全文获取类型
收费全文 | 5024篇 |
免费 | 160篇 |
国内免费 | 209篇 |
专业分类
5393篇 |
出版年
2024年 | 8篇 |
2023年 | 32篇 |
2022年 | 33篇 |
2021年 | 55篇 |
2020年 | 66篇 |
2019年 | 76篇 |
2018年 | 86篇 |
2017年 | 68篇 |
2016年 | 77篇 |
2015年 | 77篇 |
2014年 | 109篇 |
2013年 | 181篇 |
2012年 | 85篇 |
2011年 | 134篇 |
2010年 | 89篇 |
2009年 | 159篇 |
2008年 | 160篇 |
2007年 | 191篇 |
2006年 | 172篇 |
2005年 | 178篇 |
2004年 | 155篇 |
2003年 | 167篇 |
2002年 | 179篇 |
2001年 | 130篇 |
2000年 | 127篇 |
1999年 | 135篇 |
1998年 | 134篇 |
1997年 | 139篇 |
1996年 | 142篇 |
1995年 | 134篇 |
1994年 | 118篇 |
1993年 | 160篇 |
1992年 | 137篇 |
1991年 | 152篇 |
1990年 | 133篇 |
1989年 | 134篇 |
1988年 | 117篇 |
1987年 | 110篇 |
1986年 | 116篇 |
1985年 | 118篇 |
1984年 | 122篇 |
1983年 | 71篇 |
1982年 | 113篇 |
1981年 | 95篇 |
1980年 | 72篇 |
1979年 | 49篇 |
1978年 | 24篇 |
1977年 | 28篇 |
1976年 | 26篇 |
1975年 | 9篇 |
排序方式: 共有5393条查询结果,搜索用时 15 毫秒
71.
U. Katz W. Hanke 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(3):189-195
The acclimation of the clawed toad Xenopus laevis to hyperosmotic solutions of NaCl (balanced solution of sea salt), urea or mannitol was studied. The animals could not be acclimated to salt solutions more concentrated centrated than 400 mosm·l-1. Urea was tolerated till 500 mmol·l-1. Plasma osmolality was always hyperosmotic to the environmental solution, but with diminished osmotic gradient at the highest tolerated solutions. Plasma urea concentration approached 90 mmol·l-1, similar in the three solutions of acclimation. Urine volume was very small under all conditions. Serum aldosterone and corticosterone did not differ significantly, although there was a slight tendency towards lower aldosterone in the NaCl solution. In vivo water uptake in tap water acclimated animals was very small, and was higher in the other groups. Only the salt- and urea-acclimated, but not the tap water and mannitol-acclimated groups responded with a clear increase following injection of oxytocin or theophylline. In vitro urea fluxes were similar and invariable in both directions under all conditions. No significant effect of theophylline was observed. Sodium transport measured by the short-circuit technique in vitro was lower in salt- and mannitol-acclimation conditions, and was stimulated significantly under all conditions in response to serosal oxytocin or theopylline. It is concluded that Xenopus laevis can osmoregulate at a limited range of external solutions. It is limited in the increase of its plasma urea concentration; the transport properties of the skin do not change very much upon acclimation, except for the hydroosmotic response to oxytocin.Abbreviations
I
sc
short circuit current
- PD
potential difference
- SW
balanced sea water
- TW
tap water 相似文献
72.
The maximum quantum yields (a,c) for CO2 uptake in low-oxygen atmospheres were determined for 11 species of C3 vascular plants of diverse taxa, habitat and life form using an Ulbricht-sphere leaf chamber. Comparisons were also made between tissues of varied age within species. The species examined were Psilotum nudum (L.) P. Beauv., Davallia bullata Wall. ex Hook., Cycas revoluta Thunb., Araucaria heterophylla (Salisb.) Franco, Picea abies (L.) Karst., Nerium oleander L., Ruellia humilis Nutt., Pilea microphylla (L.) Karst., Beaucarnea stricta Lem., Oplismenus hirtellus (L.) P. Beauv. and Poa annua L. Quantum yields were calculated from the initial slopes of the response of CO2 uptake to the quantity of photons absorbed in conditions of diffuse lighting. Regression analysis of variance of the initial slopes of the response of CO2 uptake to photon absorption failed to show any statistically significant differences between age classes within species or between the mature photosynthetic organs of different species. The constancy of a,c was apparent despite marked variation in the light-saturated rates of CO2 uptake within and between species. The mean a,c was 0.093±0.003 for 11 species. By contrast, surface absorptance varied markedly between species from 0.90 to 0.60, producing proportional variation in the quantum yield calculated on an incidentlight basis. The ratio of variable to maximum fluorescence emission at 695 nm for the same tissues also failed to show any statistically significant variation between species, with a mean of 0.838±0.008. Mean values of a,c reported here for C3 species, in the absence of photorespiration, are higher than reported in previous surveys of vascular plants, but consistent with recent estimates of the quantum yields of O2 evolution.Abbreviations and Symbols A
rate of CO2 uptake per unit projected area (mol · m–2 · s–1)
- Fm
the maximum fluorescence emission at 695 nm in saturating excitation light when closure of PSII reaction centres is maximal (relative units)
- Fo
the ground fluorescence at 695 nm when all PSII reaction centres are assumed open (relative units)
- Fv
the difference between Fm and Fo
- JQ
rate of CO2 uptake by the sample (nmol · s–1)
- JQ
rate of photon absorption by the sample (nmol · s–1)
- Q
absorbed photon flux per unit of projected area (nmol · m–2 · s–1)
- 1
the light absorptance of photosynthetic organs (dimensionless)
- s1 and s'1
the total and projected surface areas of the photosynthetic organs examined (m2)
- a,c and i,c
the quantum yields for CO2 uptake on an absorbed- and incident-light basis, respectively (dimensionless)
- a,o
the quantum yield for O2 evolution on an absorbed-light basis (dimensionless)
This work was supported by grant PI7179-BIO, FWF, Austria to H.B-N. and by a British Council travel award to S.P.L. This work was completed under the auspices of U.S. Department of Energy under Contract No. DE-AC02-76CH00016. We also thank Dr. K.J. Parkinson of PP Systems, Hitchin, UK for the loan of a prototype of a commercial integrating-sphere leaf chamber developed from our design. 相似文献
73.
Takayoshi Yoshida Koji Yamamoto Masao Udo 《European journal of applied physiology and occupational physiology》1993,66(2):155-160
The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake
) and increased cardiac output (
) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The
was obtained by a breath-by-breath method, and
was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise,
rapidly increased during phase I with time constants of 6.8–7.3 s. The
also showed a similar rapid increment with time constants of 6.0–6.8 s with an apparent increase in stroke volume (SV). In this phase I,
increased to about 29.7%–34.1% of the steady-state value and
increased to about 58.3%–87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in
and
with time constants of 26.7–32.3 and 23.7–34.4 s, respectively. The insignificant difference between
and
time constants in phase I and the abrupt increase in both
and SV at the onset of exercise from rest provided further evidence for a cardiodynamic contribution to
following the onset of exercise from rest. 相似文献
74.
J. Iba?ez R. Rama M. Riera M. T. Prats L. Palacios 《European journal of applied physiology and occupational physiology》1993,67(1):7-13
The effect of severe acute hypoxia (fractional concentration of inspired oxygen equalled 0.104) was studied in nine male subjects performing an incremental exercise test. For power outputs over 125 W, all the subjects in a state of hypoxia showed a decrease in oxygen consumption (
O2) relative to exercise intensity compared with normoxia (P < 0.05). This would suggest an increased anaerobic metabolism as an energy source during hypoxic exercise. During submaximal exercise, for a given
O2, higher blood lactate concentrations were found in hypoxia than in normoxia (P < 0.05). In consequence, the onset of blood lactate accumulation (OBLA) was shifted to a lower
O2 (
O2 1.77 l·min–1 in hypoxia vs 3.10 l·min–1 in normoxia). Lactate concentration increases relative to minute ventilation (
E) responses were significantly higher during hypoxia than in normoxia (P < 0.05). At OBLA,
E during hypoxia was 25% lower than in the normoxic test. This study would suggest that in hypoxia subjects are able to use an increased anaerobic metabolism to maintain exercise performance. 相似文献
75.
76.
David L. Musso Nariman B. Mehta Francis E. Soroko Robert M. Ferris Elizabeth B. Hollingsworth Bernard T. Kenney 《Chirality》1993,5(7):495-500
The synthesis of the enantiomers of bupropion, (rac)-2-tert-butylamino-3′-chloropropiophenone 1 (Wellbutrin®) is described. The enantiomers were compared with the racemate in both the tetrabenazine-induced sedation model and the inhibition of uptake of biogenic amine assay. No significant differences were found in their potencies to reverse tetrabenazine-induced sedation in mice or in their IC50 values as inhibitors of biogenic amine uptake into nerve endings obtained from mouse brain. © 1993 Wiley-Liss, Inc. 相似文献
77.
The distribution of NO3? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO3?, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 mM NH4+ and 0.5, 1.5 or 10 mM NO3?. Xylem transport of NO3? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO3? reduction at all 3 levels of NO3? nutrition. However, the contribution of the shoots to the whole plant NO3? reduction increased with increasing external NO3? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO3?, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO3? translocation from roots to shoots. It is proposed that the lack of NO3? export to the shoots at low NO3? uptake rate resulted from a competition between NO3? reduction in the root epidermis/cortex and NO3? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO3? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO3? assimilation, principally occurring in the roots, was mainly incorporated in the shoots. 相似文献
78.
79.
The ability ofPseudomonas fluorescens, Escherichia coli andAcinetobacter radioresistenns to remove phosphate during growth was related to the initial biomass as well as to growth stages and bacterial species. Phosphate was removed by these bacteria under favourable conditions as well as under unfavourable conditions of growth. Experiments showed a relationship between a high initial cell density and phosphate uptake. More phosphate was released than removed when low initial cell densities (102–105 cells ml–1) were used. At a high initial biomass concentration (108 cells ml–1), phosphate was removed during the lag phase and during logarthmic growth byP. fluorescens. Escherichia coli. at high initial biomass concentrations (107 cells ml–1), accumulated most of the phosphate during the first hour of the lag phase and/or during logarithmic growth and in some cases removed a small quantily of phosphate during the stationary growth phase.Acinetobacter radioresistens, at high initial cell densities (106, 107 cells ml–1) removed most of phosphate during the first hour of the lag phase and some phosphate during the stationary growth phase.Pseudomonas fluorescens removed phosphate more thanA. radioresistens andE. coli with specific average ranges from 3.00–28.50 mg L–1 compared to average ranges of 4.92–17.14 mg L–1 forA. radioresistens and to average ranges of 0.50–8.50 mg L–1 forE. coli. 相似文献
80.
Maria Antonietta Zoroddu Michelina Fruianu Roberto Dallocchio Andreina Masia 《Biometals》1996,9(1):91-97
Vanadium uptake by whole cells and isolated cell walls of the yeast Saccharomyces cerevisiae was studied. When orthovanadate was added to wild-type S. cerevisiae cells growing in rich medium, growth was inhibited as a function of the VO4
3- concentration and the growth was completely arrested at a concentration of 20 mM of VO4
3- in YEPD. Electron paramagnetic resonance (EPR) spectroscopy was used to obtain structural and dynamic information about the cell-associated paramagnetic vanadyl ion. The presence of EPR signals indicated that vanadate was reduced by whole cells to the vanadyl ion. On the contrary, no EPR signals were detected after interaction of vanadate with isolated cell walls. A mobile and an immobile species associated in cells with small chelates and with macromolecular sites, respectively, were identified. The value of rotational correlation time
r indicated the relative motional freedom at the macromolecular site. A strongly immobilized vanadyl species bound to polar sites mainly through coulombic attractions was detected after interaction of VO2+ ions with isolated cell walls. 相似文献