Showing papers by "University of York published in 1976"

Fracture in the crossed-lamellar structure ofConus shells

Abstract: Crossed-lamellar crystal architecture is the characteristic textural pattern of the calcium carbonate shell in many kinds of molluscs. By loading specimens from shells of the genusConus in various orientations in bending tests it is shown that crossed-lamellar structure is highly anisotropic. This anisotropy is to be expected from the microscopic and submicroscopic structure, particularly the substructure of the primary lamellae and their orientation to one another, and from the paths taken by cracks travelling through layers of different orientation.

The influence of weed-cover on the mortality imposed on artificial prey by predatory ground beetles in cereal fields.

Abstract: Pitfall trapping was carried out in a field of winter wheat in the Vale of York to determine the levels of abundance of adult carabid and staphylinid beetles, (which formed the bulk of the natural predator complex) in the field. A point quadrat survey was carried out at the same time to assess the vegetation cover round each trap. Predation pressure by the beetles in the field was monitored using fruit fly (Drosophila) pupae as artificial prey. These artificial prey were attached to small cards inserted in the field adjacent to the pitfall traps.The numbers of beetles caught were found to be directly related to the frequency and density of Poa annua L. (annual meadow grass) the only abundant non-crop plant present at the time. More beetles were caught in areas where Poa was abundant than where Poa was scarce. This was probably related to the more suitable environmental factors offered by these of the wheat itself was found to be unrelated to captures.The number of fruit fly pupae taken was shown to be related to the numbers of carabid and staphylinid beetles present, and also to the type of vegetation occuring around each card.The implication of these results for the effects of predation on potential pest-outbreaks in the crop are discussed.

The stimulus control of biting inAplysia

Abstract: A number of different parameters of the biting response ofAplysia californica were examined while varying either the strength of excitatory stimuli that elicit biting, or the strength of inhibitory stimuli that suppress biting and produce satiation. Response amplitude, response latency and the interresponse interval of repetitive responses were the parameters investigated. In individual animals as well as in the mean data, all parameters of the biting response were found to be affected in a graded manner when the concentration of seaweed extract was varied, and when animals were fed to different levels of satiation. Feeding animals with non-nutritive bulk produced graded effects similar to those seen when animals were fed with seaweed. These results indicate that the biting response is modulated in a graded manner by external stimuli which elicit the response, as well as by internal stimuli which produce satiation.

The Mediating Role of the Scientific Elite

Abstract: At its inception, the sociology of science wag centrally concerned with relations between the scientific community and scientific knowledge on the one hand, and the wider society on the other hand. This was true, for instance of Merton's early work on the emergence of modern science' and on the norms governing scientific research.2 It was also true of Barber's attempt to provide a first systematic overview of the field.3 Recently, however, there has been a growing concentration upon the intemal characteristics of the research community, and particularly on the academic research community, with the wider social setting taken as given and relatively unproblematic.4 Those responsible for these more narrowly focused studies have not been unaware that the research community is subject to external influences. But they have assumed that, at least in some modem societies, it has sufficient autonomy to make a separate analysis of its internal processes acceptable as a first approximation. This research strategy has proved to be fruitful. It has enabled researchers to isolate a series of manageable problems and has produced results which have significantly improved our understanding of scientific communication,5 the reward system of science,6 social control in science,7 the development of research areas,8 and so on. However, this approach has had the unfortunate consequence of drawing attention away from the possibility of extending the analysis of the research community in such a way that connections with the

Localization on mitochondrial DNA of mutations leading to a loss of rutamycin-sensitive adenosine triphosphatase.

Abstract: Four cytoplasmic mutants of Saccharomyces cerevisiae showing loss of mitochondrial rutamycin-sensitive ATPase activity but having significant cytochrome oxidase and NADH-cytochrome c reductase have been isolated. Genetic studies indicate the mutations to be closely linked to each other and have been assigned to a new locus, PHO1. The mutations show a low frequency of recombination with the OL12 locus, suggesting a linkage to this marker. They are not, however, linked to the OLI1 locus. Linkage of the ATPase mutations to the OLI2 locus is also indicated by restoration of wild-type diploids by sigma- clones that retain the segment of mitochondrial DNA carrying OLI2. Based on the recombinants issued from crosses of the mutants with a triple drug-resistant strain and an analysis of the resistance markers present in sigma- clones that are effective in restoring a wild-type phenotype, the PHO1 locus has been placed in the segment of DNA located between PAR1 and OLI2.

Space-time singularities

Abstract: A set of conditions for the reasonableness of space-time is proposed and investigated. Using these, together with strong causality and an assumption of genericness, it is shown that future timelike or null geodesically incomplete space-times contain either curvature or intermediate singularities, or primordial singularities.

Yeast Hexokinase: Substrate‐Induced Association‐ Dissociation Reactions in the Binding of Glucose to Hexokinase P‐II

Abstract: A method is described for the purification of native hexokinases P-I and P-II from yeast using preparative isoelectric focussing to separate the isozymes. The binding of glucose to hexokinase P-II, and the effect of this on the monomer-dimer association-dissociation reaction have been investigated quantitatively by a combination of titrations of intrinsic protein fluorescence and equilibrium ultracentrifugation. Association constants for the monomer-dimer reaction decreased with increasing pH, ionic strength and concentration of glucose. Saturating concentrations of glucose did not bring about complete dissociation of the enzyme showing that both sites were occupied in the dimer. At pH 8.0 and high ionic strength, where the enzyme existed as monomer, the dissociation constant of the enzyme glucose complex was 3 × 10−4 mol1 −2 and was independent of the concentration of enzyme. Binding to the dimeric form at low pH and ionic strength (I = 0.02 mol1−1, pH < 7.5) was also independent of enzyme concentration (in the range 10–1000.μg ml−1) but was much weaker. The process could be described by a single dissociation constant, showing that the two available sites on the dimer were equivalent and non-cooperative; values of the intrinsic dissociation constant varied from 2.5 × 10−3 mol 1−1 at pH 7.0 to 6 × 10−3 at pH 6.5. Under intermediate conditions (pH 7.0, ionic strength = 0.15 mol l−1), where monomer and dimer coexisted, the binding of glucose showed weak positive cooperativity (Hill coefficient 1.2); in addition, the binding was dependent upon the concentration of enzyme in the direction of stronger binding at lower concentrations. The results show that the phenomenon of half-sites reactivity observed in the binding of glucose to crystalline hexokinase P-II does not occur in solution; the simplest explanation of our finding the two sites to be equivalent is that the dimer results from the homologous association of two identical subunits.

Sequential changes in the lipids of developing proplastids isolated from green maize leaves.

Abstract: Changes in lipid composition were followed as a proplastid develops into a chloroplast. Methods were devised for the isolation of developing proplastids from sections of five different ages from the same 7-day-old maize (Zea mays var. Kelvedon Glory) leaf. Electron micrographs illustrate the homogeneity of the five types of plastid suspension, minimal contamination with other cytoplasmic membranes, and the presence of morphologically intact plastids in the proportions 85% (youngest), 85%, 80%, 70% and 60% (oldest), respectively. Both bundle sheath and mesophyll plastids are well preserved in isolation. Plastid numbers were determined from calibration curves of the chlorophyll content of each type of suspension, and lipid values then expressed as nmoles/10(6) plastids. Monogalactosyl diglyceride (MGDG), digalactosyl diglyceride (DGDG), sulfoquinovosyl diglyceride, and phosphatidyl glycerol (PG) all increase during plastid development but the rate of increase is different for each lipid. The largest changes are in MGDG (6-fold) and DGDG (4-fold). Phosphatidyl choline shows a continuous decline during plastid development. Phosphatidyl inositol and phosphatidyl ethanolamine were found in all the suspensions in low concentrations (0.4-4.0% of total lipid): calculations showed their presence could not be accounted for by bacterial or mitochondrial contamination. The increase in PG parallels the chlorophyll changes during development and at maturity 1 molecule of PG is present per 3 molecules of chlorophyll. The results are discussed in the context of the molecular structure of the photosynthetic thylakoid membranes.

Effects of nutrient supply and competition from other species on root growth of Lolium perenne in soil

Abstract: In competition experiments with Lolium perenne and Agrostis tenuis on sandy soil with nitrogen supplied and therefore not limiting, it was found that the competitive interactions could be explained in terms of phosphate uptake, and that the ratio of root weight: length was proportional to root density. The effects of competition were then investigated in an experimental system that enabled them to be distinguished from those of nutrient supply. High levels of nutrients specifically stimulated the production of fine laterals whereas competition affected length and weight of the root system of Lolium equally. The ecological implications are discussed in the light of recent physiological work on root responses to nutrient supply. re]19750324

The tegument of Schistosoma mansoni: a histochemical investigation.

Abstract: The main components of the schistome tegument were found to be neutral glycoprotein and phospholipid; a small quantity of glycolipid was observed in the male dorsal tegument. The tegument can be differentiated from other schisotsome tissues on the basis of enzyme content; three hydrolytic enzymes were shown to be specifically localized in the tegument: alkaline phosphatase, adenosine triphosphatase and indoxyl esterase. It is suggested that these enzymes could be used as intrinsic markers for tegument structures. The subtegumental cells appear to be the major sites of biosynthetic activity since they contain large amounts of RNA and mitochondrial enzymes.

The colour polymorphism in Enoplognatha ovatum (Clerck) (araneae: theridiidae)--temporal stability and spatial variability.

Abstract: Colonies of the visually polymorphic spider Enoplognatha ovatum have been studied in Nidderdale, Yorkshire, for up to six years. Morph frequencies are found to be stable within colonies between years but to vary between colonies only short distances apart. The stability of the polymorphism argues in favour of powerful selection acting on this character. The nature of the selective factors are unknown but they are not those which influence time of maturity of the spiders or the mature female population size, both of which have varied between years. A genetic basis proposed for the polymorphism is considered and rejected as being incompatible with stable morph frequencies.

Darwin's law of natural selection and Thom's theory of catastrophes

Abstract: The fitness of a population is defined to be a real smooth function of its environment and phenotype. Darwin's law of natural selection implies that a population in equilibrium with its environment under natural selection will have a phenotype which maximizes the fitness locally. By using Thom theory it is possible under a number of assumptions to make qualitative inferences about the phenotypic change of a population subject to natural selection in a continuously varying environment. When the environment is one or two dimensional, Thom's catastrophe theorem implies that sudden and substantial changes in phenotype can only arise from the fold and cusp catastrophes. It is shown that a population in stable equilibrium with its environment exhibits genetic assimilation, and that the main modes of evolution can arise from appropriate variations in the environment.

Skin conductance recovery and pupillometrics in chronic schizophrenia.

Abstract: A group of 31 chronic male shizopherics were found to be heterogeneous with respect to skin conductance recovery, when response was made to non-signal tones. Fast skin conductance recovery was related to slow pupillary constriction in the light/dark reflex. Conversely, slow skin conductance recovery was related to fast pupillary constriction in the light/dark reflex. The cholinergic nature of pupillary constriction and the implications of these findings for a generalized cholinergic hypothesis of schizophrenia are discussed.