全文获取类型
收费全文 | 3512篇 |
免费 | 179篇 |
国内免费 | 7篇 |
专业分类
3698篇 |
出版年
2024年 | 12篇 |
2023年 | 35篇 |
2022年 | 158篇 |
2021年 | 214篇 |
2020年 | 91篇 |
2019年 | 111篇 |
2018年 | 137篇 |
2017年 | 90篇 |
2016年 | 159篇 |
2015年 | 200篇 |
2014年 | 216篇 |
2013年 | 258篇 |
2012年 | 263篇 |
2011年 | 256篇 |
2010年 | 126篇 |
2009年 | 118篇 |
2008年 | 164篇 |
2007年 | 157篇 |
2006年 | 139篇 |
2005年 | 117篇 |
2004年 | 111篇 |
2003年 | 108篇 |
2002年 | 95篇 |
2001年 | 25篇 |
2000年 | 15篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 19篇 |
1996年 | 8篇 |
1995年 | 20篇 |
1994年 | 15篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 15篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1966年 | 5篇 |
排序方式: 共有3698条查询结果,搜索用时 15 毫秒
11.
12.
Phosphorus exchange at the sediment-water interface coupled with several parameters were assessed in several reservoirs with geologically different catchment basins and different trophic status in Morocco and France.The results showed that these exchanges were regulated by a combination of factors: physical chemical variability of the environment, the geological composition of catchment basins and the trophic status of the lake.In the hypereutrophic Villerest, iron-bound phosphorus is the major form of phosphorus trapped by the sediment whereas, in Moroccan reservoirs, calcium-bound phosphorus prevailed.We suggest that a drastic control of phosphorus inputs into the waters must be done through a large program of dephosphatization of tributaries to avoid Microcystis aeruginosa bloom formation in Villerest (Aleya et al., 1993) and calcium-bound phosphorus dissociation in Moroccan reservoirs with upward release of bioavailable phosphorus.
Résumé Les échanges de phosphore au niveau de l'interface eau-sédiment couplés á la distribution temporelle de divers éléments chimiques et biologiques ont été étudiés dans divers réservoirs de niveaux trophiques différents, au Maroc et en France.Nos résultats mettent clairement en évidence une influence directe de l'environnement physico-chimique, de la nature géologique des bassins versants et de l'état trophique du lac sur la dynamique du phosphore au sein de cette interface.De plus, il apparait que dans le lac hypereutrophe de Villerest (Roanne, France), le phosphore est majoritairement complexé au fer alors que dans les retenues marocaines, ce sont les complexes phosphore-calcium qui prédominent.Nous préconisons un contrôle drastique des apports en phosphore á travers l'installation et la multiplication d'unités de déphosphatation afin d'éviter d'une part, la prolifération massive de la Cyanobactérie Microcystis aeruginosa á Villetest (Aleya et al., 1994) et d'autre part la dissociation des complexes phhosphore-calcium au sein des retenues marocaines avec libération de phosphore biodisponible.相似文献
13.
Peter F. Daels Hussni Mohammed S. M. E. Montavon G. H. Stabenfeldt J. P. Hughes Kristina Odensvik H. Kindahl 《Animal reproduction science》1995,40(4):305-321
Repeated administration of prostaglandin is the treatment of choice for the termination of pregnancy in mares more than 40 days pregnant. Even though it is well documented that PGF-2 or analogue needs to be administered every 12–24 h for successful induction of abortion, little is known about the underlying endocrine changes and the mechanism by which abortion occurs. The aim of this study was to characterize the changes in PGF-2, progesterone and estrogen secretion during prostaglandin-induced abortion. Six mares, 82–102 days pregnant, were treated daily with 250 μg cloprostenol, blood was collected at 1-h intervals until fetal expulsion and pregnancy examination was performed daily. Four mares, 92–97 days pregnant, received no treatment but were subjected to the same hourly blood collections and daily genital examinations described for cloprostenol-treated mares for 3 days. Mean time from first cloprostenol administration until fetal expulsion was 48.6 ± 5.6 h and required 2.8 ± 0.2 cloprostenol administrations. In all mares, progesterone concentrations decreased in a near linear manner after the first cloprostenol administration and were invariably low (1.3 ± 0.2 ng ml−1, mean ± SEM) at the time of fetal expulsion. Mean estrogen secretion remained unchanged until 5 h before fetal expulsion and then decreased rapidly to non-pregnant levels. Endogenous PGF-2 secretion rate increased with each cloprostenol administration and culminated in sustained PGF-2 secretion which persisted until fetal expulsion was completed. From these results we conclude that cloprostenol-induced abortion is associated with endogenous PGF-2 secretion, fetal expulsion coincides with sustained PGF-2 secretion and low progesterone concentrations and plasma estrogen concentrations remain unchanged until hours before fetal expulsion. 相似文献
14.
A conjugation system initially discovered in beta-lactamase-producing gonococci mobilized small non-selftransmissible R plasmids encoding beta-lactamase (penicillinase) production into other gonococci, Neisseria, and Escherichia coli. This conjugation system was mediated by a separate selftransmissible plasmid of 23.9 X 10(6) daltons, pFA2. Conjugative plasmids capable of mobilizing R plasmids were also found in nearly 8% of the non-penicillinase-producing gonococci. These were similar to pFA2 in size, buoyant density, and restriction endonuclease digest patterns but were less efficient than pFA2 in mobilization of the penicillinase plasmid pFA3. The presence of conjugative plasmids in gonococci isolated before the appearance of penicillinase-producing strains indicates that a conjugation system for plasmid transfer predated the appearance of R plasmids in gonococci. 相似文献
15.
As determined by their ability to exflagellate and round up, it took the macrogametocyte and the microgametocyte of Haemoproteus columbae in pigeons 68 and 116 h, respectively, after patency to reach maturity. Pigment granules appeared in the undifferentiated gametocytes 8 h after invasion of blood. Vacuoles were observed in young gametocytes and persisted in the older forms. The growth curve of H. columbae is close to the sigmoidal curve for growth in protozoa. Multiple infection was noticed in pigeons with high levels of parasitemia, but no more than 2 gametocytes reached maturity; such multiple infections were rare in relapses. The sex ratio of the gametocytes was 1:1. Strong lateral displacement of the nuclei of infected erythrocytes was the rule; hypertrophy was negligible. 相似文献
16.
Effects of edaphic factors (salinity, pH, Na+, K+, Ca++, CaCO3, water holding capacity, and grain size) on the spatial distribution of plants were investigated. Soil was sampled at 22 stands. Sixteen plant species were recorded from these stands. Relation between edaphic factors and plant distribution was investigated using correlation statistical analysis. Distribution of some plants was found to be highly correlated with edaphic factor(s). 相似文献
17.
Mohammed Sbia Marie-Francoise Diebler Nicolas Morel Maurice Israël 《Journal of neurochemistry》1992,59(4):1273-1279
The mediatophore is a presynaptic membrane protein that has been shown to translocate acetylcholine (ACh) under calcium stimulation when reconstituted into artificial membranes. The mediatophore subunit, a 15-kDa proteolipid, presents a very high sequence homology with the N,N'-dicyclohexylcarbodiimide (DCCD)-binding proteolipid subunit of the vacuolar-type H(+)-ATPase. This prompted us to study the effect of DCCD, a potent blocker of proton translocation, on calcium-dependent ACh release. The present work shows that DCCD has no effect on ACh translocation either from Torpedo synaptosomes or from proteoliposomes reconstituted with purified mediatophore. However, using [14C]DCCD, we were able to demonstrate that the drug does bind to the 15-kDa proteolipid subunit of the mediatophore. These results suggest that although the 15-kDa proteolipid subunits of the mediatophore and the vacuolar H(+)-ATPase may be identical, different domains of these proteins are involved in proton translocation and calcium-dependent ACh release and that the two proteins have a different membrane organization. 相似文献
18.
Cation transport mechanisms in Mycoplasma mycoides var. Capri cells. The nature of the link between K+ and Na+ transport 总被引:7,自引:0,他引:7 下载免费PDF全文
We have studied the links between the mechanisms of Na(+), K(+) and H(+) movements in glycolysing Mycoplasma mycoides var. Capri cells. In the light of the results reported in the preceding paper [Benyoucef, Rigaud & Leblanc (1982) Biochem. J.208, 529-538], we investigated certain properties of the membrane-bound ATPase of Mycoplasma cells, with special reference to its ionic requirements and sensitivity to specific inhibitors. Our findings show, first, that, although Na(+) stimulated ATPase activity, K(+) did not affect it, and, secondly, that NN'-dicyclocarboidi-imide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD) were potent inhibitors of the basal ATPase activity, which was unaffected by vanadate and ouabain. We also investigated the movements of Na(+) and H(+) under the experimental conditions applied to the study of the K(+) uptake reported in the preceding paper, and found that when ;Na(+)-loaded cells' previously equilibrated with (22)Na(+) were diluted in a sodium-free medium, addition of glucose induced a rapid efflux of (22)Na(+). This energy-dependent efflux was independent of the presence of KCl in the medium. Studies of the changes in internal pH by 9-aminoacridine fluorescence or [(14)C]methylamine distribution indicated that the movement of Na(+) was coupled to that of protons moving in the opposite direction, a finding that supports the presence of an Na(+)/H(+) antiport. When Na(+)-loaded cells are diluted in an Na(+)-rich medium the Na(+)/H(+) antiport is still active, but cannot decrease the intracellular Na(+) concentration. Under such conditions, net (22)Na(+) extrusion is specifically dependent on the presence of K(+) in the medium. The present results and those derived from the study of K(+) accumulation (the preceding paper) can be rationalized by assuming that Mycoplasma mycoides var. Capri cells contain two transport systems for Na(+) extrusion: an Na(+)/H(+) antiport and an ATP-consuming Na(+)/K(+)-exchange system. 相似文献
19.
Condensation of 4,6-di-O-acetyl-2,3-O-carbonyl-α-d-mannopyranosyl bromide with benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (2) gave an α-d-linked disaccharide, further transformed by removal of the carbonyl and benzylidene groups and acetylation into the previously reported benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-3-O-(2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl)-α-d-glucopyranoside. Condensation of 3,4,6-tri-O-benzyl-1,2-O-(1-ethoxyethylidene)-α-d-glucopyranose or 2-O-acetyl-3,4,6-tri-O-benzyl-α-d-glucopyranosyl bromide with 2 gave benzyl 2-acetamido-3-O-(2-O-acetyl-3,4,6-tri-O-benzyl-β-d-glucopyranosyl)-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside. Removal of the acetyl group at O-2, followed by oxidation with acetic anhydride-dimethyl sulfoxide, gave the β-d-arabino-hexosid-2-ulose 14. Reduction with sodium borohydride, and removal of the protective groups, gave 2-acetamido-2-deoxy-3-O-β-d-mannopyranosyl-d-glucose, which was characterized as the heptaacetate. The anomeric configuration of the glycosidic linkage was ascertained by comparison with the α-d-linked analog. 相似文献
20.
Parmanand Jha Shibili Nuhmani Gagan Kapoor Wafa Hashem Al Muslem Royes Joseph Shaji John Kachanathu Saad Mohammed Alsaadi 《Journal of musculoskeletal & neuronal interactions》2022,22(4):498
Objective:To determine the efficacy of a five-week core stability training program for collegiate athletes on upper extremity performance measures.Methods:Seventy healthy collegiate athletes (age 21.6±1.7years; height 175±4.63 cm; body mass 65.31±5.63 kg) were randomly allocated to experimental (n=35) and control group (n=35). The experimental group has undergone a five-week core stabilisation protocol (three days /week) and regular training, whereas the control group maintained their regular training. The upper quarter Y balance test (UQ-YBT) and Functional throwing performance index (FTPI) were assessed pre and post-training.Results:The results of mixed ANOVA show that there was a significant interaction between time and group variables on YBT (p<0.001, ηp2 =0.759) and FTPI (p<0.001, ηp2 =0.411) after five weeks of core stability training. Statistically, significant improvement was shown in YBT (mean change=15.2, p<0.001) and FTPI (mean change=14.4, p<0.001) in the experimental group; however, there was no significant change observed in both outcomes in the control groups.Conclusion:After five weeks of core stabilisation training program, the measures of UQ-YBT and FTPI were improved, thus advocating the use of a core stabilisation training program among collegiate athletes to enhance their upper extremity performance. 相似文献