全文获取类型
收费全文 | 9318篇 |
免费 | 727篇 |
国内免费 | 421篇 |
专业分类
10466篇 |
出版年
2024年 | 22篇 |
2023年 | 79篇 |
2022年 | 236篇 |
2021年 | 391篇 |
2020年 | 271篇 |
2019年 | 281篇 |
2018年 | 316篇 |
2017年 | 254篇 |
2016年 | 373篇 |
2015年 | 560篇 |
2014年 | 615篇 |
2013年 | 708篇 |
2012年 | 802篇 |
2011年 | 735篇 |
2010年 | 465篇 |
2009年 | 340篇 |
2008年 | 475篇 |
2007年 | 394篇 |
2006年 | 379篇 |
2005年 | 313篇 |
2004年 | 269篇 |
2003年 | 229篇 |
2002年 | 225篇 |
2001年 | 170篇 |
2000年 | 187篇 |
1999年 | 137篇 |
1998年 | 84篇 |
1997年 | 81篇 |
1996年 | 78篇 |
1995年 | 59篇 |
1994年 | 63篇 |
1993年 | 48篇 |
1992年 | 90篇 |
1991年 | 76篇 |
1990年 | 77篇 |
1989年 | 61篇 |
1988年 | 47篇 |
1987年 | 67篇 |
1986年 | 45篇 |
1985年 | 37篇 |
1984年 | 33篇 |
1983年 | 20篇 |
1982年 | 25篇 |
1980年 | 22篇 |
1979年 | 26篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1975年 | 18篇 |
1973年 | 21篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
肌肽对绵羊精子无氧酵解的影响 总被引:3,自引:0,他引:3
在无氧条件下,绵羊精子通过酵解途径获得能量,代谢结果产生大量乳酸,本实验通过测定精子悬液中果糖摄取以及乳酸生成量,研究肌肽、棉酚对绵羊精子酵解途径的影响,结果表明:4mM肌肽对绵羊精子酵解有显著增强作用,并能刺激精子对果糖的摄取。12μM棉酚对绵羊精子无明显抑制,棉酚能部分抑制肌肽对精子的酵解作用。 相似文献
92.
Limitations to net photosynthesis as affected by nitrogen status in jack pine (Pinus banksiana Lamb.) seedlings 总被引:2,自引:0,他引:2
Relative limitations of nitrogen (N) status on the processescontributing to photosynthetic rate (A) were investigated. Jackpine {Pinus banksiana Lamb.) seedlings from seeds grown in sandculture were supplied with four different N treatments for 6weeks, which resulted in a needle N content ranging from 5085mmol m2 (1432 mg g1 dry weight). Leaf gasexchange at varying CO2 levels was measured and limitationson A350 (A at ambient CO2 level) caused by finite, limitingcarboxylation efficiency (c.e.), maximum A (Amax)and stomatalconductance were estimated from an analysis of the responseof A to internal CO2 concentration. Although c.e. and Amax decreasedlinearly with the decline in needle N, the magnitudes of theirchanges relative to A350 differed. Amax varied with A350 andalways exceeded A350 by 3738% c.e., however, declinedfaster than A350, as needle N level decreased. Consequently,relative limitation on A350 caused by inefficient Amax remainedconstant, but limitations caused by c.e. increased by 1015%at low N levels. In contrast, the limitation by stomatal conductancedeclined initially, but remained stable when N content droppedbelow 75 mmol m2. The results suggest: (1) a decreasein biochemical capacity, but not stomatal conductance, contributedto the reduction of A350 induced by N-deficiency in jack pineseedlings; and (2) the capacity of carboxylation appeared tobe impaired more than that of electron transport and/or photophosphorylationand its reduction may be the major reason for the reductionin A350. Key words: ACi analysis, carboxylation efficiency, electron transport, nitrogen deficiency, stomatal conductance 相似文献
93.
Koh-Luar S.I. Choo G.H.B. Tan S.L.R. Tan C.W.K. 《World journal of microbiology & biotechnology》1997,14(1):17-22
We describe the use of repetitive element sequence-based PCR (rep-PCR) on the two repetitive sequences, REP and ERIC elements, to distinguish members of closely related Salmonella species. Within the species, ERIC–PCR showed a higher discriminative potential than REP–PCR, but by using a combination of the two PCR methods it was possible to distinguish all the isolates examined. The rep-PCR fingerprints of Salmonella organisms were distinctly different from some Gram-positive bacteria, for example Staphylococcus, Bacillus megaterium, and even the closely related Escherichia coli and Serratia marcescens. Identical fingerprints were observed with whole-cell preparations. Rapid specimen preparation has enhanced the value of rep-PCR in timely analysis of epidemiological relationships. 相似文献
94.
95.
催产素在脊髓水平对电针镇痛的影响 总被引:3,自引:0,他引:3
采用玻璃微电极胞外记录和脊髓表面给药的方法观察了催产素(OT)、抗催产素血清(AOTS)以及电针穴位对背角神经元伤害性诱发放电的影响。结果表明:电针穴位或脊髓表面施加OT可部分抑制脊髓背角神经元的伤害性诱发放电;在电针的基础上施加OT则明显加强电针的抑制效应;相反,用AOTS预处理后,电针的抑制作用放取消。提示OT在脊髓水平参与了对痛觉信息的调制,并与一定频率的针刺镇痛有关。 相似文献
96.
Summary Batch cultures of Saccharomycopsis lipolytica were grown in minimal medium with olive oil as carbon source. Inocula of glucose-grown cells commenced growth with little lag at rates largely unaffected by variations in the stirring rate or oil concentration. However, growth rates declined when the medium pH was below 7.0. In all cultures, media pH declined with increasing cell concentration. Cell composition during exponential growth was 42% protein and 2% fat. Carbon-limited cells maintained this composition after oil exhaustion but during nitrogen- and oxygen-limited growth, protein content decreased and fat content increased although the protein decrease was only transient with oxygen limitation. Yield coefficients for triglyceride were near unity for all cultures. Free acid concentrations rose rapidly after inoculation. As fermentations progressed, free glycerol appeared and concentrations of di- and monoglycerides passed through maximal values although peak concentrations of di- and monoglycerides persisted for extended times in oxygen- and nitrogen-limited cultures respectively. The fraction of free glycerol consumed was greater in oxygen-limited than in carbon- or nitrogen-limited culture. The basic requirements for growth of yeasts on fatty wastes are discussed with reference to these observations. 相似文献
97.
Desired family size and contraceptive behavior of 986 ever-married women aged 15-54 from Northern Mindanao, the Philippines, are described. Using the Dow and Werner typology of demographic and contraceptive patterns, it was found that a majority of the women were traditional in their demographic attitude and contraceptive behavior. A substantial proportion of the women desired medium to large families. In consistency in attitude and behavior was also manifested by women who desired small families but have never used efficient family planning methods. Sociodemographic characteristics of these women are described. A majority of the women who have never used efficient contraception also never practiced family planning. Rhythm was the primary method of birth control among demographic innovators and traditionals who have ever used inefficient methods; the pill was popular among contraceptive innovators and moderns. A considerable proportion of the demographic innovators and the traditionals were not currently using any family planning method; a lesser proportion of contraceptive innovators and moderns were also not using birth control methods. Relevance of the findings to family planning programs is pointed out. Recommendations are given. Motivation for a small family is a prerequisite for a successful family planning program. Without motivation, what family planning programs do is reduce the number of unwanted births. Family planning programs should reach out to those who are sufficiently motivated to have small families but have never used efficient family planning methods. A major finding of this research is that a majority of the women who never used efficient contraception also never used inefficient or traditional methods. Efforts should be made to change this. Additionally, efforts should be made to bring those modern and contraceptive innovators who had stopped using any family planning method to practice contraception again. 相似文献
98.
Species of Cunninghamella, Gliocladium deliquescens, Trichoderma harzianum and T. koningii were isolated from rotten wood chips. When grown on medium containing cellulose, all except Cunninghamella produced the three primary enzymes (exoglucanase, endoglucanase and -glucosidase) of the cellulase complex. The patterns for enzyme production, changes in mycelial mass and pH of the induction medium for T. harzianum and T. koningii were closely similar, and were distinguishable from those of G. deliquescens. 相似文献
99.
Uptake of d-glucosamine by rat brain synaptosomes occurs via a saturable transport process (Km 2.1 mM, V 3.0 nmol/mg per min) which was clearly distinguishable from simple diffusion. This transport process is highly sensitive to cytochalasin (Ki = 7 · 10?5 mM. d-Glucose competitively inhibits d-glucosamine uptake with a Ki value of 8 · 10?1 mM. 相似文献
100.
The effect of 2-bromopalmitate, an inhibitor of fatty acid oxidation, on progesterone synthesis in isolated luteal cells was studied. Incubation of the cells with increasing concentrations (0 – 2 mM) of the inhibitor resulted in an initial enhancement of progesterone synthesis, both in the presence and absence of LH (1 μg/m1). In the LH-treated cells, this stimulation in steroid synthesis becomes markedly impaired at 2 mM 2-bromopalmitate. In the control cells, however, progesterone synthesis was sustained at the elevated level. At high concentrations of 2-bromopalmitate, the stimulatory effect of LH (relative to the controls) on steroidogenesis was progressively diminished, until it was completely abolished at an inhibitor concentration of 2 mM. The oxidation of labelled palmitic acid by luteal cells was also effectively inhibited by 2-bromopalmitate (1 mM). The results indicate that the steroidogenic effect of LH is mediated, in part, by fatty acid oxidation, and were explained in terms of the interaction between carbohydrate and fatty acid oxidation in supporting ovarian steroidogenesis. 相似文献