全文获取类型
收费全文 | 3269篇 |
免费 | 366篇 |
国内免费 | 1篇 |
出版年
2021年 | 43篇 |
2020年 | 28篇 |
2019年 | 38篇 |
2018年 | 39篇 |
2017年 | 25篇 |
2016年 | 60篇 |
2015年 | 119篇 |
2014年 | 138篇 |
2013年 | 164篇 |
2012年 | 157篇 |
2011年 | 172篇 |
2010年 | 119篇 |
2009年 | 85篇 |
2008年 | 147篇 |
2007年 | 153篇 |
2006年 | 128篇 |
2005年 | 146篇 |
2004年 | 142篇 |
2003年 | 120篇 |
2002年 | 118篇 |
2001年 | 76篇 |
2000年 | 68篇 |
1999年 | 75篇 |
1998年 | 48篇 |
1997年 | 31篇 |
1996年 | 41篇 |
1995年 | 36篇 |
1994年 | 29篇 |
1993年 | 39篇 |
1992年 | 50篇 |
1991年 | 38篇 |
1990年 | 37篇 |
1989年 | 40篇 |
1988年 | 35篇 |
1987年 | 33篇 |
1986年 | 25篇 |
1985年 | 37篇 |
1984年 | 39篇 |
1983年 | 31篇 |
1982年 | 43篇 |
1981年 | 29篇 |
1980年 | 36篇 |
1979年 | 40篇 |
1978年 | 38篇 |
1977年 | 41篇 |
1976年 | 32篇 |
1975年 | 30篇 |
1974年 | 35篇 |
1973年 | 35篇 |
1971年 | 28篇 |
排序方式: 共有3636条查询结果,搜索用时 15 毫秒
121.
The cytochrome reductase system solubilized from microsomes exhibits monophasic reduction kinetics over the temperature range 15 ° to ?25 °C in aqueous/ethylene glycol co-solvent, whereas in intact microsomes, the process becomes increasingly heterogeneous below 0 °C, reflecting heterogeneities in membrane structure observable as distributions in reaction rates and activation energies. 相似文献
122.
The synergistic effect of exogenous cobra phospholipase A2 on the hemolysis rate of guinea pig erythrocytes by highly purified snake venom cardiotoxins was investigated. In the presence of phospholipase A2 the reaction was not only faster and had a lower activation energy but followed a sigmoidal instead of a linear time course. Similar results were obtained using porcine pancreatic phospholipase A2. Significantly, addition of even a trace of cobra phospholipase A2 (approx. 0.1%, w/w) was sufficient to bring about the full synergistic effect, emphasizing the stringent purity requirements for any meaningful investigation of cardiotoxin's own action. The possibility that the action of cardiotoxin on its own may involve the stimulation of an endogenous phospholipase is discussed in the light of the results obtained with exogenous cobra enzyme. 相似文献
123.
The destructive action of chlorine on the pathogenic Naegleria fowleri and Acanthamoeba culbertsoni, the nonpathogenic N. gruberi, and an avirulent Acanthamoeba isolate was investigated. N fowleri is somewhat more sensitive to chlorine than N. gruberi, whereas the two Acanthamoeba strains are very resistant. This study yields information needed for the destruction of amoebic cysts in drinking water and swimming pools. It also gives some explanation for the occurence of Acanthamoeba strains in these waters. 相似文献
124.
The use of sodium dithionite as the reducing agent to produce the radical cation of methyl viologen cofactor for the nitrate reductase-catalyzed reduction of nitrate to nitrite causes a negative interference in the diazonium salt azo dye determination of nitrite. The interference results from bisulfite, a reaction product of dithionite, reacting with the diazonium salt formed from nitrite and sulfanilamide. Addition of formaldehyde before the diazotization reaction masks this interference but at low acidity introduces a nonlinearity in the nitrite calibration curve. This nonlinearity is due to a competing reaction of formaldehyde with sulfanilamide to yield a Schiff base and can be eliminated by increasing the HCL content of the sulfanilamide reagent. 相似文献
125.
- 1. 1. It has previously been demonstrated that an increase in extracellular Ca2+ concentration induces a transient increase in K+ permeability and associated hyperpolarization of the red cell membrane of the giant salamander, Amphiuma means. This phenomenon is analogous to the Ca2+-induced KCl loss observed in ATP-depleted human red cells and red cell ghosts.
- 2. 2. Histamine, which enhances the Ca2+-induced K+ loss from depleted human red cells, is without effect on this Ca2+-induced hyperpolarization of Amphiuma red cells.
- 3. 3. Promethazine (10 μM) and mepyramine (1 mM), which inhibit the Ca2+-induced K+ loss in depleted human red cells, also block the Ca2+-related hyperpolarization of Amphiuma erythrocytes.
- 4. 4. Chlorpromazine (25 μM), despite being a weak antihistamine, is equally effective in blocking the Ca2+-induced hyperpolarization of Amphiuma red cells.
- 5. 5. Ionophore A23187 causes a large and sustained Ca2+/K+-dependent hyperpolarization even in the presence of normal (1.8 mM) concentrations of Ca2+. This hyperpolarization is relatively insensitive to chlorpromazine and promethazine.
- 6. 6. The inhibition of the Ca2+-induced hyperpolarization of the Amphiuma red cell membrane by chlorpromazine and promethazine may be related to their properties as local anaesthetics.
Abbreviations: MOPS, morpholinopropane sulphonate 相似文献
126.
The permeability of the blood-brain barrier to several classes of compounds was studied in rats between the ages of 15 days and 9 weeks. 14C-labelled test substances were injected simultaneously with two reference isotopes, 3H2O and 113mIn-labelled EDTA, into the common carotid artery followed by decapitation 10 s later.There was evidence that a monocarboxylic acid transport system in 15 to 23 day-old rats had a capacity at least six times greater than that present in adult animals. l-Lactate and acetate showed the highest permeability. At all ages there was a constant ratio between l-lactate and (−)d-3-hydroxybutyrate values. d-Glucose permeability increased with age, while that of several amino acids tested was the same in young and adults rats. 相似文献
127.
Harvey J. Berger Leon Speroff Steven Wolfson 《Prostaglandins & other lipid mediators》1976,11(3):499-502
Venous prostaglandins A, E, and F were determined by radioimmunoassay in 10 dogs before and one hour after administration of sodium pentobarbital (35 mg/Kg, iv). In the conscious state, PGA was 0.34 ± 0.04 ng/ml (mean ± SE), PGE 0.20 ± 0.01 ng/ml, and PGF 0.25 ± 0.03 ng/ml. During pentobarbital anesthesia, these levels were unchanged (p >0.05). Thus, pentobarbital anesthesia had no effect on peripheral venous prostaglandin levels. 相似文献
128.
The kinetics of the reaction of Golfingia gouldii hemerythrin with O2 have been studied by stopped flow spectrophotometry. For the second order oxygenation process, k1 = 7.4 X 10(6) M-1 s-1, deltaH1++ = 8.2 kcal-mol-1 and deltaS1++ = +1 e.u. at 25 degrees, pH 8.2, and I = 0.015 M. The rate constant is unchanged when protein concentration is varied from 3 to 25 muM, the ionic strength is increased to 0.07 M, and the pH moved to 6.8. The deoxygenation of oxyhemerythrin is studied with stopped flow by scavenging liberated O2 with S2O4(2-). For the first order dissociation, k-1 = 51 s-1, deltaH-1++ = 20.6 kcal-mol-1 and deltaS-1++ = +19 e.u. at 25 degrees, pH 8.2, and I = 0.015 M. The value of k-1 is independent of [protein] = 50 to 200 muM, [S2O4(2-)] = 5 to 100 mM I = 0.015 to 0.30 M and pH 6.8 to 9.0. Using myoglobin instead of S2O4(2-) as scavenger gives similar results. Combination of activation parameters for the oxygenation and deoxygenation processes gives K1 = 1.5 X 10(5) M-1, deltaH = -12.4 kcal-mol-1, and deltaS = -18 e.u., values in good agreement with independent thermodynamic data. Perchlorate ion (0.05 M) enhances k-1 about 3-fold and hardly effects k1. There is no sign of other than a single reaction in either direction, and octameric hemerythrin apparently behaves kinetically as eight single units. 相似文献
129.
The role of the adrenal gland in suppressing the onset of maternal behavior in virgin female rats was investigated. Virgin female rats were either ovariectomized, adrenalectomized, or subjected to the combined operation and tested for the induction of maternal behavior by being exposed to young pups. Females that were both ovariectomized and adrenalectomized 4 weeks prior to testing exhibited significantly shorter latencies to the onset of maternal behavior than that of females subjected to either ovariectomy or adrenalectomy alone. Replacement of either estrogen or progesterone in the group that was both adrenalectomized and ovariectomized resulted in a prolongation of the average latency to become maternal. The results indicated that both estrogen and progesterone can act to inhibit the onset of maternal behavior and that the adrenal gland may act to suppress the onset of such behavior by supplying an extra-ovarian source of endogenous progesterone and estrogen. 相似文献
130.
An obligately anaerobic bacterium, Eubacterium lentum, was shown to contain cytochromes a, b, and c and a carbon monoxide-binding pigment. Extracts of cells grown with hemin gave a typical absorption spectrum for cytochrome c with maxima at 424, 525, and 553 nm. Extracts from cells grown in the absence of hemin also had an absorption peak corresponding to cytochrome b (562 nm) in their reduced versus oxidized spectrum. Extraction of hemes and formation of pyridine hemochromes allowed quantitation of protoheme IX and heme c. Large amounts of cytochrome c masked the presence of cytochrome b in cells grown in medium containing hemin. When cells were grown in the presence of 50 mM nitrate, cytochrome A (606 nm) was detected. In anaerobic extracts of cells grown either with or without nitrate, cytochromes b and c were reduced by formate and oxidized by NO3. Cytochrome a appeared to be partially oxidized by NO3 and completely oxidized by air. 相似文献