Instabilities in the transient response of muscle

We investigate the isometric transient response of muscle using a quantitative stochastic model of the actomyosin cycle based on the swinging lever-arm hypothesis. We first consider a single pair of filaments, and show that when values of parameters such as the lever-arm displacement and the crossbridge elasticity are chosen to provide effective energy transduction, the T2 curve (the tension recovered immediately after a step displacement) displays a region of negative slope. If filament compliance and the discrete nature of the binding sites are taken into account, the negative slope is diminished, but not eliminated. This implies that there is an instability in the dynamics of individual half-sarcomeres. However, when the symmetric nature of whole sarcomeres is taken into account, filament rearrangement becomes important during the transient: as tension is recovered, some half-sarcomeres lengthen while others shorten. This leads to a flat T2 curve, as observed experimentally. In addition, we investigate the isotonic transient response and show that for a range of parameter values the model displays damped oscillations, as recently observed in experiments on single muscle fibers. We conclude that it is essential to consider the collective dynamics of many sarcomeres, rather than the dynamics of a single pair of filaments, when interpreting the transient response of muscle.Comment: 11 pages, 11 figures, Submitted to Biophysical Journa

Measuring the uncertainty associated with estimating national photovoltaic electricity generation: A Great Britain case study

Monitoring near real-time national solar PV output is an increasingly important part of operating an electricity system. Monitoring PV output requires knowledge of the output of many PV systems embedded in the distribution network whose generation are not directly visible through existing transmission system metering. In this thesis a review of 27 national solar PV monitoring services which provide national PV output estimates for 20 different countries was performed showing that every service follows the same general approach. First, the PV yield is modelled using a set of data from reference PV systems providing data in real-time. Then the modelled PV yield is scaled by an estimate of the national solar PV capacity to estimate the national PV output. National PV output is then used, along with similar measurements for other embedded technologies such as wind, to train and validate electricity forecasts which ensure efficient electricity market operation. Using Great Britain as a case study, the total error and uncertainty associated with the estimates from a national PV monitoring service are analysed. There are three main sources of error which contribute to the overall error in the national PV output estimates; the sample bias error, the statistical error in the yield model, and the error in the national capacity estimate. For the GB PV monitoring service, the domestic sample was shown to be unbiased for estimating national PV output. However, at a regional level the domestic sample used in the GB service is biased for estimating commercial/utility PV systems. The statistical error in the yield model was shown to be $\pm1\%$ providing that a sample size of at least 6000 was used. The error in the GB national capacity estimate was shown to be $\pm5\%$. I can conclude that, the capacity error, at $\pm5\%$, dominates the yield calculation error, at $< \pm1\%$ and leads to an overall error in GB solar PV output estimates of $\pm5.1\%$. I also conclude that solar PV measurements, and consequently national electricity demand forecasts, are currently limited by the state of national PV capacity registers

Studying Transition Metal Chemistry inside a Metal-Organic Framework

Metal-organic Frameworks (MOFs) are porous, crystalline materials built from metal ‘nodes’ and interconnecting organic ligands. The combination of crystallinity, porosity and building block design allows MOFs to be tailored at the nanoscale and functionalised to suit specific applications. Reactive metal complexes can be installed inside MOFs, producing well-defined reactive sites with long range order, thereby allowing metal-centred chemical processes to be studied in-situ via X-ray crystallography. Furthermore, the physical isolation of active metal complexes prevents unwanted side-reactions such as cluster formation from occurring, allowing reactive species to be trapped within the crystalline matrix while the surrounding microenvironment can be tuned, via judicious ligand design, to augment their reactivity. The Mn-based MOF [Mn3(L)2(L')] (where L = bis-(4-carboxyphenyl-3,5-dimethylpyrazolyl)methane; 1) is well suited to this application because its pores are decorated with well-defined N,N-chelation sites that bind metal complexes, which allows 1 to be post-synthetically metalated and the resulting metal complexes can be studied using X-ray crystallography. In chapter 2, a series of transition metal nitrate complexes were incorporated within 1 and their structures determined. The structural features of the complexes were compared to those of solution and solid-state analogues to elucidate the effect of the MOF pore-environment on their coordination chemistry. The distribution of metal sites within a MOF framework is important to consider for heterogenous catalysis applications; quantitative metalation can result in the MOF pores becoming ‘burdened’ with metal sites that impede mass transport through the crystal. Work in chapter 3 demonstrates that by using a mixed ligand synthesis approach, the ligand bearing the free N,N-chelating site in 1 could be partially replaced with a modified ligand that is incapable of metalation. The resulting structure possesses the same topology as 1 but is doped, specifically in the non-coordinated donor sites, with a ligand that will not readily bind to metal complexes, allowing the overall degree of metalation to be tuned. Work in chapter 4 utilised the observation that within 1 the metal complexes are site-isolated and separated by 13 Å. By incorporating a Mn(I) azide complex within 1, the site-isolation was harnessed to perform site-selective ‘click’ chemistry on small dialkynes that are shorter than the azide separation. Within 1, this ‘click’ chemistry cycle, using both simple mono- and di-alkynes, was monitored using X-ray crystallography. This work demonstrates that the nanoscale spatial control of reactive sites within MOFs, supported with X-ray crystallographic insights, can affect highly selective chemical transformations. Finally in chapter 5, advancing the concept of site-isolation within MOFs, 1 was functionalised with a Mn(I) carbonyl complex. When exposed to visible light the complex releases a portion of its carbonyl ligands which escape the porous crystalline lattice. It was envisaged that photolysis could generate site-isolated, reactive metal complexes that can activate small molecules. Preliminary experiments demonstrated that CO is successfully liberated under photolysis, while in-situ X-ray crystallography suggests that the CO ligands are replaced by weakly coordinating solvent molecules. In this way, 1 acts as a matrix for isolating and studying the reactive metal complexes formed using photolysis.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 201

Instrucción pública: lo que es i lo que debería ser

Reflexiona en torno a la educación popular, sus principios y administración. Evalúa las perspectivas de sitintos sectores de la sociedad respecto de la expansión de la educación popular. Desarrolla la idea de educación pública por niveles: primaria, secundaria y universitaria

Force Dependence of the Michaelis Constant in a Two-State Ratchet Model for Molecular Motors

We present a quantitative analysis of recent data on the kinetics of ATP hydrolysis, which has presented a puzzle regarding the load dependence of the Michaelis constant. Within the framework of coarse grained two-state ratchet models, our analysis not only explains the puzzling data, but provides a modified Michaelis law, which could be useful as a guide for future experiments.Comment: 4 pages, 3 eps figures, accepted for publication on Physical Review Letter

Force-Velocity Relations of a Two-State Crossbridge Model for Molecular Motors

We discuss the force-velocity relations obtained in a two-state crossbridge model for molecular motors. They can be calculated analytically in two limiting cases: for a large number and for one pair of motors. The effect of the strain-dependent detachment rate on the motor characteristics is studied. It can lead to linear, myosin-like, kinesin-like and anomalous curves. In particular, we specify the conditions under which oscillatory behavior may be found.Comment: 5 pages, 4 figures, REVTeX; thoroughly revised version; also available at http://www.physik.tu-muenchen.de/~frey

Quotations

A series of quotations by Thomas H. Huxley and J. J. Rousseau

Lameness in cattle: recent research to inform clinical practice

Lameness in cattle has significant consequences for welfare, health and productivity. More research is now being done on lameness and this article, the first in a two-part series, provides an update on research-based advances in the field published from around the world over the past five years. These developments have improved our understanding of lameness in cattle and can inform clinical practice and the control of lameness on-farm. The second article, to be published in a subsequent issue of In Practice, will cover the understanding and management of claw health and claw horn lesions

Hume, with helps to the study of Berkeley.

"Authorized edition."Mode of access: Internet