全文获取类型
收费全文 | 1624036篇 |
免费 | 152849篇 |
国内免费 | 1721篇 |
出版年
2021年 | 18268篇 |
2019年 | 16303篇 |
2018年 | 19170篇 |
2017年 | 18034篇 |
2016年 | 29256篇 |
2015年 | 43300篇 |
2014年 | 51410篇 |
2013年 | 77624篇 |
2012年 | 47003篇 |
2011年 | 37919篇 |
2010年 | 47091篇 |
2009年 | 47330篇 |
2008年 | 34827篇 |
2007年 | 33444篇 |
2006年 | 36720篇 |
2005年 | 37761篇 |
2004年 | 36907篇 |
2003年 | 34204篇 |
2002年 | 32021篇 |
2001年 | 51877篇 |
2000年 | 49703篇 |
1999年 | 44959篇 |
1998年 | 27584篇 |
1997年 | 27409篇 |
1996年 | 26560篇 |
1995年 | 24715篇 |
1994年 | 24345篇 |
1993年 | 23571篇 |
1992年 | 37854篇 |
1991年 | 35912篇 |
1990年 | 34534篇 |
1989年 | 34971篇 |
1988年 | 32220篇 |
1987年 | 30236篇 |
1986年 | 28609篇 |
1985年 | 30024篇 |
1984年 | 28149篇 |
1983年 | 24373篇 |
1982年 | 23124篇 |
1981年 | 21985篇 |
1980年 | 20477篇 |
1979年 | 23790篇 |
1978年 | 21277篇 |
1977年 | 20134篇 |
1976年 | 18859篇 |
1975年 | 18926篇 |
1974年 | 19707篇 |
1973年 | 19971篇 |
1972年 | 17130篇 |
1971年 | 15646篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
791.
Lucia Di Giambattista P. Grimaldi S. Gaudenzi D. Pozzi M. Grandi S. Morrone I. Silvestri A. Congiu Castellano 《European biophysics journal : EBJ》2010,39(6):929-934
We have made a preliminary analysis of the results about the effects on tumoral cell line (lymphoid T cell line Jurkat) induced
by UVB radiation (dose of 310 mJ/cm2) with and without a vegetable mixture. In the present study, we have used two techniques: Fourier transform infrared spectroscopy
(FTIR) and flow cytometry. FTIR spectroscopy has the potential to provide the identification of the vibrational modes of some
of the major compounds (lipid, proteins and nucleic acids) without being invasive in the biomaterials. The second technique
has allowed us to perform measurements of cytotoxicity and to assess the percentage of apoptosis. We already studied the induction
of apoptotic process in the same cell line by UVB radiation; in particular, we looked for correspondences and correlations
between FTIR spetroscopy and flow cytometry data finding three highly probable spectroscopic markers of apoptosis (Pozzi et
al. in Radiat Res 168:698–705, 2007). In the present work, the results have shown significant changes in the absorbance and
spectral pattern in the wavenumber protein and nucleic acids regions after the treatments. 相似文献
792.
793.
M Traczewska D Chmielewska M Ochocka 《Folia haematologica (Leipzig, Germany : 1928)》1985,112(4):525-536
The in vivo effect of thymus factor X (TFX) on the E-rosetting capacity, the absolute peripheral blood lymphocytes and T-cell number per microliter, the skin test reactivity to recall antigens, and the immunoglobulin production was evaluated in 20 children with acute lymphoblastic leukaemia. The in vitro effect of TFX was also tested. The mean percentage of E-rosettes in these patients increased from 50 to 64%. Although absolute peripheral blood lymphocyte and T-cell number per microliter rose significantly, the mean values did not reach those in healthy children. The tests with recall antigens were positive in 13% of patients prior to immunotherapy and 32% following the therapy. The influence of immunotherapy on infectious episodes and on the stabilisation of remission was also evaluated. TFX in vivo appears to restore immunocompetence, decrease infections, and prolong remissions in children with ALL in remission. 相似文献
794.
795.
Crystals of the steroid-metabolizing enzyme, delta 5-3-ketosteroid isomerase (EC 5.3.3.1) from Pseudomonas testosteroni, exhibit many enzymatic properties. Each enzyme subunit in the lattice binds a competitive inhibitor, progesterone, with the same stoichiometry (1:1) and affinity (KD = 6 X 10(-6) M) as the enzyme in solution. Another competitive inhibitor, 19-nortestosterone, competes with progesterone for the same binding sites in the crystal. The enzyme crystals catalyze the conversion of delta 5- to delta 4-ketosteroids, but because the enzyme is so efficient, and substrate diffusion into the crystal is so slow, substrate cannot penetrate deeply into the crystal before being converted to product. A general theoretical formulation is presented to account for the effects of substrate diffusion into enzyme crystals of different shapes and sizes. The dependence of apparent mean enzyme activity in steroid isomerase crystals as a function of crystal size is shown to be consistent with this theoretical formulation. These inhibitor binding and catalytic properties suggest that the enzyme is in an active conformation within these crystals. 相似文献
796.
We present an analysis of the diffusion of a tracer in a model of a cell-intercellular space system. The problem reduces to
the resolution of a system of a linear partial differential equation and of a linear integral differential equation. The mathematical
results have been obtained in terms of their Laplace transforms, which have been inverted by a numerical procedure for some
parameter values. The importance of considering gradients of concentrations in intercellular spaces instead of lumping them
with the external mediums has been discussed together with the possibility of extending Ussing's relation to transient cases,
in order to detect active transports. Some possible implementations of the model to take into account more general situations
have been considered. 相似文献
797.
The neuropeptide thyrotropin releasing hormone (TRH) is capable of influencing both neuronal mechanisms in the brain and the activity of the pituitary-thyroid endocrine axis. By the use of immunocytochemical techniques, first the ultrastructural features of TRH-immunoreactive (IR) perikarya and neuronal processes were studied, and then the relationship between TRH-IR neuronal elements and dopamine-beta-hydroxylase (DBH) or phenylethanolamine-N-methyltransferase (PNMT)-IR catecholaminergic axons was analyzed in the parvocellular subnuclei of the hypothalamic paraventricular nucleus (PVN). In control animals, only TRH-IR axons were detected and some of them seemed to follow the contour of immunonegative neurons. Colchicine treatment resulted in the appearance of TRH-IR material in parvocellular neurons of the PVN. At the ultrastructural level, immunolabel was associated with rough endoplasmic reticulum, free ribosomes and neurosecretory granules. Non-labelled axons formed synaptic specializations with both dendrites and perikarya of the TRH-synthesizing neurons. TRH-IR axons located in the parvocellular units of the PVN exhibited numerous intensely labelled dense-core and fewer small electron lucent vesicles. These axons were frequently observed to terminate on parvocellular neurons, forming both bouton- and en passant-type connections. The simultaneous light microscopic localization of DBH or PNMT-IR axons and TRH-synthesizing neurons demonstrated that catecholaminergic fibers established contacts with the dendrites and cell bodies of TRH-IR neurons. Ultrastructural analysis revealed the formation of asymmetric axo-somatic and axo-dendritic synaptic specializations between PNMT-immunopositive, adrenergic axons and TRH-IR neurons in the periventricular and medial parvocellular subnuclei of the PVN. These morphological data indicate that the hypophysiotrophic, thyrotropin releasing hormone synthesizing neurons of the PVN are directly influenced by the central epinephrine system and that TRH may act as a neurotransmitter or neuromodulator upon other paraventricular neurons. 相似文献
798.
Abstract Permeabilized cells of Haemophilus influenzae incorporate wall precursors into murein material in an ampicillin-sensitive reaction. In resistant transformants that contain the low antibiotic affinity penicillin-binding proteins (PBPs) 4 and 5, the sensitivity of this incorporation reaction to ampicillin is proportionally lower, suggesting a catalytic role for these proteins in wall synthesis. We conclude that, analogous to the reaction in Escherichia coli , PBPs 4 and 5 of H. influenzae have transpeptidase activity. 相似文献
799.
Barbara A. Booth Jouni Uitto 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,675(1):117-122
Human skin fibroblasts were cultured under conditions optimized for collagen synthesis, and the effects of ascorbic acid on procollagen production, proline hydroxylation and the activity of prolyl hydroxylase were examined in cultures. The results indicated that addition of ascorbic acid to confluent monolayer cultures of adult human skin fibroblasts markedly increased tha amount of [3H]hydroxyproline syntehsized. Ascorbic acid, however, did not increase the synthesis of 3H-labeled collagenous polypeptides assayed independently of hydroxylation of proline residues, nor did it affect the amount of prolyl hydroxylase detectable by an in vitro enzyme assay. Also long-term cultures of the cells or initiation of fibroblast cultures in the presence of ascorbic acid did not lead to an apparent selection of a cell population which might be abnormally responsive to ascorbic acid. Thus, ascorbic acid appears to have one primary action on the synthesis of procollagen by cultured human skin fibroblasts: it is necessary for synthesis of hydroxyproline, and consequently for proper triple helix formation and selection of procollagen. 相似文献
800.
R.J. Ulvik I. Romslo F. Roland R.R. Crichton 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,677(1):50-56
Mitochondria mobilize iron from ferritin by a mechanism that depends on external FMN. With rat liver mitochondria, the rate of mobilization of iron is higher from rat liver ferritin than from horse spleen ferritin. With horse liver mitochondria, the rate of iron mobilization is higher from horse spleen ferritin than from rat liver ferritin. The results are explained by a higher affinity between mitochondria and ferritins of the same species. The mobilization of iron increases with the iron content of the ferritin and then levels off. A maximum is reached with ferritins containing about 1 200 iron atoms per molecule. The results represent further evidence that ferritin may function as a direct iron donor to the mitochondria. 相似文献