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1.
J. Van House A. Rich P. W. Zitzewitz 《Origins of life and evolution of the biosphere》1984,14(1-4):413-420
The proposed connection between the parity-violating handedness of beta particles in radioactive decay and the sign (L) of biological chirality (the Vester-Ulbricht [V-U] hypothesis) is being investigated by measuring the theoretically predicted asymmetry in the formation of triplet positronium in amino acid enantiomers by low energy positrons under reversal of the helicity of the positrons. We find the asymmetry in leucine to be (0.8±1.0)×10–4, i.e. consistent with the theoretical, prediction of 10–6 to 10–7. The apparatus is now sensitive enough to test the predicted asymmetry in optically active molecules which have heavy atoms at their chiral centers. The connection between these results and asymmetry in radiolysis by beta-decay electrons is made, and the implications of our limits for the V-U hypothesis discussed. Although the above limits are 106 times lower than direct measurements of radiolysis, they are still not small enough to allow us to rule out the V-U hypothesis. 相似文献
2.
The plant growth retardant paclobutrazol, (PP333) (2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol, inhibits specifically the three steps in the oxidation of the gibberellin-precursorent-kaurene toent-kaurenoic acid in a cell-free system fromCucurbita maxima endosperm. The KI50 for this inhibition is 2×10–8 M. The KI50 values for the separated2S, 3S, and2R, 3R enantiomers of paclobutrazol in this system are 2×10–8 M and 7×10–7 M, respectively. A cell-free preparation from immatureMalus pumila embryos convertsent-kaurene to gibberellin A9, whereas no conversion occurs in a similar preparation fromMalus endosperm. The conversion ofent-kaurene by the embryo preparation is inhibited by paclobutrazol with KI50 values for the2S,3S and2R,3R enantiomers of 2×10–8 M and 6×10–8 M, respectively. 相似文献
3.
The effects of the polyene antibiotics nystatin (2 × 10–5–10–4 mol/l), mycoheptin (1.3 × 10–6–10–5 mol/l) and levorin (10–8–5 × 10–5 mol/l)on isolated frog skeletal muscle fibres and whole sartorius muscles of the frog have been investigated. Cation conductance was measured under current clamp conditions using a double sucrosegap technique. Cation effluxes were studied by means of flame emission photometry. All three antibiotics increased the cation conductance and efflux rates; however, differences between the polyenes were found in the steady state values of induced cation transport at a given concentration. The values of both induced conductance gA
and efflux rate constants KA formed the following sequence: levorin > mycoheptin > nystatin, demonstrating a correlation with the order of antifungal activities. The dose-response curves of lg polyene-induced cation transport against lg of antibiotic concentration in our experiments had slope values which were much lower than those in bilayers: 1.7 and 1.3 for nystatin and mycoheptin, respectively, whereas the aromatic heptaene levorin had an even smaller concentration dependence. The decline in the equilibrium conductance caused by nystatin- and mycoheptin removal was very fast (during the first minute = 0.74 and 2.39 min, respectively). In contrast, levorin-induced conductance was irreversible. It is proposed that the processes which limit the rate of channel formation are different in biological and model membranes.
Correspondence to: N. E. Shvinka 相似文献
4.
Effects of downcomer-to-riser cross sectional area ratio on operation behaviour of external-loop airlift bioreactors 总被引:1,自引:0,他引:1
Experiments performed in two external-loop airlift bioreactors of laboratory and pilot scale, (1.880–1.189) · 10–3 m3 and (0.170-0.157)m3, respectively, are reported. The A
D
/A
R
ratio was varied between 0.111–1.000 and 0.040–0.1225 in the laboratory and pilot contractor respectively.Water and solutions of different coalescence (2-propanol 2% vol, 1 M Na (glucose 50% wt/vol) and rheological behaviour (non-Newtonian starch solutions with consistency index K=0.061–3.518 Pas
n
and flow behaviour index n=0.86-0.39), respectively, were used as liquid phase. Compressed air at superficial velocities v
SGR
=0.016–0.178 ms–1 in the laboratory contactor and v
SGR
=0.010–0.120 ms–1 in the pilot contactor, respectively was used as gaseous phase.The A
D
/A
R
ratio affect gas-holdup behaviour as a result of the influence of A
D
/A
R
on liquid circulation velocity.Experimental results show that A
D
/A
R
ratio affect circulation liquid velocity by modifying he resistence to flow and by varying the fraction of the total volume contained in downcomer and riser. A
D
/A
R
ratio has proven to be the main factor which determines the friction in the reactor. Mixing time increases with increasing of the reactor size and decreases with A
D
/A
R
decreasing.The volumetric gas-liquid mass transfer coefficient increases with A
D
/A
R
ratio decreasing, as a result of variations of the liquid velocity with A
D
/A
R
, which affect interfacial areas.Correlations applicable to the investigated contactors have been presented, together with the fit of some experimental data to existing correlation in literature.List of Symbols
A
D
downcomer cross sectional area (m2)
-
A
R
riser cross sectional area (m2)
-
a
coefficient in Eq. (9) (-)
-
a
L
gas-liquid interfacial area per unit volume (m–1)
-
b
coefficient in Eq. (9) (-)
-
C
tracer concentration (kg m–3)
-
C
tracer concentration at the state of complete mixing (kg m–3)
-
c
coefficient in Eq. (12)
-
c
S
coefficient in Eq. (5)
-
D
D
downcomer diameter (m)
-
D
R
riser diameter (m)
-
d
B
bubble size (m)
-
H
D
downcomer height (m)
-
H
d
dispersion height (m)
-
H
L
gas-free liquid height (m)
-
H
R
riser height (m)
-
I
inhomogeneity (-)
-
K
consistency index (Pa s
n
)
-
k
L
a
volumetric gas-liquid oxygen mass transfer coefficient (s–1)
-
m
exponent in Eq. (12) (-)
-
n
flow behaviour index (-)
-
P
G
power input due to gassing (W)
-
t
M
mixing time (s)
-
V
A
connecting pipe volume (m3)
-
V
D
downcomer volume (m3)
-
V
d
volume of dispersion (m3)
-
V
R
riser volume (m3)
-
V
T
total reactor liquid volume (m3)
-
v
SGR
riser gas superficial velocity (m s–1)
-
GR
riser gas holdup (-)
-
shear rate (m s–1)
-
app
apparent viscosity (Pa s)
-
shear stress 相似文献
5.
Delayed fluorescence dark decays in the time interval from 0.35 to 5.5ms are measured during dark to light adaptation in whole barley leaves and isolated thylakoid membranes, using a disc phosphoroscope. The changes in delayed fluorescence features are compared with variable chlorophyll fluorescence simultaneously registered with the same apparatus as well as in parallel by Handy PEA (Hansatech Instruments Ltd.), and absorbance changes at 820 nm. The registered delayed fluorescence signal is a sum of three components – submillisecond with lifetime of about 0.6 ms, millisecond decayed 2–4 ms and slow component with lifetime > >5.5 ms. The submillisecond delayed fluorescence component is proposed to be a result of radiative charge recombination in Photosystem II reaction centers in the state Z+PQA−QB−, and its lifetime is determined by the rate of electron transfer from QA− to QB−. The millisecond delayed fluorescence component is associated with recombination in Z+PQA−QB= centers with a lifetime determined by the sum of the rate constants of electron transfer from the oxygen-evolving complex to Z+ and of the exchange between the reduced and oxidized plastoquinone pool in the QB-site. On the basis of these assumptions and of the different share of the three components in the integral delayed fluorescence during induction, an attempt has been made to interpret the changes in the delayed fluorescence intensity during the transition of the photosynthetic apparatus from dark to light adapted state. 相似文献
6.
Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation (A
c) and ecosystem respiration (R
E), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A
c and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m–2 a–1) due to a minor imbalance between large A
c (–1,202 g C m–2 a–1) and R
E (1,299 g C m–2 a–1) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (<0.2 m3m–3), which influenced A
c and thereby NEE by decreasing stomatal conductance. For this experiment, low had minor impact on R
E. Thus, stomatal limitations to A
c were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of –65 g C m–2 a–1 assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems. 相似文献
7.
G. Antonutto M. Girardis D. Tuniz E. Petri C. Capelli 《European journal of applied physiology and occupational physiology》1994,69(3):183-188
The stroke volume of the left ventricle (SV) was calculated from noninvasive recordings of the arterial pressure using a finger photoplethysmograph and compared to the values obtained by pulsed Doppler echocardiography (PDE). A group of 19 healthy men and 12 women [mean ages: 20.8 (SD 1.6) and 22.2 (SD 1.6) years respectively] were studied at rest in the supine position. The ratio of the area below the ejection phase of the arterial pressure wave (A
s) to SV, as obtained by PDE, yielded a calibration factor dimensionally equal to the hydraulic impedance of the system (Z
ao =A
s ·SV
–1). TheZ
ao amounted on average to 0.062 (SD 0.018) mmHg · s · cm–3 for the men and to 0.104 (SD 0.024) mmHg · s · cm–3 for the women. TheZ
ao was also estimated from the equation:Z
ao = a · (d + b ·HR + c ·PP + e ·MAP)–1, whereHR was the heart rate,PP the pulse pressure,MAP the mean arterial pressure and the coefficients of the equation were obtained by an iterating statistical package. The value ofZ
ao thus obtained allowed the calculation of SV from measurements derived from the photoplethysmograph only. The mean percentage error between the SV thus obtained and those experimentally determined by PDE amounted to 14.8 and 15.6 for the men and the women, respectively. The error of the estimate was reduced to 12.3 and to 11.1, respectively, if the factorZ
ao, experimentally obtained from a given heart beat, was subsequently applied to other beats to obtain SV from theA
s measurement in the same subject. 相似文献
8.
Summary The relations between leaf conductance (gl) transpiration rate and root permeability to water (Rp) of three sunflower (Helianthus annuus L.) cultivars grown in a controlled environment cabinet are described.No differences in transpiration rates were found but it was shown that plants with low values of Rp have active stomatal closure with favourable consequences for water use efficiency under water limiting conditions.Rp was estimated by applying hydrostatic pressure on the root system. Values of Rp per unit root volume ranged from 0.34×10–5 to 16.75×10–5 (s MPa–1). There were significant inter-cultivar differences (P<0.05) in Rp and gl and an inverse correlation between Rp and the maximum values cf gl within cultivars.Pressure applied on the root system is proposed as a useful tool for the determination of differences in the root permeability to water amongst sunflower cultivars. 相似文献
9.
Summary Diffusion of auxin (indole-3-acetic acid) through planar lipid bilayer membranes was studied as a function of pH and auxin concentration. Membranes were made of egg or soybean lecithin or phosphatidyl serine inn-decane (25–35 mg/ml). Tracer and electrical techniques were used to estimate the permeabilities to nonionized (HA) and ionized (A–) auxin. The auxin tracer flux is unstirred layer limited at low pH and membrane limited at high pH, i.e., when [A–][HA]. The tracer flux is not affected by the transmembrane voltage and is much higher than the flux predicted from the membrane conductance. Thus, only nonionized auxin crosses the membrane at a significant rate. Auxin transport shows saturation kinetics, but this is due entirely to unstirred layer effects rather than to the existence of an auxin carrier in the membrane. A rapid interconversion of A– and HA at the membrane surface allows A– to facilitate the auxin flux through the unstirred layer. Thus, the total flux is higher than that expected for the simple diffusion of HA alone. The relation between flux (J
A), concentrations and permeabilities is: 1/J
A=1/P
UL([A–]+[HA])+1/P
HA
M
[HA]. By fitting this equation to our data we find thatP
UL=6.9×10–4 cm/sec andP
HA
M
=3.3×10–3 cm/sec for egg lecithin-decane bilayers. Similar membrane permeabilities were observed with phosphatidyl serine or soybean lipids. Thus, auxin permeability is not affected by a net surface charge on the membrane. Our model describing diffusion and reaction in the unstirred layers can explain the anomolous relationship between pH and weak acid (or weak base) uptake observed in many plant cells. 相似文献
10.
Total stem, branch, twig, and coarse root respiration (Rt) of an adult Pinus cembra tree at the alpine timberline was measured continuously at ten positions from 7 October 2001 to 21 January 2003 with an automated multiplexing gas exchange system. There was a significant spatial variability in woody tissue respiration when expressed per unit surface area or per unit sapwood volume. Surface area related maintenance (Rm) respiration at 0°C ranged between 0.109 and 0.643 mol m–2 s–1 and there was no clear trend with respect to tissue type and diameter. Sapwood volume based Rm at 0°C by contrast, varied between 2.5 mol m–3 s–1 in the stem and 193.2 mol m–3 s–1 in thin twigs in the upper crown. Estimated Q10 values ranged from 1.7 to 3.1. These Q10 values were used along with Rm at 0°C and annual woody tissue temperature records to predict annual total Rm. Annual total Rm accounted for 73±6% of annual Rt in 2002. 相似文献
11.
The decay of tyrosine cation radical was found to be biphasic at 253 K. The fast phase corresponds to the YZ
component while the slow phase corresponds to the tyrosine D radical (YD
) component. At 253 K, the t1/2 value was 28.6 s for the fast phase and 190.7 s for the slow phase. The fast phase is attributed to the recombination of charges between YZ
and QA
–. The activation energy for the reaction of YZ with QA
– between 253 and 293 K was 48 kJ mol–1 in Cl–-depleted photosystem 2 (PS2) membranes. Both the decay rate and the amplitude of the PAR -induced signal of YZ
were affected by addition of chloride anion. Change in the decay rate and the amplitude of the PAR-induced signal of YZ
was observed when other anions like Br–, I–, F, HCO3
–, NO3
–, PO4
3– were substituted in the Cl–-depleted PS2. 相似文献
12.
The kinetics of CNProto- and CNDeutero-hemin binding to apohemoglobin A2 was investigated in a stopped-flow device in 0.05 M potassium phosphate buffer, pH 7, at 10°C. The overall kinetic profile exhibited multiple phases: Phases I–IV corresponding with heme insertion (8.5–13 × 107 M–1 s–1), local structural rearrangement (0.21–0.23 s–1), global structural event (0.071–0.098 s–1), and formation of the Fe–His bond (0.009–0.012 s–1), respectively. Kinetic differences observed between apohemoglobin A2 and apohemoglobin A (previously studied) prompted an analysis of the structures of and chains through molecular modeling. This revealed a structural repositioning of the residues not only at, but also distant from the site of the amino acid substitutions, specifically those involved in the heme contact and subunit interface. A significant global change was observed in the structure of the exon-coded 3 region and provided additional evidence for the designation of this as the subunit assembly domain. 相似文献
13.
CO2 exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO2 uptake and release, soil respiration rate (R
s), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO2 uptake and release were –3.240 and 1.903 mol m–2 s–1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum R
s were 1.470 and 0.226 mol(CO2) m–2 s–1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than R
s, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type. 相似文献
14.
The effects of the selective adenosine (ADO) A3receptor agonist IB-MECA (N6-(3-iodobenzyl)adenosine-5′-N-methylcarboxamide) on cultured newborn rat cardiomyocytes were examined in comparison with ADO, the ADO A1receptor-selective agonistR-PIA (N6-R-phenylisopropyladenosine), or the ADO A3selective antagonist MRS 1191 (3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynyl-1,4-(±)-dihydropyridine-3,5 dicarboxylate), using digital image analysis of Feulgen-stained nuclei. At high concentration, IB-MECA (10 μM ) and ADO (200 μM) induced apoptosis; however,R-PIA or MRS 1191 did not have any detectable effects on cardiac cells. In addition, DNA breaks in cardiomyocytes undergoing apoptosis following treatment by IB-MECA were identifiedin situusing the nick end labeling of DNA (“TUNEL”-like) assay. In the presence of 10 μM IB-MECA, disorder in the contraction waves appeared, and a decrease in the frequency of beats was observed. Analysis with light microscopy revealed that the number of contracting cells decreased in a concentration-dependent manner. The A3receptor agonist IB-MECA caused an increase in intracellular free calcium concentration ([Ca2+]i). The drug produced a rapid rise followed by a sustained increase in [Ca2+]i, which lasted for 40–60 s. Finally, cessation of beating and Ca2+transients were observed. Full recovery of contractile activity and rhythmical Ca2+transients were observed 15–20 min after IB-MECA treatment. The induction of apoptosis in the cardiocytes by IB-MECA led to the appearance of features of apoptotic nuclei: the onset of condensation, compacting, and margination of nuclear chromatin. These effects were accompanied by the disintegration of the structural framework of the nucleus and nuclear breakdown. The results suggest that activation of the A3adenosine receptor may participate in the process of apoptosis in cardiomyocytes. 相似文献
15.
Paul R. Sorenson Paul O. Fromm 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1976,112(3):345-357
1. | Heat transfer was evaluated in isolated-perfused second gill arches and in isolated-perfused heads of rainbow trout. |
2. | At a perfusion flow of 0.5 ml min–1 the second gill arch exchanges 0.774±0.024 ( [`(X)]\bar X ±SE,n=6) cal min–1°C–1. This value can be increased by 11% with the infusion of 10–5 M epinephrine. |
3. | With perfusion flows of 16 and 20 ml min–1, isolated-perfused heads had a transfer maximum (hAmax) of 21.27±0.57 (21) and 24.79±0.77 (21) cal min–1 °C–1 and a ventilatory flow ( [(V)\dot]\dot V g) resulting in 0.5hAmax transfer ( [(V)\dot]\dot V 0.5 max) of 144±17 (21) and 183±23 (21) ml min–1 respectively. |
4. | The values of [(V)\dot]\dot V 0.5 max were unaffected by the administration of 10–5 M epinephrine, 10–8 M or 10–7 M acetylcholine.hAmax was increased only in the presence of 10–5 M epinephrine. |
5. | Increasing perfusion flow increasedhAmax without affecting [(V)\dot]\dot V 0.5 max. At a given perfusion flow there were no changes in heat transfer when heart rate and stroke volume were varied. |
6. | Analysis of a simple model for whole body heat exchange indicated that the gills may account for as much as 60% of the total heat exchanged by the animal. |
16.
M. Teresa García-López Ibon Alkorta M. José Domínguez Rosario González-Mu?iz Rosario Herranz Nils L. Johansen Kjeld Madsen Henning Th?gersen Peter Suzdak 《Letters in Peptide Science》1995,1(6):269-276
Summary In order to enforce different spatial orientations in the C-terminal hexapeptide of neurotensin (NT8–13) and to gain information about the importance of the 10–11 peptide bond for binding to NT receptors, the Pro10-Tyr11 fragment has been replaced with (2R,8S,8aR)-, (2S,8S,8aR)-, (2S,8S,8aS)-, (2S,8R,8aS)- and (2R,8R,8aS)-8-amino-2-benzyl-3-oxoindolizidine-2-carboxylic acid. Molecular dynamics calculations and energy minimization studies have shown that, contrarily to the Pro-Tyr moiety, none of these indolizidines display a tendency to adopt type I and III -turns, but those having (8S,8aR) or (8R,8aS) stereochemistry essentially adopt extended conformations and the (8S,8aS) stereoisomer prefers a nonstandard folding. The four diastereomeric NT8–13 analogues incorporating (8S,8aR) or (8R,8aS) indolizidines displayed binding affinities for the brain NT receptor similar to that of [Ala11]-NT8–13 and only five- to ninefold lower than that of the corresponding analogue, [Phe11]NT8–13. Although this slight decrease could be attributed to differences in conformational behavior between these constrained NT8–13 analogues and [Phe11]NT8–13 or NT8–13, it is not clear whether the -turn around Pro10-AA11 (AA=Phe, Tyr) is conserved upon receptor binding. An excessive restriction in the motions of the aromatic side chain, imposed by the highly steric constraint of the indolizidine moiety, emerges as an alternative explanation. The findings reported here demonstrate the possibility of replacing the Pro10-Tyr11 dipeptide in NT8–13 with a non-peptide residue without affecting considerably the affinity for brain NT receptors. 相似文献
17.
Characterization of the light field in laboratory scale enclosures of eutrophic lake water (Lake Loosdrecht,The Netherlands) 总被引:3,自引:3,他引:0
Light conditions in laboratory scale enclosures (LSE) of shallow, eutrophic Lake Loosdrecht (The Netherlands), including a method for simulating a natural incident light course, are described. Total PAR (400–700 nm) and spectral irradiance distribution were measured at sestonic chlorophyll a and dry weight concentrations 100 mg m–3 and 16 g m–3, respectively. Phytoplankton was dominated by Oscillatoria spp. The euphotic depth (Z
eu) was 0.7–1.0 m. Shortly after filling the LSE with lake water, diffuse attenuation coefficients ranged from 14 m–1 for blue to 5 m–1 for red light. Around Z
eu, attenuation in the blue region was markedly lower and irradiance reflectance (R) continued to increase; these anomalies were caused by lateral incident light from the LSE's waterbath. Spectrophotometry indicated that absorption was mainly by particles, but dissolved humic substances (gilvin) were also important. The particles were likely to be dominated by detritus absorbing more blue relative to red light. Subsurface R in lake water in the LSE had a maximum around 705 nm and low values in the blue band, but was lower than that previously reported for measurements in situ. Wash-out of detritus, presumably both dissolved and particulate fractions, by flow-through with synthetic medium, greatly affected the spectral reflectance measured outside the LSE. The maximum value of R decreased from 0.022 to 0.009, and the peak shifted to 550 nm. 相似文献
18.
Summary The charge-pulse technique has been used previously for the study of quasistationary processes in membranes which required only a moderate time resolution. It is shown here that a time resolution of about 400 nsec may be achieved with this technique and that it may be applied to the kinetic analysis of carrier-mediated ion transport. By this method we have studied the transport of alkali ions through optically black monoolein membranes in the presence of the ion carrier valinomycin. All three relaxation processes that are predicted by theory have been resolved. From the relaxation times and the relaxation amplitudes the rate constants for the association (k
R
) and the dissociation (k
D
) of the ioncarrier complex, as well as the translocation rate constants of the complex (k
MS
) and the free carrier (k
S
) could be obtained. For 1m Rb+ at 25° C the values arek
R
=3×105
m
–1 sec–1,k
D
=2×105 sec–1,k
MS
=3×105 sec–1,k
S
=4×104 sec–1. The activation energies of the single rate constants which have been estimated from experiments at two different temperatures range between 50 and 90 kJ/mol. 相似文献
19.
Dr. Paul Hirsch-Ayalon 《The Journal of membrane biology》1979,51(1):1-6
Summary The conditioned state of a precipitation membrane with its particular properties exists within a limited range of membrane potentials and requires certain minimum concentrations,C
lim, of the generating ions in the adjoining solutions. We investigated these quantities for the BaSO4 cellophane membrane and foundC
lim to be 10×10–5
n (0.5×10–4
m), equally for Ba++ and SO
4
––
. Beyond these limits, the membrane becomes deconditioned. This transformation is a reversible process provided the limits have not been surpassed too far. The capability for de- and reconditioning is a characteristic and unique property of precipitation membranes, not found in other membrane systems. The phenomenon is explained by the adsorption theory for precipitation membranes. It allows wide modifications and quick variations of the electrical properties and permeability of the membrane in an easy and reversible manner. 相似文献
20.
Influence of differences in leaf anatomy on net photosynthetic rates of some cultivars of Cassava 总被引:1,自引:0,他引:1
Gas exchange measurements and leaf anatomy of 10 cassava cultivars were conducted to study the interrelationship between the relatively high photosynthetic rates and the factors limiting internal CO2 diffusion. The internal mesophyll surface area per unit leaf surface area (Ames/A) and the intracellular components of CO2 diffusion and fixation resistance (RcellCO2) were determined. Among the group of cultivars tested net CO2 exchange rates were 26±2.5 mol CO2 m–2 s–1 in normal air and intense light and Ames/A ranged from 14 to 38. Estimated RcellCO2 ranged from 4300 to 13,000 s m–1. The combined and compensating effects of Ames/A and RcellCO2 accounted for both the high net photosynthetic rates (Pn) and the lack of large differences in Pn among cultivars. 相似文献