An axiomatic method for goal-dependent allocation in life cycle assessment

The International Journal of Life Cycle Assessment - How to apply allocation in an life cycle assessment (LCA) is a long-running and controversial debate. Consensus seems to exist on the fact that...     

Self-maintenance and self-reproduction in an abstract cell model

Living cells must maintain their membranes by active metabolism. The membrane is not static but a dynamic structure that has evolved along with its internal reactions. When we reflect on the emergence and evolution of primitive cells, we should not forget the mutual dependency between membranes and metabolic cycles inside the cell. In this paper, we present a simple abstract model of the self-maintaining cell. A metabolic cycle will produce a self-assembling membrane that will enclose the metabolic cycle. We show that a self-maintaining cell has the potential to reproduce itself spontaneously. Further, we have demonstrated two different ways of cellular reproduction depending on the mobility of chemicals. In the first case, a cell releases autocatalytic chemicals that create new cells outside the mother cell. In the second case, a cell grows larger and divides itself into daughter cells by creating a new internal dividing membrane.     

Synthetic Turing protocells: vesicle self-reproduction through symmetry-breaking instabilities

The reproduction of a living cell requires a repeatable set of chemical events to be properly coordinated. Such events define a replication cycle, coupling the growth and shape change of the cell membrane with internal metabolic reactions. Although the logic of such process is determined by potentially simple physico-chemical laws, modelling of a full, self-maintained cell cycle is not trivial. Here we present a novel approach to the problem that makes use of so-called symmetry breaking instabilities as the engine of cell growth and division. It is shown that the process occurs as a consequence of the breaking of spatial symmetry and provides a reliable mechanism of vesicle growth and reproduction. Our model opens the possibility of a synthetic protocell lacking information but displaying self-reproduction under a very simple set of chemical reactions.     

Significance of chiral purity of biomolecules for self-reproduction of organisms]

Living systems mainly syntesise and utilise chiral pure polimers which have only one form of enantiomers (l, l,..., l-proteins, d, d, d,..., d-sugars). The biologycal meaning of chiral purity of molecules in organisms is discussed. It is shown that chiral purity of biomolecules is necessary to ensure ordering of organisms during selfreproduction. Even when formation of chiral pure form does not have any advantages over formation of a set of other stereoisomers, only chiral pure molecules may act as regulators of further syntheses. Molecules with distorded chiral purity are not able to maintain the initial order during selfreproduction.     

An explanation of gastric hypersecretion in the pylorus-ligated rat

The hypersecretion of gastric acid in the pylorus-ligated rat has been shown to be of vagal origin. The present series of experiments were performed to identify the stimulus. The pyloric sphincter was ligated in a series of Sprague Dawley rats. Along with pylorus ligation, various other surgical manipulations were performed. Intestinal obstruction by ligation approximately 20 cm aboral to the cecum reduced unstimulated gastric secretion in the pylorus-ligated rat. However, perfusion of the lower small intestine with bicarbonate (143 mEq/L) stimulated secretion. Perfusion with either saline or deoxycholic acid (20 mEq/L) did not alter secretion. This supports a role for bicarbonate in the hypersecretion of gastric acid in the pylorus-ligated rat. The reflex appears to involve the myenteric plexus, since section of the pylorus seemed to attenuate gastric secretion. Plasma from animals with pylorus ligation, either alone or with intestinal ligation, equally inhibited gastric secretion. This suggests that while some factor inhibiting gastric secretion may be present, it appears to be unrelated to pylorus ligation.     

Lipase-catalyzed reactions in vesicles as an approach to vesicle self-reproduction

Abstract

As a continuation of our endeavor to find conditions under which bounded aggregate structures are able to self-reproduce, we have investigated the reactivity of lipase, both in free solution and vesicle-entrapped, against mixed oleic acid/oleate/ethyl oleate vesicles. Three types of vesicles have been prepared and characterized: (A) oleic acid/oleate vesicles; (B) oleic acid/oleate/ethyl oleate vesicles; and (C) lipase containing oleic acid/oleate vesicles. Long time stability studies by quasi elastic light scattering show that whereas (B) and (C) vesicles remain stable with a diameter of 110-130 nm and monodisperse for over a period of one month, vesicles (A) separated from an initial single population of 105 nm diameter into two populations, having respectively 70 nm diameter (more than 95% of the particles) and 180-210 nm diameter (less than 5% of the total population). In the case of vesicles (C), it could be shown that the enzyme remains localized inside the vesicles and it does not protrude into the water bulk phase. The enzymatic hydrolysis of ethyl oleate (which is water-insoluble) incorporated in the B-vesicles was studied under two configurations: (I) by adding lipase externally to the B-vesicles; (II) by mixing vesicles (B) and vesicles (C). In both cases, the reaction progressed to 100% hydrolysis. In the first case, the reaction was attended by an increase of the number of vesicles, and since this hydrolysis reaction takes place within the boundary of the parent vesicles, the criteria of autopoietic self-reproduction of vesicles are satisfied. In the case (II) instead, no increase of the population number of particles could be detected. The possible reasons for this difference are discussed.     

Life, self-reproduction and information: beyond the machine metaphor

The problem of representing information in automation models of self-replication is considered. It is shown that, unlike in the natural reproduction process, in a computable model the reproduced entities do not contain all the information necessary for guiding the process. Current theoretical understanding of life and its replication, based on such models, is argued to be essentially inadequate. The solution to this problem is claimed to require recognition of the theoretical fact that information in living systems is different from that subsumed under the category of "knowledge", which is representable as computer programs or triggers of state transitions. A discussion of fundamentals of a new theory of information and its relationship to replication models is given and a new direction of further developments of biological theories is envisioned.     

An evolutionary explanation of the aggregation model of species coexistence

In ecology, the 'aggregation model of coexistence' provides a powerful concept to explain the unexpectedly high species richness of insects on ephemeral resources like dung pats, fruits, etc. It suggests that females aggregate their eggs across resource patches, which leads to an increased intraspecific competition within occupied patches and a relatively large number of patches that remain unoccupied. This provides competitor-free patches for heterospecifics, facilitating species coexistence. At first glance, deliberately causing competition among the females' own offspring and leaving resources to heterospecific competitors seems altruistic and incompatible with individual fitness maximization, raising the question of how natural selection operates in favour of egg aggregation on ephemeral resource patches. Allee effects that lead to fitness maxima at intermediate egg densities have been suggested, but not yet detected. Using drosophilid flies on decaying fruits as a study system, we demonstrate a hump-shaped relationship between egg density and individual survival probability, with maximum survivorship at intermediate densities. This pattern clearly selects for egg aggregation and resolves the possible conflict between the ecological concept of species coexistence on ephemeral resources and evolutionary theory.     

Evolution: an axiomatic perspective.

In this paper we present an axiomatic theory of evolution which is inspired by the reading of a paper written by G.V. Schiaparelli in 1898. Schiaparelli was a famous astronomer, but he also studied the Darwinian ideas. We propose five axioms which can characterize the theory of evolution. We have also written these axioms using the language of the logic of predicates of first order with some constant monadic and dyadic predicates and appropriate functionals. But we can use other types of logic. So we can examine the concepts of species and of speciation. Then we introduce the interesting notion of generation distance. Moreover we give a theorem which establishes a geometrical model of our theory. If we analyze further the fourth axiom (which concerns the notion of generation distance) we can propose an elementary dynamical model by which we can represent possible evolutionary dynamics. These dynamics partially depend on random quantities.     

An evolutionary explanation for seasonal trends in avian sex ratios

We present an extensive set of data for five species of raptorialbirds to demonstrate that some raptor species produce an excessof daughters early in the season and an excess of sons in latenests, while others show the reverse. By means of a simulationmodel we investigate an evolutionary explanation for this phenomenonin terms of sex-specific differences in the relation betweenage at first breeding and date of birth. The model predictsthat that gender should be produced first in the season whoseage of first breeding is more strongly accelerated by an earlybirth date. We argue that this tends to be the male gender inraptor species, such as the common kestrel (Falco tinnunculus),which tend to breed early in life, while it is the female genderin larger species with later onset of breeding, such as themarsh harrier (Circus aeruginosus). The empirical evidence isqualitatively consistent with this hypothesis. Our model isquite general in that it makes no assumptions about the mechanism(primary sex-ratio bias at egg laying or secondary sex-differentialmortality before fledging) by which the bias is generated. Yetit is able to create quantitative predictions for species wheresufficient demographic and life-history data are available.From the available data set in the common kestrel we derivea quantitative prediction for the seasonal trend in brood sexratio. The observed trend is in good agreement with this prediction.[BehavEcol 7: 426–430 (1996)]     

The role of methylation in chemotaxis. An explanation of outstanding anomalies

The role of methylation in chemotaxis is understood generally, but several anomalies exist which bring into question the timing of methylation relative to sensing. A double mutant bacterium, deficient in both methyltransferase and methylesterase (Tr-Es-) is capable of chemotaxis even though the respective single mutants (Tr- and Es-) are not. This Tr-Es- mutant will accumulate in capillaries containing aspartic acid but not in capillaries containing serine despite the fact that both the aspartate and serine receptors are part of the methylation-dependent pathway. To understand these anomalies, a combination of theoretical analyses and experimental studies was performed. A mathematical analysis of the gradients of aspartate and serine in the capillary assay shows that outside the capillary the gradients are shallow, but just inside the mouth of the capillary they are very steep. Also, when the number of bacteria accumulated in the capillary is at a maximum, the range of attractant concentrations in the steep gradient just inside the mouth of the capillary is optimal for response and partial adaptation by the Tr-Es- mutant. We postulate that random motion brings the Tr-Es- mutant into the capillary, where it is able to move up the steep gradient. The difference in timing of the responses to serine and aspartate explains why the Tr-Es- mutant accumulates in aspartate- but not in serine-containing capillaries. A simple diffusion-capture model incorporating these concepts can account for experimental values of the number of Tr-Es- bacteria accumulating in the capillary. These studies provide a rational explanation for all of the apparent anomalies and lead to the conclusion that methylation/demethylation plays a crucial role in sensing as well as setting the zero point of the receptor.     

An explanation of fine structure map expansion in terms of excision repair

Summary Fine structure map expansion is a marker effect which can be explained on the basis of the repair of mismatched bases in hybrid DNA. The chance of a mismatched base pair being corrected independently of a closely linked mismatched pair will sharply increase as the distance between the two sites becomes greater than the length of the DNA segment involved in the correction process. The consequences of this explanation are worked out and it is shown that, if it is true, the mapping curve should show three phases: an initial additive phase when the recombining sites are closely linked, a phase of increased slope—map expansion-and a final additive phase of reduced slope beyond the expansion region. Comparison of the initial and final slopes should yield information on the relation between gene conversion and crossing-over. Many sets of experimental data show a clear transition from the initial additive region to that of map expansion, but evidence for the predicted final phase is scanty, possibly because fine structure maps cover too small a region of the chromosome. Using data from genes with known products, estimates can be made of the minimum length of the DNA segments involved in correction. These are calculated as about 40 nucleotides in fission yeast and at least 130 nucleotides in Neurospora.     

Dissident remarks on the origin of optical activity. An intrabiological explanation

The attempts to explain the origin of natural optical activity through external, extrabiological, agents, typically ionizing rays, i.e. as a purely incidental effect without deeper significance, have failed and should be abandoned. An alternative, intrabiological, Darwinian, explanation seeks to explain optical activity as a necessary phenomenon in early evolution. This explanation is based on the idea that organisms with the existing stereospecificity initially happened, by chance, to be more efficient than organisms with the opposite stereospecificity on independent grounds that have nothing to do with their stereospecificity. If cells based on L aminoacids are called L cells, it is to be assumed that cosmic bodies carrying L cells are about equally frequent as those carrying D cells.