Kinetic models using enzyme kinetics are developed for the three ways that Louie proved that Rosen’s minimal (M-R)-System can be closed to efficient cause; i.e., how the “replication” component can itself be entailed from within the system. The kinetic models are developed using the techniques of network thermodynamics. As a demonstration, each model is simulated using a SPICE circuit simulator using arbitrarily chosen rate constants. The models are built from SPICE sub-circuits representing the key terms (...) in the chemical rate equations. The models include the addition of an ad hoc semi-permeable membrane so the system can achieve steady state fluxes and also to illustrate the need for all the efficient cause agents to be continually replaced. Comments are made about exactly what is being simulated. (shrink)

Smarandache Hypothesis states that there is no speed limit of anything, including light and particles. While the idea is quite simple and based on known hypothesis of quantum mechanics, called Einstein-Podolski-Rosen paradox, in reality such a superluminal physics seems still hard to accept by majority of physicists. Here we review some experiments to support superluminal physics and also findings to explain Smarandache Quantum Paradoxes and Quantum Sorites Paradox. We also touch briefly on new experiment on magneton, supporting SubQuantum Kinetic (...) Model of Electron. (shrink)

Fluid-phase endocytosis (pinocytosis) kinetics were studied inDictyostelium discoideum amoebae from the axenic strain Ax-2 that exhibits high rates of fluid-phase endocytosis when cultured in liquid nutrient media. Fluorescein-labelled dextran (FITC-dextran) was used as a marker in continuous uptake- and in pulse-chase exocytosis experiments. In the latter case, efflux of the marker was monitored on cells loaded for short periods of time and resuspended in marker-free medium. A multicompartmental model was developed which describes satisfactorily fluid-phase endocytosis kinetics. In particular, it accounts (...) correctly for the extended latency period before exocytosis in pulse-chase experiments and it suggests the existence of some sorts of maturation stages in the pathway. (shrink)

We assume the existence of a specific G1 protein which is an initiator of DNA replication. This initiator is supposed to be synthesized according to Michaelis-Menten kinetics. In order to start DNA replication, it is assumed that this G1 specific protein must be produced in a required amount. Intra-cellular growth inhibitors and extra-cellular growth factors control the production of the initiator. This model allows to calculate the average G1 phase time as a function of the various chemical concentrations of nutrients, (...) enzymes, growth inhibitors and growth factors. This model is compared to cell kinetics experiments on a leukemic cell line responding to Interleukin 3 deprivation. The curves giving the experimental average G1 phase times with respect to Interleukin-3 concentrations are fitted by the mathematical model with a quite good agreement. (shrink)

Development of the shapes of living organisms and their parts is a field of science in which there are no generally accepted theoretical principles. What form these principles are likely to take, when they emerge, is a subject in which there is a wide gulf of disagreement between physical scientists and experimental biologists. This book contains both an extensive philosophical commentary on this dichotomy in views and an exposition of the type of theory most favoured by physical scientists. In this (...) theory living form is a manifestation of the dynamics of chemical change and physical transport or other physics of spatial communication. The reaction-diffusion theory, as initiated by Turing in 1952 and since elaborated by Prigogine and by Gierer and Meinhardt among others, is discussed in detail at a level that requires a good knowledge of a first course in calculus, but no more than that. The great growth of molecular biology has tended to overshadow the increase in our understanding of the nature of these kinetic processes. This book seeks to reawaken interest in dynamics in the hope that a better balance between the importance of things and the importance of actions may emerge in the biology of the 21st century. (shrink)

When rates of transcription from specific genes change, delays of variable length intervene before the corresponding mRNAs and proteins attain new levels. For most mammalian genes, the time required to complete transcription, processing, and transport of mRNA is much shorter than the period needed to achieve a new, steady‐state level of protein. Studies of inducible genes have shown that the period required to attain new levels of individual mRNAs and proteins is related to their unique half‐lives. The basis for this (...) is a physical principle that predicts rates of accumulation of particles in compartmental systems. The minimum period required to achieve a new level is directly proportional to product half‐lives because rates of decay control the ratio between the rate of synthesis and the concentration of gene products at steady state. This kinetic model suggests that sensitivity of gene products to degradation by ribonucleases and proteinases is an important determinant of the time scale of gene expression. (shrink)

A model for kinetics of circular substrate cleavage by restriction endonuclease was formulated. The aim of the analysis of the model was to extract kinetic constants for all target sites from time- dependence of fragment concentration in reaction products. That was proved to be possible for molecules with an odd number of fragments only. A symmetry of the molecules with an even number of fragment is the cause. A solution for molecules with an odd number of fragments was found (...) and methods for dealing with the other molecules were suggested. (shrink)

Throughout its matrix of atoms, myoglobin has a network of cavities that are inhabited for short lengths of time by ligands released by photolysis from the myoglobin heme. The purpose or effect of this cavity network is not clear. A recently published kinetic scheme that fits data from many native and mutant myoglobin oxygen photolysis experiments can be modified easily into a kinetic scheme that includes kinetic proofreading. Proofreading would provide protection against contaminants and, specifically, might help (...) protect the cell from carbon monoxide poisoning. Here we present a two‐part model: (1) myoglobin represented by a kinetic description, which includes proofreading reactions associated with the cavities, and (2) a reaction–diffusion description of a myocyte model in which the part 1 myoglobin acts as a mobile buffer in simultaneous carbon monoxide and oxygen gradients. The non‐equilibrium nature of part 2 should promote the proofreading function of part 1. A simulation using the model demonstrates that the cavity system can in principle proofread, reducing mitochondrial enzyme contamination. BioEssays 26:422–433, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Stability properties of kinetic model equations (including discrete versions of the master equation and Boltzmann's equation) are derived by means of Lyapunov's direct method. The construction of suitable Lyapunov functions leads to results about the structural stability of the dynamical systems and makes it possible to compose more complicated systems from the given ones, preserving automatically some form of stability.

A commonly used experimental procedure for the study of granulocyte kinetics involves the labelling and subsequent tracing of granulocyte DNA. Following the introduction of a label into the system, observations are made periodically on the concentration of label in the DNA of granulocytes taken from the circulating blood. A mathematical model for the expected value of this concentration has been derived, studied, and related to experimental observations from studies using P32 as a label. Insofar as the derivation of the model (...) accurately incorporates the relevant aspects of granulocyte kinetics, the model will be useful in the interpretation of the experimental observations in terms of these kinetics.Among other things, study of the model indicates that the assumption of “flash” labelling made with respect to some labels is quite crucial and needs to be examined critically. It has also been necessary to make adjustments to allow for the observed emergence of labelled cells from the marrow soon after label introduction. In addition, despite a high degree of confounding of parameter effects it has been possible to suggest bounds on some of the parameters of the model for some species. The bounds will be refined as the model is improved and more data become available. Study and development of the model continues with particular interest in generalizations to include the diseased state chronic granulocytic leukemia. (shrink)

The Enzymatic control reactions model was presented within the scope of fractional calculus. In order to accommodate the usual initial conditions, the fractional derivative used is in Caputo sense. The methodologies of the three analytical methods were used to derive approximate solution of the fractional nonlinear system of differential equations. Two methods use integral operator and the other one uses just an integral. Numerical results obtained exhibit biological behavior of real world problem.

The Enzymatic control reactions model was presented within the scope of fractional calculus. In order to accommodate the usual initial conditions, the fractional derivative used is in Caputo sense. The methodologies of the three analytical methods were used to derive approximate solution of the fractional nonlinear system of differential equations. Two methods use integral operator and the other one uses just an integral. Numerical results obtained exhibit biological behavior of real world problem.

Recent accounts of the speed/accuracy relation for motor tasks have focused on the concept of motor output variability. We outline the advantages of this approach and the limitation of Plamondon's model in explaining movement error. We also examine and present complimentary data for rapid timing tasks. While these tasks do not meet the presented assumptions, the data still fit the model predictions.

Observed biological growth curves generally are sigmoid in appearance. It is common practice to fit such data with either a Verhulst logistic or a Gompertz curve. This paper critically considers the conceptual bases underlying these descriptive models.The logistic model was developed by Verhulst to accommodate the common sense observation that populations cannot keep growing indefinitely. A justification for using the same equation to describe the growth of individuals, based on considerations from chemical kinetics (autocatalysis of a monomolecular reaction), was (...) put forward by Richardson, but met with heavy criticism as a result of his erroneous derivation of the basic equation (Snell, 1929). It errs on the side of over-simplicity (Priestley & Pearsall, 1922). Von Bertalanffy (1957) subsequently based a justification on the assumption that, as a first approximation, the rates of catabolism and anabolism may be assumed to be proportional to weight and power (still vacant places, between the maximal possible and the already accumulated population sizes. This point of view was fiercely challenged by Nicholson (1933), Milne (1962), Smith (1954) and Rubinov (1973). And indeed, what is meant by vacant places has never become entirely clear. Finally Lotka (1925) devised a third leading approach by just truncating a Taylor expansion around zero of the differential law for autonomous growth after the second degree term. (shrink)

This article explores the use of interactive and kinetic technologies in contemporary art practice as a means by which artists engage with conditions of social and ecological crisis. In a context in which the perpetual exploitation of human and natural resources threatens the sustainability of the planet and all earthy life, the language of interactivity provides perspective into the interconnectivity of organisms and the interdependence of biological, social, economic and political systems. The interactive, kinetic work affords a distilled (...) set of relationships and movements that rely on connection and participation and, as such, serve as simple and relatable models of complex and interrelated systems. By engendering a sense of fragile suspension between sustainability and collapse – a felt experience of the present ecological crisis – the artists discussed incite affective responses through their works that are argued to be distinctly empathic in nature. Contemporary environmental theory provides insight into a shifting relationship between human and ‘nature’ that dissolves the previously instated binary in favour of an empathic consciousness, at the core of which is a recognition of the interdependence of living organisms. Art historical insight of interactivity as a means of production reveals a notable link between interactive art, destruction and trauma. The affective response engendered in contemporary works that grapple with concepts of transience and mortality relies on the very human knowledge of the body as a relatable site of experience, particularly pain and trauma. The idea of empathy as prefigured by violence and loss is particularly resonant in relation to contemporary ecological trauma. Works by artists including Rebecca Horn, Theo Jansen, Natalie Jeremijenko, Alan Rath, Lygia Clark, Alexandra Karakashian and Robert Krishner are discussed in terms of interactivity and kineticism as prompting empathic engagement in the context of environmental entropy. (shrink)

Econophysics is a new and exciting cross-disciplinary research field that applies models and modelling techniques from statistical physics to economic systems. It is not, however, without its critics: prominent figures in more mainstream economic theory have criticized some elements of the methodology of econophysics. One of the main lines of criticism concerns the nature of the modelling assumptions and idealizations involved, and a particular target are ‘kinetic exchange’ approaches used to model the emergence of inequality within the distribution (...) of individual monetary income. This article will consider such models in detail, and assess the warrant of the criticisms drawing upon the philosophical literature on modelling and idealization. Our aim is to provide the first steps towards informed mediation of this important and interesting interdisciplinary debate, and our hope is to offer guidance with regard to both the practice of modelling inequality, and the inequality of modelling practice. _1_ Introduction _1.1_ Econophysics and its discontents _1.2_ Against burglar economics _2_ Modelling Inequality _2.1_ Mainstream economic models for income distribution _2.2_ Econophysics models for income distribution _3_ Idealizations in Kinetic Exchange Models _3.1_ Binary interactions _3.2_ Conservation principles _3.3_ Exchange dynamics _ 4 _ Fat Tails and Savings _ 5 _ Evaluation. (shrink)

A summary is presented of the subquantum kinetics ether methodology, a type of unified field theory that successfully predicts a large number of physical phenomena. This utilizes a new approach to theory development that emphasizes the use of system theory and places theory development as primary and observation as secondary. The rationale is given for adopting an open system, process-based view of the physical universe and for choosing the Model G reaction system as a prospective “genetic code” for the physical (...) universe. Model G is shown to provide a viable model for the creation of subatomic particles from zero-point energy fluctuations. (shrink)

Ion channels are transport proteins present in the lipid bilayers of biological membranes. They are involved in many physiological processes, such as the generation of nerve impulses, hormonal secretion, and heartbeat. Conformational changes in the ion channel-forming protein allow the opening or closing of pores to control the ionic flux through the cell membranes. The opening and closing of the ion channel have been classically treated as a random kinetic process, known as a Markov process. Here the time the (...) channel remains in a given state is assumed to be independent of the condition it had in the previous state. More recently, however, several studies have shown that this process is not random but a deterministic one, where both the open and closed dwell-times and the ionic current flowing through the channel are history-dependent. This property is called long memory or long-range correlation. However, there is still much controversy regarding how this memory originates, which region of the channel is responsible for this property, and which models could best reproduce the memory effect. In this article, we provide a review of what is, where it is, its possible origin, and the mathematical methods used to analyze the long-term memory present in the kinetic process of ion channels. (shrink)

In addition to its obvious successes within the kinetic theory the ideal gas law and the modeling assumptions associated with it have been used to treat phenomena in domains as diverse as economics and biology. One reason for this is that it is useful to model these systems using aggregates and statistical relationships. The issue I deal with here is the way R. A. Fisher used the model of an ideal gas as a methodological device for examining the causal (...) role of selection in producing variation in Mendelian populations. The model enabled him to create the kind of population where one could measure the effects of selection in a way that could not be done empirically. Consequently we are able to see how the model of an ideal gas was transformed into a biological model that functioned as an instrument for both investigating nature and developing a new theory of genetics. (shrink)

Background: The regulation of muscle force is a vital aspect of sensorimotor control, requiring intricate neural processes. While neural activity associated with upper extremity force control has been documented, extrapolation to lower extremity force control is limited. Knowledge of how the brain regulates force control for knee extension and flexion may provide insights as to how pathology or intervention impacts central control of movement.Objectives: To develop and implement a neuroimaging-compatible force control paradigm for knee extension and flexion.Methods: A magnetic resonance (...) imaging safe load cell was used in a customized apparatus to quantify force during neuroimaging. Visual biofeedback and a target sinusoidal wave that fluctuated between 0 and 5 N was provided via an MRI-safe virtual reality display. Fifteen right leg dominant female participants completed a knee extension and flexion force matching paradigm during neuroimaging. The force-matching error was calculated based on the difference between the visual target and actual performance. Brain activation patterns were calculated and associated with force-matching error and the difference between quadriceps and hamstring force-matching tasks were evaluated with a mixed-effects model.Results: Knee extension and flexion force-matching tasks increased BOLD signal among cerebellar, sensorimotor, and visual-processing regions. Increased knee extension force-matching error was associated with greater right frontal cortex and left parietal cortex activity and reduced left lingual gyrus activity. Increased knee flexion force-matching error was associated with reduced left frontal and right parietal region activity. Knee flexion force control increased bilateral premotor, secondary somatosensory, and right anterior temporal activity relative to knee extension. The force-matching error was not statistically different between tasks.Conclusion: Lower extremity force control results in unique activation strategies depending on if engaging knee extension or flexion, with knee flexion requiring increased neural activity for the same level of force and no difference in relative error. These fMRI compatible force control paradigms allow precise behavioral quantification of motor performance concurrent with brain activity for lower extremity sensorimotor function and may serve as a method for future research to investigate how pathologies affect lower extremity neuromuscular function. (shrink)

Nitric oxide (NO) is involved in synaptic long-term potentiation (LTP) by multiple signaling pathways. Here, we show that LTP of synaptic transmission can be explained as a feature of signal transduction—bistable behavior in a chain of biochemical reactions with positive feedback, formed by diffusion of NO to the presynaptic site and facilitating the release of glutamate (Glu). The dynamics of Glu, calcium (Ca2+) and NO is described by a system of nonlinear reaction–diffusion equations with modified Michaelis–Menten (MM) kinetics. Numerical investigation (...) reveals that the chain of biochemical reactions analyzed can exhibit a bistable behavior under physiological conditions when production of Glu is described by MM kinetics and decay of NO is modeled by means of two enzymatic pathways with different kinetic properties. Our finding extends understanding of the role of NO in LTP: a short high-intensity stimulus is “memorized” as a long-lasting elevation of NO concentration. The conclusions obtained by analysis of the chain of biochemical reactions describing LTP can be generalized to other chains of interactions or for creating the logical elements for biological computers. (shrink)

Esta introdução descreve os mais importantes dados biográficos da vida e da obra do físico teórico austríaco Ludwig Boltzmann (1844-1906). As principais contribuições científicas de Boltzmann situam-se nos domínios da teoria cinética dos gases e mecânica estatística, da qual ele foi um dos fundadores. A tese de que as teorias científicas são representações dos fenômenos naturais é encontrada em todos os artigos de Boltzmann. No verbete "modelo", ela é apresentada de modo mais organizado, o que o torna uma peça fundamental (...) para a compreensão do pensamento filosófico do físico austríaco. O verbete, publicado na famosa enciclopédia Britannica, foi escrito para o público em geral. Ainda assim, Boltzmann não se eximiu de tomar partido em favor do uso de modelos. Estes últimos não apenas seriam inevitáveis, mas necessários, uma vez que tornavam possível o aperfeiçoamento contínuo da ciência. This introduction describes the most important biographical details of the life and work of the Austrian theoretical physicist Ludwig Boltzmann (1844-1906). The main scientific contributions of Boltzmann can be found in areas such as the kinetic theory of gases and statistical mechanics, of which he was one of the co-founders. The thesis that scientific theories are representations of natural phenomena is found in all philosophical articles by Boltzmann. In the entry "model", published in the celebrated Encyclopedia Britannica, the thesis is presented in a more organized way, making it the keystone to understand the philosophical thought of the Austrian physicist. Although the entry was written for the general public, in it Boltzmann did not refrain from taking sides as far as the use of models is concerned. According to him, the latter would not only be inevitable, but in fact necessary, since they would made possible the continuous improvement of science. (shrink)

Heterocyst spacing in blue -green bacteria is widely assumed to be due to a diffusible inhibitor. The inhibitor, a nitrogen-rich compound, probably glutamine, is produced via the N2-fixing enzymes of the heterocyst and in turn serves to suppress the induction of these enzymes and of the differentiation of vegetative cells to heterocysts. This simple morphogenetic mechanism operating in growing cellular filaments ofAnabaena species is investigated on the basis of a continuous and a discrete cellular model, as well as by cell-by-cell (...) simulation of the inhibitor transport. The resulting distances between heterocysts and kinetics of their production are compared with observations, and the values of physical parameters are estimated from the models. (shrink)

This work is a qualitative study of an organism''s physiological adaptative response to stress. The experimental data were selected from a previous study leading to the conclusion that stress may be considered as a topological retraction within a vital space that must be more precisely defined. The experimental methodology uses rat poisoning by neurotoxins. The control parameter is the intensity of the toxic doses. Measured parameters are the animals'' survival rate and the kinetics of cerebral acetylcholinesterase activity. The results, when (...) expressed as a function of the inverted doses, show a characteristic evolution. The pattern of the curve closely resembles a vortex profile. This analogy is studied more extensively in both the physical and biological domains. These findings help to clarify the concept of biological stress which presents the same vectorial properties as hydrodynamic vorticity. In particular, the dissipation of stress and the dissipation of vorticity seem to obey the same laws. This observation is valid for both diffusion and convection processes. The decompensation phase of stress could be compared with the instability and turbulence in flows.Our approach in this paper is mainly to establish a general and phenomenological description of the stress response fitting experimental observations. (shrink)

The discovery of so-called ‘mirror neurons’ - found to respond both to own actions and the observation of similar actions performed by others - has been enormously influential in the cognitive sciences and beyond. Given the self-other symmetry these neurons have been hypothesized as underlying a ‘mirror mechanism’ that lets us share representations and thereby ground core social cognitive functions from intention understanding to linguistic abilities and empathy. I argue that mirror neurons are important for very different reasons. Rather than (...) a symmetric ubiquitous or context- independent mechanism, I propose that these neurons are part of broader sensorimotor circuits, which help us navigate and predict the social affordance space that we meet others in. To develop both the critical and positive project I analyze the interpretive choices and the debate surrounding the mirror neuron research and show how the field is marred by highly questionable assumptions about respectively motor and social cognition. The discovery of mirror neurons - and the sensorimotor circuits of which these neurons are a part – actually empirically challenge many of these tacit assumptions. Findings of sensorimotor goal representations at levels of abstraction well beyond actual sensory information and kinetic movements challenge the idea of motor cognition as primarily output production. Additionally, the focus on 3rd person mindreading of hidden mental states is misguiding the field of social cognition. Much ‘mind-reading’ seems rooted in sensorimotor representations and a developmentally primary 2nd person understanding of actions and the mental lives of others, which precisely breaks the assumed dichotomy between mind and behavior. I propose a Social Affordance model where parallel fronto-parietal sensorimotor circuits support representations not just of other people’s actions but of the overall social affordance space. It is a process that monitors concrete goals and teleological possibilities that the environment affords respectively oneself and other present agents. With this model I hypothesize that the complex spectrum of sensorimotor integrations are indeed essential not only to normal action choice calibration but also to social cognitive abilities, as the sensorimotor teleological representations let us relate to others and understand their action choices in a shared pragmatic and intentional context. (shrink)

We give an explicit solution of a model Boltzmann kinetic equation describing a gas between two walls maintained at different temperatures. In the model, which is essentially one-dimensional, there is a probability for collisions to reverse the velocities of particles traveling in opposite directions. Particle number and speeds (but not momentum) are collision invariants. The solution, which depends on the stochastic collision kernels at the walls, has a linear density profile and the energy flux satisfies Fourier's law.

This paper approaches the scientific realism question from a naturalistic perspective. On the basis of a historical case study of the work of James Clerk Maxwell and Ludwig Boltzmann on the kinetic theory of gases, it shows that scientists’ views about the epistemological status of theories and models typically interact with their scientific results. Subsequently, the implications of this result for the current realism debate are analysed. The case study supports Giere’s moderately realist view of scientific models (...) and theories, based on the notion of similarity, and it highlights the crucial role of model users. The paper concludes with a discussion of Boltzmann’s Bildtheorie, the sophisticated form of realism that he developed in response to the scientific problems of kinetic theory. (shrink)

This paper approaches the scientific realism question from a naturalistic perspective. On the basis of a historical case study of the work of James Clerk Maxwell and Ludwig Boltzmann on the kinetic theory of gases, it shows that scientists’ views about the epistemological status of theories and models typically interact with their scientific results. Subsequently, the implications of this result for the current realism debate are analysed. The case study supports Giere’s moderately realist view of scientific models (...) and theories, based on the notion of similarity, and it highlights the crucial role of model users. The paper concludes with a discussion of Boltzmann’s Bildtheorie, the sophisticated form of realism that he developed in response to the scientific problems of kinetic theory. (shrink)

This paper explores the consequences of the theoretical forward activation enzymatic pathway A 0 A 1 A 2 A 3 where E 1 convents A 0 to A 1, E 2 converts A 1 to A 2 and E 3 converts A 2 to A 3. A 0, which is environmentally determined, also serves to activate (or modulate) the activity of E 3 in such a way as to keep the concentration of A 2 ([A 2]) constant at a particular (...) set-point value. For mathematical simplicity, first order rate kinetics are used where k 1, k 2 and k 3 are the rate constants for E 1, E 2, and E 3 respectively. It is shown that if k 3 is modulated appropriately so as keep [A 2] at the setpoint value with a changing upstream [A 0], then the modulation of k 3 must be anticipatory of the dynamics of the biochemical pathway. In other words, the rate of change of k 3, will be a function of [A 0], k 1, k 2, k 3, and the set-point value of [A 2]. If the modulation of k 3 does not perfectly model (anticipate) the reaction pathway of which it is a part, then the actual [A 2] will deviate from the set-point value. This type of anticipatory feed-forward activation may represent an important aspect of biological organization. (shrink)

The ability of predicting upcoming events or conditions in advance offers substantial selective advantage to living beings. The most successful systematic tool for fairly reliable prognoses is the use of dynamical causal models in combination with memorised experience. Surprisingly, causality is a fundamental but rather controversially disputed concept. For both models and memory, symbol processing is requisite. Symbols are a necessary and sufficient attribute of life from its very beginning; the process of their evolutionary emergence was discovered by (...) Julian Huxley a century ago. In behavioural biology, this universal symmetry-breaking kinetic phase transition became known as ritualisation. Symbol use for predicting future dynamical processes has culminated in the unprecedented complexity of mental models used in science and technology, coining the historical ascent of modern humans. Observation and measurement transform structural information of physical exchange processes into symbolic information from which state quantities are derived by means of mental models. However, phylogenetically inherited models such as naïve realism do not necessarily explain the sophisticated insights revealed by modern experiments with, say, entangled quantum states. It is suggested to carefully distinguish observed exchange quantities from predicted unobservable state quantities, and physical reality from mental models thereof. (shrink)

Tetrahymena has been used as a model cell system in many studies of morphogenesis, conjugation, gene mapping, cell division and growth kinetics. In this article, we consider some advances which have resulted from the successful development of a chemically defined medium (CDM), and how subsequent work has extended the contribution that this organism has made to our understanding of different aspects of growth, nutrition, cell cycle control, cytokinesis and intercellular signalling. Finally, we discuss the considerable potential that has arisen for (...) the biotechnological exploitation of this big and rapidly growing eukaryotic cell. (shrink)