The involvement of a rhodopsin-like photopigment in the photoperiodic response of the Japanese quail

Summary A technique has been developed for the investigation of the photopigment involved in the photoperiodic control of reproduction in Japanese quail,Coturnix coturnix. When these photoreceptors were exposed to white or monochromatic light a clear relationship was found between light intensity and the extent of photo-induced luteinizing hormone (LH) secretion. A spectroradiometric investigation of the passage of light through the skull and brain enabled us to illuminate the hypothalamic region with equal numbers of photons at a range of wavelengths. Action spectra were then conducted and showed a photopigment with a peak sensitivity at wavelengths near 500 nm. An excellent match was obtained when the standard absorption spectrum for a rhodopsin was fitted to the action spectrum, suggesting a rhodopsin maximally sensitive at 492 nm. The absolute sensitivity of the photoreceptors was calculated at a range of wavelengths: with light at 500 nm, 2.85×10^–12 E·cm^–2·s^–1 triggered the photoperiodic response. This level of sensitivity is matched only by the rhodopsin visual pigments.Abbreviations LH luteinizing hormone - T transmission     

Exchange of reduced sulfur gases between lichens and the atmosphere

Fourteen lichens, 10 green algal lichens and four cyanolichens, as well as a cyanobacterium emitted significant quantities of H₂S (0.01–0.04 pmol g dw^–1 s^–1) and DMS (0.005–0.025 pmol g dw^–1 s^–1) but were sinks for COS (0.015–0.14 pmol g dw^–1 s^–1). In contrast, exchange of CH₃SH and CS₂ were sporatic and inconsistent. Although some interspecific variation occurred for the first three gases, exchange rates were relatively uniform and were not influenced by irradiance conditions. In contrast to DMS and H₂S emission, COS uptake was strongly influenced by degree of thallus hydration. Because lichen dominated systems cover extensive terrestrial habitats, COS uptake is potentially important in the world's sulfur budget.     

Growth yield determination in a chemostat culture of the marine microalgaIsochrysis galbana

The growth yield of the PUFA-producing marine microalgaIsochrysis galbana ALII-4 grown in a light limited chemostat, was measured under a wide variety of conditions of incident irradiance (I _O ) and dilution rates (D). The experiments were conducted under laboratory conditions at 20 °C under continuous light. D ranged from 0.0024 to 0.0410 h^–1 at three intensities of Io (820, 1620 and 3270 µmol photon m^–2 s^–1) close to those found in outdoor cultures. A maximum efficiency _max = 0.616 g mol photon^–1 was obtained at I _O = 820 µmol photon m^–2 s^–1 and D = 0.030 h^–1 and the maximum capacity of the biomass to metabolize the light harvested was found to be 13.1 µmol photon g^–1 s^–1. Above this value, a significant drop in the system efficiency was observed. A new approach based in the averaged irradiance is used to assess the photon flux absorbed by the biomass.     

Effect of Irradiance, Sugars and Nitrogen on Leaf Size of In Vitro Grown Ceratonia Siliqua L.

Carob (Ceratonia siliqua L.) has compound pinnate leaves consisting of 4 – 6 pairs of leaflets. However, in conditions of in vitro culture only one pair of leaflets develops. With increasing irradiance from 9.3 to 74.1 µmol m^–2 s^–1, leaf area increased 5-fold. Sucrose also significantly increased leaf area and the maxima were at concentration 147 mM at high irradiance and 233.6 mM at low irradiance. Sucrose was superior to fructose, glucose and combination of both in increasing leaf area. Decreasing concentration of KNO₃ and NH₄NO₃ caused a 3-fold decline of leaf area.     

Polarized-light sensitivity in rainbow trout (Oncorhynchus mykiss): characterization from multi-unit responses in the optic nerve

Integrated spike activity of axons from the optic nerve was measured in an investigation of the e-vector sensitive mechanism underlying the ability of rainbow trout (Oncorhynchus mykiss) for orientation in downwelling, linearly-polarized light. In anaesthetized, immobilized fish, one eye was exposed to incremental light flashes which were superimposed over closely controlled background conditions. Under scotopic and various photopic conditions, intensity/response curves were generated from the on-response of the optic nerve. Relative sensitivity curves were then obtained as a function of e-vector direction for the 5 kinds of receptor cells in this trout's retina: rods, ultraviolet cones (UV), short wavelength cones (S), medium wavelength cones (M), and long wavelength cones (L).Under scotopic conditions, no sensitivity to e-vector was apparent: thus, rods do not mediate polarization sensitivity. Under photopic conditions, parr weighing 8–10 g exhibited e-vector sensitivity in two orthogonal channels. A UV stimulus (380 nm) on a white background evoked a three-peaked response (0°, 90°, and 180°) to the e-vector orientations presented in 30° increments between 0° and 180°. In contrast, when the background was illuminated with appropriate short and long wavelength cut-off filters, M-and L-cones showed maximum responses only to the horizontal (90°) plane whether they were stimulated at their -absorption band or their -absorption band in the near UV. Isolated UV-cones gave maximum responses to the vertical (0° and 180°) e-vector, thus corresponding to a second channel. The blue sensitive, S-cones, did not show evidence of polarization sensitivity. As well, no evidence of the polarization sensitivity was observed under UV isolating background conditions in larger individuals, 50–78 g smolts, although the other cone mechanisms responded as in smaller individuals.     

Photosynthetic light-use by three bromeliads originating from shaded sites (<Emphasis Type="Italic">Ananas ananassoides,Ananas comosus</Emphasis> cv. Panare) and exposed sites (<Emphasis Type="Italic">Pitcairnia pruinosa</Emphasis>) in the medium Orinoco basin,Venezuela

Three Bromeliaceae species of the medium Orinoco basin, Venezuela, were compared in their light-use characteristics. The bromeliads studied were two species of pineapple, i.e. the wild species Ananas ananassoides originating from the floor of covered moist forest, and the primitive cultivar Panare of Ananas comosus mostly cultivated in semi-shaded palm swamps, and Pitcairnia pruinosa, a species abundant in highly sun exposed sites on rock outcrops. Ananas species are Crassulacean acid metabolism (CAM) plants, P. pruinosa is C₃ plant. Plants were grown at low daily irradiance (LL = 1.3 mol m^–2 d^–1 corresponding to an incident irradiance of 30 mol m^–2 s^–1) and at high irradiance (HL = 14.7 mol m^–2 d^–1 or 340 mol m^–2 s^–1), and CO₂ and H₂O-vapour gas exchange and photochemical (q_P) and non-photochemical quenching (q_NP) of chlorophyll a fluorescence of photosystem 2 (PS2) were measured after transfer to LL, medium irradiance (ML = 4.1 mol m^–2 d^–1 or 95 mol m^–2 s^–1) and HL. All plants showed flexible light-use, and q_P was kept high under all conditions. LL-grown plants of Ananas showed particularly high rates of CAM-photosynthesis when transferred to HL and were not photoinhibited.     

Photomovement of the red alga Porphyridium cruentum (Ag.) Naegeli

Phototaxis of the unicellular red alga Porphyridium cruentum was studied by staining the slime tracks of individual cells as well as with the aid of a population method. Because of the increased straightness of the movement the mean linear velocity of a unilaterally illuminated population exceeds considerably that of an only photokinetically stimulated one. In white light the phototactic reaction is saturated already at 100 lx. The zero threshold lies at about 1 lx. Spectral sensitivity curves of phototaxis obtained at high photon fluence rates (>=10^–11 mol cm^–2 s^–1) display two main peaks which shift against each other at intermediate irradiances and, finally, form a single maximum in the blue range (443 nm) at low photon fluence rates (10^–12 and 10^–13 mol cm^–2 s^–1). Photon fluence rate-response curves reveal that supraoptimal irradiances decrease the phototactic reaction, especially in the range of the highest sensitivity of the cells. The action spectrum of phototaxis was calculated on the basis of the photon fluence rate-response curves. It shows a maximum at 443 nm and shoulder at 416 nm and between 467 and 477 nm. Wavelengths longer than 540 nm are phototactically inactive even at very high irradiances (25 W m^–2). Thus, this is the first phototactic action spectrum of a biliprotein-containing organism which does not indicate the participation of biliproteins in the absorption of phototactically active light. DCMU and potassium iodide have no specific effects on phototaxis.Abbreviation DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

Combined Effect of Irradiance and Water Regime on Sorghum Photosynthesis

Grain sorghum [Sorghum bicolor (L.) Moench. cvs. TX430 and KS82] was grown in a Haynie very fine sandy loam (coarse-silty, mixed, superactive, calcareous, mesic Mollic Udifluvents) under constant 47 % shade or full irradiance in a greenhouse under two watering regimes to see the combined and individual effects of low irradiance (LI) and low water (LW) on the sorghum genotypes. Under the high-irradiance (HI) and high-water (HW) treatment (control) and the LI-HW treatment, TX430 grew taller than KS82. Both LI and LW reduced several times the fresh and dry masses. Under the control conditions, TX430 reached its maximum net photosynthetic rate (P _Nmax) of 28.93 mol m^–2 s^–1 at a photosynthetic photon flux density (PPFD) of 1 707 mol m^–2 s^–1, and KS82 reached its P _Nmax of 28.32 mol m^–2 s^–1 at a PPFD of 2 973 mol m^–2 s^–1. The fact that TX430 had P _Nmax under a lower PPFD than KS82 may relate to its taller growth under LI conditions. Hence genotypes of sorghum might be selected for low irradiance using curves relating P _N to PPFD.     

The effect of temperature,irradiance and animal size on incorporation rates of Simocephalus vetulus

Carbon incorporation rates of Simocephalus vetulus were measured to study the effects of the physical state of the animals, size of the animal, varying temperature and light conditions. Physical state of the animal showed little effect on incorporation rates after the first hour. Incorporation rates increased in proportion to the third power of animal size. Experimental animals collected at temperatures of 12, 20 or 25°C fed maximally at 10, 15 and 25°C respectively, when subjected to a feeding temperature range of 5 to 30°C. We have interpreted this as an indication that S. vetulus is able to acclimate and incorporate maximally at various temperatures after prolonged exposure to that temperature. When fed over an irradiation range of 0 to 4.68 × 10^–3 cal cm^–2 s^–1 incorporation rates were indirectly proportional to irradiance. This suggests a response to decreased irradiance in the weedy, littoral habitat of these animals.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with “antisense” rbcS

Tobacco (Nicotiana tabacum L.) plants transformed with antisense rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO₂ at 20°C at 100, 300 and 750 mol·m^–2·s^–1 irradiance, and at 28°C at 100, 300 and 1000 mol·m^–2·s^–1 irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C _infRubisco ^supA ) was very low (0.01–0.03) at low growth irradiance, and still fairly low (0.24–0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that C _infRubisco ^supA increases as incident irradiance rises, (iii) When low-light (100 mol·m^–2·s^–1)-grown plants are exposed to high (750–1000 mol·m^–2·s^–1) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750–1000 mol·m^–2·s^–1) are suddenly exposed to high and saturating irradiance (1500–2000 mol·m^–2·s^–1), C _infRubisco ^supA remained relatively low (0.23–0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in sun leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the light reactions and Rubisco; this was shown by the low value of C _infRubisco ^supA (see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) Antisense plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light.Abbreviations A rate of photosynthesis - C _infRubisco ^supA flux control coefficient of Rubisco for photosynthesis - c_i internal CO₂ concentration - qE energy-dependent quenching of chlorophyll fluorescense - qQ photochemical quenching of chlorophyll fluorescence - NADP-MDH NADP-dependent malate dehydrogenase - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose-1,5-bisphosphate This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137).     

Leaf Structure of Tobacco In Vitro Grown Plantlets as Affected by Saccharose and Irradiance

Tobacco plantlets were cultured in vitro under high (200 µmol m^–2 s^–1) or low (60 µmol m^–2 s^–1) irradiance with or without saccharose in the medium. Light microscopy and image analysis were used to evaluate the effect of these culture conditions on leaf anatomy. Addition of saccharose resulted in thicker leaves (all leaf layers) and larger mesophyll cells under both growth irradiances. Various irradiance affected leaf anatomy differently when plantlets had been cultivated in presence or absence of saccharose in the medium. While under high irradiance in presence of saccharose leaf thickness and number of chloroplasts per cell section were increased, plantlets grown under high irradiance in absence of saccharose had thinner leaves and less chloroplasts per cell section. The changes were more pronounced in palisade parenchyma layer.     

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (P _max) was 4.41 mol photon m^–2 s^–1 at 26 °C and 4.07 mol photon m^–2 s^–1 at 28 °C. Saturating irradiance occurred at 95 mol photon m^–2 s^–1 at 26 °C and 65 mol photon m^–2 s^–1 at 28 °C. In contrast, growth experiments at 41.7 mol photon m^–2 s^–1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day^–1 at 28 °C and 25 mol photon m^–2 s^–1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.     

S-Potentials in the Skate Retina : Intracellular recordings during light and dark adaptation

The S-potentials recorded intracellularly from the all-rod retina of the skate probably arise from the large horizontal cells situated directly below the layer of receptors. These cells hyperpolarize in response to light, irrespective of stimulus wavelength, and the responses in photopic as well as scotopic conditions were found to be subserved by a single photopigment with λ_max = 500 nm. The process of adaptation was studied by recording simultaneously the threshold responses and membrane potentials of S-units during both light and dark adaptation. The findings indicate that the sensitivity of S-units, whether measured upon steady background fields or in the course of dark adaptation, exhibits changes similar to those demonstrated previously for the ERG b-wave and ganglion cell discharge. However, the membrane potential level of the S-unit and its sensitivity to photic stimulation varied independently for all the adapting conditions tested. It appears, therefore, that visual adaptation in the skate retina occurs before the S-unit is reached, i.e., at the receptors themselves.     

The cell composition of Nannochloropsis sp. changes under different irradiances in semicontinuous culture

Nannochloropsis sp. was grown semicontinuously with a rate of daily renewal of the culture media of 40% of the volume of the culture under different irradiances (40, 60, 80, 220 and 480 mol quanta m^–2 s^–1). Under the conditions tested, light saturation was achieved at 220 mol quanta m^–2 s^–1 with no significant increase in steady-state cell density or of dry weight productivity with higher irradiance, reaching values of 115 × 10⁶ cells ml^–1 and 375 mg l^–1 day^–1 respectively. C/N ratios clearly indicated the point of light saturation, decreasing with increasing irradiance for light-limited conditions and increasing for light-saturated conditions. Under light-limited conditions, an increase in the irradiance produced an increase in the protein percentage of the organic fraction to the detriment of lipids and carbohydrates, while small changes were recorded under light-saturated conditions. The degree of unsaturation of fatty acids was lower with increasing irradiance, with a three-fold decrease of the percentage of total n–3 fatty acids, from 29 to 8% of total fatty acids, caused mainly by a decrease of eicosapentaenoic acid (EPA) (20:5n–3). The microalga reached its maximal value of dry weight productivity (375 mg l^–1 day^–1), EPA productivity (3.2 mg l^–1 day^–1) and maximal protein content (36% of the organic content) at the point at which light saturation was achieved. Results demonstrate the efficiency of the use of the irradiance for the modification of the biochemical composition of Nannochloropsis sp.     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

Significance of the Leaf Area Ratio in Hevea brasiliensis Under High Irradiance and Low Temperature Stress

Adjustment in leaf area : mass ratio called leaf area ratio (LAR) is one of the strategies to optimize photon harvesting. LAR was recorded for 10 genotypes of Hevea brasiliensis under high irradiance and low temperature and the genotypes were categorized into two groups, i.e. high LAR and low LAR types. Simultaneously, the growth during summer as well as winter periods, photosynthetic characteristics, and in-vitro oxidative damage were studied. Low LAR (19.86±0.52 m² kg^–1) types, recorded an average of 18.0 % chlorophyll (Chl) degradation under high irradiance and 7.1 % Chl degradation under low temperature. These genotypes maintained significantly higher net photosynthetic rate (P _N) of 10.4 mol(CO₂) m^–2 s^–1 during winter season. On the contrary, the high LAR (24.33±0.27 m² kg^–1) types recorded significantly lower P _N of 4 mol(CO₂) m^–2 s^–1 and greater Chl degradation of 37.7 and 13.9 % under high irradiance and low temperature stress, respectively. Thus LAR may be one of the physiological traits, which are possibly involved in plant acclimation process under both stresses studied.     

Light response of D1 turnover and photosystem II efficiency in the seagrassZostera capricorni

Biochemical and biophysical parameters, including D1-protein turnover, chlorophyll fluorescence, oxygen evolution activity and zeaxanthin formation were measured in the marine seagrassZostera capricorni (Aschers) in response to limiting (100 mol·m^–2·^–1), saturating (350 mol·m^–2·s^–1) or photoinhibitory (1100 mol·m^–2·s^–1) irradiances. Synthesis of D1 was maximal at 350 mol·m^–2·s^–1 which was also the irradiance at which the rate of photosynthetic O₂ evolution was maximal. Degradation of D1 was saturated at 350 mol·m^–2·s^–1. The rate of D1 synthesis at 1100 mol·m^–2·s^–1 was very similar to that at 350 mol·m^–2·s^–1 for the first 90 min but then declined. At limiting or saturating irradiance little change was observed in the ratio of variable to maximal fluorescence (Fv/Fm) measured after dark adaptation of the leaves, while significant photoinhibition occurred at 1100 mol·m^–2·s^–1. The proportion of zeaxanthin in the total xanthophyll pool increased with increasing irradiance, indicative of the presence of a photoprotective xanthophyll cycle in this seagrass. These results are consistent with a high level of regulatory D1 turnover inZostera under non-photoinhibitory irradiance conditions, as has been found previously for terrestrial plants.We would like to thank Professor Peter Böger (Department of Plant Biochemistry, University of Konstanz, Germany) for the kind gift of D1 antibodies. This work was partly supported by a University of Queensland Enabling Grant to CC.     

Differential Photosynthetic Acclimation Pattern to Limiting Growth-Irradiance in Two Types of C4 Plants

Photosynthetic acclimation to reduced growth irradiances (650 and 200 µmol m^–2 s^–1) in Eleusine coracana (L.) Garten, a nicotinamide adenine dinucleotide-malic enzyme (NAD-ME) C₄ species and Gomphrena globosa L., a nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ species were investigated. E. coracana plants acclimated in 4 and 8 d to 650 and 200 µmol m^–2 s^–1, respectively, whereas G. globosa plants took 8 and 10 d, respectively, to acclimate to the same irradiances. The acclimation to reduced irradiance was achieved in both species by greater partitioning of chlorophyll towards the light-harvesting antennae at the expense of functional components. However, magnitude of increase in the light-harvesting antenna was higher in E. coracana as compared to G. globosa. Superior photosynthetic acclimation to reduced irradiance in G. globosa was due to the smaller change in functions of the cytochrome b ₆/f complex, photosystem (PS) 1 and PS2 leading to the higher carbon fixation rates compared to E. coracana.     

The basal permeability to water of human red blood cells evaluated by a nuclear magnetic resonance technique

The characteristics of water diffusional permeability (P) of human red blood cells were studied on isolated erythrocytes by a doping nuclear magnetic resonance technique. In order to estimate the basal permeability the maximal inhibition of water diffusion was induced by exposure of red blood cells to p-chloromercuribenzene sulfonate (PCMBS) under various conditions (concentration, duration, temperature). The lowest values of P were around 0.7×10^–3 cm s^–1 at 10°C, 1.2×10^–3 cm s^–1 at 15°C, 1.4×10^–3 cm s^–1 at 20°C, 1.8×10^–3 cm s^–1 at 25°C, 2.1×10^–3 cm s^–1 at 30°C and 3.5×10^–3 cm s^–1 at 37°C. The mean value of the activation energy of water diffusion (E_a,d) was 25 kJ/mol for control and 43.7 kJ/mol for PCMBS-inhibited erythrocytes. The values of P and E_a,d obtained after induction of maximal inhibition of water diffusion by PCMBS can be taken as references for the basal permeability to water of the human red blood cell membrane.     

Diffusion of sucrose in xanthan gum solutions

Molecular diffusion of solutes, like sucrose in the xanthan gum fermentation, is important in order to understand the complex behavior of mass transfer mechanisms during the process. This work was focused to determine the diffusion coefficient of sucrose, a carbon source for xanthan production, using similar sucrose and xanthan concentrations to those occurring in a typical fermentation. The diaphragm cell method was used in experimental determinations. The data showed that diffusion coefficient of sucrose significantly decreases when xanthan gum concentration increases. Theoretical and semiempirical models were used to predict sucrose diffusivity in xanthan solutions. Molecular properties and rheological behavior of the system were considered in the modeling. The models tested fitted well the behavior of experimental data and that reported for oxygen in the same system.List of Symbols A constant in eq. (5) - C _pg cm^–3 polymer concentration - D cm² s^–1 diffusivity - D _ABcm² s^–1 diffusivity of A through liquid solvent - D _APcm² s^–1 diffusivity of A in polymer solution - D _AWcm² s^–1 diffusivity of A in water - D _Pcm² s^–1 diffusivity of polymer in liquid solvent - E _D gradient of the activation energy for diffusion - H _P hydratation factor of the polymer in water (g of bound water/g of polymer) - K dyn sⁿ cm^–2 consistency index - K ₁ constant in eq. (5) - K _P overall binding coefficient [g of bound solute/cm³ of solution]/[g of free solute/cm³ of polymer free solution] - n flow behavior index - M _Bg g mol^–1 molucular weight of liquid solvent - M _Pg g mol^–1 molecular weight of the polymer - M _Sg g mol^–1 Molecular weight of polymer solution (= M _BX_B+M_PX_P) - R cm³ atm g mol^–1 K^–1 ideal gas law constant - T K absolute temperature - V _Bcm³ g mol^–1 molar volume of liquid solvent - V _Pcm³ g mol^–1 molar volume of polymer - V _Scm³ g mol^–1 molar volume of polymer solution - X _B solvent molar fraction - X _P polymer molar fraction - polymer blockage shape factor - _P volume fraction of polymer in polymer solution - g cm^–1 s^–1 viscosity - _ag cm^–1 s^–1 apparent viscosity of the polymer solution - _icm³ g^–1 intrinsic viscosity - ₀ g cm^–1 s^–1 solvent viscosity - _Pg cm^–1 s^–1 polymer solution viscosity - _R relative viscosity (= / ₀) - ₌₀ g cm^–1 s^–1 viscosity of polymer solution obtained at zero shear rate - ₀ g cm^–3 water density