Pushmepullyou: An efficient micro-swimmer

The swimming of a pair of spherical bladders that change their volumes and mutual distance is efficient at low Reynolds numbers and is superior to other models of artificial swimmers. The change of shape resembles the wriggling motion known as {\it metaboly} of certain protozoa.Comment: Minor rephrasing and changes in style; short explanations adde

Exposure factors manual

ABSTRACT: Assessing health risks associated with potential exposure to chemicals from petroleum or petrochemical operations requires the consideration of multiple exposure pathways. These pathways include ingestion of water, food, or soil, inhalation of vapors or airborne particulate, and dermal absorption from contaminated soil, water, or by direct skin contact. To estimate the exposures for each pathway, a number of variables related to exposure, that is, exposure factors, are needed. Some categories of exposure factors include physiologic factors (e.g., body weight), time-activity factors (e.g., time spent at home), and contact rate factors (e.g., soil ingestion rate). This manual is organized by exposure factor category and includes a description of selected exposure factors commonly used in risk assessments, a brief summary, and an evaluation of the current scientific data supporting a recommended point value for each factor, and available information on the known distributions. It is hoped that this information will promote consistency and quality among various risk assessment activities

Phosphatase localization in the endomembrane system of the dinoflagellate Crypthecodinium cohnii.

The distribution of four enzymes within the endomembrane system of the protist Crypthecodinium cohnii has been determined using cytochemical localizations with lead as a capture agent. Nucleoside diphosphatase (NDPase) activity, using inosine diphosphate (IDP) and thiamine pyrophosphate (TPP) as substrates, was observed in the Golgi apparatus, with a gradient of increasing reaction product noted in some cells from the cis to trans cisternae. Tubules and vesicles associated with the trans cisternae also contained reaction product. The endoplasmic reticulum exhibited a high activity of glucose-6-phosphatase [with glucose-6-phosphate (G-6-P) as substrate]. Traces of reaction product were also observed in the cis-most and trans-most cisternae of the dictyosomes. Activity of acid phosphatase (AcPase) was observed in Golgi cisternae as well as in associated cytoplasmic vesicles. Heaviest deposition was localized in medial and trans dictyosome cisternae. The cytoplasmic system of flattened vesicles subtending the surface membranes in these cells did not exhibit reactivity with any of the substrates used. The distribution of these enzymes in this algal cell appears similar to that observed in animal cells and suggests that these enzymes may represent markers for algal cell endomembrane compartments. </jats:p

Chromosome structure and mitosis in the dinoflagellates: an ultrastructural approach to an evolutionary problem

Chromosome structure and mitosis have been examined in three evolutionarily diverse members of the Pyrrophyta. Chromosome uncoiling, revealing the chromonema, has been correlated with the uptake of [3H]thymidine. In addition, chromosome uncoiling has been observed during gamete formation, gamete fusion, and in the nucleolar organizing region of the chromosomes suggesting that dinoflagellate chromosomes undergoing duplication, transcription or pairing have a morphology different from the characteristic tightly banded structure generally observed during most of interphase and mitosis. The dinoflagellate chromonema is composed of 2.5-nm fibers and 9.0-nm granules coiled into a helix around a central core of 9.0-nm fibers. Chromosome attachment to nuclear channels and kinetochore division and separation have been examined in several dinoflagellates. After evaluating many nuclear and cytoplasmic characteristics of the dinoflagellates it appears that this group of organisms are true eukaryotes which may be on the main line to the evolution of the mitotic spindle typical of higher plant and animals cells.