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1.
Water permeability of isolated cuticular membranes: The effect of cuticular waxes on diffusion of water 总被引:24,自引:0,他引:24
J. Schönherr 《Planta》1976,131(2):159-164
Summary The water permeability of astomatous cuticular membranes isolated from Citrus aurantium L. leaves, pear (Pyrus communis L.) leaves and onion (Allium cepa L.) bulb scales was determined before and after extraction of cuticular waxes with lipid solvents. In pear, the permeability coefficients for diffusion of tritiated water across cuticular membranes (CM) prior to extraction [P
d(CM)] decreased by a factor of four during leaf expansion. In all three species investigated P
d(CM) values of cuticular membranes from fully expanded leaves varied between 1 to 2×10-7 cm-3 s-1·P
d(CM) values were not affected by pH. Extraction of cuticular waxes from the membranes increased their water permeability by a factor of 300 to 500. Permeability coefficients for diffusion of THO across the cutin matrix (MX) after extraction [P
d(MX)] increased with increasing pH. P
dvalues were not inversely proportional to the thickness of cuticular membranes. By treating the cutin matrix and cuticular waxes as two resistances acting in series it was shown that the water permeability of cuticles is completely determined by the waxes. The lack of the P
d(CM) values to respond to pH appeared to be due to structural effects of waxes in the cutin matrix. Cuticular membranes from the submerse leaves of the aquatic plant Potamogeton lucens L. were three orders of magnitude more permeable to water than the cuticular membranes of the terrestrial species investigated.Abbreviations CM
cuticular membrane
- MX
cutin matrix
- WAX
waxes
This study was supported by a grant from the Deutsche Forschungsgemeinschaft. 相似文献
2.
Water permeability of thin lipid membranes 总被引:18,自引:11,他引:7
The osmotic permeability coefficient, Pf, and the tagged water permeability coefficient, Pd, were determined for thin (<100 A) lipid membranes formed from ox brain lipids plus DL-α-tocopherol; their value of approximately 1 x 10-3 cm/sec is within the range reported for plasma membranes. It was established that Pf = Pd. Other reports that Pf > Pd can be attributed to the presence of unstirred layers in the experimental determination of Pd. Thus, there is no evidence for the existence of aqueous pores in these thin phospholipid membranes. The adsorption onto the membrane of a protein that lowers its electrical resistance by a factor of 103 was found not to affect its water permeability; however, glucose and sucrose were found to interact with the membrane to modify Pf. Possible mechanisms of water transport across these films are discussed, together with the implications of data obtained on these structures for plasma membranes. 相似文献
3.
Diecke FP Cacace VI Montalbetti N Ma L Kuang K Iserovich P Fischbarg J 《The Journal of membrane biology》2011,242(1):41-51
Layers of rabbit corneal endothelial cells were cultured on permeable inserts. We characterized the diffusional permeability
of the cell layer to nonelectrolyte and charged molecules and compared the diffusional and filtration permeabilities of the
paracellular and transcellular pathways. We determined the rates of diffusion of 3H- and 14C-labeled nonelectrolyte test molecules and estimated the equivalent pore radius of the tight junction. Negatively charged
molecules permeate slower than neutral molecules, while positively charged molecules permeate faster. Palmitoyl-dl-carnitine, which opens tight junctions, caused an increase of permeability and equivalent pore radius. Diffusional water
permeability was determined with 3H-labeled water; the permeabilities of the tight junction and lateral intercellular space were calculated using tissue geometry
and the Renkin equation. The diffusional permeability (P
d
) of the paracellular pathway to water is 0.57 μm s−1 and that of the transcellular path is 2.52 μm s−1. From the P
d
data we calculated the filtration permeabilities (P
f
) for the paracellular and transcellular pathways as 41.3 and 30.2 μm s−1, respectively. In conclusion, the movement of hydrophilic molecules through tight junctions corresponds to diffusion through
negatively charged pores (r = 2.1 ± 0.35 nm). The paracellular water permeability represents 58% of the filtration permeability
of the layer, which points to that route as the site of sizable water transport. In addition, we calculated for NaCl a reflection
coefficient of 0.16 ≤ σNaCl ≤ 0.33, which militates against osmosis through the junctions and, hence, indirectly supports the electro-osmosis hypothesis. 相似文献
4.
The water permeability of periderm membranes stripped from mature trees of Betula pendula Roth was investigated. The diffusion of water was studied using the system water/membrane/water, and transpiration was measured using the system water/membrane/water vapor. Betula periderm consists of successive periderm layers each made up of about 5 heavily suberized cell layers and a varying number of cell layers that are little suberized, if at all. It is shown that these layers act as resistances in series. The permeability coefficient of the diffusion of water (P
d) can be predicted with 79% accuracy from the reciprocal of the membrane weight (x in mg cm-2) by means of the linear equation P
d=14.69·10-7
x-0.73·10-7. For example, the P
d of a periderm membrane having a weight of 10 mg cm-2 (approx. 250 m thick) is 7.4·10-8 cm s-1, which is comparable to the permeability of cuticles. This comparison shows that on a basis of unit thickness, Betula periderm is quite permeable to water as cuticles have the same resistance with a thickness of only 0.5 to 3 m. It is argued that this comparatively high water permeability of birch periderm is due to the fact that middle lamellae and the primary walls of periderm cells are not at all, or only incompletely suberized and, therefore, form a hydrophilic network within which the water can flow. This conclusion is based on the following observations: (1) Middle lamellae and primary walls stain strongly with toluidine blue, which shows them to be polar. (2) If silver ions are added as tracer for the flow of water, they are found only in the middle lamellae, primary walls, and in plasmodesmata, while no silver can be detected in the suberized walls. (3) Permeability coefficients of transpiration strongly depend on water activity. This shows conclusively that water flows across Betula periderm via a polar pathway. It is further argued that liquid continuity is likely to be maintained under all physiological conditions in the network formed by middle lamellae and primary walls. On the other hand, the lumina of periderm cells, intercellular air spaces in the lenticels, and even the pores in the suberized walls (remainders of plasmodesmata) will drain at a humidity of 95% and below. Due to the presence of intercellulars the permeability coefficient of lenticels is much greater than that of the periderm. A substantial amount of the total water, therefore, flows as vapor through lenticels even though they cover only 3% of the surface.Abbreviations PM
perideron membrane
-
P
d
permeability coefficient for diffusion of water
-
P
tt
permeability coefficient of transpiration
- MES
(N-morpholino)ethane sulfonic acid 相似文献
5.
Yong-Fang Li Yu-Fen Zhao In-Sup So Ok Young Lee-Stadelmann 《Journal of Plant Biology》1999,42(3):232-238
The effects of amino acids (aa) and N-(diisopropyloxyphosphoryl)-amino acids (DIPP-aa) on cell membranes were investigated
by evaluating water and methyl urea permeability. Permeability coefficients Pf and Ps were determined by standard osmotic methods for cells ofPisum sativum stem base epidermis after 20 min exposure to a 5 mM solution of each aa and DIPP-aa. The Pf value ofP. sativum epidermal cells (untreated controls) was 1.3 ± 0.4 × 10-3 μm s-1. Treat ments with the diisopropyl-oxyphosphoryl derivatives of three one charged and three polar amino acids (serine, threonine,
asparagine, and aspartic acid) and unsubstituted (free) serine and threonine increased water permeability up to about two
fold of the control value. Serine and threonine and their DIPP-derivatives increased methyl urea permeability (controls 1.03
± 0.09 × 10-3 μm s-1) 30 to 80 percent Other amino acids and their DIPP-derivatives caused small or insignificant changes of water permeability.
Only certain polar amino acids and their DIPP-derivatives increased the osmotic water and methyl urea permeation through the
plasma membrane. The specificity of these molecules on plasma membranes suggests that the active amino acids (serine and threonine)
and their DIPP-derivatives interact with charged membrane molecules. The relatively small changes in water and methyl urea
permeability may indicate that the effective aa’s and their DIPP-derivatives interact with phospholipids rather than aquaporin.
A concurring alteration of water channel proteins, however, cannot excluded. 相似文献
6.
Water permeability and composition of soluble cuticular lipids of isolated cuticular membranes from leaves of Citrus aurantium L. were investigated for 3 successive years. The average water permeability coefficient determined using 169 cuticular membranes was 1.09·10–7 cm s–1 with a standard deviation of 0.78·10–7 cm s–1. There were no significant differences in water permeability between years. Cuticular membranes are characterized by a great variability in water permeability both within and between years. Both water permeability of individual membranes and variability between membranes are shown to be determined by soluble cuticular lipids contained within the cuticular membranes. The soluble cuticular lipids of Citrus leaves are composed of fatty acids, primary alcohols, esters, and hydrocarbons. They occur in amounts of 9.84 g cm–2, which represents approx. 3% of the total mass of isolated cuticular membranes. The specific weight of cuticular membranes (365.4 g cm–1) and total amount of soluble cuticular lipids did not vary significantly between years. Significant differences were observed for the amounts and composition of the constituent classes of lipids. Six homologues comprise 86% of the fatty acids (C16; C18; C19; C21; C24; C26), 83% of the primary alcohols (C24; C26; C28; C30; C32; C34) and 88% of the esters (C36; C38; C40; C41; C42; C44). Eleven major homologues amount only to 62% of the total hydrocarbons (C16; C17; C18; C20; C26; C27; C29; C30; C31; C32; C33). Variability in the composition of soluble cuticular lipids between years was much smaller than variability of water permeability and, therefore, no relation between composition of soluble cuticular lipids and water permeability could be found. It is suggested that this may be due to the fact that the lipid composition observed represents the averages of 20 to 30 membranes analyzed so that differences between individual membranes may have been leveled out.Abbreviations CM
cuticular membranes
- MX
polymer matrix
- Pd
permeability coefficient for diffusion of water
- SCL
soluble cuticular lipids
- MES
morpholinoethane sulphonic acid 相似文献
7.
The osmotic permeability coefficient (Pf) for water movement across Novikoff hepatoma cells was found to be 82 ± 3 (S.E.) · 10?5 cm · s?1 at 20°C. The corresponding diffusional permeability coefficient for 3HHO (Pd) was 97 ± 10 (S.E.) · 10?5 cm · s?1, therefore the ratio is close to unity. The apparent activation energy for water filtration was 10.4 ± 0.4 (S.E.) kcal · mol?1. This value is significantly greater than the activation energy for the self diffusion of water. The product of the hydraulic permeability coefficient and the viscosity coefficient for water was temperature-dependent. However, the product of the hydraulic permeability coefficient and the viscosity coefficient for membrane lipid did not vary with temperature. These data are interpreted as evidence for water movement across a lipid membrane barrier rather than through aqueous channels. 相似文献
8.
Gheorghe Benga Tudor Borza 《Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology》1995,112(4)
An extensive programme of comparative nuclear magnetic resonance measurements of the membrane diffusional permeability for water (Pd) and of the activation energy (Ea,d) of this process in red blood cells (RBCs) from 21 mammalian species was carried out. On the basis of Pd, these species could be divided into three groups. First, the RBC's from humans, cow, sheep and “large” kangaroos (Macropus giganteus and Macropus rufus) had Pd values 5 × 10−3 cm/s at 25°C and 7 × 10−3 cm/s at 37°C. The RBCs from other marsupial species, mouse, rat, guinea pig and rabbit, had Pd values roughly twice higher, whereas echidna RBCs were twice lower than human RBCs. The value of Ea,d was in most cases correlated with the values of Pd. A value of Ea,d -26 kJ/mol was found for the RBCs from humans and the species having similar Pd values. Low values of Ea,d (ranging from 15 to 22 kJ/mol) appeared to be associated with relatively high values of Pd. The highest value of Ea,d (33 kJ/mol) was found in echidna RBCs. This points to specialized channels for water diffusion incorporated in membrane proteins; a relatively high water permeability of the RBC membrane could be due to a greater number of channel proteins. There are, however, situations where a very high water permeability of RBCs is associated with a high value of Ea,d (above 25 kJ/mol) as in the case of RBCs from mouse, rat and tree kangaroo. Moreover, it was found that Pd in different species was positively correlated to the RBC membrane phosphatidylcholine and negatively correlated to the sphingomyelin content. This suggests that in addition to the number of channel proteins, other factors are involved in the water permeability of the RBC membrane. 相似文献
9.
Paola Carpi-Medina Vladimir León Joussef Espidel Guillermo Whittembury 《The Journal of membrane biology》1988,104(1):35-43
Summary The diffusive water permeability (P
d
) of the plasma membrane of proximal kidney tubule cells was measured using a1H-NMR technique. The values obtained for the exchange time (T
ex) across the membrane were independent of the cytocrit and of the Mn2+ concentration (in the range 2.5 to 5mm). At 25°C the calculatedP
d
value was (per cm2 of outer surface area without taking into account membrane invaginations) 197±17 m/sec. This value equals 22.3±1.9 m/sec when the invaginations are taken into account. Cell exposure to 2.5mm parachloromercuribenzenesulfonic acid,pCMBS, (for 20 to 35 min) reducedP
d
to 45% of its control value. Fivemm dithiothreitol, DTT, reverted this effect. The activation energy for the diffusive water flux was 5.2±1.0 kcal/mol under control conditions. It increased to 9.1±2.2 kcal/mol in the presence of 2.5mm
pCMBS. Using our previous values for the osmotic water permeability (P
os) in proximal straight tubular cells theP
os/P
d
ratio equals 18±1, under control conditions, and 3.2±0.3 in the presence ofpCMBS. These experimental results indicate the presence of pathways for water, formed by proteins, crossing these membranes, which are closed bypCMBS. Assuming laminar flow (within the pore), fromP
os/P
d
of 13 to 18 an unreasonably large pore radius of 12 to 15 Å is calculated which would not hinder cell entry of known extracellular markers. Alternatively, for a single-file pore, 11 to 20 would be the number of water molecules which would be in tandem inside the pore. The water permeability remaining in the presence ofpCMBS indicates water permeation through the lipid bilayer. There are similarities between these results and those obtained in human red blood cells and in the apical cell membrane of the toad urinary bladder. 相似文献
10.
Water can pass through biological membranes via two pathways: simple diffusion through the lipid bilayer, or water-selective facilitated diffusion through aquaporins (AQPs). Although AQPs play an important role in osmotic water permeability (Pf), the role of AQPs in diffusional water permeability remains unclear because of the difficulty of measuring diffusional water permeability (Pd). Here, we report an accurate and instantaneous method for measuring the Pd of a single HeLa S3 cell using coherent anti-Stokes Raman scattering (CARS) microscopy with a quick perfusion device for H2O/D2O exchange. Ultra-high-speed line-scan CARS images were obtained every 0.488 ms. The average decay time constant of CARS intensities (τCARS) for the external solution H2O/D2O exchange was 16.1 ms, whereas the intracellular H2O/D2O exchange was 100.7 ± 19.6 ms. To evaluate the roles of AQP in diffusional water permeability, AQP4 fused with enhanced green fluorescent protein (AQP4-EGFP) was transiently expressed in HeLa S3 cells. The average τCARS for the intracellular H2O/D2O exchange in the AQP4-EGFP-HeLa S3 cells was 43.1 ± 15.8 ms. We also assessed the cell volume and the cell surface area to calculate Pd. The average Pd values for the AQP4-EGFP-HeLa S3 cells and the control EGFP-HeLa S3 cells were 2.7 ± 1.0 × 10−3 and 8.3 ± 2.6 × 10−4 cm/s, respectively. AQP4-mediated water diffusion was independent of the temperature but was dependent on the expression level of the protein at the plasma membrane. These results suggest the possibility of using CARS imaging to investigate the hydrodynamics of single mammalian cells as well as the regulation of AQPs. 相似文献
11.
Cristina Ibarra Pierre Ripoche Mario Parisi Jacques Bourguet 《The Journal of membrane biology》1990,116(1):57-64
Summary It has been reported that PCMBS (p-chloromercuribenzene sulfonate) blocks the water permeability of red cells and of the tubular kidney membranes. In this study we compare the effects of this mercurial compound on the permeability of water and other small solutes in the frog urinary bladder.We observed that: (i) 5mm PCMBS applied at pH 5.0 to the mucosal side inhibited the net and unidirectional water fluxes induced by oxytocin without changing the P
f/P
d ratio. (ii) The oxytocin-induced urea and Na+ influxes were also inhibited by PCMBS. (iii) The unidirectional Cl– movement was first reduced and then increased during the course of PCMBS treatment. (iv) The short-circuit measured at low mucosal Na+ concentration (10mm), diminished continuously, whereas the transepithelial resistance first increased and then diminished. (v) Mannitol, raffinose, -methyl-glucose, antipyrine, caffeine and Rb+ movements were not changed significantly during the first 26 min of the water permeability inhibition. In conclusion: (i) The ADH-sensitive water, urea and Na+ transport systems were inhibited by PCMBS, (ii) PCMBS did not induce a nonspecific and general effect on the permeability of the membrane during the development of the water permeability inhibition, and (iii) in terms of water channels, the inhibition of water transport with the maintenance of a highP
f/P
d ratio suggests that PCMBS closes the water channels in an all or none manner, reducing their operative number in the apical border of frog bladder. 相似文献
12.
Hydroxyl radicals (*OH) as produced in the Fenton reaction (Fe2+ + H2O2 = Fe3+ + OH– + *OH) have been used to reversibly inhibit aquaporins in the plasma membrane of internodes of Chara corallina. Compared to conventional agents such as HgCl2, *OH proved to be more effective in blocking water channels and was less toxic to the cell. When internodes were treated for 30 min, cell hydraulic conductivity (Lp) decreased by 90% or even more. This effect was reversed within a few minutes after removing the radicals from the medium. In contrast to HgCl2, radical treatment reduced membrane permeability of small lipophilic organic solutes (ethanol, acetone, 1‐propanol, and 2‐propanol) by only 24 to 52%, indicating some continued limited movement of these solutes across aquaporins. The biggest effect of *OH treatment on solute permeability was found for isotopic water (HDO), which largely used water channels to cross the membrane. Inhibition of aquaporins reduced the diffusional water permeability (Pd) by about 70%. For the organic test solutes, which mainly use the bilayer to cross the membrane, channel closure caused anomalous (negative) osmosis; that is, cells had negative reflection coefficients (σs) and were transiently swelling in a hypertonic medium. From the ratio of bulk (Lp or osmotic permeability coefficient, Pf) to diffusional (Pd) permeability of water, the number (N) of water molecules that align in water channels was estimated to be N = Pf/Pd = 46 (on average). Radical treatment decreased N from 46 to 11, a value still larger than unity, which would be expected for a membrane lacking pores. The gating of aquaporins by *OH radicals is discussed in terms of a direct action of the radicals when passing the pores or by an indirect action via the bilayer. The rapid recovery of inhibited channels may indicate an easy access of cytoplasmic antioxidants to closed water channels. As hydrogen peroxide is a major signalling substance during different biotic and abiotic stresses, the reversible closure of water channels by *OH (as produced from H2O2 in the apoplast in the presence of transition metals such as Fe2+ or Cu+) may be downstream of the H2O2 signalling. This may provide appropriate adjustments in water relations (hydraulic conductivity), and a common response to different kinds of stresses. 相似文献
13.
14.
Gheorghe Benga Bogdan E. Chapman Guy C. Cox Philip W. Kuchel 《Cell biology international》2010,34(7):703-708
As part of a programme of comparative measurements of P d (diffusional water permeability) the RBCs (red blood cells) from an aquatic monotreme, platypus (Ornithorhynchus anatinus), and an aquatic reptile, saltwater crocodile (Crocodylus porosus) were studied. The mean diameter of platypus RBCs was estimated by light microscopy and found to be ~6.3 μm. P d was measured by using an Mn2+‐doping 1H NMR (nuclear magnetic resonance) technique. The P d (cm/s) values were relatively low: ~2.1×10?3 at 25°C, 2.5×10?3 at 30°C, 3.4×10?3 at 37°C and 4.5 at 42°C for the platypus RBCs and ~2.8×10?3 at 25°C, 3.2×10?3 at 30°C, 4.5×10?3 at 37°C and 5.7×10?3 at 42°C for the crocodile RBCs. In parallel with the low water permeability, the E a,d (activation energy of water diffusion) was relatively high, ~35 kJ/mol. These results suggest that “conventional” WCPs (water channel proteins), or AQPs (aquaporins), are probably absent from the plasma membranes of RBCs from both the platypus and the saltwater crocodile. 相似文献
15.
Water permeability of the plasma membrane (PM) and the vacuolar membrane (VM) is important for intracellular and transcellular
water movement in plants, because mature plant cells have large central vacuoles. We have developed a new method for measuring
the osmotic water permeability of the PM and VM (P
f1 and P
f2, respectively) in individual plant cells. Here, the theoretical basis and procedure of the method are discussed. Protoplasts
isolated from higher plant tissues are used to measure P
f1 and P
f2. Because of the semi-permeability (selective permeability) of cellular membranes, protoplasts swell or shrink under hypotonic
or hypertonic conditions. A theoretical three-compartment model is presented for simulating time-dependent volume changes in the vacuolar and cytoplasmic spaces in a protoplast during osmotic
excursions. The model describes the theoretical relationships between P
f1, P
f2 and the bulk osmotic water permeability of protoplasts (P
f(bulk)). The procedure for measuring the osmotic water permeability is: (1) P
f(bulk) is calculated from the time when half of the total change in protoplast volume is completed, by assuming that the protoplast
has a single barrier to water movement across it (two-compartment model); (2) P
f2 of vacuoles isolated from protoplasts is obtained in the same manner; and (3) P
f1 is determined from P
f(bulk) and P
f2 according to the three-compartment model. The theoretical relationship between P
fl
(m s−1) and L
Pl
(hydraulic conductivity, l=1, 2) (m s−1 Pa−1) is also discussed.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorised users.
Tsuneo Kuwagata and Mari Murai-Hatano contributed equally to the paper. 相似文献
16.
Summary When present at nanomolar concentrations on one side of a lipid bilayer membrane,helianthus toxin (a protein of mol wt16,000) increases enormously membrane permeability to ions and nonelectrolytes by forming channels in the membrane. Membranes containing sphingomyelin are especially sensitive to toxin, but sphingomyelin isnot required for toxin action. Conductance is proportional to about the 4th power of toxin concentration. Single channel conductances are approximately 2×10–10 mho in 0.1m KCl. Toxin-treated membranes are more permeable to K+ and Na+ than to Cl– and SO
4
=
, but the degree of selectivity is pH dependent. Above pH 7 membranes are almost ideally selective for K+ with respect to SO
4
=
, whereas below pH 4 they are poorly selective. The channels show classical molecular sieving for urea, glycerol, glucose, and sucrose — implying a channel radius >5 Å. In symmetrical salt solutions above pH 7, theI–V characteristic of the channel shows significant rectification: below pH 5 there is very little rectfication. Because of the effects of pH on ion selectivity and channel conductance, and also because of the rectification in symmetrical salt solutions and the effect of pH on this, we conclude that there are titratable negative charge groups in the channel modulating ion permeability and selectivity. Since pH changes on the side containing the toxin are effective whereas pH changes on the opposite side are not, we place these negative charges near the mouth of the channel facing the solution to which toxin was added. 相似文献
17.
Osmotic water permeability of isolated vacuoles 总被引:5,自引:0,他引:5
We measured the osmotic water permeability (P
os) of vacuoles isolated from onion (Allium cepa L.), rape (Brassica napus L.), petunia (Petunia hybrida Hook.) and red beet (Beta vulgaris L.). For all the vacuolar types investigated, P
os values were in the range 200–1000 μm s−1. The change in membrane surface area induced by an osmotic gradient was smaller than 2–6%. The vacuolar P
os values for red beet and onion were reduced by 1 mM HgCl2, to 14% and 30% of the control values, respectively, but were partially restored to 51% and 76% by 5 mM β-mercaptoethanol.
These results suggest that aquaporins were present in all the vacuoles tested. In HgCl2-treated onion vacuoles, the reduced P
os (56 μm s−1) had a low activation energy (approx. 6 kJ mol−1), indicating that water permeation was still occurring mainly via aquaporins, and that the water permeability of the lipid
part of the vacuolar membrane is probably very low.
Received: 18 February 1999 / Accepted: 21 June 1999 相似文献
18.
L. Adamec 《Biologia Plantarum》1984,26(2):128-131
The overall washing out of ions, especially86Rb+ (as the tracer for K+), from hypocotyl segments of pumpkin (Cucurbita pepo L.) into distilled water or a CaCl2 solution was studied, during plasmolysis with a saccharose solution and during deplasmolysis. Compartimental analysis was
used to evaluate the86Rb+ washing out kinetics. During plasmolysis, the washing out of86Rb+ increases, due to two processes whose half-times are lower than those during washing out into the CaCl2 solution. During deplasmolysis, the permeability of plasmalemma and tonoplast is substantially descreased, leading to washing
out of most86Rb+ from the cells. Plasmolysis differs from a mere decrease in the turgor pressure in the fact that after exchange for a hypotonic
solution the membranes are irreversibly damaged.
The aim of this work was to monitor the changes in the cell membrane permeability due to a change in the water potential of
the cells, especially during plasmolysis and deplasmolysis. 相似文献
19.
The diffusional water permeability (Pd) in various gut structures of the fleshfly, Sarcophaga bullata, was measured using tritiated water. Water Pd in the larval hindgut was 3.91 × 10?6 cm/sec, whereas in the adult hindgut it was 4.4 × 10?4 cm/sec. The presence of cuticle in various parts of the gut apparently controls the water permeabilities of these structures. Furthermore, the water permeability of the cuticle may be correlated with the mechanism for the production of a hyperosmotic excretion in the hindgut. 相似文献
20.
Vesicle suspensions of up to 5 % egg lecithin and 2.5 % cholesterol have been found to have no effect on the NMR relaxation times of 17O from water. Addition of 1–5 mM Mn2+ to an equimolar vesicle suspension of egg lecithin and cholesterol permitted resolution of the free induction decay into two exponential components, a fast one arising from the external water and a slow one arising from the intravesicular fluid. From the rates of relaxation the mean life time of the water molecules within the vesicles was calculated to be 1±0.1 ms at 22°C. The size of the vesicle was estimated from electron micrographs to be about 500 Å in diameter. These data yield an equilibrium water permeability, Pw, of about 8 μs−1 for the vesicle membranes. From the temperature dependence of Pw an activation energy of 12±2 kcal/mol was obtained. The longitudinal relaxation time (T1) of water within vesicles remained the same as in pure water. 相似文献