Properties of the sugar carrier in baker's yeast

A total of 16 hexoses and pentoses were investigated with respect to transport intoSaccharomyces cerevisiae cells. All monosaccharides were transported across the cytoplasmic membrane but only those with an equatorial hydroxyl group in positions 1 and 4 of theC1 chair conformation and those with an equatorial hydroxyl group in position 2 and an equatorial −CH₂OH group in position 5 of the1C chair conformation reached an equilibrium distribution in the entire cell water volume. Other monosaccharides reached a distribution in only 20–66% of the intracellular water. The two groups of sugars are apparently transported by different carriers (either in parallel or in series), each of them showing countertransport and an apparent activation energy of 6,700–7,800 cal/mol. The carrier transporting the perfectly distributing sugars (Group 1) is affected by uranyl nitrate but not by 2,4-dinitrophenol, the other carrier (Group 2) is apparently not susceptible to uranyl ions but is influenced by 2,4-dinitrophenol. The space of distribution of the Group 1 sugars is reduced in hypertonic media in accordance with changes of intracellular water, that of the Group 2 sugars is altered only very slightly. The carriers differ in their kinetic parametres (mobility of the loaded carriers, maximum rate of transport). There is only a very indistinct competition for transport between representatives of the two groups. Preincubation with d-galactose induces the formation or unmasking of a transport system whereafter even the Group 2 sugars reach equilibrium in the entire cell water. Part I. Fol. microbiol. 10: 30, 1965.     

In vivo Visualization of Synaptic Vesicles Within Drosophila Larval Segmental Axons

Elucidating the mechanisms of axonal transport has shown to be very important in determining how defects in long distance transport affect different neurological diseases. Defects in this essential process can have detrimental effects on neuronal functioning and development. We have developed a dissection protocol that is designed to expose the Drosophila larval segmental nerves to view axonal transport in real time. We have adapted this protocol for live imaging from the one published by Hurd and Saxton (1996) used for immunolocalizatin of larval segmental nerves. Careful dissection and proper buffer conditions are critical for maximizing the lifespan of the dissected larvae. When properly done, dissected larvae have shown robust vesicle transport for 2-3 hours under physiological conditions. We use the UAS-GAL4 method ¹ to express GFP-tagged APP or synaptotagmin vesicles within a single axon or many axons in larval segmental nerves by using different neuronal GAL4 drivers. Other fluorescently tagged markers, for example mitochrondria (MitoTracker) or lysosomes (LysoTracker), can be also applied to the larvae before viewing. GFP-vesicle movement and particle movement can be viewed simultaneously using separate wavelengths.Download video file.^{(34M, mov)}     

Application of the Entropy-Enthalpy Relationship in Water Transport through Plant Roots

Water transport through plant roots is determined by a single layer of cells, so that water passes through a plasmamembrane-cytoplasm-plasmamembrane system. The water transport shows an exponential relationship with temperature in two phases with an abrupt transition. The Arrhenius activation parameters log A and E are calculated for the two phases of water transport below and above the transition temperature. Between log A and E two linear and parallel relationships are observed, one for each phase of water transport. The difference of log A between these two relationships is a measure for a change in entropy in cell water structure at the transition temperature. The change in entropy was small (13.4 J · mol^?1· K^?1) in comparison to the difference in activation energy E for water transport above and below the transition temperature. The role of the plasmamembrane and cytoplasm in determining the cell water structure is discussed.     

Mechanisms of hyperosmotic acclimation in Xenopus laevis (salt,urea or mannitol)

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     

Abscisic acid prevents pollen abortion under high‐temperature stress by mediating sugar metabolism in rice spikelets

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near‐isogenic line) were subjected to heat stress of 40°C after pre‐foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non‐structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.     

The ultrastructure of the posterior midgut caecum of Pachygrapsus crassipes (Decapoda, Brachyura) adapted to low salinity

The effects of salinity adaptation and of composition and tonicity of fixatives upon the ultrastructure of the posterior midgut caecum (PMC) of Pachygrapsus crassipes have been studied. The PMC epithelium consists of a single layer of columnar cells with a microvillous border. The apical cytoplasm contains numerous mitochondria, lysosomes, and much smooth endoplasmic reticulum. Rough endoplasmic reticulum and Golgi apparatus are situated in the perinuclear cytoplasm. This epithelium resembles other transporting epithelia in that the basal cytoplasm has an extensive system of branched tubules formed from invaginations of the lateral and basal plasma membrane. Numerous mitochondria are associated with the basal tubular system. To determine the possible contribution of the PMC to the osmoregulatory ability of Pachygrapsus, the ultrastructure of the PMC from animals adapted to 40, 50, 100 and 150% sea water was investigated. Enlargement of basal tubules and intercellular spaces at low salinity, suggestive of fluid-transport activity, was found to be an artifact of fixation. The most consistent response when animals were acclimated to dilute salinities was that some basal mitochondria assume a more complex shape, usually appearing as rings in cross sections of the caecum. A hypothesis concerning the functional significance of these mitochondria is proposed.     

Comparison of the permeability properties and post-thaw motility of ejaculated and epididymal bovine spermatozoa

There are very few experimental reports on the comparative water transport (membrane permeability) characteristics of ejaculated and epididymal mammalian spermatozoa during freezing. In the present study, we report the effects of cooling ejaculated and epididymal bovine sperm from the same males with and without the presence of a cryoprotective agent, glycerol. Water transport data during freezing of ejaculated and epididymal bovine sperm suspensions were obtained at a cooling rate of 20 °C/min under two different conditions: (1) in the absence of any cryoprotective agents, CPAs and, (2) in the presence of 0.7 M glycerol. Using values published in the literature, we modeled the spermatozoa as a cylinder of length 39.8 μm and a radius of 0.4 μm with an osmotically inactive cell volume, V_b, of 0.61V_o, where V_o is the isotonic cell volume. The subzero water transport response is analyzed to determine the variables governing the rate of water loss during cooling of bovine spermatozoa, i.e. the membrane permeability parameters (reference membrane permeability, L_pg and activation energy, E_Lp). The predicted best-fit permeability parameters ranged from, L_pg = 0.021–0.038 μm/min-atm and E_Lp = 27.8–41.1 kcal/mol. The subzero water transport response and consequently the subzero water transport parameters are not significantly different between the ejaculated and epididymal bovine spermatozoa under corresponding cooling conditions. If this observation is found to be more generally valid for other mammalian species as well, then in the future the sperm extracted from the testes of a postmortem male could be optimally cryopreserved using procedures similar to those derived for ejaculated sperm.     

Focus on Water: Biodesalination: A Case Study for Applications of Photosynthetic Bacteria in Water Treatment

Shortage of freshwater is a serious problem in many regions worldwide, and is expected to become even more urgent over the next decades as a result of increased demand for food production and adverse effects of climate change. Vast water resources in the oceans can only be tapped into if sustainable, energy-efficient technologies for desalination are developed. Energization of desalination by sunlight through photosynthetic organisms offers a potential opportunity to exploit biological processes for this purpose. Cyanobacterial cultures in particular can generate a large biomass in brackish and seawater, thereby forming a low-salt reservoir within the saline water. The latter could be used as an ion exchanger through manipulation of transport proteins in the cell membrane. In this article, we use the example of biodesalination as a vehicle to review the availability of tools and methods for the exploitation of cyanobacteria in water biotechnology. Issues discussed relate to strain selection, environmental factors, genetic manipulation, ion transport, cell-water separation, process design, safety, and public acceptance.Bacteria are commonly employed for the purification of municipal and industrial wastewater but until now, established water treatment technologies have not taken advantage of photosynthetic bacteria (i.e. cyanobacteria). The ability of cyanobacterial cultures to grow at high cell densities with minimal nutritional requirements (e.g. sunlight, carbon dioxide, and minerals) opens up many future avenues for sustainable water treatment applications.Water security is an urgent global issue, especially because many regions of the world are experiencing, or are predicted to experience, water shortage conditions: More than one in six people globally are water stressed, in that they do not have access to safe drinking water (United Nations, 2006). Ninety-seven percent of the Earth’s water is in the oceans; consequently, there are many efforts to develop efficient methods for converting saltwater into freshwater. Various processes using synthetic membranes, such as reverse osmosis, are successfully used for large-scale desalination. However, the high energy consumption of these technologies has limited their application predominantly to countries with both relatively limited freshwater resources and high availability of energy, for example, in the form of oil reserves.The development of an innovative, low-energy biological desalination process, using biological membranes of cyanobacteria, would thus be both attractive and pertinent. The core of the proposed biodesalination process (Fig. 1) is a low-salt biological reservoir within seawater that can serve as an ion exchanger. Its development can be separated into several complementary steps. The first step comprises the selection of a cyanobacterial strain that can be grown to high cell densities in seawater with minimal requirement for energy sources other than those that are naturally available. The environmental conditions during growth can be manipulated to enhance natural extrusion of sodium (Na⁺) by cyanobacteria. In the second step, cyanobacterial ion transport mechanisms must be manipulated to generate cells in which sodium export is replaced with intracellular sodium accumulation. This will involve inhibition of endogenous Na⁺ export and expression of synthetic molecular units that facilitate light-driven sodium flux into the cells. A robust control system built from biological switches will be required to achieve precisely timed expression of the salt-accumulating molecular units. The third step consists of engineering efficient separation of the cyanobacterial cells from the desalinated water, using knowledge of physicochemical properties of the cell surface and their natural ability to produce extracellular polymeric substances (EPSs), which aid cell separation while preserving cell integrity. The fourth step integrates the first three steps into a manageable and scalable engineering process. The fifth and final step assesses potential risks and public acceptance issues linked to the new technology.Open in a separate windowFigure 1.Proposed usage of cyanobacterial cultures for water treatment. A, Hypothetical water treatment station. Situated in basins next to the water source, sun-powered cell cultures remove unwanted elements from the water. The clean water is separated from the cells for human uses. The produced biomass is available for other industries. The proposed biodesalination process is based on the following steps. B, Photoautotrophic cells divide to generate high-density cultures. C, The combined cell volume is low in salt as a result of transport proteins in the cell membrane that export sodium using photosynthetically generated energy. D, Through environmental and genetic manipulation, salt export is inhibited and replaced with transport modules that accumulate salt inside the cells. This process is again fueled by light energy. E, Manipulation of cell surface properties separates the salt-enriched cells from the desalinated water.In this review, we outline the state of knowledge and available technology for each of the steps, as well as summarize the current knowledge gaps and technical limitations in employing a large-scale water treatment process using cyanobacteria. Before discussing these issues, we provide some background information on the usage of cyanobacteria in biotechnology and the impact of sodium on cellular functions of cyanobacteria. The example of biodesalination provides a good vehicle to discuss the suitability of photosynthetic bacteria for water treatment more generally. The issues addressed in this review are relevant for a wide range of biotechnological applications of cyanobacteria, including bioremediation and biodegradation as well as the generation of biofuels, natural medicines, or cosmetics.     

Resistance to Water Transfer in Desert Shrubs Native to Death Valley, California

Differences in plant resistance to water flow, patterns of water transport through stems, and stomatal behavior were studied on three species native to the exceptionally hot and dry habitat of Death Valley, California (—, and Larrea divaricata). Dawn xylem water potentials in July for Atriplex were — 27.5 bar under natural conditions. Corresponding values for Tidestromia and Larrea were respectively — 8.0 bar and -32.0 bar (natural) and — 7.5 bar and — 18.0 bar (irrigated). Recovery of xylem water potential in covered field plants of an irrigated transplant garden reached a maximum value in July of — 9.5 bar in Atriplex, — 5.7 bar in Tidestromia and — 7.0 bar in Larrea. Resistance to free-energy transfer was used to study resistance to water transport through the plants. Under field conditions irrigated Atriplex plants gave a whole plant resistance of 20.70 × 10⁶ s cm^-1, as compared lo 18.37 × 10⁶ s cm^-1 for Larrea and 10.01 × 10⁶ s cm^-1 for Tidestromia. Plant resistance to water How computed by this method on Atriplex plants grown under laboratory conditions gave a value of 3.73 × 10⁶ s cm^-1 at 35C. Paths of water flow in field plants as investigated with injected acid fuchsin indicated a sectorial straight type vessel. The relationship between transpiration rates and xylem water potentials in Atriplex hymenelytra was linear between transpiration 1.28 μg cm^-2 s^-1 and 2.35 μg cm^-2 s^-1 at 35°C. These results indicate that according to the Van den Honert model for water transport, plant resistance to water flow remained rather constant at this temperature. In Atriplex grown under laboratory conditions there was an adjustment of plant resistance so change in water flux at 9.5°C and 25°C. When laboratory-grown plants of Atriplex and Tidestromia were subjected to water stress by withholding water. Tidestromia closed stomata and reduced transpiration rates at higher water potentials than in Atriplex. The ratio of vapor pressure gradients of leaf/air to leaf diffusion resistance was proportional lo transpiration rates. It is suggested that Atriplex hymenelytra is a species that combines strong regulation of water loss by stomata with low efficiency of the water transport system. These plants are unable to prevent depression of plant water potential as transpiration increases. On the other hand. Tidestromia oblongifolia has little stomatal regulation of transpiration and a highly efficient water transport system. These plants sustain very high rates of transpiration without significant decrease in plant water potential.     

The dispersal and deposition of hydrochorous plant seeds in drainage ditches

1. Surface water is an important dispersal vector for wetland plant species. However, most previous studies on hydrochory (i.e. water dispersal) have focused on ecosystems with relatively rapid water flow. Therefore, there is a need to study such dispersal in slow‐flowing or stagnant waterbodies, such as drainage ditches, which might act as dispersal corridors between habitat patches. 2. To gain insight into the mechanisms by which seeds are transported in drainage ditches, the effect of the velocity of wind and water on the rate of transport of floating seeds of three wetland species (Carex pseudocyperus L., Iris pseudacorus L. and Sparganium erectum L.) was investigated. Furthermore, in release and retrace experiments with painted C. pseudocyperus seeds, a number of factors potentially determining the probability of seed deposition were investigated. 3. Net wind speed was found to be the main factor determining the rate at which seeds are transported in drainage ditches. No relation between water flow at middepth in the ditches and seed transport was found. Wind speed and flow at the water surface were positively related. The effect of wind speed on the rate of transport of floating seeds was greater for S. erectum seeds, because a greater ratio of their volume protrudes from the water, than for C. pseudocyperus and I. pseudacorus seeds. 4. The principal factors that determine seed deposition were aquatic plant cover, ditch slope and indentations in the ditch bank. Seeds changed direction if the wind direction changed, or if there was a bend in the ditch. The final pattern of deposition was related to mean net wind speed. Mean transport distance after 2 days varied between 34 and 451 m. 5. Unlike in rivers, seed transport in ditches was determined by wind speed and direction, enabling multidirectional seed dispersal. We conclude that in slow‐flowing waters, wind is a more important driver for hydrochorous seed transport than the flow of water. This sheds a new light on hydrochory and has important consequences for the management of otherwise fragmented wetland remnants.     

Window on a Microworld: Simple Microfluidic Systems for Studying Microbial Transport in Porous Media

Microbial growth and transport in porous media have important implications for the quality of groundwater and surface water, the recycling of nutrients in the environment, as well as directly for the transmission of pathogens to drinking water supplies. Natural porous media is composed of an intricate physical topology, varied surface chemistries, dynamic gradients of nutrients and electron acceptors, and a patchy distribution of microbes. These features vary substantially over a length scale of microns, making the results of macro-scale investigations of microbial transport difficult to interpret, and the validation of mechanistic models challenging. Here we demonstrate how simple microfluidic devices can be used to visualize microbial interactions with micro-structured habitats, to identify key processes influencing the observed phenomena, and to systematically validate predictive models. Simple, easy-to-use flow cells were constructed out of the transparent, biocompatible and oxygen-permeable material poly(dimethyl siloxane). Standard methods of photolithography were used to make micro-structured masters, and replica molding was used to cast micro-structured flow cells from the masters. The physical design of the flow cell chamber is adaptable to the experimental requirements: microchannels can vary from simple linear connections to complex topologies with feature sizes as small as 2 μm. Our modular EcoChip flow cell array features dozens of identical chambers and flow control by a gravity-driven flow module. We demonstrate that through use of EcoChip devices, physical structures and pressure heads can be held constant or varied systematically while the influence of surface chemistry, fluid properties, or the characteristics of the microbial population is investigated. Through transport experiments using a non-pathogenic, green fluorescent protein-expressing Vibrio bacterial strain, we illustrate the importance of habitat structure, flow conditions, and inoculums size on fundamental transport phenomena, and with real-time particle-scale observations, demonstrate that microfluidics offer a compelling view of a hidden world.Download video file.^{(163M, mp4)}     

Effect of cell water amount on photosynthetic yield in the cyanobacterium Nostoc flagelliforme and involvement of NADPH dehydrogenase-mediated cyclic electron transport

Nostoc flagelliforme is a terrestrial cyanobacterium, and water is one of the most important factors limiting its photosynthetic yield. The aims of the present study were to investigate the effect of cell water amount on photosynhetic yield and the role of NADPH dehydrogenase (NDH-1)-mediated cyclic electron transport in this effect. The role of NDH-1-mediated cyclic electron transport was assessed by measuring NDH-1 expression, several chlorophyll fluorescence parameters, and photosynthetic O₂ evolution at several time points after cell water had been redried. The results indicated that the highest rate of NDH-1-mediated cyclic electron transport, reflected by post-illumination increase in chlorophyll fluorescence and NDH-1 amount, was only obtained when the cells contained about 1.8 times water relative to dry weight. This was consistent with observed changes in photosynthetic yield, reflected by O₂ evolution. However, the highest photochemical activity of photosystem II, reflected by F _v/F _m and qP, could be maintained when N. flagelliforme cells included water in a broad range. This implies that the effect of cell water amount on photosynthetic yield is related to NDH-1-mediated cyclic electron transport. The possible mechanisms of this effect are discussed.     

The PIP and TIP aquaporins in wheat form a large and diverse family with unique gene structures and functionally important features

Aquaporins, members of major intrinsic proteins (MIPs), transport water across cellular membranes and play vital roles in all organisms. Adversities such as drought, salinity, or chilling affect water uptake and transport, and numerous plant MIPs are reported to be differentially regulated under such stresses. However, MIP genes have been not yet been characterized in wheat, the largest cereal crop. We have identified 24 PIP and 11 TIP aquaporin genes from wheat by gene isolation and database searches. They vary extensively in lengths, numbers, and sequences of exons and introns, and sequences and cellular locations of predicted proteins, but the intron positions (if present) are characteristic. The putative PIP proteins show a high degree of conservation of signature sequences or residues for membrane integration, water transport, and regulation. The TIPs are more diverse, some with potential for water transport and others with various selectivity filters including a new combination. Most genes appear to be expressed as expressed sequence tags, while two are likely pseudogenes. Many of the genes are highly identical to rice but some are unique, and many correspond to genes that show differential expression under salinity and/or drought. The results provide extensive information for functional studies and developing markers for stress tolerance. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seasonal and long term changes of the phytoplankton in the lake Tortum in relation to environmental factors, Erzurum, Turkey

Seasonal changes in phytoplankton community structure of the lake Tortum were studied over one year period, from March 2002 to February 2003. The collected data were compared with the data collected 21 years ago. Chlamydomonas microsphaerella, Cyclotella krammeri, C. glomerata, and Ceratium hirundinella were identified to be dominant several times during the study period. Species diversity and biomass of the phytoplankton were very low in spite of sufficient and high levels of nutrient concentrations. Maximum phytoplankton density levels were observed during summer and late autumn. Phytoplankton density was positively correlated with nutrients, temperature and pH, and it was negatively correlated with Secchi depth and dissolved oxygen. Phytoplankton growths were negatively affected from water transparency and high levels of water mass transport (circulation) and velocity in the lake.     

Functional morphology of scale hinges used to transport water: convergent drinking adaptations in desert lizards (<Emphasis Type="Italic">Moloch horridus</Emphasis> and <Emphasis Type="Italic">Phrynosoma cornutum</Emphasis>)

The Australian thorny devil, Moloch horridus Gray, 1841, and the Texas horned lizard, Phrynosoma cornutum Harlan, 1825, have the remarkable ability to rapidly move water through interscalar spaces on their skin’s surface to their mouth for drinking. The morphology of these scale hinges has not been studied. We used histological and SEM techniques to examine and compare the scale hinges of both species. Additional taxa in their respective lineages were examined in order to evaluate the potential that convergent evolution has occurred. In the two species that transport water, each scale hinge has a basally expanded and semi-enclosed channel formed by the hinge joint that is interconnected with all scale hinges on the body. We hypothesize that it is within this semi-tubular channel system of hinge joints, where the β-layer keratin of the integument is very thin, that water is transported. Hinge joint walls are covered by a complex topography of fractured surfaces that greatly expand the channel’s surface area and probably enhance capillary transport of water. In addition, we note differing morphology of scale surfaces at the rear of the jaws of both species. We hypothesize that capillary forces fill the scale-hinge system and additional forces, generated within the mouth by observed motions during drinking, depress local water-pressure to pull water through the channels of the hinge-joint system. We conclude that the combined features in the two species, semi-tubular hinge-joint channels with convoluted walls and a jaw-buccal cavity pumping-mechanism, have convergently evolved for capture, transport, and drinking of water from sporadic rainfall.     

Significance of plasmalemma aquaporins for water‐transport in Arabidopsis thaliana

The plant plasma membrane intrinsic protein, PIP1b, facilitates water transport. These features were characterized in Xenopus oocytes and it has asked whether aquaporins are relevant for water transport in plants. In order to elucidate this uncertainty Arabidopsis thaliana was transformed with an anti-sense construct targeted to the PIP1b gene. Molecular analysis revealed that the anti-sense lines have reduced steady-state levels of PIP1b and the highly homologous PIP1a mRNA. The cell membrane water permeability was analyzed by swelling of protoplasts, which had been transferred into hypotonic conditions. The results indicate that the reduced expression of the specific aquaporins decreases the cellular osmotic water permeability coefficient approximately three times. The morphology and development of the anti-sense lines resembles that of control plants, with the exception of the root system, which is five times as abundant as that of control plants. Xylem pressure measurement suggests that the increase of root mass compensates the reduced cellular water permeability in order to ensure a sufficient water supply to the plant. The results obtained by this study, therefore, clearly demonstrate that aquaporins are important for plant water transport.     

Permeability and Reflection Coefficients of Urea and Small Amides in the Human Red Cell

Measurement of the transport parameters that govern the passage of urea and amides across the red cell membrane leads to important questions about transport of water. It had initially been thought that small protein channels, permeable to water and small solutes, traversed the membrane (see Solomon, 1987). Recently, however, very strong evidence has been presented that the 28 kDa protein, CHIP28, found in the red cell membrane, is the locus of the water channel (see Agre et al., 1993). CHIP28 transports water very rapidly but does not transport small nonelectrolytes such as urea. The irreversible thermodynamic parameter, σ _i , the reflection coefficient, is a measure of the relationship between the permeability of the solute and that of water. If a solute permeates by dissolution in the membrane, σ _i = 1.0; if it permeates by passage through an aqueous channel, σ _i < 1.0. For urea, Goldstein and Solomon (1960) found that σ_urea= 0.62 ± 0.03 which meant that urea crosses the red cell membrane in a water-filled channel. This result and many subsequent observations that showed that σ_urea < 1.0 are at variance with the observation that CHIP28 is impermeable to urea. In view of this problem, we have made a new series of measurements of σ _i for urea and other small solutes by a different method, which obviates many of the criticisms Macey and Karan (1993) have made of our earlier method. The new method (Chen et al., 1988), which relies upon fluorescence of the intracellular dye, fluorescein sulfonate, leads to the corrected value, σ_urea,corr= 0.64 ± 0.03 for ghosts, in good agreement with earlier data for red cells. Thus, the conclusion on irreversible thermodynamic and other grounds that urea and water share a common channel is in disagreement with the view that CHIP28 provides the sole channel for water entrance into the cell. Received: 6 February 1996/Revised: 20 May 1996     

Morphology,distribution and significance of the manganese-accumulating microorganism metallogenium in lakes

The manganese-accumulating microorganism Metallogenium is very common in the deep water and on the sediment surface in lakes of the Oslo district, southern Norway. Metallogenium accumulates manganese in its star-shaped coenobia; other trace metals are also greatly concentrated. The taxonomic position of the organism is uncertain. Sections of coenobia suggests that the encrusted parts are not enclosing living structures.In lake Nordbytjern, the occurrence of Metallogenium coincides with periods of turbulence in the lake. By sinking, the population accumulates close to the chemocline, becoming heavily encrusted. Plankton traps indicate that Metallogenium adds to the downward transport and accumulation of manganese and iron in the lake.     

Development of photosynthetic electron transport in Pinus silvestris

Photosynthetic electron transport activity has been measured in chloroplasts isolated from dark-grown seedlings of Pinus silvestris L. and in chloroplasts isolated from seedlings subjected to illumination for periods of up to 48 h. Activities of photosystem 2, photosystem 1 and photosystem 2 plus 1 have been measured. Chloroplasts isolated from dark-grown seedlings showed significant electron transport activity through both photosystems and through the entire electron transport chain from water to NADP. Illumination of the seedlings for only 5 min markedly promoted photosystem 2 activity. The artificial electron donor, diphenylcarbazide. promoted activity in chloroplasts from dark-grown seedlings and in chloroplasts from seedlings illuminated for up to 30 min. In comparison to photosystem 2 and overall electron transport from water to NADP, photosystem 1 activity increased only slightly during illumination. Measurements of electron transport and fluorescence kinetics have confirmed that photosynthetic electron transport capacity is limited on the water splitting side of photosystem 2 in dark-grown seedlings, whereas the primary and secondary electron acceptors of photosystem 2 are fully synthesized and functioning in darkness. Polyethylene glycol must be used as a protective agent when isolating photoactive chloroplasts from secondary needles of conifers. However, the presence of polyethylene glycol, when isolating chloroplasts from dark-grown pine cotyledons, caused a total inhibition of the activity of photosystem 2. The failure of others to show a substantial electron transport activity in chloroplasts from dark-grown Pinus silvestris might depend on their use of polyethylene glycol in the preparation medium and/or on their use of suboptimal reaction conditions for the electron transport measurements.     

Potential for passive internal dispersal: eggs of an aquatic leaf beetle survive passage through the digestive system of mallards

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