全文获取类型
收费全文 | 5918篇 |
免费 | 400篇 |
国内免费 | 557篇 |
专业分类
6875篇 |
出版年
2024年 | 16篇 |
2023年 | 54篇 |
2022年 | 62篇 |
2021年 | 90篇 |
2020年 | 154篇 |
2019年 | 140篇 |
2018年 | 112篇 |
2017年 | 149篇 |
2016年 | 143篇 |
2015年 | 171篇 |
2014年 | 192篇 |
2013年 | 230篇 |
2012年 | 167篇 |
2011年 | 189篇 |
2010年 | 151篇 |
2009年 | 221篇 |
2008年 | 258篇 |
2007年 | 272篇 |
2006年 | 233篇 |
2005年 | 230篇 |
2004年 | 235篇 |
2003年 | 251篇 |
2002年 | 244篇 |
2001年 | 218篇 |
2000年 | 184篇 |
1999年 | 179篇 |
1998年 | 188篇 |
1997年 | 172篇 |
1996年 | 187篇 |
1995年 | 161篇 |
1994年 | 159篇 |
1993年 | 147篇 |
1992年 | 156篇 |
1991年 | 125篇 |
1990年 | 144篇 |
1989年 | 114篇 |
1988年 | 106篇 |
1987年 | 105篇 |
1986年 | 90篇 |
1985年 | 86篇 |
1984年 | 71篇 |
1983年 | 65篇 |
1982年 | 79篇 |
1981年 | 54篇 |
1980年 | 43篇 |
1979年 | 30篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1976年 | 12篇 |
1975年 | 10篇 |
排序方式: 共有6875条查询结果,搜索用时 0 毫秒
11.
Michael E. Salvucci Archie R. Portis Jr. William L. Ogren 《Photosynthesis research》1985,7(2):193-201
Ribulosebisphosphate carboxylase/oxygenase (EC 4.1.1.39) (rubisco) must be fully activated in order to catalyze the maximum rates of photosynthesis observed in plants. Activation of the isolated enzyme occurs spontaneously, but conditions required to observe full activation are inconsistent with those known to occur in illuminated chloroplasts. Genetic studies with a nutant of Arabidopsis
thaliana incapable of activating rubisco linked two chloroplast polypeptides to the activation process in vivo. Using a reconstituted light activation system, it was possible to demonstrate the participation of a chloroplast protein in rubisco activation. These results indicate that a specific chloroplast enzyme, rubisco activase, catalyzes the activation of rubisco in vivo. 相似文献
12.
13.
The green marine macroalga Ulva lactuca L. was found to be able to utilize HCO3? from sea water in two ways. When grown in flowing natural sea water at 16°C under constant dim irradiance, photosynthesis at pH8.4 was suppressed by acetazolamide but unaffected by 4,4′-diisothiocyanostilbene-2,2′-disulphonate. These responses indicate that photosynthetic HCO3? utilization was via extracellular carbonic anhydrase (CA) -mediated dehydration followed by CO2 uptake. The algae were therefore described as being in a ‘CA state’. If treated for more than 10 h in a sea water flow-through system at pH9.8, these thalli became insensitive to acetazolamide but sensitive to 4,4′-diisothiocyanostilbene-2,2′-disulphonate. This suggests the involvement of an anion exchanger (AE) in the direct uptake of HCO3?, and these plants were accordingly described as being in an ‘AE state’. Such thalli showed an approximately 10-fold higher apparent affinity for HCO3? (at pH9.4) than those in the ‘CA state’, while thalli of both states showed a very high apparent affinity for CO2. These results suggest that the two modes of HCO3? utilization constitute two ways in which inorganic carbon may enter the Ulva lactuca cells, with the direct entry of HCO3?, characterizing the ‘AE state’, being inducible and possibly functioning as a complementary uptake system at high external pH values (e.g. under conditions conducive to high photosynthetic rates). Both mechanisms of entry appear to be connected to concentrating CO2 inside the cell, probably via a separate mechanism operating intracellularly. 相似文献
14.
15.
Photosynthesis, transpiration and salt fluxes through leaves of Leptochloa fusca L. Kunth 总被引:3,自引:0,他引:3
J. GORHAM 《Plant, cell & environment》1987,10(2):191-196
Abstract Salt excretion by glands on the leaves of Leptochloa fusca was studied. The rate of excretion was strongly dependent on temperature up to 39°C, which is near the optimum for photosynthesis in this thermophilic C4 grass. The concentration of salt in the xylem required to sustain the observed rate of excretion was low (about two orders of magnitude less than the external concentration). Salt excretion is concluded to be a secondary mechanism of salt tolerance, with exclusion at the roots being the major mechanism. The rate of salt excretion was strongly dependent on temperature. 相似文献
16.
A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake 总被引:3,自引:0,他引:3
Andrew L. Herczeg 《Biogeochemistry》1987,4(3):231-263
The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,12C and13C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in 13CDIC), due to preferential uptake of12C by phytoplankton and a change in the dominant CO2 source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in 13CDIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion.Mass balance calculations of DI12C and DI13C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (Corg) of 430 ± 150 moles d-1 (6.5 ± 1.8 m moles m-2 d-1. Net CO2 invasion from the atmosphere was 420 ± 120 moles d-1 (6.2 ± 1.8 m moles m-2 d-1) with an exchange coefficient of 0.6 ± 0.3m d-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO2. About 50% of CO2 invasion and conversion to Corg for the summer occurred during a two week interval in mid-summer.DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to Corg in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The 13C of DIC added to the deep water (-22.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO2 (-7) was produced by a combination of POC oxidation and as a by-product of methanogenesis. 相似文献
17.
Photosynthetically active vesicles prepared from Chlamydomonas reinhardtii retained a light-dependent glutamate synthase activity which was highly specific for 2-oxoglutarate (Km=2.1 mM) and L-glutamine (Km=0.9 mM) as amido group acceptor and donor respectively. This activity was inhibited by azaserine, p-hydroxymercuribenzoate and 3-(p-chlorophenyl)-1,1-dimethyl urea.Light-dependent synthesis of glutamate was also obtained by coupling Chlamydomonas photosynthetic particles to purified ferredoxin-glutamate synthase, using ascorbate and 2,6-dichlorophenol-indophenol as electron donor. This system was also specific for 2-oxoglutarate (Km=1 mM) and L-glutamine (Km=0.8 mM) as substrates, and was stimulated by dithioerythritol. Azaserine and p-hydroxymercuribenzoate, but not 3-(p-chlorophenyl)-1,1-dimethyl urea, inhibited the reconstituted activity; high concentrations of 2-oxoglutarate were inhibitory.Abbreviations A
Absorbance
- CCP
p-Trichlorometoxi-carbonylcyanide-phenylhydrazone
- Chl
Chlorophyll
- CMU
3-(p-Chlorophenyl)-1,1-dimethyl urea
- DPIP
2,6-Dichlorophenol-indophenol
- DTE
Dithioerythritol
- MSX
L-Methionine, D-L, sulfoximine
- MV
Methyl viologen 相似文献
18.
19.
20.
Abstract. Portulacaria afra (L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C3 to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from -0.1 to -0.5 MPa, predawn and midday, when the C3 pathway of photosynthesis was utilized. Osmotic potentials were constant, from -0.7 to - 0.8 MPa, indicating very little change in turgor. P. afra shifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m−2 ) despite C3 -like predawn water potentials. Midday water potentials usually decreased 0.2-0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three-to five-fold over C3 levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other inducible CAM species. 相似文献