Role of lipids in theNeurospora crassa membrane. I. Influence of fatty acid composition on membrane lipid phase transitions

Summary The relationship between lipid composition and phase transition was investigated by differential scanning calorimetry for intact and membrane phospholipid extracts of wild-type (w/t) and thecel ^– (Tw 40) mutant ofNeurospora crassa. Thecel ^– (Tw 40) mutant (grown on minimal, sucrose medium supplemented with Tween 40 at 34 °C) had approximately twice the saturated fatty acid content ofw/t organisms grown at 22 °C. The gel-liquid crystal phase transitions of ergosterol-free extracts derived fromw/t andcel ^– (Tw 40) occur at –31 and –11 °C, respectively. The heats of transition (H) of these extracts were 1 and 13 cal/g, respectively. The addition of ergosterol (the predominant sterol inNeurospora) to the phospholipid extracts decreased the observed heats of transition, but did not alter the transition temperature. IntactNeurospora, whetherw/t orcel ^– (Tw 40) did not manifest similar gel-liquid crystal phase transitions in the differential scanning calorimeter. However, an endothermic peak at approximately 30 °C was observed in intact cells and extracted phospholipids of bothw/t andcel ^– (Tw 40) organisms. This peak was insensitive to the addition of ergosterol, had a low heat content (H1 cal/g), and was reversible.     

Seasonal and spatial variation of woody tissue respiration in a <Emphasis Type="Italic">Pinus cembra</Emphasis> tree at the alpine timberline in the central Austrian Alps

Total stem, branch, twig, and coarse root respiration (R_t) of an adult Pinus cembra tree at the alpine timberline was measured continuously at ten positions from 7 October 2001 to 21 January 2003 with an automated multiplexing gas exchange system. There was a significant spatial variability in woody tissue respiration when expressed per unit surface area or per unit sapwood volume. Surface area related maintenance (R_m) respiration at 0°C ranged between 0.109 and 0.643 mol m^–2 s^–1 and there was no clear trend with respect to tissue type and diameter. Sapwood volume based R_m at 0°C by contrast, varied between 2.5 mol m^–3 s^–1 in the stem and 193.2 mol m^–3 s^–1 in thin twigs in the upper crown. Estimated Q₁₀ values ranged from 1.7 to 3.1. These Q₁₀ values were used along with R_m at 0°C and annual woody tissue temperature records to predict annual total R_m. Annual total R_m accounted for 73±6% of annual R_t in 2002.     

Regulation of local sweating in sleep-deprived exercising humans

Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

Electrophysiological properties ofDictyostelium derived from membrane potential measurements with microelectrodes

Summary Electrical membrane properties of the cellular slime moldDictyostelium discoideum were investigated with the use of intracellular microelectrodes. The rapid potential transients (1 msec) upon microelectrode penetration of normal cells had a negative-going peak-shaped time course. This indicates that penetration of a cell with a microelectrode causes a rapid depolarization, which can just be recorded by the microelectrode itself. Therefore, the initial (negative) peak potential transient valueE _p (–19 mV) should be used as an indicator of the resting membrane potentialE _m ofD. discoideum before impalement, rather than the subsequent semistationary depolarized valueE _n (–5 mV). Using enlarged cells such as giant mutant cells (E _p=–39 mV) and electrofused normal cells (E _p=–30 mV) improved the reliability ofE _p as an indicator ofE _m. From the data we concluded thatE _m ofD. discoideum cells bathed in (mm) 40 NaCl, 5 KCl and 1 CaCl₂ is at least –50 mV. This potential was shown to be dependent on extracellular potassium. The average input resistanceR _i of the impaled cells was 56 M for normalD. discoideum. However, our analysis indicates that the membrane resistance of these cells before impalement is >1 G. Specific membrane capacitance was 1–3 pF/cm². Long-term recording of the membrane potential showed the existence of a transient hyperpolarization following the rapid impalement transient. This hyperpolarization was associated with an increase inR _i of the impaled cell. It was followed by a depolarization, which was associated with a decrease inR _i. The depolarization time was dependent on the filling of the microelectrode. The present characterization of the electrical membrane properties ofDictyostelium cells is a first step in a membrane electrophysiological analysis of signal transduction in cellular slime molds.     

Determination of the overall volumetric mass transfer coefficient kLa,by a temperature dependent change of gas solubility

Summary The solubility of oxygen in the liquid phase of a bioreactor was changed by a ramp change of temperature, and k_La was determined from the resulting return to equilibrium of dissolved oxygen activity. The maximum k_La that can be measured by this method in a standard laboratory scale bioreactor is 145 h^–1 corresponding to a temperature change rate of 320°C h^–1.Nomenclature p Difference between p_G and p_L (% saturation) - T Ramp change of temperature (°C) - E Temperature-compensated output from the oxygen electrode (A) - E_u Uncompensated output from the oxygen electrode (A) - k_La Overall volumetric mass transfer coefficient (h^–1) - k_La_Tm Overall volumetric mass transfer coefficient at temperature T_m (h^–1) - P_G Dissolved oxygen activity in equilibrium with the gas phase (% saturation) - p_L Dissolved oxygen activity (% saturation) - p_Lm Dissolved oxygen activity at time t_m (% saturation) - t Time (h) - t_m Time of maximum p (h) - T Temperature (°C) - T_cal Calibration temperature of the oxygen electrode (°C) - T_m Final temperature after a temperature shift (°C) - T_n Temperature at time t_n     

Role of lipids in the Neurospora crassa membrane. II. Membrane potential and resistance studies; the effect of altered fatty acid composition on the electrical properties of the cell membrane.

The effect of doubling the saturated fatty acid content on the electrophysiology of Neurospora crassa membranes was studied. Intracellular membrane input resistance (Rm) and potential (Em) were measured for wild-type (w/t) and cel- (Tween 40) organisms as a function of temperature. Over the 0 to 40 degrees C temperature range studied, mean Em values of both w/t and cel- (Tw 40) organisms increased from -160 to -210 mV. This difference is greater than that expected from Nernst potential considerations, indicating an active component of Em. This active component is insensitive to a doubling of the saturated fatty acid content. Rm exhibits a temperature dependence and hysteresis. Averaged data indicate an increase in Rm with decreased temperature. The slope of the temperature dependence varies among individual hyphae. Above 17.5 degrees C cel- (Tw 40) hyphae averaged greater than 70% higher values of Rm than w/t. Below 17.5 degrees C w/t Rm data divided into low and high temperature dependence groups, while cel- data exhibited a low temperature dependence. The results are discussed in relation to gel-liquid crystal phase transitions, membrane fluidity, and the contribution of fatty acid structure to membrane electrical properties.     

Pygoscelid penguins in a swim canal

Summary In Antarctica, we investigated the energy consumption of Adélie (Pygoscelis adeliae), Gentoo (P. papua) and Chinstrap (P. antarctica) penguins while resting in the water (8.4 W-kg^–1) and swimming underwater at various speeds, using a 21m long canal filled with sea-water at 4°C in conjunction with respirometry. The birds swam at will and consumed 15.7, 16.1 and 10 W·kg^–1 at the speed where cost of transport was minimal (2.1, 2.3 and 2.5 m·s^–1 in Adélie, Gentoo and Chinstrap penguins, respectively). Thermal conductance in pygoscelid penguins was 3.3 W·°C^–1. m^–2 and energy expenditure (P_i, W·kg^–1) while resting in the water is given by P_j = -0.3 t_a+9.6, where t_a is water temperature in °C. During the breeding season, pygoscelid penguins spend 25–40% of their daily energy expenditure while foraging at sea. The importance of accurate estimates of at-sea activity and energy consumption is discussed.     

Respiration of individual honeybee larvae in relation to age and ambient temperature

The CO₂ production of individual larvae of Apis mellifera carnica, which were incubated within their cells at a natural air humidity of 60–80%, was determined by an open-flow gas analyzer in relation to larval age and ambient temperature. In larvae incubated at 34 °C the amount of CO₂ produced appeared to fall only moderately from 3.89±1.57 µl mg^–1 h^–1 in 0.5-day-old larvae to 2.98±0.57 µl mg^–1 h^–1 in 3.5-day-old larvae. The decline was steeper up to an age of 5.5 days (0.95±1.15 µl mg^–1 h^–1). Our measurements show that the respiration and energy turnover of larvae younger than about 80 h is considerably lower (up to 35%) than expected from extrapolations of data determined in older larvae. The temperature dependency of CO₂ production was determined in 3.5-day-old larvae, which were incubated at temperatures varying from 18 to 38 °C in steps of 4 °C. The larvae generated 0.48±0.03 µl mg^–1 h^–1 CO₂ at 18 °C, and 3.97±0.50 µl mg^–1 h^–1 CO₂ at 38 °C. The temperature-dependent respiration rate was fitted to a logistic curve. We found that the inflection point of this curve (32.5 °C) is below the normal brood nest temperature (33–36 °C). The average Q₁₀ was 3.13, which is higher than in freshly emerged resting honeybees but similar to adult bees. This strong temperature dependency enables the bees to speed up brood development by achieving high temperatures. On the other hand, the results suggest that the strong temperature dependency forces the bees to maintain thermal homeostasis of the brood nest to avoid delayed brood development during periods of low temperature.Abbreviations m body mass - R rate of development or respiration - T_I inflexion point of a logistic (sigmoid) curve - T_L lethal temperature - T_O temperature of optimum (maximum) developmentCommunicated by G. Heldmaier     

Regeneration of resistance and ion transport in rabbit corneal epithelium after induced surface cell exfoliation

Summary Exposure of the rabbit corneal surface to a 20-m digitonin-0.9% NaCl solution leads to permeabilization of the most superficial cells of the stratified epithelium. The devitalized cells exfoliate spontaneously from the corneal surface. Detergent exposure limited to 4–8 min leads to permeabilization and rapid exfoliation of a monolayer of surface cells. Consistent with the presence of the epithelial paracellular permeability barrier in this cell layer, their permeabilization results in complete loss of transepithelial resistance (R _t ). Within minutes after detergent removal an initial recovery ofR _t can be noticed indicating generation of a new paracellular permeability barrier by the viable subsurface cells. This recovery proceeds rapidly andR _t reaches within 70 min a maximum equal to > 90% of the preexfoliation values (=2.43 k·cm²,n=22). TheR _t recovery is fully blocked in a reversible manner by 10 m dihydrocytochalasin B. The recovery is not affected by inhibition of protein synthesis with 5 m cycloheximide. When the ocular surface is treated again with digitonin the permeabilization and exfoliation of a monolayer of cells and loss ofR _t are repeated. After the second detergent exposure an initial recovery ofR _t occurs as before within minutes. However, the pace ofR _t recovery is much slower: 4–5 hr are required to reach a stable maximalR _t values amounting to about 73% of initial control. This recovery can be fully blocked by 5 m cycloheximide indicating that protein synthesis is required for generation of tight junctions by the second subcellular layer. With only a fraction ofR _t recovered, short-circuit currents amounting to, at least, 50% of control values and attributable in part to cell-to-tear movement of Cl^– through the apical surface can be measured. This suggests that apical-type Cl^– channels are either present in the apically facing membrane of subsurface cells or that they are rapidly inserted in it from preexisting intracellular pools immediately following the devitalization of the surface cells by digitonin.     

Measurement of axonal membrane conductances and capacity by means of a varying potential control voltage clamp

Summary A new mode of voltage clamping in the squid giant axon is introduced and its advantages are analyzed, tested, and utilized to investigate membrane conductances and capacity. This method replaces the constant command potentials of the standard voltage clamp with potentials which vary with time. Some of the advantages in using the varying potential clamp are: (1) slowly varying potentials generate practically pureI _K ; (2) rapidly varying potentials generate practically pureI _Na; (3) triangular waves generate, under proper conditions, pure capacity currents and easy-to-analyze leakage currents; (4) the method gives direct, on-line display of sodium or potassium I–V characteristics within milliseconds; (5) it enables rapid and accurateE _Na andE _K determinations; and (6) it enables simple and accurate determination ofC _m. The method was utilized to study the effects of various ions on membrane conductances and the effects of ionic composition, ionic strength, and temperature on membrane capacity. Membrane capacity was found to be practically independent of frequency in the 200 to 2,000 Hz range. Replacement of external sodium by Ca⁺⁺, by impermeable Tris⁺, or even by dextrose or sucrose (low ionic-strength solutions) had negligible effects onC _m.C _m showed a small, positive temperature coefficient of 1.39% per °C in the 3 to 21°C range, and little change with temperature in the 20 to 40°C range. Above 40°C, bothC _m andg _L increased considerably with temperature.     

Temperature Effect on Carbachol-Induced Depression of Spontaneous Quantal Transmitter Release in Frog Neuromuscular Junction

The effects of carbachol (CCh) on the frequency (f) of the miniature endplate potentials were tested at temperatures between 5 and 30°C. Higher CCh concentrations, 1 × 10^–5 and 5 × 10^–6 M, reduced the f to 60% and the temperature dependence was negligible. However, an inverse temperature dependence was found when low concentrations 3 × 10^–7 and 6 × 10^–7 M were applied. The depression of f was 40–50% in 5–10°C but only 10–20% of the control in the 25 and 30°C. During application of CCh, the new steady of f was reached at temperatures between 5 and 30°C within 17–20 min (Q₁₀ = 1.07). Much greater temperature dependence of recovery was observed during washing out CCh (Q₁₀ = 1.6). The temperature-independence of the steady state effects of CCh, good agreement with Langmuir adsorption-desorption theory and non-steady kinetics indicate that physical rather than receptor-mediated events are responsible for the depression of f.     

Response of chloride efflux from skeletal muscle ofRana pipiens to changes of temperature and membrane potential and diethylpyrocarbonate treatment

Summary Efflux of³⁶Cl^– from frog sartorius muscles equilibrated in two depolarizing solutions was measured. Cl^– efflux consists of a component present at low pH and a pH-dependent component which increases as external pH increases.For temperatures between 0 and 20°C, the measured activation energy is 7.5 kcal/mol for Cl^– efflux at pH 5 and 12.6 kcal/mol for the pH-dependent Cl^– efflux. The pH-dependent Cl^– efflux can be described by the relationu=1/(1+10n(pK _a -pH)), whereu is the Cl^– efflux increment obtained on stepping from pH 5 to the test pH, normalized with respect to the increment obtained on stepping from pH 5 to 8.5 or 9.0. For muscles equilibrated in solutions containing 150mm KCl plus 120mm NaCl (internal potential about –15 mV), the apparent pK _a is 6.5 at both 0 and 20°C, andn=2.5 for 0°C and 1.5 for 20°C. For muscles equilibrated in solutions containing 7.5mm KCl plus 120mm NaCl (internal potential about –65 mV), the apparent pK _a at 0°C is 6.9 andn is 1.5. The voltage dependence of the apparent pK _a suggests that the critical pH-sensitive moiety producing the pH-dependent Cl^– efflux is sensitive to the membrane electric field, while the insensitivity to temperature suggests that the apparent heat of ionization of this moiety is zero. The fact thatn is greater than 1 suggests that cooperativity between pH-sensitive moieties is involved in determining the Cl^– efflux increment on raising external pH.The histidine-modifying reagent diethylpyrocarbonate (DEPC) applied at pH 6 reduces the pH-dependent Cl^– efflux according to the relation, efflux=exp(–k·[DEPC]·t), wheret is the exposure time (min) to DEPC at a prepared initial concentration of [DEPC] (mm). At 17°C,k ^–1=188mm·min. For temperatures between 10 and 23°C,k has an apparent Q₁₀ of 2.5. The Cl^– efflux inhibitor SCN^– at a concentration of 20mm substantially retards the reduction of the pH-dependent Cl^– efflux by DEPC. The findings that the apparent pK _a is 6.5 in depolarized muscles, that DEPC eliminates the pH-dependent Cl^– efflux, and that this action is retarded by SCN^– supports the notion that protonation of histidine groups associated with Cl^– channels is the controlling reaction for the pH-dependent Cl^– efflux.     

Ion channel involvement in the temperature-sensitive response of the rabbit corneal endothelial cell resting membrane potential

Previous studies have shown that the resting potential (E _m) of the corneal endothelium hyperpolarizes following an increase in temperature above 24°C. Whole-cell studies using the perforated-patch technique were used to compare currents and E _mvalues from isolated corneal endothelial cells at 24 and 32°C. These studies revealed a small, outwardly rectifying, slowly activating, weakly voltage-dependent current with a reversal potential showing K⁺ selectivity (E _rev = –80 mV). This current had features similar to the whole-cell current seen following addition of HCO₃ ^– to these cells. E _mmeasurements found an average 24 mV hyperpolarization following temperature elevation in NaCl Ringer. Single channel studies found the only change in channel activity following an elevation in temperature to be an increase in the open probability (P _o) of a K⁺ channel previously reported in this cell type to be activated by external anions. P _o(–30 mV) at 24 and 32°C equaled 0.003 and 0.06, respectively. Increases in P _owere found at all voltages examined. This increased P _ocan account for the magnitude of the hyperpolarization seen in these cells following temperature elevation. Addition of HCO₃ ^– along with elevated temperature produced a synergistic effect on the increase in P _oalong with an increased hyperpolarization of the cell, pointing to separate mechanisms of activation from these two stimuli.The authors would like to thank Ms. Helen Hendrickson for her technical support and Drs. Gianrico Farrugia and Adam Rich for their helpful comments. This work was supported by NIH grants EY09673, EY03282, EY06005, and an unrestricted award from Research to Prevent Blindness.     

Different arachidonate and palmitate binding capacities of the human red cell membrane

Human red cell membrane bindings of arachidonate and palmitate at pH 7.3 are investigated at temperatures between 0 and 38°C by equilibrating ghosts with the long-chain fatty acids bound to bovine serum albumin in molar ratios (v) within the physiological range (<1.7). Linearized relations of ghost uptakes and fatty acid monomer concentrations in buffer provide estimates of the binding capacities and corresponding equilibrium dissociation constants (K _dm ). The temperature-independent arachidonate binding capacity, 5.5 ± 0.5 nmol g^–1 packed ghosts, is approximately fivefold smaller than that of palmitate, 26.6 ± 2.0 nmol g^–1. While K _dm of arachidonate binding 5.1 ± 0.5 nm is temperature independent, K _dm of palmitate increases with temperature from 3.7 nm at 0°C to 12.7 nm at 38°C.The large difference in binding capacities suggests the presence of at least two different fatty acid binding domains in human red cell membranes.     

Voltage dependence of the Ca2+-activated K+ conductance of human red cell membranes is strongly dependent on the extracellular K+ concentration

Summary The conductance of the Ca²⁺-activated K⁺ channel (g _K(Ca)) of the human red cell membrane was studied as a function of membrane potential (V _m ) and extracellular K⁺ concentration ([K⁺]_ex). ATP-depleted cells, with fixed values of cellular K⁺ (145mm) and pH (7.1), and preloaded with 27 m ionized Ca were transferred, with open K⁺ channels, to buffer-free salt solutions with given K⁺ concentrations. Outward-current conductances were calculated from initial net effluxes of K⁺, correspondingV _m , monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (E _K) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of⁴²K at (V _m –E _K)0, using a single-file flux ratio exponent of 2.7. Within a [K⁺]_ex range of 5.5 to 60mm and at (V _m –E _K) 20 mV a basic conductance, which was independent of [K⁺]_ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 S/cm² between 0 and –100 mV. As (V _m –E _K) decreased from 20 towards zero mVg _K(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 S/cm². The zero-current conductance was not significantly dependent on [K⁺]_ex (30 to 156mm) corresponding toV _m (–50 mV to 0). A further increase ing _K(Ca) symmetrically aroundE _K is suggested as (V _m –E _K) becomes positive. Increasing the extracellular K⁺ concentration from zero and up to 3mm resulted in an increase ing _K(Ca) from 50 to 70 S/cm². Since the driving force (V _m –E _K) was larger than 20 mV within this range of [K⁺]_ex this was probably a specific K⁺ activation ofg _K(Ca). In conclusion: The Ca²⁺-activated K⁺ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.     

Optimum temperature policy for batch reactors performing biochemical reactions in the presence of enzyme deactivation

A necessary condition is found for the optimum temperature policy which leads to the minimum reaction time for a given final conversion of substrate in a well stirred, enzymatic batch reactor performing an enzyme-catalyzed reaction following Michaelis-Menten kinetics in the presence of first order enzyme decay. The reasoning, which is based on Euler's classical approach to variational calculus, is relevant for the predesign steps because it indicates in a simple fashion which temperature program should be followed in order to obtain the maximum advantage of existing enzyme using the type of reactor usually elected by technologists in the fine biochemistry field. In order to highlight the relevance and applicability of the work reported here, the case of optimality under isothermal operating conditions is considered and a practical example is worked out.List of Symbols C _E mol.m^–3 concentration of active enzyme - C _E ^* dimensionless counterpart of C_E - C _E,0 mol.m^–3 initial concentration of active enzyme - C _E,b mol.m^–3 final concentration of active enzyme - C _E,opt ^* optimal dimensionless counterpart of C_E - C _smol.m^–3 concentration of substrate - C _S ^Emphasis>/* dimensionless counterpart of C_S - C _S,0mol.m^–3 initial concentration of substrate - C _S,bmol.m^–3 final concentration of substrate - E enzyme in active form - E ₃ ^* dimensionless counterpart of E_a,3 - E _a,1J.mol^–1 activation energy associated with k₁ - E _a,3J.mol^–1 activation energy associated with k₃ - E _d enzyme in deactivated form - ES enzyme/substrate complex - k ₁ s^–1 kinetic constant associated with the enzyme-catalyzed transformation of substrate - k _1,0 s^–1 preexponential factor associated with k₁ - k ₂ mol^–1.m³s^–1 kinetic constant associated with the binding of substrate to the enzyme - k _–2 s^–1 kinetic constant associated with the dissociation of the enzyme/substrate complex - K _2,0 mol.m^–3 constant value of K₂ - K _2,0 ^* dimensionless counterpart of K_2,0 - k ₃ s^–1 kinetic constant associated with the deactivation of enzyme - k _3,0 s^–1 preexponential factor associated with k₃ - k _3,0 ^* dimensionless counterpart of k_3,0 - P product - R J.K^–1.mol^–1 ideal gas constant - S substrate - t s time since start-up of reaction - T K absolute temperature - T ^* dimensionless absolute temperature - T _i,opt ^* optimal dimensionless isothermal temperature of operation - T _opt ^* optimal dimensionless temperature of operation - t _b s time of a batch - t _b ^* dimensionless counterpart of t_b - t _b,min ^* minimum value of the dimensionless counterpart of t_b Greek Symbols dimensionless counterpart of C_E,0 - dimensionless counterpart of C_E,b - dummmy variable of integration - dummy variable of integration - auxiliary dimensionless variable - ^* dimensionless variation of k₁ with temperature - _i ^* dimensionless value of k₁ under isothermal conditions - _opt ^* optimal dimensionless variation of k₁ with temperature     

Optimization of a packed bed bioreactor with immobilized cells using experimental design

A central composite design was employed for the optimization of heterogeneous enzymatic hydrolysis of sucrose. The reaction was catalyzed by whole yeast cells of Saccharomyces cerevisiae immobilized in Ca-pectate gel. Bioreactor volumetric productivity was chosen as an optimization criterion, while temperature and gel biomass concentration were optimization parameters. Sucrose inlet concentration of 700 kg m^–3 and outlet conversion of 65% were constant in all experiments. In the temperature range 51–73 °C and biomass concentration range 11–39 kg m^–3 (dry mass of cells), the dependence of bioreactor productivity on the two factors was described by a second order polynom regression equation. No simple optimum was revealed by the experimental design. The bioreactor productivity increased within the whole experimental range of biomass concentration, whereas a temperature optimum was found to be between 60 and 65 °C.List of Symbols b _j jth regression coefficient - c _Si kg m^–3 inlet sucrose concentration - F m³ min^–1 flow rate - F F distribution - f _LF degrees of freedom of lack of fit variance - f _P degrees of freedom of pure error variance - N total number of runs - n ₀ number of runs in the centre of design - P kg m^–3 min^–1 productivity - s _LF ² lack of fit variance - SS _LF lack of fit sum of squares - S _p ² pure error variance - SS _P pure error sum of squares - SS _R total residual sum of squares - V _b m³ bioreactor bed volume - X _O outlet conversion - x ₁ 1st factor - coded temperature - x ₂ 2nd factor - coded biomass concentration - y kgm^–3min^–1 measured response (productivity) - kg m^–3 min^–1 estimated response (productivity) - y _Oi kg m^–3 min^–1 measured response in the centre of design - ¯y ₀ kg m^–3 min^–1 average of response in the centre of design     

Planktonic rotifers and temperature

The influence of temperature (t) upon rotifer embryonic development rate (D_e) has been analysed using data from the literature, and the author's own results from experimental and natural populations. For Keratella cochlearis (Gosse), within the temperature range of 1–28°C, this relationship is best expressed by the equation: 1/D_e = 0.002 + 0.00025t + 0.000065t².For Brachionus calyciflorus Pallas, between 8°C and 35°C, the best relationship is given by the equation: 1/D_e = 0.005 + 0.00013t + 0.00013t².Increasing the incubation temperature to 37–40°C resulted in a decrease in development rate and a sharp reduction in life length.Analysis of the relationship between respiration rate and temperature in experimental and natural populations of Brachionus calyciflorus and Hexarthra mira (Hudson) showed that the maximum rate of oxygen consumption occurred at 32–33°C.The effects of temperature upon the ingestion rates of rotifers is greatly influenced by food concentration. Consequently, this factor also influences the secondary production of experimental populations at different temperatures.     

Carbon dioxide and water vapor exchange in a warm temperate grassland

Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation (A _c) and ecosystem respiration (R _E), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A _c and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m^–2 a^–1) due to a minor imbalance between large A _c (–1,202 g C m^–2 a^–1) and R _E (1,299 g C m^–2 a^–1) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (<0.2 m³m^–3), which influenced A _c and thereby NEE by decreasing stomatal conductance. For this experiment, low had minor impact on R _E. Thus, stomatal limitations to A _c were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of –65 g C m^–2 a^–1 assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.     

Analog circuit of theAcetabularia membrane

Summary The high membrane potential ofAcetabularia (E _m=–170 mV) is due to an electrogenic pump in parallel with the passive diffusion system (E _d=–80 mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive forceE _p with two elementsP ₁ andP ₂ in series;P ₂ is shunted by a large capacitance (C _p=3 mF cm^–2). The nonlinear current-voltage relationship ofP ₁ (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value ofE _p (–190 mV) indicates a stoichiometry of 21 between electrogenically transported charges and ATP. The electrical energy, normally stored inC _p, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship ofP ₂ (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 W cm^–2) amounts to some percent of the total energy turnover. The high Cl^– fluxes (1 nmol cm^–2 sec^–1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl^– pathway between the vacuole and outside is postulated to exist.