude the possible co-existence of the α-aminoadipic acid route, the results imply a closer evolutionary relationship of pennate diatoms to bacteria and “classical” photosynthetic plants rather than to heterotrophic or mixotrophic fungi and atypical algal strains such as the Euglenophyta. 相似文献
22.
A. Ma Fernández-Peralta J. J. González-Aguilera 《Plant Systematics and Evolution》1986,152(3-4):167-183
Six closely related taxa of the sect.Eusideritis of the genusSideritis (S. leucantha, S. pusilla, S. flavovirens, S. granatensis, S. biflora andS. osteoxylla) are analysed to elucidate their phylogenetic relationships and position within the sect.Eusideritis. Meiotic behaviour, karyotype features, size and fertility of pollen grains, DNA amounts and seed protein profiles are reviewed. A polyploid origin of the group (from x = 7) and the further diversification through dysploidy and chromosome repatterning is postulated.S. osteoxylla is apparently of hybrid origin. 相似文献
23.
24.
Stuart J. Hamill David Y. Cooper Heinz Schleyer Otto Rosenthal 《Archives of biochemistry and biophysics》1983,224(2):614-624
The time-course kinetics of the cytochrome P-450-catalyzed dealkylations of the exogenous compounds benzphetamine, ethylmorphine, codeine, and 7-ethoxycoumarin were compared to the hydroxylation of the endogenous compound testosterone. Using liver microsomes from phenobarbital-induced rats, the time course of the demethylations of ethylmorphine, codeine, and especially benzphetamine was characterized by a fast initial phase of enzymatic activity and then a steady decline in the rate throughout the remainder of the reaction. In contrast, under the same experimental conditions, both the dealkylation of 7-ethoxycoumarin and the hydroxylation of testosterone showed no initial fast phase of activity and a constant rate of product formation for most of the remainder of the time course. The difference also held for the carbon monoxide inhibition studies in which the degree of inhibition of the demethylation reactions by a variety of CO:O2 mixtures was time dependent, in contrast to the constant, time-independent degree of CO inhibition of the other two reactions. The kinetics of the demethylation reactions could not be explained by enzyme destruction, back reaction, or product adduct formation and were further confirmed by measurements of the rate of O2 utilization and NADPH oxidation. The complexity of the demethylation reaction should be taken into consideration in any detailed studies of the monooxygenation reaction system. 相似文献
25.
The isolation of a brain peptide identical to the intestinal peptide PHI (peptide HI) is described. The peptide was isolated from porcine brain extract using a chemical assay method based on its C-terminal isoleucine amide structure. The complete amino acid sequence of the peptide was found to be: His-Ala-Asp-Gly-Val-Phe-Thr-Ser-Asp-Phe-Ser-Arg-Leu-Leu-Gly-Gln-Leu-Ser-Ala- Lys-Lys-Tyr-Leu-Glu-Ser-Leu-Ile-NH2. This sequence is identical to the intestinal peptide thus demonstrating PHI to be a brain-gut peptide. The role of PHI in the central nervous system as a neurotransmitter or neuromodulator is discussed. 相似文献
26.
Microbial fermentative preparation of L-[15N2]lysine and its tracer: application to serum amino acid kinetic studies 总被引:1,自引:0,他引:1
C S Irving C L Cooney L T Brown D Gold J Gordon P D Klein 《Analytical biochemistry》1983,131(1):93-98
The microorganism Brevibacterium flavum 21129 has been used to produce multigram batches of L-[15N2]lysine of high purity and isotopic enrichment by supplementation of the growth medium with (15NH4)2SO4 of 98.0 atom% excess. The doubly 15N-labeled lysine can be detected at dilutions 10 times greater than singly labeled lysine when isotope dilution curves are analyzed by gas chromatography-mass spectrometry. This enhanced sensitivity permits kinetic measurements of plasma free-lysine isotope content over a 300-fold dilution during 6 h following a single oral bolus of 5 mg/kg body wt. This inexpensive preparation method lends itself to the production of highly useful biochemical compounds for kinetic studies of human nutrition. 相似文献
27.
A simple procedure for obtaining useful narrow-pH-range ampholytes from inexpensive laboratory-synthesized ampholytes by preparative isoelectric focusing in Pevikon is described. The narrow range ampholytes prepared in this way are comparable to commercial ampholyte preparation as judged by conductivity, buffer capacity, pH gradient formation, and resolving power. These inexpensive narrow-range ampholytes are particularly well suited to preparative isoelectric focusing applications requiring large quantities of ampholytes. 相似文献
28.
Eleven patients with Raynaud's syndrome accompanied by monospecific IgG ANA, nine patients with Raynaud's syndrome in the absence of ANA, and nine normal volunteers were exposed to an ambient cold challenge during which time venous blood was continuously sampled. ANA negative patients were shown to have significantly higher levels of cortisol during a cold challenge than either ANA positive patients or normal controls, and exhibited significantly lower levels of plasma norepinephrine compared with normal controls. ANA positive patients did not differ significantly from normals in their neuroendocrine response to cold. It is suggested that the high plasma cortisol found in Raynaud's syndrome in the absence of ANA may be responsible for the vasospasticity in this group of patients. 相似文献
29.
Administration of somatostatin intracerebroventricularly to mice produced a dose-dependent mydriasis, whereas intravenous injections were ineffective. Naloxone could prevent or abolish this effect. It is suggested that somatostatin either directly excites opiate receptors or activates endopioid pathways involved in the regulation of pupillary size. 相似文献
30.
Thymus cell differentiation and in vivo T-cell migration. I. Migration of lectin-selected thymocytes
The in vivo quantitative distribution and tissue positioning of mouse thymocytes selected in vitro by Lyt phenotype and lectin binding properties were examined. Lyt 1+2- thymocytes were selected for by cytotoxic elimination; peanut agglutinin (PNA) and soybean agglutinin (SBA) binding and nonbinding thymocyte fractions were separated by an agglutinin technique. Selected cell suspensions were labelled in vitro with 51chromium (51Cr) or [3H]adenosine. Labeled washed cells were injected intravenously into syngeneic recipients which were killed at 1, 24 or 48 hr. In recipients of 51Cr-labeled cells, tissues were collected for gamma counting, and the overall percentage recovery of injected radiolabel from the various tissues was assessed. Tissues collected from recipients of [3H]adenosine-labeled cells were fixed, sectioned, and processed for autoradiography; the positioning of labeled cells within the tissues was determined. Selected Lyt 1+2-, PNA-, and SBA- sets all showed significantly enhanced entry into lymph nodes and intestinal lymphoid tissues. Entry of SBA+ cells into these tissues was comparable to that of peripheral T cells. PNA- and SBA- selected sets, but not Lyt 1+2- selected cells, also showed increased localization to the spleen and lungs, and decreased localization to the liver. By autoradiography, PNA- cells entered lymphoid tissues much more than PNA+ cells, and at 1 hr fewer PNA+ cells in spleen were associated with lymphoid follicles. At 24 and 48 hr almost all labeled cells in lymphoid tissues were positioned in T-dependent areas. These results suggest that enrichment for thymocyte subpopulations described as "mature" also enriches for cells with the ability to enter lymphoid tissue. They also suggest that interactions at other tissue sites are important in the determination of in vivo migration, and that surface carbohydrate composition is an important factor in this determination. 相似文献