全文获取类型
收费全文 | 19557篇 |
免费 | 1684篇 |
国内免费 | 1篇 |
出版年
2023年 | 79篇 |
2021年 | 162篇 |
2020年 | 124篇 |
2019年 | 144篇 |
2018年 | 370篇 |
2017年 | 384篇 |
2016年 | 422篇 |
2015年 | 356篇 |
2014年 | 473篇 |
2013年 | 741篇 |
2012年 | 1355篇 |
2011年 | 1466篇 |
2010年 | 782篇 |
2009年 | 518篇 |
2008年 | 1248篇 |
2007年 | 1241篇 |
2006年 | 1169篇 |
2005年 | 1064篇 |
2004年 | 1013篇 |
2003年 | 962篇 |
2002年 | 940篇 |
2001年 | 783篇 |
2000年 | 901篇 |
1999年 | 420篇 |
1998年 | 199篇 |
1997年 | 145篇 |
1996年 | 187篇 |
1995年 | 168篇 |
1994年 | 145篇 |
1993年 | 149篇 |
1992年 | 138篇 |
1991年 | 136篇 |
1990年 | 131篇 |
1989年 | 120篇 |
1988年 | 131篇 |
1987年 | 129篇 |
1986年 | 112篇 |
1985年 | 150篇 |
1984年 | 147篇 |
1983年 | 126篇 |
1982年 | 162篇 |
1981年 | 150篇 |
1980年 | 115篇 |
1979年 | 128篇 |
1978年 | 99篇 |
1977年 | 93篇 |
1976年 | 82篇 |
1975年 | 80篇 |
1974年 | 82篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Influx of na, k, and ca into roots of salt-stressed cotton seedlings : effects of supplemental ca 下载免费PDF全文
High Na+ concentrations may disrupt K+ and Ca2+ transport and interfere with growth of many plant species, cotton (Gossypium hirsutum L.) included. Elevated Ca2+ levels often counteract these consequences of salinity. The effect of supplemental Ca2+ on influx of Ca2+, K+, and Na+ in roots of intact, salt-stressed cotton seedlings was therefore investigated. Eight-day-old seedlings were exposed to treatments ranging from 0 to 250 millimolar NaCl in the presence of nutrient solutions containing 0.4 or 10 millimolar Ca2+. Sodium influx increased proportionally to increasing salinity. At high external Ca2+, Na+ influx was less than at low Ca2+. Calcium influx was complex and exhibited two different responses to salinity. At low salt concentrations, influx decreased curvilinearly with increasing salt concentration. At 150 to 250 millimolar NaCl, 45Ca2+ influx increased in proportion to salt concentrations, especially with high Ca2+. Potassium influx declined significantly with increasing salinity, but was unaffected by external Ca2+. The rate of K+ uptake was dependent upon root weight, although influx was normalized for root weight. We conclude that the protection of root growth from salt stress by supplemental Ca2+ is related to improved Ca-status and maintenance of K+/Na+ selectivity. 相似文献
52.
Vacuoles of tobacco mesophyll and of suspension-cultured cells were isolated in order to study the localization of peroxidase isoenzymes. Only basic peroxidases were detectable by electrophoretic separation of the vacuolar sap. Some of the basic peroxidases have formerly been described as an ionically bound cell-wall fraction. This fraction, however, was found to be an artifact produced by incomplete cell breakage. Reinvestigation of isolated cell walls confirmed that mainly acidic peroxidases are localized in the cell walls where they move freely or are bound. As a consequence of former and present results we think it probable that all of the peroxidase isoenzymes are secretory proteins because they have to be transported from the sites of synthesis in the cytoplasm to the sites of function, the extracytoplasmic spaces, cell wall (acidic peroxidases), and vacuole (basic peroxidases).Abbreviation ER
endoplasmic reticulum
- PAGE
polyacrylamide gel electrophoresis 相似文献
53.
54.
Hippocampal EEG was recorded in behaving rats in which the brain stem afferents to the septal region were previously damaged. In these animals rhythmic slow activity (RSA or theta) was continuously present, including drinking and immobility. The average frequency of RSA, however, was significantly higher during running (8-9 Hz) than during drinking or awake motionless state (6-8 Hz). In normal rats irregular sharp waves, rather than RSA were present during drinking and immobility. The results suggest that brain stem afferents are necessary to suppress the rhythmic firing of septal "pacemaker" cells. 相似文献
55.
56.
Isolation and characterization of a substitution mutation in the ompR gene of Salmonella typhimurium LT2. 总被引:4,自引:1,他引:3 下载免费PDF全文
The expression of the genes ompC and ompF encoding major outer membrane proteins is dependent on the ompR-envZ operon. Here we describe the isolation and characterization of an ompR mutation, a single-base-pair change, that results in an Arg-to-Cys substitution. When present in multiple copies, the mutant allele conferred a dominant OmpC- OmpF+ phenotype. Furthermore, the mutant allele exhibited allele-specific negative complementation with other ompR mutations. This ability, together with its dominant character, suggested that the OmpR protein is capable of multimerization. 相似文献
57.
Summary Nonstationary pump currents which have been observed in K+-free Na+ media after activation of the Na,K-ATPase by an ATP-concentration jump (see the preceding paper) are analyzed on the basis of microscopic reaction models. It is shown that the behavior of the current signal at short times is governed by electrically silent reactions preceding phosphorylation of the protein; accordingly, the main information on charge-translocating processes is contained in the declining phase of the pump current. The experimental results support the Albers-Post reaction scheme of the Na,K-pump, in which the translocation of Na+ precedes translocation of K+. The transient pump current is represented as the sum of contributions of the individual transitions in the reaction cycle. Each term in the sum is the product of a net transition rate times a dielectric coefficient describing the amount of charge translocated in a given reaction step. Charge translocation may result from the motion of ion-binding sites in the course of conformational changes, as well as from movement of ions in access channels connecting the binding sites to the aqueous media. A likely interpretation of the observed nonstationary currents consists in the assumption that the principal electrogenic step is the E1-P/P-E2 conformational transition of the protein, followed by a release of Na+ to the extracellular side. This conclusion is supported by kinetic data from the literature, as well as on the finding that chymotrypsin treatment which is known to block the E1-P/P-E2 transition abolishes the current transient. By numerical simulation of the Albers-Post reaction cycle, the proposed mechanism of charge translocation has been shown to reproduce the experimentally observed time behavior of pump currents. 相似文献
58.
Kirsi-Marja Marnela Vince Varga Gábor Dibó Pekka Lähdesmäki 《Journal of neurochemistry》1987,48(4):1090-1092
To elucidate the position of the peptide bond in glutamyl-taurine this dipeptide was extracted from calf brain synaptic vesicles and subjected to paper electrophoresis. It was analyzed further in an automatic amino acid analyzer prior and subsequent to acid hydrolysis. Both alpha- and gamma-forms were found to be present in approximately equal amounts. 相似文献
59.
Roland von Bothmer Maija Kotimäki Ib Linde-Laursen 《Plant Systematics and Evolution》1987,156(3-4):183-188
Hybrids between the Chinese endemic speciesPsathyrostachys huashanica
Keng and the SW. Asian speciesP. fragilis (Boiss.)Nevski (all 2n = 14) developed normally but were completely sterile. Meiotic analyses revealed a high chiasma frequency indicating that the two species as well asP. juncea (Fisch.)Nevski share the same basic genome (called N). The hybrid nature of the plants was established through karyotype analysis and Giemsa C-banding. 相似文献
60.
InCallithrix, Saguinus, Aotus, andCallicebus other group members than the mother participate in infant care. Differences among these species are obvious in respect to the time of being off any caregiver, and in respect to the carrying position (Aotus andCallicebus). Alloparental care, a basic behaviour pattern in these species, has evolved from the parking behaviour of the prosimians. The infants are «parked» at other group members. 相似文献