全文获取类型
收费全文 | 14221篇 |
免费 | 914篇 |
国内免费 | 2篇 |
出版年
2022年 | 87篇 |
2021年 | 153篇 |
2019年 | 104篇 |
2018年 | 140篇 |
2017年 | 111篇 |
2016年 | 214篇 |
2015年 | 342篇 |
2014年 | 380篇 |
2013年 | 831篇 |
2012年 | 692篇 |
2011年 | 655篇 |
2010年 | 360篇 |
2009年 | 393篇 |
2008年 | 627篇 |
2007年 | 605篇 |
2006年 | 640篇 |
2005年 | 630篇 |
2004年 | 656篇 |
2003年 | 637篇 |
2002年 | 580篇 |
2001年 | 545篇 |
2000年 | 532篇 |
1999年 | 442篇 |
1998年 | 177篇 |
1997年 | 160篇 |
1996年 | 148篇 |
1995年 | 112篇 |
1994年 | 127篇 |
1993年 | 133篇 |
1992年 | 332篇 |
1991年 | 307篇 |
1990年 | 312篇 |
1989年 | 285篇 |
1988年 | 242篇 |
1987年 | 244篇 |
1986年 | 217篇 |
1985年 | 186篇 |
1984年 | 143篇 |
1983年 | 137篇 |
1982年 | 123篇 |
1981年 | 113篇 |
1980年 | 101篇 |
1979年 | 117篇 |
1978年 | 99篇 |
1977年 | 83篇 |
1976年 | 82篇 |
1975年 | 75篇 |
1974年 | 86篇 |
1973年 | 97篇 |
1971年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 30 毫秒
31.
Photosynthetically competent chloroplasts were isolated fromcells of Euglena gracilis Z grown photoautotrophically in 1.5%CO2. The isolated chloroplasts were intact and substantiallyfree from cytosolic, mitochondrial and microbody materials.The effects of some compounds on the activity of photosynthetic14CO2 fixation were examined. The optimal pH and sorbitol concentrationwere 8.0 and 0.33 M, respectively. The chloroplasts requireda high level of P, (5 to 20 mM) for the maximal rate of photosynthesis.They were insusceptible to 10 mM of free Mg2+. ATP, ADP andAMP at 1 to 5 mM notably stimulated photosynthesis, althoughhigh concentrations of AMP were unfavorable. In the assay mediumdeveloped for this study, the chloroplasts exhibited photosyntheticactivity of 120µmoles-mg1 Chl-h1 at 30?C. Chloroplasts could also be isolated from cells grown under ordinaryair. The rate of photosynthetic 14CO2 fixation at 1 mM NaHl4CO3was higher in these chloroplasts than in those isolated fromcells grown in 1.5% CO2, whereas at 10 mM NaHl4CO3, the ratesof the two types of chloroplasts were nearly the same. Theseresults suggest that the CO2 concentration given during growthof the algal cells affects the affinity for dissolved inorganiccarbon at the chloroplast level. (Received March 30, 1987; Accepted August 17, 1987) 相似文献
32.
33.
Summary During imbibition of whole tea seeds (6 days) two purine alkaloids, caffeine and theobromine, did not decrease in the seed
coats and there was no increase in the seeds. In parallel with and after the breaking of seed coats there was a gradual release
of caffeine from coats of germinating seeds. By contrast, when the seed was freed from the outer seed coat and soaked, imbibition
of the seed required only 2 days and simultaneously caffeine was released from the inner seed coat. In such seeds, but not
in whole seeds, growth of embryonic tissues (roots and shoots) was inhibited after the breaking of the inner seed coats. Nevertheless,
caffeine increased more in such roots of the seedlings of decoated seeds than in roots of normal seedlings. 相似文献
34.
Influence of leaf age on photosynthesis, enzyme activity, and metabolite levels in wheat 总被引:9,自引:2,他引:7
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25°C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25°C) was limited by utilization of photosynthate. 相似文献
35.
Induction and subsequent repair of DNA damage by fast neutrons in cultured mammalian cells 总被引:1,自引:0,他引:1
The induction and repair of DNA damage were studied by a DNA unwinding method in mouse L5178Y cells exposed to fast neutrons. DNA lesions induced by fast neutrons were classified into three types from their repair profiles: fast-reparable breaks (T1/2 = 3-5 min), slow-reparable breaks (T1/2 = 70 min), and nonreparable breaks. The repair rates of both fast-reparable and slow-reparable breaks were almost the same as those of corresponding damage induced by low-LET radiation. Neutrons induced a smaller amount of fast-reparable damage, an almost equal amount of slow-reparable damage, and a larger amount of nonreparable damage than those induced by equal doses of gamma rays or X rays. RBEs for fast- and slow-reparable damage were 0.3 and 0.9, respectively. The RBE for nonreparable damage was dose dependent and was 1.4 at the level of 100 breaks/10(12) Da DNA. Among the three types of lesions, only the nonreparable damage levels correlated with the linear-quadratic shape of the survival curves and with the enhanced killing effectiveness of neutrons (RBE = 1.7 at D0). 相似文献
36.
Driven by the same Ig enhancer and SV40 T promoter ras induced lung adenomatous tumors, myc induced pre-B cell lymphomas and SV40 large T gene a variety of tumors in transgenic mice. 总被引:24,自引:7,他引:17
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Y Suda S Aizawa S Hirai T Inoue Y Furuta M Suzuki S Hirohashi Y Ikawa 《The EMBO journal》1987,6(13):4055-4065
37.
Positional heterogeneity of interaction between fragments of avian limb bud in organ culture 总被引:2,自引:0,他引:2
We have previously succeeded in culturing whole leg bud from stage 21-23 chick embryos and observed a leg structure with typical cartilage pattern in vitro. In the present study, we have attempted the organ culture of the fragmented leg bud and investigated its capacity to form cartilage. Leg buds from stages 17-21 chick embryos were dissected into four pieces in the anteroposterior sequence (named 1, 2, 3, and 4, respectively) and cultured on a membrane filter in a medium consisting of Ham's F-12, chick serum, and chick embryo extract. After 6 days in culture, two central fragments (2 and 3) developed into large cartilaginous masses, while anterior (1) and posterior (4) fragments formed few or small cartilaginous masses. In addition, when these less chondrogenic fragments were combined, pinned together, and cultured, large cartilaginous masses were formed from 1 + 4 combinations but not from 1 + 1 or 4 + 4 combinations. These observations were analyzed quantitatively by measurement of 35SO4 incorporation into the sulfated glycosaminoglycan (S-GAG) and of final DNA content per explant, and by histological reconstruction of the chick-quail chimera explant. The results showed that (a) the 1 + 4 combination resulted in higher S-GAG synthesis and final DNA content than the 1 + 1 or 4 + 4 combinations in stage 18 and 21 leg buds (P less than 5%); (b) removal of ectoderm from the leg bud inhibited the increase observed for the 1 + 4 combination; c) in chick-quail chimera explants the cartilage formed from the 1 + 4 combination was largely of fragment 1 origin. These results demonstrate, first, the presence of a difference in chondrogenic capacity along the anteroposterior axis in the leg bud and, second, the occurrence of an interaction between anterior and posterior fragments which mimics the effects of grafting a zone of polarizing activity (ZPA). The mechanism of ZPA function is still unknown but the ectoderm may play some role. Some roles for ectoderm in ZPA function and differences in mesodermal responsiveness to ZPA factor(s) are suggested. 相似文献
38.
Primary structures of human protein kinase C beta I and beta II differ only in their C-terminal sequences 总被引:5,自引:0,他引:5
Two types of cDNA clones encoding human protein kinase C (PKC) were isolated from a spleen cDNA library using rabbit protein kinase C beta I/beta II cDNA as a hybridization probe. Nucleotide sequence analyses of these cDNA inserts revealed complete primary structures of two distinct types of human protein kinase C beta I and beta II which differ only in their C-terminal 50 or 52 amino acid residues. It was concluded that there exist four distinct types of PKC, PKC alpha, beta I, beta II and gamma, in human as well as rabbit, and that the corresponding sequences are strictly conserved among mammalian species. 相似文献
39.
H Suzuki M Obara H Kuwayama T Kanazawa 《The Journal of biological chemistry》1987,262(32):15448-15456
Sarcoplasmic reticulum vesicles were modified with a fluorescent thiol reagent, N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine. One mol of readily reactive thiols per mol of the Ca2+-ATPase was labeled without a loss of the catalytic activity. The fluorescence of the label increased by 8% upon binding of Ca2+ to the high affinity sites of the enzyme. This fluorescence enhancement probably reflects a conformational change responsible for Ca2+-induced enzyme activation. Upon addition of ATP to the Ca2+-activated enzyme, the fluorescence decreased by 15%. This fluorescence drop and formation of the phosphoenzyme intermediate were determined under the same conditions with a stopped-flow apparatus and a rapid quenching system. The amplitude of the fluorescence drop thus determined was saturated with 3 microM ATP. This shows that the fluorescence drop was caused by ATP binding to the catalytic site. In contrast, the rate of the fluorescence drop was not saturated even with 50 microM ATP. The fluorescence drop coincided with phosphoenzyme formation at 0.5 or 3 microM ATP, but it became much faster than phosphoenzyme formation when the ATP concentration was raised to 100 microM. These results indicate that the ATP-induced fluorescence drop reflects a conformational change in the enzyme.ATP complex. The fluorescence drop was accompanied by a red spectrum shift, which suggests that the label was exposed to a more hydrophilic environment. The electrophoretic analysis of the tryptic digest of the labeled enzyme (10.9 kDa) showed that almost all of the label was located on the 5.2-kDa fragment which includes the carboxyl terminus and the putative ATP-binding domain. The sequencing of the two major labeled peptides, which were isolated from the thermolytic digest of the labeled enzyme, revealed that the labeled site in either of these peptides was Cys674. It seems likely that the label bound to this Cys674 could be involved in the observed fluorescence changes. 相似文献
40.
W S Argraves S Suzuki H Arai K Thompson M D Pierschbacher E Ruoslahti 《The Journal of cell biology》1987,105(3):1183-1190
The amino acid sequence deduced from cDNA of the human placental fibronectin receptor is reported. The receptor is composed of two subunits: an alpha subunit of 1,008 amino acids which is processed into two polypeptides disulfide bonded to one another, and a beta subunit of 778 amino acids. Each subunit has near its COOH terminus a hydrophobic segment. This and other sequence features suggest a structure for the receptor in which the hydrophobic segments serve as transmembrane domains anchoring each subunit to the membrane and dividing each into a large ectodomain and a short cytoplasmic domain. The alpha subunit ectodomain has five sequence elements homologous to consensus Ca2+-binding sites of several calcium-binding proteins, and the beta subunit contains a fourfold repeat strikingly rich in cysteine. The alpha subunit sequence is 46% homologous to the alpha subunit of the vitronectin receptor. The beta subunit is 44% homologous to the human platelet adhesion receptor subunit IIIa and 47% homologous to a leukocyte adhesion receptor beta subunit. The high degree of homology (85%) of the beta subunit with one of the polypeptides of a chicken adhesion receptor complex referred to as integrin complex strongly suggests that the latter polypeptide is the chicken homologue of the fibronectin receptor beta subunit. These receptor subunit homologies define a superfamily of adhesion receptors. The availability of the entire protein sequence for the fibronectin receptor will facilitate studies on the functions of these receptors. 相似文献