全文获取类型
收费全文 | 10066篇 |
免费 | 818篇 |
国内免费 | 414篇 |
专业分类
11298篇 |
出版年
2024年 | 34篇 |
2023年 | 128篇 |
2022年 | 181篇 |
2021年 | 245篇 |
2020年 | 287篇 |
2019年 | 353篇 |
2018年 | 342篇 |
2017年 | 308篇 |
2016年 | 298篇 |
2015年 | 378篇 |
2014年 | 420篇 |
2013年 | 773篇 |
2012年 | 362篇 |
2011年 | 418篇 |
2010年 | 318篇 |
2009年 | 449篇 |
2008年 | 469篇 |
2007年 | 523篇 |
2006年 | 473篇 |
2005年 | 485篇 |
2004年 | 422篇 |
2003年 | 388篇 |
2002年 | 352篇 |
2001年 | 281篇 |
2000年 | 209篇 |
1999年 | 282篇 |
1998年 | 207篇 |
1997年 | 156篇 |
1996年 | 145篇 |
1995年 | 138篇 |
1994年 | 147篇 |
1993年 | 174篇 |
1992年 | 136篇 |
1991年 | 99篇 |
1990年 | 88篇 |
1989年 | 83篇 |
1988年 | 59篇 |
1987年 | 52篇 |
1986年 | 54篇 |
1985年 | 85篇 |
1984年 | 91篇 |
1983年 | 63篇 |
1982年 | 95篇 |
1981年 | 60篇 |
1980年 | 54篇 |
1979年 | 46篇 |
1978年 | 32篇 |
1977年 | 21篇 |
1976年 | 11篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Summary Palmitate binding to human erythrocyte ghost membranes has been investigated with ghost preparations suspended in 0.2% albumin solutions. Free unbound palmitate in the extracellular water phase was measured in equilibrium studies using albumin-filled acid loaded ghosts as small semipermeable bags. The apparent dissociation constant of binding to the membrane is 13.5 nM and the binding capacity 19 nmoles per 7.2 × 109 cells.The 0°C exchange efflux kinetics of palmitate from albumin-filled ghosts is described by a model, which provides estimates of the rate constant of membrane transfer, k3 = 0.024 s–1, independent of the molar ratio of palmitate to albumin () and of a mean dissociation rate constant of the palmitate-albumin complex, k1 = 0.0015 s–1 at 0.2, allowing for a heterogeneity of the palmitate binding to albumin.The values of a third kinetically determined dependent model constant, Q, the ratio of palmitate bound to the membrane inner surface to palmitate on intracellular albumin, are not different from the Q values obtained by equilibrium experiments.The temperature dependences of k1 and k3 in the interval 0°C to 15°C give activation energies of 96 and 103 kJ/mole, respectively. The 0°C exchange efflux increases about 2 fold in response to a rise of pH from 6 to 9. The results suggest a carrier mediated palmitate flux at low with a Vmax about 2 pmoles min–1 cm–2 at 0°C pH 7.3. 相似文献
42.
The enzymatic mechanism and the kinetic parameters of GABA-transaminase extracted from cultured mouse cerebral cortex neurons and astrocytes were studied. Neuronal as well as astrocytic GABA-transaminase obeyed a bi bi ping-pong reaction mechanism. The estimated Km-values for -ketoglutarate and GABA were significantly lower for astroglial GABA-transaminase compared to the neuronal enzyme suggesting a possible existence of cell specific isozymes of GABA-transaminase. The observed enzymatic mechanism and the magnitude of the estimated kinetic parameters imply that GABA-transaminase synthesized in the two types of cultured neural cells is mechanistically and kinetically equivalent to the enzyme synthesized in the brainin vivo. 相似文献
43.
WALTER C. QUEVEDO JACOB DYCKMAN RUTH HALABAN GISELA E. MOELLMANN JANET M. COWAN THOMAS J. HOLSTEIN 《Pigment cell & melanoma research》1988,1(Z1):124-131
The BULT melanoma originated at Brown University as a spontaneous, small black nodule on the tail of an adult female mouse of the LT/Ch strain. Histological examination of a portion of the tumor indicated that it was intradermal and consisted predominantly of heavily melanized, ovoid to fusiform cells with melanin-laden macrophages scattered among them. The BULT melanoma has been maintained in LT/Ch mice for approximately 5 years by periodic transplantation, at first subcutaneously on the flanks and, more recently, intramuscularly in the hind legs. The shift in transplantation site was made following a marked decline in the growth of subcutaneous grafts. The transplants have retained the uniform deep-black melanization and general histology of the primary melanoma. Numerous melanosomes at all stages of development are found within the melanoma cells. DOPA-positive cytoplasmic vesicles are abundant. Occasional autophagic vacuoles containing clusters of melanosomes are also present. A few metastases from the transplanted melanoma have been observed in lymph nodes and on one occasion in the lungs. When grown in vitro, BULT melanoma cells do not require special growth promoting agents (e.g., TPA; cAMP) in order to proliferate. The BULT melanoma differs in one or more respects from each of the other three transplantable spontaneous mouse melanomas widely used in cancer research. In addition, it arose in a strain of mice characterized by the spontaneous death of melanocytes while the latter are engaged in synthesizing eumelanin within hair follicles. Karyotypic analysis of cultured cells showed a modal chromosome number of 68 with a range of 58–72 chromosomes. 相似文献
44.
Tohru Nakashizuka 《Ecological Research》1988,3(1):21-35
Seedling recruitment and survivorship of beech (Fagus crenata) were studied with special reference to the simultaneous death of undergrowing bamboo (Sasa kurilensis). The survival rate of beech seedlings on the floor whereSasa had withered was much higher than that on the floor whereSasa survived. Damping off caused the largest mortality among beech seedlings. However, the allocation pattern of matter to different
parts of the seedlings indicated that their survival was greatly affected by production economy. The dense cover of dwarf
bamboo prevented the establishment of beech seedling banks on the forest floor. The interval between the times when simultaneous
death ofSasa occur and the length of its recovery period are thus important factors controlling the dynamics of beech forests in Japan. 相似文献
45.
Merten OW 《Cytotechnology》1988,1(2):113-121
Batch cultures of mouse-mouse hybridoma cell lines were carried out and their growth and production kinetics investigated. Three main cell specific production patterns (expressed as pg IgG/cell x hour) were found, which can be used as a classification system for hybridoma cell lines (groups I–III). Cells showing the highest IgG-production at the beginning of the batch culture (during the lag and the onset of the log-phase) were classified as either group I and II. The difference was that cell lines of group II showed a second high cell specific production at the onset of the stationary and death phases. Cell lines of group III had a quite constant production of antibodies during their growth; but IgG secretion completely stopped after the beginning of the stationary phase. The implications of these three production patterns on the design of a production process are discussed. 相似文献
46.
Desensitization of the insulin-secreting beta cell. 总被引:2,自引:0,他引:2
In human diabetes, inherent impaired insulin secretion can be exacerbated by desensitization of the beta cell by chronic hyperglycemia. Interest in this phenomenon has generated extensive studies in genetic or experimentally induced diabetes in animals and in fully in vitro systems, with often conflicting results. In general, although chronic glucose causes decreased beta-cell response to this carbohydrate, basal response and response to alternate stimulating agents are enhanced. Glucose-stimulated insulin synthesis can be increased or decreased depending on the system studied. Using a two-compartment beta-cell model of phasic insulin secretion, a unifying hypothesis is described which can explain some of the apparent conflicting data. This hypothesis suggests that glucose-desensitization is caused by an impairment in stimulation of a hypothetical potentiator singularly responsible for: 1) some of the characteristic phases of insulin secretion; 2) basal release; 3) potentiation of non-glucose stimulators; and 4) apparent "recovery" from desensitization. Review of some of the pathways that regulate insulin secretion suggest that phosphoinositol metabolism and protein kinase-C production are regulated similarly to the theoretical potentiator and their impairment is a major contributor to glucose desensitization in the beta cell. 相似文献
47.
It has been known for several decades that cultured murine cells undergo a defined series of changes, i.e., anin vitro evolution, which includes crisis, spontaneous transformation (immortalization), aneuploidy, and spontaneous neoplastic transformation. These changes have been shown to be caused by thein vitro environment rather than an inherent instability of the murine phenotype or genotype. Serum amine oxidases were recently identified as a predominant cause of crisis. These enzymes generate hydrogen peroxide from polyamine substrates that enter the extracellular milieu. This finding implicates free-radical toxicity as the underlying cause ofin vitro evolution. We propose an oxyradical hypothesis to explain each of the stages ofin vitro evolution and discuss its significance for cytotechnology and long-term cultivation of mammalian cell types.ORR, CDER, FDA Mod-1, Room 2023, 8301 Muirkirk Road, Laurel MD 20708, USA 相似文献
48.
A. A. Serikov 《Journal of biological physics》1991,18(1):65-77
Model equations for the kinetics of the synthesis and decay of molecular aggregates are used to show the high sensitivity of equilibrium concentrations of high-molecular aggregates to external radiation. This phenomenon is used to explain the effects of low-intensity microwave fields on the functioning of biological systems. The experimental results on the influence of SHF-radiation on ferricyanide reduction by erythrocytes are interpreted in detail. 相似文献
49.
Nuclear genome characterization of the carrageenophyteAgardhiella subulata (Rhodophyta) 总被引:2,自引:0,他引:2
Donald F. Kapraun Julie A. Dutcher Juan Lopez-Bautista 《Journal of applied phycology》1992,4(2):129-137
DNA reassociation kinetics were used to determine nuclear genome organization and complexity inAgardhiella subulata (Gigartinales, Rhodophyta). Results indicate the presence of three second-order components corresponding to fast (22%), intermediate
(68%) and slow (10%) fractions. Thus, the genome consists of 90% repetitive sequences. Microspectrophotoometry with the DNA-localizing
fluorochrome DAPI was used to confirm ploidy level differences in the gametophytic and tetrasporophytic phases. Results indicate
that meiosis occurs during tetrasporogenesis. Comparison of mean nuclear DNA (If) values to chicken erythrocytes (RBC) resulted in an estimate of 0.9 pg/2C genome forAgardhiella. Karyological studies using aceto-orcein revealed a chromosome complement of 2N = 44 in carposporangia and the presence of 22 bivalents during diakinesis of tetraspore mother cells. 相似文献
50.
D. A. Schultz R. L. Baldwin 《Protein science : a publication of the Protein Society》1992,1(7):910-916
A chemically synthesized gene for ribonuclease A has been expressed in Escherichia coli using a T7 expression system (Studier, F.W., Rosenberg, A.H., Dunn, J.J., & Dubendorff, J.W., 1990, Methods Enzymol. 185, 60-89). The expressed protein, which contains an additional N-terminal methionine residue, has physical and catalytic properties close to those of bovine ribonuclease A. The expressed protein accumulates in inclusion bodies and has scrambled disulfide bonds; the native disulfide bonds are regenerated during purification. Site-directed mutations have been made at each of the two cis proline residues, 93 and 114, and a double mutant has been made. In contrast to results reported for replacement of trans proline residues, replacement of either cis proline is strongly destabilizing. Thermal unfolding experiments on four single mutants give delta Tm approximately equal to 10 degrees C and delta delta G0 (apparent) = 2-3 kcal/mol. The reason is that either the substituted amino acid goes in cis, and cis<==>trans isomerization after unfolding pulls the unfolding equilibrium toward the unfolded state, or else there is a conformational change, which by itself is destabilizing relative to the wild-type conformation, that allows the substituted amino acid to form a trans peptide bond. 相似文献