全文获取类型
收费全文 | 11698篇 |
免费 | 572篇 |
国内免费 | 1089篇 |
出版年
2024年 | 35篇 |
2023年 | 206篇 |
2022年 | 294篇 |
2021年 | 352篇 |
2020年 | 368篇 |
2019年 | 322篇 |
2018年 | 332篇 |
2017年 | 258篇 |
2016年 | 341篇 |
2015年 | 478篇 |
2014年 | 525篇 |
2013年 | 663篇 |
2012年 | 360篇 |
2011年 | 425篇 |
2010年 | 393篇 |
2009年 | 515篇 |
2008年 | 502篇 |
2007年 | 502篇 |
2006年 | 544篇 |
2005年 | 513篇 |
2004年 | 479篇 |
2003年 | 388篇 |
2002年 | 449篇 |
2001年 | 294篇 |
2000年 | 272篇 |
1999年 | 273篇 |
1998年 | 255篇 |
1997年 | 254篇 |
1996年 | 244篇 |
1995年 | 240篇 |
1994年 | 259篇 |
1993年 | 226篇 |
1992年 | 209篇 |
1991年 | 185篇 |
1990年 | 159篇 |
1989年 | 144篇 |
1988年 | 149篇 |
1987年 | 105篇 |
1986年 | 117篇 |
1985年 | 114篇 |
1984年 | 142篇 |
1983年 | 79篇 |
1982年 | 89篇 |
1981年 | 60篇 |
1980年 | 70篇 |
1979年 | 56篇 |
1978年 | 32篇 |
1977年 | 26篇 |
1976年 | 21篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
In order to regulate cell volume during hyperosmotic stress, the intertidal copepod Tigriopus californicus, like other aquatic crustaceans, rapidly accumulates high levels of intracellular alanine, proline, and glycine. Glutamate-pyruvate transaminase (GPT; EC 2.6.1.2), which catalyzes the final step of alanine synthesis, is genetically polymorphic in T. californicus populations at Santa Cruz, California. Spectrophotometric studies of homogenates derived from a homozygous isofemale line of each of the two common GPT alleles indicated that the GPTF allozyme has a significantly higher specific activity than the GPTS allozyme. Under conditions of hyperosmotic stress, individual adult copepods of GPTF and GPTF/S genotypes accumulated alanine, but not glycine or proline, more rapidly than GPTS homozygotes. When young larvae were subjected to the same hyperosmotic conditions, GPTS larvae suffered a significantly higher mortality than GPTF or GPTF/S larvae. These results suggest that the biochemical differences among GPT allozymes result in specific physiological variation among GPT genotypes and that this physiological variation is manifested in differential genotypic survivorships under some naturally occurring environmental conditions.This work was supported in part by a grant from the Lerner Fund for Marine Research of the American Museum of Natural History, an NIH Training Grant in Integrative Biology, and NIH Grants GM 28016 and GM 10452. 相似文献
162.
David Dichek Thomas Quertermous 《In vitro cellular & developmental biology. Plant》1989,25(3):289-292
Summary Levels of seven messenger RNA species were compared in human umbilical vein endothelial cells of different lineage and time
in culture. Specifically, cells obtained from the American Type Culture Collection (ATCC) and subcultured were compared to
early passage cells from cultures produced in our laboratory. Messenger RNA for tissue plasminogen activator, plaminogen activator
inhibitor 1, urokinase, and thrombomodulin were expressed at higher levels in the ATCC cells. Thrombospondin, von Willebrand's
Factor, and protein S messenger RNA were expressed at higher levels in the cells that we isolated. In addition, in the ATCC
cells a shift in the proportion of plasminogen activator inhibitor messenger RNA from the 3.4 to the 2.4 kilobase species
was found. We conclude that specific messenger RNA levels can vary considerably between cultured human umbilical vein endothelial
cells. The large variation in mRNA levels which we describe has important implications for experiments involving gene expression
in cultured endothelium. 相似文献
163.
Tai C. Chen Norman P. Curthoys Carl F. Lagenaur Jules B. Puschett 《In vitro cellular & developmental biology. Plant》1989,25(8):714-722
Summary Proximal tubules were prepared from rat kidney cortex by collagenase digestion and purified by percoll gradient centrifugation.
Their enrichment was estimated by comparing the specific activities of various cell-specific enzymes in homogenates of renal
cortex and of the isolated tubules. The tubules were cultured in a 50:50 mixture of Dulbecco’s modified Eagle’s and Ham’s
F12 media supplemented with insulin, transferrin, epidermal growth factor, hydrocortisone, and prostaglandin E1. After 2 to 3 d an extensive outgrowth of epithelial cells developed from the attached tubules. After 5 to 7 d near confluent
monolayers were obtained. Hormonal responsiveness, marker enzyme activities, and transport properties were determined to further
characterize the primary cultures. The cultured cells exhibited increased cyclic AMP production in response to parathyroid
hormone but not calcitonin or vasopressin, consistent with the absence of cells derived from distal and collecting tubules.
The cells also retained significant levels of 25-hydroxyvitamin D3-lα-hydroxylase, alkaline phosphatase, and ψ-glytamyltranspeptidase, three enzymes that are primarily associated with the
proximal tubule. The cultured epithelial cells also exhibit a Na+-dependent phosphate and glucose transport systems. Therefore, the cells retain many functional properties that are characteristic
of proximal tubules. Thus, the primary cultures should be suitable for the study of processes that occur specifically within
this segment of the rat nephron.
This work was supported in part by the Veterans Administration (JBP), Washington, DC, by grant DK-37124 (NPC) from the National
Institutes of Health, Bethesda, MD, and by grant BNS-86-17004 (CFL) from the National Science Foundation, Washington, DC. 相似文献
164.
Kin Sing Lam Jacqueline Mattei Salvatore Forenza 《Journal of industrial microbiology & biotechnology》1989,4(2):105-108
Summary A new antitumor antibiotic named rebeccamycin was isolated from fermentations of an actinomycete,Saccharothrix aerocolonigenes. A defined medium was developed to study the regulation of synthesis of rebeccamycin byS. aerocolonigenes. In glucose medium formation of rebeccamycin was detected only after glucose was depleted. Examination of eleven different carbon sources revealed that carbon catabolite regulation is a major control mechanism for rebeccamycin production. 相似文献
165.
166.
渗透胁迫对水稻幼苗膜脂过氧化及体内保护系统的影响 总被引:5,自引:0,他引:5
两个不同抗旱性的水稻品种对PEG6000渗透胁迫(-0.5MPa,-0.8MPa)的反应具有一定差异。渗透胁迫下SOD,POD和CAT活性及Vc,Car含量与膜脂过氧化水平及膜透性呈一定负相关性,表明这些指标可作为水稻抗旱育种的参考依据。 相似文献
167.
Modulation of IL-4-induced human IgE production in vitro by IFN-gamma and IL-5: the role of soluble CD23 (s-CD23) 总被引:10,自引:0,他引:10
J Pène I Chrétien F Rousset F Brière J Y Bonnefoy J E de Vries 《Journal of cellular biochemistry》1989,39(3):253-264
IL-4 specifically induced IgE production by peripheral blood lymphocytes or by tonsil or spleen cells from healthy donors. IL-4-induced IgE synthesis was dependent on CD4+ T cells and monocytes and was blocked by IFN-gamma, IFN-alpha, and prostaglandin E-2 (PGE-2). These substances also inhibited IL-4-induced CD23 expression and subsequent release of soluble CD23 (s-CD23). In addition, IgE production was blocked by F(ab')2 fragments of an mAb against CD23. In contrast, IL-5 enhanced IL-4-induced IgE production, provided IL-4 was added at nonsaturating concentrations. This increase in IgE production correlated quantitatively with an enhanced release of s-CD23. Collectively, these results indicate that there is a correlation between s-CD23 release and IgE production. However, s-CD23 fractionated from supernatants of the lymphoblastoid cell line RPMI-8866 was ineffective in inducing IgE production in the absence of IL-4, but acted synergistically with suboptimal concentrations of IL-4. In addition, it is demonstrated that alloreactive T-cell clones produced varying concentrations of IL-4, IL-2, or IFN-gamma upon stimulation. Only supernatants of 2/4 of these T-cell clones induced a low degree of IgE synthesis, but in the presence of anti-IFN-gamma antibodies, all four supernatants induced a strong induction of IgE production. This IgE synthesis was blocked specifically by anti-IL-4 antibodies, indicating that IL-4 is the sole inducer of IgE synthesis. Our findings demonstrate that IL-4-induced IgE production involves complex interactions of T cells, B cells, and monocytes and is positively modulated by IL-5 and s-CD23 but down-regulated by IFN-gamma, IFN-alpha, and PGE-2, respectively. 相似文献
168.
Kurt R. Brunden Anthony J. Windebank Joseph F. Poduslo 《Journal of neurochemistry》1990,54(2):459-466
Previous studies have suggested that neonatal Schwann cell cultures deprived of axonal contact do not express components of the myelin membrane, including the major myelin glycoprotein, P0. In contrast, Schwann cells from permanently transected, adult nerve exhibit continued biosynthesis of P0 after culture, suggesting that the ability to express the myelin glycoprotein may depend on the degree of cellular differentiation. To examine further the ability of Schwann cell cultures to express P0 as a function of age, we have performed precursor incorporation studies on endoneurial explants from 4- to 12-day-old rat sciatic nerves after 5 days in culture. The data reveal that explants from 12-day-old animals synthesize detectable levels of this integral myelin protein when assayed by [3H]mannose incorporation, even though there is no apparent myelin assembly in the cultures. Pulse-chase analysis of cultures from 12-day-old rats demonstrates that [3H]mannose-labeled P0 is substantially degraded within 3 h. This catabolism largely can be prevented by the addition of swainsonine, ammonium chloride, or L-methionine methyl ester to the pulse-chase media. The former agent alters oligosaccharide processing whereas the latter two compounds inhibit lysosomal function. The P0 synthesized by the 12-day explant cultures following the addition of swainsonine is readily fucosylated, implying that the protein has progressed at least as far as the medial Golgi before its exit and subsequent catabolism. If cultures from 4-, 6-, and 8-day-old animals are analyzed for P0 biosynthesis by [3H]mannose incorporation in the presence of swainsonine, detectable levels of the glycoprotein are seen.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
169.
Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C 总被引:2,自引:0,他引:2
Carlo Brugnara 《The Journal of membrane biology》1989,111(1):69-81
Summary In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1mm) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell × hr) and in the affinity for external K (from 77±16mm to 28±3mm) of K influx. When the external K concentration is varied from 0 to 140mm, K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride. 相似文献
170.