Accommodating the difference in students’ prior knowledge of cell growth kinetics

This paper describes the development and benefits of an adaptive digital module on cell growth to tackle the problem of educating a heterogeneous group of students at the beginning of an undergraduate course on process engineering. Aim of the digital module is to provide students with the minimal level of knowledge on cell growth kinetics they need to comprehend the content knowledge of the subsequent lectures and pass the exam. The module was organised to offer the subject matter in a differentiated manner, so that students could follow different learning paths. Two student groups were investigated, one consisting of students who had received their prior education abroad and one of students that had not. Exam scores, questionnaires, and logged user data of the two student groups were analysed to discover whether the digital module had the intended effect. The results indicate that students did indeed follow different learning paths. Also, the differences in exam scores between the two student groups that was present before the introduction of the digital module was found to have decreased afterwards. In general, students appreciated the use of the material regardless of their prior education. We therefore conclude that the use of adaptive digital learning material is a possible way to solve the problem of differences in prior education of students entering a course

On the Mass-Loss Rates of Massive Stars in the Low-Metallicity Galaxies IC 1613, WLM and NGC 3109

We present a spectroscopic analysis of VLT/X-Shooter observations of six O-type stars in the low-metallicity (Z ~ 1/7 Z\odot) galaxies IC 1613, WLM and NGC 3109. The stellar and wind parameters of these sources allow us, for the first time, to probe the mass-loss versus metallicity dependence of stellar winds below that of the Small Magellanic Cloud (at Z ~ 1/5Z\odot) by means of a modified wind momentum versus luminosity diagram. The wind strengths that we obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not agree with theoretical predictions. The objects in IC 1613 tend towards a higher than expected mass-loss rate, but remain consistent with predictions within their error bars. We discuss potential systematic uncertainties in the mass-loss determinations to explain our results. However, if further scrutinization of these findings point towards an intrinsic cause for this unexpected sub-SMC mass-loss behavior, implications would include a higher than anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity environments, but a reduced number of long-duration gamma-ray bursts produced through a single-star evolutionary channel.Comment: 9 pages, 3 figures; accepted for publication in The Astrophysical Journal Letter

Oxidation of azaheterocycles by free and immobilized xanthine oxidase and xanthine dehydrogenase

The objective of the study described in this thesis was to show that enzymes, especially immobilized enzymes, can be advantageously used in synthetic organic chemistry.In Chapter 1 enzymes are introduced and the advantages discussed when these highly active and specific biocatalysts are immobilized, i.e. , attached to a solid support. Criteria for general acceptation of a specific enzyme as a routine catalyst in the organic-chemistry laboratory are stated. The reasons for the choice of xanthine oxidase as a model enzyme and some relevant properties are given.The solid support used for the immobilization determines to a large extent the ultimate characteristics of the immobilized enzyme. In Chapter 2 properties of an ideal support and guidelines for the evaluation of a specific support arediscussed. Various supports are treated in some detail.The mode of immobilization is also very important, especially with respect to the eventual specific activity of the immobilized enzyme. In Chapter 3 the various methods of immobilization are discussed. Just as with the supports (Chapter 2), special attention is given to the ones investigated in this study.In Chapter 4 various properties of immobilized milk xantine oxidase are described, as well as the stabilization by coimmobilization with protein, superoxide dismutase and catalase. It is concluded that conversion of substrate on a preparativescale can be performed conveniently, but that for efficient application a more stable enzyme preparation is desired.Chicken-liver xanthine dehydrogenase is an enzyme very similar to milk xanthine oxidase. In Chapter 5 the reasons are given why it was expected that the liver enzyme would be more stable and practical for synthetic application. The immobilization and the properties of the free and immobilized enzyme are described. Analogous to the milk enzyme, the operational stability of xanthine dehydrogenase is much lower than the storage stability.In order to determine the substrate limits of xanthine oxidase more systematical ly, so that more-reliable predictions would be possible, series of substrates were synthesized for this purpose. In Chapter 6 the synthesis of 7-(p-X-phenyl) pteridin-4-ones and the oxidation by free and immobilized xanthine oxidase are presented. As X becomes more electron withdrawing, the rate of oxidation decreases. It is therefore concluded that the electron donation from substrate to enzyme must be the rate-limiting step.Convenient oxidation of the above substrates and easy product isolation was possible on a small preparative scale. The limited stability of xanthine oxidase demanded however a relatively large amount of enzyme. In Chapter 7 an easy and mild immobilization procedure, using gelatin as support and glutaraldehyde as cross-linking agent, is presented. Whole milk can be used as starting enzyme solution and no isolation is required. The resulting preparation of immobilized xanthine oxidase is very cheap, highly active, relatively stable and suitable for application in organic synthesis in a continuous manner.In Chapter 8 the improvement of the immobilization method with gelatin is described and the extension to the immobilization of whole cells, i.e. , Arthrobacter cells containing xanthine-oxidase activity. These cells were chosen because of their high specific activity, their substrate activation instead of inhibition and their expected higher stability. Immobilized Arthrobacter xanthineoxidase is indeed highly active, more stable and has a different substrate specificity than milk xanthine oxidase. Therefore, a wider spectrum of substrates can be conveniently and efficiently oxidized.In the final Chapter 9 additional information, mostly gathered at a later stage than the time of publication, is given and discussed.The end conclusion is: Xanthine oxidase, either from whole milk or Arthrobacter, when immobilized in glutaraldehyde-crosslinked gelatin, can be conveniently used for the oxidation of a wide range of substrates in a continuous fashion.<p/

The influence of student characteristics on the use of adaptive e-learning material

Adaptive e-learning materials can help teachers to educate heterogeneous student groups. This study provides empirical data about the way academic students differ in their learning when using adaptive elearning materials. Ninety-four students participated in the study. We determined characteristics in a heterogeneous student group by collecting demographic data and measuring motivation and prior knowledge. We also measured the learning paths students followed and learning strategies they used when working with adaptive e-learning material in a molecular biology course. We then combined these data to study if and how student characteristics relate to the learning paths and strategies they used. We observed that students did follow different learning paths. Gender did not have an effect, but (mainly Dutch) BSc students differed from (international) MSc students in the intrinsic motivation they had and the learning paths and strategies they followed when using the adaptive e-learning materia

The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109

Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution. In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models. We have obtained intermediate-resolution VLT/X-Shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram. The derived parameters of our target stars indicate stellar masses that reach values of up to 50 $M_{\odot}$. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is lessened, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8 figure

A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?

The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre- and near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive star forming region M17 to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods: We compute the RV dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the simulated sigma_rv distributions to the observed value. We specifically investigate two scenarios: a low binary fraction and a dearth of short-period binary systems. Results: Simulated populations with low binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv observed within their 68%-confidence intervals. Parent populations with f_bin > 0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level. Both constraints are contrast with the high binary fraction and plethora of short-period systems found in few Myr-old, OB-type populations. To explain the difference, the first scenario requires a variation of the outcome of the massive star formation process. In the the second scenario, compact binaries must form later on, and the cut-off period may be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. Conclusions: If the obtained constraints are representative of the overall properties of massive young stellar objects, our results may provide support to a formation process in which binaries are initially formed at larger separations, then harden or migrate to produce the typical (untruncated) power-law period distribution observed in few Myr-old OB binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics Letter

O-star mass-loss rates at low metallicity

Mass fluxes J are computed for the extragalactic O stars investigated by Tramper et al. (2011; TSKK). For one early-type O star, computed and observed rates agree within errors. However, for two late-type O stars, theoretical mass-loss rates underpredict observed rates by ~ 1.6 dex, far exceeding observational errors. A likely cause of the discrepancy is overestimated observed rates due to the neglect of wind-clumping. A less likely but intriguing possibility is that, in observing O stars with Z/Z_sun ~ 1/7, TSKK have serendipitously discovered an additional mass-loss mechanism not evident in the spectra of Galactic O stars with powerful radiation-driven winds. Constraints on this unknown mechanism are discussed. In establishing that the discrepancies, if real, are inescapable for purely radiation-driven winds, failed searches for high-J solutions are reported and the importance of a numerical technique that cannot spuriously create or destroy momentum stressed. The Z-dependences of the computed rates for Z/Z_sun in the interval (1/30, 2) show significant departures from a single power law, and these are attributed to curve-of-growth effects in the differentially-expanding reversing layers. The best-fitting power-law exponents range from 0.68-0.97.Comment: 6 pages, 2 figure

The mass of the very massive binary WR21a

We present multi-epoch spectroscopic observations of the massive binary system WR21a, which include the January 2011 periastron passage. Our spectra reveal multiple SB2 lines and facilitate an accurate determination of the orbit and the spectral types of the components. We obtain minimum masses of $64.4\pm4.8 \ M_{\odot}$ and $36.3\pm1.7 \ M_{\odot}$ for the two components of WR21a. Using disentangled spectra of the individual components, we derive spectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary, respectively. Using the spectral type of the secondary as an indication for its mass, we estimate an orbital inclination of $i=58.8\pm2.5^{\mathrm{o}}$ and absolute masses of $103.6\pm10.2 \ M_{\odot}$ and $58.3\pm3.7 \ M_{\odot}$, in agreement with the luminosity of the system. The spectral types of the WR21a components indicate that the stars are very young (1$-$2 Myr), similar to the age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine the mass-luminosity relation for the total system mass. We find that for a distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in good agreement with those from evolutionary predictions.Comment: 7 pages, 4 figures; accepted for publication in MNRA

Development and Evaluation of an Adaptive Digital Module on Enzyme Kinetics

An adaptive module on basic enzyme kinetics was developed for first- and second-year university students. The module offers more assignments to students who have less knowledge of the theory than to more advanced students. The aim of the research was to investigate what influence students’ backgrounds have on their use and appreciation of this module. Both freshmen and second-year students showed a large variation in the number of assignments they needed to perform in order to finish the module, indicating that the module’s adaptive feature was exploited by all the students. Findings indicated that the prior knowledge was of influence of students’ motivation and perception of difficulty of the modul