全文获取类型
收费全文 | 1254篇 |
免费 | 138篇 |
国内免费 | 2篇 |
出版年
2021年 | 13篇 |
2019年 | 11篇 |
2016年 | 17篇 |
2015年 | 19篇 |
2014年 | 25篇 |
2013年 | 35篇 |
2012年 | 61篇 |
2011年 | 41篇 |
2010年 | 27篇 |
2009年 | 33篇 |
2008年 | 25篇 |
2007年 | 36篇 |
2006年 | 33篇 |
2005年 | 46篇 |
2004年 | 35篇 |
2003年 | 40篇 |
2002年 | 36篇 |
2001年 | 44篇 |
2000年 | 49篇 |
1999年 | 35篇 |
1996年 | 11篇 |
1994年 | 13篇 |
1992年 | 19篇 |
1991年 | 29篇 |
1990年 | 20篇 |
1989年 | 25篇 |
1988年 | 37篇 |
1987年 | 42篇 |
1986年 | 28篇 |
1985年 | 28篇 |
1984年 | 18篇 |
1983年 | 21篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 31篇 |
1978年 | 19篇 |
1977年 | 38篇 |
1976年 | 22篇 |
1975年 | 20篇 |
1974年 | 26篇 |
1973年 | 36篇 |
1972年 | 29篇 |
1971年 | 19篇 |
1970年 | 15篇 |
1969年 | 16篇 |
1968年 | 15篇 |
1967年 | 11篇 |
1966年 | 14篇 |
1965年 | 11篇 |
排序方式: 共有1394条查询结果,搜索用时 46 毫秒
71.
Summary The use of an Ussing chamber with well-defined mixing characteristics coupled to a mass spectrometer permits the concurrent evaluation of transepithelial current and oxidative metabolism with improved temporal resolution. The time-course of the amiloride-sensitive currentI
a and the rate of suprabasal CO2 productionJ
CO2
sb
were observed in 10 toad urinary bladders at short-circuit and after clamping at 100 mV, serosa positive. Following perturbation of (0100mV),I
a declined sharply within 1/2 min, remained near constant 15 min, and then increased slightly.J
CO2
sb
declined more gradually, remained near constant at 4–7 min, and then declined further. Detailed analysis revealed an early quasi-steady state with near constancy ofJ
CO2
sb
starting at 2.9±1.1 (sd) min and lasting 4.7 ±1.8 (sd) min, followed by relaxation to a later steady state at about 15 min. During the early quasi-steady state,I
a was also nearly constant. Considering that in steady statesI
a/FJ
Na
a
, the rate of transepithelial active Na transport, during the early quasi-steady state mean values ±se ofJ
Na
a
,J
CO2
sb
and (J
Na
a
/J
CO2
sb
) were, respectively, 29.9±1.7%, 59.4 ±3.2%, and 56.4±5.7% of values at short-circuit. Corresponding values during the late steady state were 41.4±6.0%, 38.2±6.1%, and 111.3±8.6%. Thus the flow ratioJ
Na
a
/J
CO2
sb
was depressed significantly during the early quasi-steady state, but returned later to the original value. The results of measurements ofI
a andJ
CO2
sb
in three hemibladders were qualitatively similar. In terms of a phenomenological black-box treatment the findings are consistent with earlier studies indicating incomplete coupling between transport and metabolism. Further studies will be required to clarify the molecular basis for these observations. 相似文献
72.
The theory of computational complexity and certain explicitly-stated hypotheses imply limitations on the information processing
power of biological systems. Parallelism, special purpose organization, and analog mechanisms may provide speedup critical
for life processes, but have little power in the face of exponential growth. We show that “polynomially simulatable” biological
systems cannot exhibit dynamic behavior which produces the solution of an intractable problem. The argument implies that parallelism
does not allow biological systems to defeat the exponential explosion, but rather is important because it allows polynomial
time algorithms to be used more efficiently. 相似文献
73.
Joseph C. LaManna Myron Rosenthal Roger Novack David F. Moffett Frans F. Jöbsis† 《Journal of neurochemistry》1980,34(1):203-209
Temperature coefficients of both cat and toad brain have been calculated for the active metabolic state induced by electrical stimulation. Values are higher than most of the values previously reported for "rest" metabolism, whether calculated from Arrhenius plots or from linear graphs. Relative rates of oxidative metabolism were obtained by measuring the time course of the transient changes in NADH fluorescence and cytochrome aa3 absorption by reflectance techniques directly from the surface of the exposed cat cerebral cortex in vivo and from the isolated intact toad brain mounted in a cuvet. These findings demonstrate that such optical methods accurately record metabolic processes. 相似文献
74.
Defective specialized SP beta transducing bacteriophages of Bacillus subtilis that carry the sup-3 or sup-44 gene 总被引:6,自引:4,他引:2 下载免费PDF全文
We isolated defective specialized transducing phages of SP beta that carry one of the extracistronic suppressors, sup-3 or sup-44. Lysates containing these phages can be used in a simple spot test to determine whether an auxotrophic mutation can be suppressed. The sup-3 and sup-44 mutations are distinct, in that their suppression patterns differ for the markers hisA1, metC3, and thr-5; and they are not alleles. 相似文献
75.
Intravenous bolus injection of prostaglandin I2 in the Inactin-anaesthetised rat produces a slow dose-dependant vasodepression which reaches maximum approximately 15 s. after injection. Administration of 9 beta-[3H1]-prostaglandin I2 by the same route followed by serial arterial sampling and TLC analysis revealed a slow conversion into one less polar metabolite starting after 20 s and reaching 40% by two minutes in the circulation. These experiments indicate that prostaglandin I2 survives pulmonary transit for a sufficiently long time to elicit a biological action. Thus its continuous systemic vascular synthesis could play an important role in the control of hypertension. 相似文献
76.
The Prophage of SPβc2DcitK1, a Defective Specialized Transducing Phage of BACILLUS SUBTILIS 总被引:4,自引:0,他引:4 下载免费PDF全文
The defective specialized transducing phage SP beta c2dcitK1 carries two known bacterial genes, kauA and citK, as well as SP beta hage markers including the heat-sensitive repressor allele, c2. Some phage genes (including essential ones) are missing. When SP beta c2dcitK1 transduces SP beta-sensitive cells of Bacillus subtilis, the defective prophage is inserted into sites in the homologous bacterial DNA of the attSP beta-kauA-citK region of the recipient chromosome. During the growth of these transductants, occasional excisions occur that result in the loss of the phage genes and of the heterogenotic state. These excisions increase greatly in frequency during growth at repressor-inactivating temperatures. The kinds of insertions and excisions seen suggest that a Campbell-type (CAMPBELL 1962) circular phage genome may occur transiently. If the transductants are superinfected by SP beta c2 or by the clear-plaque mutant SP beta c1, the resulting double lysogen can be heat induced to release high-frequency-of-transduction (HFT) lysates for kauA and citK. 相似文献
77.
Light Effects in Yeast: Evidence for Participation of Cytochromes in Photoinhibition of Growth and Transport in Saccharomyces cerevisiae Cultured at Low Temperatures 下载免费PDF全文
Stanislaw Uaszewski Theofanis Mamouneas Win-Kuang Shen Philip J. Rosenthal John R. Woodward Vincent P. Cirillo Leland N. Edmunds Jr 《Journal of bacteriology》1979,138(2):523-529
Visible light of moderate intensity inhibits growth, respiration, protein synthesis, and membrane transport in bakers' yeast and has a deleterious effect on membrane integrity. The results of this study indicate that these effects require the presence of cytochromes b and a/a(3). The light sensitivities of growth rate and [(14)C]histidine uptake in wild-type rho(+) Y185 and D225-5A strains of Saccharomyces cerevisiae were compared with those in a variety of mutants lacking cytochrome b or a/a(3) or both; a close correlation was found between the presence of these respiratory pigments and photosensitivity. Thus, strain TL5-3C, a nuclear petite lacking cytochromes b, a, and a(3), was resistant to light; strain GL5-6A, another nuclear petite having reduced amounts of cytochromes a and a(3), was partially resistant; strains MB127-20C and MB1-6C, nuclear petites lacking only cytochrome b, were also only partially resistant to light; whereas mutants containing all three cytochromes but having their respiratory chain either nonfunctional (strain ZK3-6B) or uncoupled (strain 18-27/t12) were fully sensitive to light. Finally, an equal-energy, broad-band action spectrum for the light inhibition of growth and transport indicated that blue light (408 nm) was most effective; these wavelengths correspond to the Soret region of the cytochrome absorption spectrum. The results suggest, therefore, that the yeast cytochromes b, a, and a(3) are the primary photoreceptors for the inhibitory effects of light and, perhaps, for other processes, such as the entrainment of biological rhythms in this species. 相似文献
78.
In an enzyme-specific drug screening system nalidixic acid and 3'-FTdR, inhibitors of DNA synthesis, both reduce the growth of wild type and temperature-sensitive point mutants of phage T3 with different efficiencies. The wild type shows the strongest sensitivity against the drugs, while an exonuclease mutant is the most insensitive variant. The DNA polymerase mutants exhibit an intermediate degree of inhibition. The anthracycline antibiotics violamycin BI and adriblastin which preferentially inhibit RNA synthesis show the same degree of inhibition for all mutants. This is true also for the RNA synthesis inhibitor lambdamycin, which is identical with chartreusin. The protein synthesis inhibitors chloramphenicol and o-phenanthroline, a chelating agent, impair all mutants to the same extent. Our data confirm the hypothesis that structural variants of essential viral enzymes, when compared with the wild type should reveal different sensitivities against specific inhibitors and show that this T3 system could be used for the indication of specific inhibitors of DNA synthesis. 相似文献
79.
80.
Eggs of Baltic herring were incubated (10°C; 16S) in sea water containing mixtures of Cd, Cu and Pb at concentrations of 0.56–5.0, 0.0167–0.15, 0.56–5.0 mg metal/l; embryonic survival until hatching, viable hatch and uptake of metals by embryos and early larvae were measured. Negative effects of metals on embryonic survival and viable hatch were additive in the case of Cu and Cd. Pb did not exer detrimental effects. Uptake of metals with exposure time was non-linear in eggs and linear in larvae. Total uptake of Cu and Cd by eggs was subjected to antagonistic or synergistic action of the other two metals present. Accumulation of Pb by eggs was enhanced when Cu was also present. 相似文献