全文获取类型
收费全文 | 8298篇 |
免费 | 668篇 |
国内免费 | 3篇 |
专业分类
8969篇 |
出版年
2023年 | 83篇 |
2022年 | 144篇 |
2021年 | 283篇 |
2020年 | 230篇 |
2019年 | 227篇 |
2018年 | 340篇 |
2017年 | 263篇 |
2016年 | 398篇 |
2015年 | 488篇 |
2014年 | 510篇 |
2013年 | 595篇 |
2012年 | 686篇 |
2011年 | 637篇 |
2010年 | 388篇 |
2009年 | 350篇 |
2008年 | 415篇 |
2007年 | 418篇 |
2006年 | 390篇 |
2005年 | 251篇 |
2004年 | 260篇 |
2003年 | 226篇 |
2002年 | 198篇 |
2001年 | 158篇 |
2000年 | 156篇 |
1999年 | 128篇 |
1998年 | 62篇 |
1997年 | 56篇 |
1996年 | 51篇 |
1995年 | 38篇 |
1994年 | 38篇 |
1993年 | 32篇 |
1992年 | 56篇 |
1991年 | 40篇 |
1990年 | 25篇 |
1989年 | 51篇 |
1988年 | 27篇 |
1987年 | 25篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 20篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1975年 | 8篇 |
1974年 | 12篇 |
1971年 | 11篇 |
1968年 | 7篇 |
排序方式: 共有8969条查询结果,搜索用时 0 毫秒
91.
J. A. L. Santos J. M. S. Cabral C. L. Cooney 《Bioprocess and biosystems engineering》1992,7(5):205-211
This work describes the recovery of an extracellular alkaline protease from fermentation broths of a Bacillus sp ATCC 21536, at pH=10.0 using ultrafiltration (MWCO 100,000) and microfiltration (0.1 m) membranes in hollow fiber devices. The influence of membrane pore size and polymeric material and membrane filtration performance was studied. High protein recoveries and high average flux rates were obtained with polysulfone membranes. A decrease of concentration polarization was obtained, simultaneously with enhancement of filtration flux rate and enzyme recovery by using submicron sized charged particles. These polymers lead to flocculation and adsorption of whole cells and soluble factors from the fermentation broth. The best results were obtaiend by combination of cationic (0.1%) and anionic (0.04%) polymers. 相似文献
92.
C E Negrao E D Moreira M C Santos V M Farah E M Krieger 《Journal of applied physiology》1992,72(5):1749-1753
The present investigation was undertaken to evaluate the vagal function of trained (T) and sedentary (S) rats by use of different approaches in the same animal. After 13 wk of exercise training (treadmill for 1 h 5 times/wk at 26.8 m/min and 15% grade), T rats had a resting heart rate (HR) slightly but significantly lower than S rats (299 +/- 3 vs. 308 +/- 3 beats/min). T rats had marked reduction of the intrinsic HR (329 +/- 4 vs. 369 +/- 5 beats/min) after blockade by methylatropine and propranolol. They also exhibited depressed vagal and sympathetic tonus. Baroreflex bradycardia (phenylephrine injections) was reduced, bradycardic responses produced by electrical stimulation of the vagus were depressed, and responses to methacholine injection were decreased in T rats. Therefore several evidences of vagal function impairment were observed in T rats. The resting bradycardia after exercise training is more likely to be dependent on alterations of the pacemaker cells, inasmuch as the intrinsic HR was markedly reduced. 相似文献
93.
In situ pancreatic DNaseI digestions were used as probes to study the structural organization of facultative and constitutive heterochromatin during both mitotic and meiotic divisions. Three different types of heterochromatic regions from three insect species were chosen for this study. These regions had been previously characterized by in situ treatments with restriction endonucleases (AT and GC rich DNA sequences). Progressive increase in DNaseI concentration (from 10 to 200 ng/ml) or in incubation time (from 5 to 30 min) revealed a specific pattern of sequential digestion of the constitutive heterochromatic regions, the centromeric ones (AT-rich DNA) being the most resistant to DNaseI action. The interstitial C-bands (with AT or GC-rich DNA) were more sensitive to DNaseI, and the band 4.4 from Baetica ustalata was the most resistant of the non-centromeric bands. Similar results were obtained during meiosis, but increased accessibility to DNAseI was observed compared to mitosis. DNA methylation in the non-centromeric band 4.4 of B. ustulata could be responsible for its differential digestion with respect to the remaining intercalar heterochromatin. Facultatively heterochromatic regions (X chromosomes) were found to exhibit a differential response to DNaseI attack from mitosis to meiosis. While they behaved as cuchromatin during mitosis, they were the most resistant together with centromeric heterochromatin regions, during metaphase I and II. The different responses to digestion of the X chromosome and X-derived regions between somatic and meiotic divisions are probably a consequence of the changes in the organization of this chromosome during the facultative heterochromatinization process. 相似文献
94.
Dulce E. Oliveira Ana Lúcia C. Santos Neto Anita D. Panek 《Analytical biochemistry》1981,113(1):188-192
A quantitative in situ assay of yeast α-glucosidase involving permeabilization of the cells by freezing and thawing is described. The assay was applied to different strains in different physiological states and was shown to give results comparable to those obtained with total cell homogenates. The primary advantage of the in situ assay was the possibility of analyzing a large number of samples from the same culture during a growth curve using a very reduced cell mass. 相似文献
95.
96.
The isolation of Saccharomyces cerevisiae plasma membrane was carried out after hypotonic lysis of yeast protoplasts treated with concanavalin A by two independent methods: a, at low speed centrifugation and b, at high speed centrifugation in a density gradient. Several techniques (electron microscopic, enzymic, tagging, etc.) were used to ascertain the degree of purification of the plasma membranes obtained. The low speed centrifugation technique as compared with the other method gave a higher yield of plasma membranes with a similar degree of purification. Analysis of the yeast plasma membrane of normally growing cells by sodium dodecyl sulphate polyacrylamide gel electrophoresis showed at least 25 polypeptide bands. Twelve glycoprotein bands were also found, and their apparent molecular weights were determined. Treatment of the protoplasts with cycloheximide resulted in a significant decrease in the carbohydrate and protein content of the plasma membrane. The electrophoretic pattern of the plasma membrane of cycloheximide-treated cells showed a redistribution of the relative amounts of each protein band and a drastic reduction in the number of Schiff-positive bands. The isoelectric point of the most abundant proteins was low (pI 4) or lower than expected from previous data. A large part of the mannosyl transferase activity found in the cell (80%) was associated with the internal membranes, the remaining activity (20%) was located in the plasma membrane preparation. Part of the mannosyl transferase activity of the cells is located at the plasma membrane surface. Invertase (an external mannoprotein) is found in both the plasma and internal membranes, and as the specific activity dropped significantly following cycloheximide treatment of the cells, it is suggested that these membranes systems are the structures for the glycosylation of a precursor invertase and its subsequent release into the periplasmic space. Other transferase found in the plasma membrane preparation transfers glucose residues from UDPglucose to a poly(alpha(1 leads to 4) polymer identified as glycogen. 相似文献
97.
J D Ashbrook A A Spector E C Santos J E Fletcher 《The Journal of biological chemistry》1975,250(6):2333-2338
The binding of six physiologically important long chain fatty acids to defatted human plasma albumin was measured at 37 degrees in a calcium-free Krebs-Ringer phosphate buffer, pH 7.4. The data were analyzed in terms of multiple stepwise equilibria. With the saturated acids, the magnitude of the equilibrium (association) constants, Ki, increased as the chain length increased: laurate smaller than myristate smaller than palmitate smaller than stearate. Oleate was bound more tightly than stearate; by contrast, linoleate was bound less tightly than stearate. The equilibrium constants, K1 through K12, ranged from 2.4 times 10-6 - 3.5 times 10-3 m-1 for laurate to 2.6 times 10-8 - 3.5 times 10-5 m-1 for oleate. Successive values of Ki decrease for each of the acids, indicating that major cooperative binding effects do not occur over the physiological range of fatty acid concentrations. In no case could the Ki be segregated into distinct classes, suggesting that any grouping of albumin binding sites is somewhat arbitrary. The results were inconclusive concerning whether premicellar association of unbound fatty acid occurs. Although corrections for premicellar association produced very little change in the Ki values for myristate, they raised the Ki for palmitate and stearate by 300 to 700 per cent. A sigmoidal relationship was obtained when the logarithm of Ki was plotted against chain length for the saturated fatty acids containing 6 to 18 carbon atoms, indicating that the binding energy is not simply a statistical process dependent only on the fatty acid chain length. This selectivity that albumin contributes to the binding process may be due to varying degrees of configurational adaptability of its binding sites as the fatty acid increases in length. 相似文献
98.
P G Santos F Grande 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1975,149(3):652-655
The lipolytic effect of glucagon was measured in vitro with adipose tissue of "young" (4-8 wk) and "old" (over 1 yr) geese. The response of the young geese tissue was about twice that observed with tissue of old geese, for glucagon concentrations of 0.05, 0.5, and 5.0 mug/ml. Our estimates indicate that the number of adipose cells per g of adipose tissue of young geese was three times that of the old geese tissue. This suggests that the greater lipolytic response to glucagon, observed in young geese adipose tissue, may possibly be due to its greater cellularity, rather than to a greater lipolytic response of the individual adipocyte. The lipolytic effect of glucagon in vivo, for each of the doses between 1.0 and 20.0 mug/kg, was significantly greater in the old than in the young geese. The slope of the linear equation relating log10 of glucagon dose and elevation of plasma FFA 5 min after injection, was significantly greater for the old than for the young geese. In the goose, therefore, the influence of age on the adipokinetic effect of glucagon appears to be mediated by factors operating in the whole animal, more than by changes in the adipose cell itself. A slower removal rate of circulating FFA by the old geese, could be one of these factors. 相似文献
99.
Marina Rovani Drummond Luciene Silva dos Santos Renata Soalheiro Fvaro Raquel Silveira Bello Stucchi Ilka de Ftima Santana Ferreira Boin Paulo Eduardo Neves Ferreira Velho 《PLoS neglected tropical diseases》2022,16(7)
BackgroundThis study aimed to assess the prevalence of Bartonella sp.-DNA detection in blood and skin samples from patients with non-viral end-stage liver disease awaiting liver transplantation.Methodology/Principal findingsBlood samples and healthy skin fragments from 50 patients were tested using microbiological and molecular methods. Fifteen patients had cryptogenic hepatitis (CH) and 35 had alcoholic, drug-induced or autoimmune liver disease. DNA was extracted from whole blood and liquid culture samples, isolates, and skin fragments. Thirteen of the 50 patients (26%) had Bartonella henselae DNA detection in their blood (9/50) and/or skin (5/50) samples. Colonies were isolated in 3/50 (6%) and infection was detected in 7/50 (14%) of the 50 patients. B. henselae-DNA detection was more prevalent in patients with CH than in other patients (p = 0.040). Of 39 patients followed-up for at least two years, a higher mortality rate was observed among patients with CH infected with B. henselae (p = 0.039).Conclusions/SignificanceFurther studies assessing the role of B. henselae infection in the pathogenesis of hepatitis patients must be urgently conducted. 相似文献
100.
Uncoupling effect of nitrite during denitrification by Pseudomonas fluorescens: An in vivo (31)P-NMR study 总被引:1,自引:0,他引:1
In vivo (31)P-NMR was used to investigate the basis for the inhibition of denitrification by nitrite accumulated endogenously by Pseudomonas fluorescens ATCC 17822 (biotype II) at pH 7.0. Cells were immobilized in kappa-carrageenan to obtain high cell concentrations in the NMR tube. Acetate and nitrate in two concentration ratios were supplied as electron donor and acceptor, respectively, to achieve different levels of nitrite accumulation. During denitrification, cells were able to maintain a pH gradient of approximately 0.4 to 0.5 units, but when nitrite accumulation reached values approximating 27 mM the transmembrane DeltapH collapsed sharply. Nitrite stimulated the reduction rate of nitrate; furthermore, at nitrite concentrations below 1 mM, activation of oxygen respiratory rates was observed in cells grown under aerobic conditions. The results provide evidence for nitrite acting as a protonophore (an uncoupler that increases the proton permeability of membranes by a shuttling mechanism). (c) 1996 John Wiley & Sons, Inc. 相似文献