Membrane Arrangement Research Articles

Scale-Up Design Analysis and Modelling of Cobalt Oxide Silica Membrane Module for Hydrogen Processing

This work shows the application of a validated mathematical model for gas permeation at high temperatures focusing on demonstrated scale-up design for H2 processing. The model considered the driving force variation with spatial coordinates and the mass transfer across the molecular sieve cobalt oxide silica membrane to predict the separation performance. The model was used to study the process of H2 separation at 500 °C in single and multi-tube membrane modules. Parameters of interest included the H2 purity in the permeate stream, H2 recovery and H2 yield as a function of the membrane length, number of tubes in a membrane module, space velocity and H2 feed molar fraction. For a single tubular membrane, increasing the length of a membrane tube led to higher H2 yield and H2 recovery, owing to the increase of the membrane area. However, the H2 purity decreased as H2 fraction was depleted, thus reducing the driving force for H2 permeation. By keeping the membrane length constant in a multi-tube arrangement, the H2 yield and H2 recovery increase was attributed to the higher membrane area, but the H2 purity was again compromised. Increasing the space velocity avoided the reduction of H2 purity and still delivered higher H2 yield and H2 recovery than in a single membrane arrangement. Essentially, if the membrane surface is too large, the driving force becomes lower at the expense of H2 purity. In this case, the membrane module is over designed. Hence, maintaining a driving force is of utmost importance to deliver the functionality of process separation.

Nanoscale Phase Separation in DSPC–Cholesterol Systems

The lipid arrangement of eukaryotic cell membranes has been shown to be heterogeneous, with domains enriched in cholesterol and saturated phospholipids, coexisting with a continuous phase that is enriched in unsaturated phospholipids. While the existence of these domains is well-established, there is still a lack of consensus regarding domain size and the factors influencing it. In this work, we investigate model membranes consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-cholesterol (Chol) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC, 18:1-16:0)-DSPC-Chol with a steady-state fluorescence assay and report the influence of phospholipid chain saturation and chain length on domain size. The spectral shifts of 1-myristoyl-2-[12-[(5-dimethylamino-1-naphthalenesulfonyl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (DAN-PC) and a Förster resonance energy transfer (FRET) assay were used, along with an analytical model, to estimate domain sizes. A region of nanoscale domain existence was observed in both ternary systems; however, the domains formed in the system containing the asymmetric lipid (POPC, 18:1-16:0) were larger than those formed in the diunsaturated lipid (DOPC, 18:1-18:1). This is a new finding, as domains were not previously known to exist in similar POPC-based systems.

Experimental study on the hydrodynamic effects of gas permeation through horizontal membrane tubes in fluidized beds

Abstract Fluidized Bed Membrane Reactors gain worldwide increasing interest for various applications. Nevertheless, fundamental research on the hydrodynamics of these reactors is required in order to improve the predictive capabilities of numerical models and to improve reactor performance. This study comprises an experimental investigation in a pseudo-2D gas–solid fluidized bed containing horizontal submerged membrane tubes, through which gas can be added to – or extracted from – the fluidized suspension. Employing Particle Image Velocimetry (PIV) combined with a novel Digital Image Analysis (DIA) technique, the effect of the presence of, and permeation of gas through the membranes, on both the solids flux profiles as well as the bubble properties has been investigated in great detail for different membrane configurations. It has been shown that merely the presence of the membrane tubes decreases the solid fluxes by about a factor three, but does not significantly change the overall circulation patterns in the bed. The permeation ratio and the membrane tube arrangement (in-line vs. staggered, size and number of membranes) hardly influence the time-averaged solid flux profiles. In addition, it has been found that, compared to a fluidized bed without inserts, the average bubble size with horizontally submerged membrane tubes decreases by a factor of approximately three, but is hardly dependent on the permeation ratio. The average total superficial gas velocity is more important than the permeation ratio. With respect to bubble size and number of bubbles, the staggered membrane arrangement with 9.6 mm tubes outperforms the in-line arrangement, the arrangement with smaller tubes and the arrangement with fewer membrane tubes, which is of great benefit to reduce bubble-to-emulsion phase mass transfer limitations in fluidized bed membrane reactors under reactive conditions.

Two Oxytrichid Ciliates, Cyrtohymena primicirrata and Oxytricha granulifera (Ciliophora: Sporadotrichida: Oxytrichidae) Unknown from Korea

This study reports the discovery of two oxytrichid ciliates, Cyrtohymena primicirrata (Berger and Foissner, 1987) and Oxytricha granulifera Foissner and Adam, 1983, in Jeju Island, Korea. The morphology of the two species was studied using live observation and protargol impregnation. These species are described as follows: Cyrtohymena primicirrata has a body size in live specimens <TEX>$90-140{\times}40-60{\mu}m$</TEX>, length : width ratio 2.3 : 1 on average; elongated and slender obovate in outline of body. Cortical granules are shiny yellow on the ventral and dorsal side. Adoral zone of membranelles (AZM) is covering about 48% of the cell with about 38 adoral membranelles. Arrangement of undulating membranes is ordinary Cyrtohymena pattern. Dorsal kineties is six rows with <TEX>$5{\mu}m$</TEX> long bristles. Oxytricha granulifera has a body size in live specimens <TEX>$90-115{\times}25-38{\mu}m$</TEX>, length : width ratio 3.31 on average; elongated ellipsoidal in outline of body. Cortical granules are colorless on the ventral and dorsal side. AZM is covering 28% of the cell length in vivo with about 24 adoral membranelles. Arrangement of undulating membranes is Oxytricha pattern. Dorsal kineties is five rows with about <TEX>$3{\mu}m$</TEX> long dorsal bristles.

Cholesterol Regulates Insulin Secretion through Protein Prenylation and Membrane Arrangement in INS-1E Cells

Cholesterol plays a key role in membrane structure and secretory function of pancreatic B cells. Prenyl molecules derived from the cholesterol biosynthesis pathway, such as geranylgeranyl pyrophosphate (GGPP), serve as substrates for post-translational modifications of small GTPases involved in insulin secretion. However, appropriate cholesterol content is required for the maintenance of the membrane physicochemical properties such as fluidity that directly impact insulin granule fusion. Since statins reduce both prenyl groups and cholesterol, our aim was to study the relative contribution of these lipid lowering compounds on glucose-stimulated insulin secretion (GSIS) in insulin secreting cells.INS-1E cells were cultured in the absence or presence of simvastatin (SIM, 1μM) or zaragozic acid (ZGA, inhibitor of cholesterol synthesis but not of prenyl groups, 20μM), in acute (2h) and chronic (24h) treatments; rat islets were acutely treated with SIM. To evaluate the role of protein prenylation, cells were treated for 24h with GGPP (20μM). Secreted insulin during 2h incubation at 2.8 and 16.7mM glucose was quantified by radioimmunoassay. Cellular cholesterol after treatment was measured by a colorimetric assay. Membrane lipid rafts after 2h SIM treatment were analyzed by immunofluorescence. Plasma membrane fluidity was measured by EPR using 5-DSA as probe.Acute SIM reduces GSIS, but does not affect total cholesterol content, nor membrane microdomain distribution or membrane fluidity. GSIS inhibition was not observed after acute ZGA treatment nor after acute SIM treatment on cells previously loaded with GGPP. Chronic SIM inhibits GSIS but enhances cholesterol reduction and increases plasma membrane fluidity. Our results show that protein prenylation contributes to GSIS and that plasma membrane cholesterol content may be a critical parameter for the maintenance of physicochemical properties that regulate cellular functions such as granule fusion and cargo exocytosis.

Protein Flotation Assay to Isolate Lipids Rafts from Soft Tissue or Cells

The arrangement in eukaryotic cell membranes of liquid-ordered states surrounded by liquid-disordered phases allows for the existence of organized membrane microdomains called rafts. Rafts play a crucial role in signal-transduction events, in lipid transport and in various internalization processes, and for all these reasons need to be purified for further characterization., 在由液体无序相包围的液体有序状态的真核细胞膜中的排列允许存在称为筏的有组织的膜微结构域。 木筏在信号转导事件，脂质运输和各种内化过程中发挥关键作用，并且由于所有这些原因，需要进行纯化以进一步表征。

Reconstruction of the proximal ends of retiolitid rhabdosomes (Graptolithina) from the Upper Wenlock and the Lower Ludlow

Detailed study of the ultrastructures on the cortical lists of retiolitids occurring after the lundgreni event has been undertaken. The extremely well preserved material from the Goldap borehole of Poland (Baltica) enables classification of clathrial lists into two types—with single and double seams. Significantly different seam development is observed between two groups of retiolitids occurring before and after the lundgreni event. The study of seams and increments on the lists enables reconstruction of the membrane arrangement of the rhabdosomes. Reconstruction of the proximal ends, the arrangement of the ancora umbrella, and the layout of the proximal ventral and lateral orifices of five genera, Neogothograptus, Holoretiolites, Plectograptus, Spinograptus, and Quattuorgraptus n. gen., is presented. The new genus Quattuorgraptus is separated from Spinograptus because of the different development of the ancora umbrella and the width of the lateral walls of the rhabdosome.

Curvature-driven effective attraction in multicomponent membranes

We study closed liquid membranes that segregate into three phases due to differences in the chemical and physical properties of its components. The shape and in-plane membrane arrangement of the phases are coupled through phase-specific bending energies and line tensions. We use simulated annealing Monte Carlo simulations to find low-energy structures, allowing both phase arrangement and membrane shape to relax. The three-phase system is the simplest one in which there are multiple interface pairs, allowing us to analyze interfacial preferences and pairwise distinct line tensions. We observe the system's preference for interface pairs that maximize differences in spontaneous curvature. From a pattern selection perspective, this acts as an effective attraction between phases of most disparate spontaneous curvature. We show that this effective attraction is robust enough to persist even when the interface between these phases is the most penalized by line tension. This effect is driven by geometry and not by any explicit component-component interaction.

Numerical Investigations to Quantify the Effect of Horizontal Membranes on the Performance of a Fluidized Bed Reactor

Abstract Reactive simulations of gas-solid flows occurring in fluidized beds with horizontal membrane insertion were carried out using computational fluid dynamics based on the kinetic theory of granular flows. The effect of altering the membrane arrangement on the overall reactor performance (degree of conversion achieved) was investigated by means of fractional factorial designs. When membranes served only as hydrodynamic modifiers to the flow, it was found that significant improvements could be obtained by optimising the membrane arrangement. A slightly larger improvement could be attained by injecting some of the reacting gas through the membranes. When compared to the improvements that can be attained by simply scaling up the reactor height and especially the reactor width, however, these improvements were of less significance. The use of membranes solely for altering reactor hydrodynamics by serving as obstructions and gas injection points can therefore not be merited. Further optimisation studies into membrane arrangement are therefore only recommended for specific processes in which the membranes play a central role by extracting some of the process gasses.

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes.

BackgroundThe thylakoid system in plant chloroplasts is organized into two distinct domains: grana arranged in stacks of appressed membranes and non-appressed membranes consisting of stroma thylakoids and margins of granal stacks. It is argued that the reason for the development of appressed membranes in plants is that their photosynthetic apparatus need to cope with and survive ever-changing environmental conditions. It is not known however, why different plant species have different arrangements of grana within their chloroplasts. It is important to elucidate whether a different arrangement and distribution of appressed and non-appressed thylakoids in chloroplasts are linked with different qualitative and/or quantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranes and whether this arrangement influences the photosynthetic efficiency.ResultsOur results from TEM and in situ CLSM strongly indicate the existence of different arrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids are regularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, while irregular appressed thylakoid membranes within bean chloroplasts correspond to smaller and less distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show a distinct spatial separation of stacked thylakoids from stromal spaces whereas spatial division of stroma and thylakoid areas in bean chloroplasts are more complex. Structural differences influenced the PSII photochemistry, however without significant changes in photosynthetic efficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well as spectroscopic investigations indicated a similar proportion between PSI and PSII core complexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones. Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSI supercomplexes between species are suggested.ConclusionsBased on proteomic and spectroscopic investigations we postulate that the differences in the chloroplast structure between the analyzed species are a consequence of quantitative proportions between the individual CP complexes and its arrangement inside membranes. Such a structure of membranes induced the formation of large stacked domains in pea, or smaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with each other and not always parallel to each other.

SIMULATION AND PROCESS OPTIMIZATION OF A MEMBRANE-BASED DENSE GAS EXTRACTION USING HOLLOW FIBER CONTACTORS

Supercritical fluid and membrane technology coupling is a relatively new concept applicable to solvent separation and solute extraction. In these processes a hydrophobic or hydrophilic macroporous membrane is used as a two-different-nature solutions contactor. This methodology is an alternative to conventional liquid solution supercritical fluid extraction processes, which are associated with high investment costs. In the present work, a membrane-based supercritical fluid extraction module is modeled, simulated, and optimized as an independent industrial-scale operational unit. UniSim design suite R390 software from Honeywell was used as the platform for the simulation. Acetone and ethanol literature extraction results and methanol experimental extraction results (27.6% to 14.5% with a 10 wt.% aqueous solution; 7.1% to 5.9% with a 500 ppm aqueous solution) were used for validation of the model and definition of the semi-empirical equation parameters. The generated industrial-scale system optimization, which used a modular membrane arrangement, was strongly dependent on thermodynamic, economic, and energetic variables (higher mass transfer resistance in the carbon dioxide phase increased the number of membranes needed; process feasibility was affected by the number of membrane units, carbon dioxide flow rate, and product added value; compression energy requirements affected the optimization result). The modeled system proved to be an important aid in the design, scaling, and optimization of systems that use membranes as phase contactors in liquid solution supercritical carbon dioxide extraction.

A look through ‘lens’ cubic mitochondria

Cell membranes may fold up into three-dimensional nanoperiodic cubic structures in biological systems. Similar geometries are well studied in other disciplines such as mathematics, physics and polymer chemistry. The fundamental function of cubic membranes in biological systems has not been uncovered yet; however, their appearance in specialized cell types indicates a role as structural templates or perhaps direct physical entities with specialized biophysical properties. The mitochondria located at the inner segment of the retinal cones of tree shrew (Tupaia glis and Tupaia belangeri) contain unique patterns of concentric cristae with a highly ordered membrane arrangement in three dimensions similar to the photonic nanostructures observed in butterfly wing scales. Using a direct template matching method, we show that the inner mitochondrial membrane folds into multi-layered (8 to 12 layers) gyroid cubic membrane arrangements in the photoreceptor cells. Three-dimensional simulation data demonstrate that such multi-layer gyroid membrane arrangements in the retinal cones of a tree shrew's eye can potentially function as: (i) multi-focal lens; (ii) angle-independent interference filters to block UV light; and (iii) a waveguide photonic crystal. These theoretical results highlight for the first time the significance of multi-layer cubic membrane arrangements to achieve near-quasi-photonic crystal properties through the simple and reversible biological process of continuous membrane folding.

Ultrastructure of Erythrophores and Xanthophores of the Siamese Fighting Fish, Betta splendens

The ultrastructural morphology of brightly colored pigment cells (chromatophores) of Siamese fighting fish, Betta splendens, was investigated by transmission electron microscopy. The major pigment cells in the epidermis and dermis of the red and golden strains were erythrophores and xanthophores, respectively. Specific combinations of these chromatophores formed the basis of pigmentation patterns in both strains. The ultrastructure of the erythrophores was characterized by ellipsoidal electron-lucent vesicles that had limiting membranes and inner lamellae. The latter appeared whorl-like due to a concentric arrangement of parallel membranes. The xanthophores contained small and large cytoplasmic vesicles that appeared hollow and electron-lucent, with some vesicles displaying slightly electron dense particles. Sections of some large vesicles also revealed a very thin membrane enveloping these droplet-like vesicles.

Effects of Hidrophobic Surfactant Proteins SP-B and SP-C on the Mechanical Properties and Structural Stability of Phospholipid Bilayers

The respiratory surface is stabilized by pulmonary surfactant, a complex mixture of lipids and proteins, whose main function is reducing surface tension at the alveolar air-liquid interface in order to facilitate the work of breathing. It is composed by around 90% lipids and 8-10% specific proteins, including the hydrophobic polypeptides SP-B and SP-C. A combined action of both proteins is essential for a proper organization of functional membrane arrays in surfactant complexes.SP-B and SP-C have dramatical effects on membrane structure and dynamics. In the present study we compare some structural, mechanical and dynamical properties of model POPC vesicles in the absence and presence of them, either in their physiological combined proportion or each protein by itself. Structural effects caused by hydrophobic surfactant proteins were noticed both by optical microscopy of giant proteolipid vesicles and by electron microscopy of 100 nm diameter extruded vesicles. Also, impermeable POPC membranes became permeable when supplemented with any of these proteins. Significant differences were noticed between the effect of SP-B and SP-C on giant vesicles: suspensions containing only SP-B were stable but those containing only SP-C were quite dynamic, undergoing frequent fluctuations, reorganizations and ruptures as observed under the microscope. In order to investigate the physical explanation of these phenomena, mechanical studies have been carried out with giant phospholipid and proteolipid vesicles.The differences found between the effect of each protein separately and their physiological mixture support the concept that SP-B and SP-C mutually modulate their membrane-perturbing properties, which may be crucial for the structure, arrangement and dynamics of pulmonary surfactant membranes.

Methylomonas koyamae sp. nov., a type I methane-oxidizing bacterium from floodwater of a rice paddy field

A novel methane-oxidizing bacterium, strain Fw12E-Y(T), was isolated from floodwater of a rice paddy field in Japan. Cells of strain Fw12E-Y(T) were Gram-negative, motile rods with a single polar flagellum and type I intracytoplasmic membrane arrangement. The strain grew only on methane or methanol as sole carbon and energy source. It was able to grow at 10-40 °C (optimum 30 °C), at pH 5.5-7.0 (optimum 6.5) and with 0-0.1% (w/w) NaCl (no growth above 0.5% NaCl). 16S rRNA gene sequence analysis showed that strain Fw12E-Y(T) is related most closely to members of the genus Methylomonas, but at low levels of similarity (95.0-95.4%). Phylogenetic analysis of pmoA and mxaF genes indicated that the strain belongs to the genus Methylomonas (97 and 92 % deduced amino acid sequence identities to Methylomonas methanica S1(T), respectively). The DNA G+C content of strain Fw12E-Y(T) was 57.1 mol%. Chemotaxonomic data regarding the major quinone (MQ-8) and major fatty acids (C(16:1) and C(14:0)) also supported its affiliation to the genus Methylomonas. Based on phenotypic, genomic and phylogenetic data, strain Fw12E-Y(T) is considered to represent a novel species of the genus Methylomonas, for which the name Methylomonas koyamae sp. nov. is proposed. The type strain is Fw12E-Y(T) ( = JCM 16701(T) = NBRC 105905(T) = NCIMB 14606(T)).

Synchrotron SAXS and WAXS Study of the Interactions of NSAIDs with Lipid Membranes

Cell membranes often constitute the first biological structure encountered by drugs, and binding or interactions of drugs with lipid components of the membrane may explain part of their mechanism of activity or their side effects. The present study provides evidence of alterations in the structural properties of phospholipid bilayers at acidic conditions that can be correlated with the mechanism of action of nonsteroidal anti-inflammatory drugs (NSAIDs) and with their local action effect on the gastrointestinal tract lipids, aiming a molecular biophysical approach to the interaction of these drugs with lipid membranes. In this context, the structural modifications of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers at pH 5.0, induced by increasing concentrations of five NSAIDs (piroxicam, meloxicam, tolmetin, indomethacin, and nimesulide), were studied by small-angle and wide-angle X-ray scattering. Results obtained highlight the effect of each NSAID in modulating the membrane structure properties. All the NSAIDs promoted distinct biophysical effects by perturbing the membrane arrangement to different degrees that are intimately related to their different physicochemical properties as well as with the initial organization of the lipids, depending if they are in the gel (L(β')) or in the liquid-crystalline phase (L(α)).

Methylothermus subterraneus sp. nov., a moderately thermophilic methanotroph isolated from a terrestrial subsurface hot aquifer

A novel methane-oxidizing bacterium, strain HTM55(T), was isolated from subsurface hot aquifer water from a Japanese gold mine. Strain HTM55(T) was a Gram-negative, aerobic, motile, coccoid bacterium with a single polar flagellum and the distinctive intracytoplasmic membrane arrangement of a type I methanotroph. Strain HTM55(T) was a moderately thermophilic, obligate methanotroph that grew on methane and methanol at 37-65 °C (optimum 55-60 °C). The isolate grew at pH 5.2-7.5 (optimum 5.8-6.3) and with 0-1 % NaCl (optimum 0-0.3 %). The ribulose monophosphate pathway was operative for carbon assimilation. The DNA G+C content was 54.4 mol% and the major fatty acids were C(16 : 0) (52.0 %) and C(18 : 1)ω7c (34.8 %). Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain HTM55(T) was closely related to Methylothermus thermalis MYHT(T) (99.2 % 16S rRNA gene sequence similarity), which is within the class Gammaproteobacteria. However, DNA-DNA relatedness between strain HTM55(T) and Methylothermus thermalis MYHT(T) was ≤ 39 %. On the basis of distinct phylogenetic, chemotaxonomic and physiological characteristics, strain HTM55(T) represents a novel species of the genus Methylothermus, for which the name Methylothermus subterraneus sp. nov. is proposed. The type strain is HTM55(T) ( = JCM 13664(T) = DSM 19750(T)).

Spatial organization of transmembrane receptor signalling

The spatial organization of transmembrane receptors is a critical step in signal transduction and receptor trafficking in cells. Transmembrane receptors engage in lateral homotypic and heterotypic cis-interactions as well as intercellular trans-interactions that result in the formation of signalling foci for the initiation of different signalling networks. Several aspects of ligand-induced receptor clustering and association with signalling proteins are also influenced by the lipid composition of membranes. Thus, lipid microdomains have a function in tuning the activity of many transmembrane receptors by positively or negatively affecting receptor clustering and signal transduction. We review the current knowledge about the functions of clustering of transmembrane receptors and lipid-protein interactions important for the spatial organization of signalling at the membrane.

3-D modelling of chloroplast structure under (Mg 2+) magnesium ion treatment. Relationship between thylakoid membrane arrangement and stacking

We performed for the first time three-dimensional (3D) modelling of the entire chloroplast structure. Stacks of optical slices obtained by confocal laser scanning microscope (CLSM) provided a basis for construction of 3D images of individual chloroplasts. We selected pea ( Pisum sativum) and bean ( Phaseolus vulgaris) chloroplasts since we found that they differ in thylakoid organization. Pea chloroplasts contain large distinctly separated appressed domains while less distinguished appressed regions are present in bean chloroplasts. Different magnesium ion treatments were used to study thylakoid membrane stacking and arrangement. In pea chloroplasts, as demonstrated by 3D modelling, the increase of magnesium ion concentration changed the degree of membrane appression from wrinkled continuous surface to many distinguished stacked areas and significant increase of the inter-grana area. On the other hand 3D models of bean chloroplasts exhibited similar but less pronounced tendencies towards formation of appressed regions. Additionally, we studied arrangements of thylakoid membranes and chlorophyll–protein complexes by various spectroscopic methods, Fourier-transform infrared spectroscopy (FTIR) among others. Based on microscopic and spectroscopic data we suggested that the range of chloroplast structure alterations under magnesium ions treatment is a consequence of the arrangement of supercomplexes. Moreover, we showed that stacking processes always affect the structural changes of chloroplast as a whole.

A quick introduction to membrane computing

Membrane computing is a branch of natural computing inspired from the architecture and the functioning of biological cells. The obtained computing models are distributed parallel devices, called P systems, processing multisets of objects in the compartments defined by hierarchical or more general arrangements of membranes. Many classes of P systems were investigated – mainly from the point of view of computing power and computing efficiency; also, a series of applications (especially in modeling biological processes) were reported. This note is a short and informal introduction to this research area, introducing a few basic notions, research topics, types of results, and pointing out to some relevant references.