全文获取类型
收费全文 | 6888篇 |
免费 | 596篇 |
专业分类
7484篇 |
出版年
2022年 | 38篇 |
2021年 | 74篇 |
2020年 | 70篇 |
2019年 | 88篇 |
2018年 | 113篇 |
2017年 | 102篇 |
2016年 | 167篇 |
2015年 | 221篇 |
2014年 | 228篇 |
2013年 | 349篇 |
2012年 | 383篇 |
2011年 | 385篇 |
2010年 | 245篇 |
2009年 | 247篇 |
2008年 | 343篇 |
2007年 | 341篇 |
2006年 | 334篇 |
2005年 | 332篇 |
2004年 | 324篇 |
2003年 | 375篇 |
2002年 | 375篇 |
2001年 | 166篇 |
2000年 | 116篇 |
1999年 | 143篇 |
1998年 | 123篇 |
1997年 | 103篇 |
1996年 | 91篇 |
1995年 | 73篇 |
1994年 | 77篇 |
1993年 | 80篇 |
1992年 | 120篇 |
1991年 | 88篇 |
1990年 | 67篇 |
1989年 | 70篇 |
1988年 | 79篇 |
1987年 | 56篇 |
1986年 | 56篇 |
1985年 | 65篇 |
1984年 | 55篇 |
1983年 | 52篇 |
1982年 | 69篇 |
1981年 | 65篇 |
1980年 | 58篇 |
1979年 | 46篇 |
1978年 | 45篇 |
1977年 | 44篇 |
1976年 | 51篇 |
1975年 | 24篇 |
1974年 | 26篇 |
1973年 | 21篇 |
排序方式: 共有7484条查询结果,搜索用时 15 毫秒
41.
A 126 bp fragment of a plant histone gene promoter confers preferential expression in meristems of transgenic Arabidopsis 总被引:3,自引:0,他引:3
Rossitza Atanassova Nicole Chaubet Claude Gigot 《The Plant journal : for cell and molecular biology》1992,2(3):291-300
The tissue-specific pattern of expression directed by the H4A748 Arabidopsis histone promoter was investigated by analysis of beta-glucuronidase (GUS) activity in transgenic Arabidopsis containing H4A748-GUS gene fusions. As determined by fluorimetric and histochemical tests, the H4A748 promoter directs preferential expression in meristems of young seedlings and adult plants. The low activity found in nonproliferating tissues may relate to basal constitutive expression of the histone promoter and/or to endoreduplication occurring in some tissues. The endogenous histone mRNA levels parallel the GUS activity found in different tissues. Analysis of the regulatory properties of 5' deleted promoters showed that multiple positive elements exist between -900 and -219 and that the proximal region of the promoter to -219 is sufficient to establish the full tissue-specific pattern of expression. Further deletion to -93 nearly abolished the promoter activity thus suggesting that the 126 bp fragment located between -219 and -93 contains the elements responsible for the specific expression pattern. The presence of several remarkable sequences within this fragment is discussed. 相似文献
42.
Valérie Maxime Jean-Pierre Pennec Claude Peyraud 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(6):557-568
Summary The effects of increased ambient salinity (35 mg · ml-1) were studied at 1, 6, and 24 h after direct transfer of rainbow trout from freshwater to seawater. Two series of experiments were carried out successively. The first series was designed to simultaneously study all the respiratory (except Hb affinity for O2), circulatory, and acid-base variables in each fish. In this series, fish were fitted with catheters chronically inserted into the cardiac bulbus, the dorsal aorta, and the opercular and buccal cavities. In the second series, designed to study haemoglobin O2 affinity, fish were fitted with only a dorsal aorta catheter. The ventilatory flow (
) was markedly increased just after transfer (by 55% at 1 h), then more moderately (by 20% at 6 h and 32% at 24 h). The initial hyperventilation peak was associated with frequent couphing motions. These ventilatory changes resulted essentially from increase in ventilatory amplitude. Initially, standard oxygen consumption (MM}O2) decreased slightly, the moderately increased (by 12% at 24 h), so that the oxygen convection requirement (
) increased substantially. In spite of an increased ventilation, the partial pressure of oxygen in arterial blood (P
aO2) decreased slightly at 1 h, prior to returning to control levels, while partial pressure of carbon dioxide in arterial blood (P
aCO2) was not significantly decreased. Gill oxygen transfer factor decreased substantially at 1 h (by 35%) then more moderately (by 7% at 1 h and 12% at 24 h). These results suggest a decrease in gas diffusing capacity of the gills. As P
aCO2 remained approximatively unchanged, the gradual decrease in arterial pH (pHa) from 7.94 to 7.67 at 24 h must therefore be regarded as a metabolic acidosis. The strong ion difference decreased markedly because the concentration of plasma chloride increased more than that of sodium. Arterial O2 content (C
aO2) gradually decreased (by 38% at 24 h) simultaneously with the decrease in pHa, while the ratio P
aO2/C
aO2 increased. In parallel, seawater exposure induced a marked decrease in affinity of haemoglobin for O2, so that at 24 h, P50 was increased by 26% above the value obtained in freshwater-adapted trout. The increase in
could be ascribed initially (at 1 h) to the decrease of P
aO2 and later to a stimulation of respiratory neurons resulting from the lowered medullary interstitial pH. The decrease in C
aO2 could be interpreted mainly as a consequence of a decreased affinity of haemoglobin for O2, likely to be due to the blood acidosis and a predictable increase in chloride concentration within erythrocytes. Cardiac output (
) slightly decreased at 1 h, then progressively increased by 30% at 24 h. Branchial vascular resistance increased at 1 h by 28%, then decreased by 18% of the control value at 24 h. Systemic vascular resistance decreased markedly by 40% at 24 h. As heart rate (HR) remained significantly unchanged, the cardiac stroke volume initially decreased then increased in relation to the changes in
. The increase of
, allowing compensation for the effect of decreased C
aO2 in tissue O2 supply, was interpreted as a passive consequence of the decrease in total vascular resistance occurring during seawater exposure.Abbreviations a.u.
arbitrary units
-
C
aO2
arterial oxygen content
- pH50
arterial pH at P50
-
C
vO2
venous oxygen content
- Hb
haemoglobin
- HR
heart rate
- Hct
hematocrit
- nHill
Hill coefficient
- O2
standard oxygen consumption
-
P
aCO2
arterial partial pressure of carbon dioxide
-
P
aO2
arterial partial pressure of oxygen
-
P
vO2
oxygen partial pressure in mixed venous blood
- P50
oxygen tension at half saturation of haemoglobin
-
P
VA, P
DA
blood pressure in ventral and dorsal aorta
- pHa
arterial pH
-
PIO2, PEO2
oxygen partial pressure of inspired and expired water
-
PO2
oxygen partial pressure
-
cardiac output
- SEM
standard error of mean
- S.I.D.
strong ion difference
- SV
cardiac stroke volume
- TO2
gill oxygen transfer factor
- U
oxygen extraction coefficient
- VA
ventilatory amplitude
- VF
ventilatory frequency
- VRG, VRS
branchial and systemic vascular resistances
-
ventilatory flow
-
ventilatory oxygen convection requirement 相似文献
43.
The sperm nuclei of Aulacomya ater, family Mitylidae, contain three proteins (X, Aa5 and Aa6) which are specific to this cell type coexisting with a set of five somatic-type histones. Information about the chromatin structure resulting from this kind of association is scarce. Therefore, we have probed the structure of this sperm chromatin through digestion with micrococcal nuclease in combination with salt fractionation. The data obtained have allowed us to propose a nucleosomal arrangement for this chromatin. However, two types of nucleosomes would be present in agreement with their protein components. 相似文献
44.
Claude Sauvage Jean-François Rumigny Michel Maitre 《Molecular and cellular biochemistry》1991,107(1):65-77
Summary Three G proteins from human brain membranes were purified to near homogeneity by conventional techniques including preparative
electrophoresis. These G proteins were characterized by their ability to bind GTP, GDP and GTP analogs. Two of these proteins
have molecular weights of 50,000 (G50) and 36,000 (G36), as determined on SDS-gels. G36 was ADP-ribosylated by pertussis toxin. Thus, G50 could represent a Gsα subunit, whereas G36 could be Giα or Goα. G50 was phosphorylated by cAMP dependent protein kinase and protein kinase C. G36 was phosphorylated by a protein kinase independent of calcium and phospholipid, a proteolytic product of protein kinase C,
analogous to protein kinase M. Phosphorylation of G36 by this protein kinase induced a dramatic decrease in its GTPase activity. The third G protein, of molecular weight 22,000
probably belongs to the group of monomeric G proteins possessing functional similarities withras gene products. The regulation of G proteins involving calcium-dependent and independent pathways is delineated. 相似文献
45.
Marta E. Farías Ana M. Strasser de Saad Aída A. Pesce de Ruiz Holgado Dr. Guillermo Oliver 《Current microbiology》1991,22(4):205-211
l-Serine dehydratase fromLactobacillus fermentum was purified 100-fold. It was stabilized by the presence of 1 mM
l-cysteine in 50 mM phosphate buffer. Mr=150,000 was determined by gel filtration. The enzyme consists of four apparently identical subunits (Mr=40,000) that were observed after treatment with sodium dodecyl sulfate. The apparent Km forl-serine was 65 mM. Fe++ was required for the enzymatic activity, and the apparent Km value for this reaction was 0.55 mM. Maximum enzymatic activity was observed at 45°C and pH 8.0 in 50 mM phosphate buffer. At pH values different from the optimum, a positive cooperativity between substrate molecules was observed. The activation energy of the reaction was 11,400 and 22,800 cal × mol–1 for temperature values more than and less than 35°C respectively. The purified enzyme showed a maximum absorption between 400 and 420 nm, indicating the presence of pyridoxal-5-phosphate (PLP) as a prosthetic group. The PLP concentration was 0.027 µmoles per milligram of protein. The data suggest that there is 1 mol of PLP for each protein subunit. 相似文献
46.
A backpack system is described whereby osmotic minipumps are used to infuse gonadotrophin-releasing hormone (GnRH) subcutaneously in a pulsatile manner into infertile socially subordinate female marmoset monkeys (Callithrix jacchus jacchus). This procedure enables long-term infusion of GnRH without the necessity of repeated subcutaneous implantation of pumps and GnRH reservoirs. The backpack and cannulae system is inexpensive and can be constructed from commonly available materials. The GnRH treatment successfully overcame the suppression of pituitary luteinizing hormone secretion imposed by the low social status of female marmoset monkeys. 相似文献
47.
J L Lacuara R D Ruiz M Fiol de Cuneo 《Canadian journal of physiology and pharmacology》1985,63(4):283-286
The effects of piretanide upon mechanical activity and pharmacological reactivity of vascular and myocardial tissues from normotensive rats were investigated. Magnitude of phasic contractions of isolated rat portal vein was diminished by the drug in a dose-related manner; contractile depression induced by piretanide (10(-4)M) was less in the presence of insulin (0.1 U/mL), glucose (22 mM) or pyruvate (5 mM). Responses to KCl (90 mM), or norepinephrine (2.5 X 10(-5)M) were also reduced. Contractile activity of atria and ventricle strips was diminished only when piretanide reached 10(-4)M. Results support direct actions of piretanide upon cardiac and vascular tissues. Possible mechanisms of action are discussed. 相似文献
48.
Effect of Drying Medium on Residual Moisture Content and Viability of Freeze-Dried Lactic Acid Bacteria 总被引:5,自引:1,他引:4 下载免费PDF全文
Graciela F. de Valdez Graciela S. de Giori Aida P. de Ruiz Holgado Guillermo Oliver 《Applied microbiology》1985,49(2):413-415
The effect of various substances on the relationship between residual moisture content and the viability of freeze-dried lactic acid bacteria has been studied. Compounds such as polymers, which display considerable ability in displacing water, showed no protective action during freeze-drying. Adonitol, on the other hand, produced the smallest change in water content at various times during drying and allowed the highest rate of survival. 相似文献
49.
Effect of the rehydration medium on the recovery of freeze-dried lactic acid bacteria. 总被引:7,自引:1,他引:6 下载免费PDF全文
Sixteen cultures of lactic acid bacteria were freeze-dried in 10% nonfat skim milk plus 0.75 M adonitol and rehydrated by using different rehydration media. Marked variations in their capacity to repair cellular damage after freeze-drying were observed among the species and strains under consideration. 相似文献
50.
Isolation and characterization of Pediococcus halophilus from salted anchovies (Engraulis anchoita) 总被引:3,自引:0,他引:3
M Villar A P de Ruiz Holgado J J Sanchez R E Trucco G Oliver 《Applied and environmental microbiology》1985,49(3):664-666
The presence of bacteria in salted anchovies during and at the end of the curing process was investigated. Attempts to isolate bacteria under aerobic or anaerobic conditions led to the isolation of only bacteria of the genus Pediococcus which were identified as Pediococcus halophilus. The isolates correspond to a rather heterogeneous group in which some of the members differ in some biochemical tests from the types described in the literature. 相似文献