Bead Populations Research Articles

CONTINUOUS FRACTIONATION OF GLASS MICROSPHERES BY GRAVITATIONAL SEDIMENTATION IN SPLIT-FLOW THIN (SPLITT) CELLS

ABSTRACT Split-flow thin (SPLITT) separation cells, consisting of submillimeter thick rectangular channels having now splitters at both inlet and outlet ends, were operated continuously using the earth's gravitational field as a driving force to prepare narrow fractions from polydisperse micron-size glass bead populations. Equations arc shown that make it possible to achieve a binary fractionation around a specified cutoff particle diameter by the control of inlet and outlet flowrates. Using a single separation cell, each narrow fraction was obtained by a two step fractionation. one dividing the panicle population around the upper desired limit and the other around the lower desired limit of particle diameters. The clean fractionation by SPLITT cell operation was verified by scanning electron microscopy, which also provided the mean particle diameter and the coefficient of variation for each fraction. The consistency of size distribution results was also examined by steric field-flow fractionation.

A simple and rapid method for determining the linearity of a flow cytometer amplification system.

We describe a simple and rapid method for determining the linearity of a flow cytometer amplification system. The method is based on a fundamental characteristic of linear amplifiers: The difference between two amplified signals increases linearly with increasing amplifier gain. Two populations of beads or cells, differing slightly in fluorescence intensity, are analyzed by the flow cytometer at increasing photomultiplier tube high-voltage settings. The distribution of the populations' mean difference versus mean position is a straight line intersecting the origin for linear amplifiers. Although some types of nonlinearities cannot be detected with this technique, deviations from linearity indicate nonlinear components in the flow cytometer amplification system. The correlation coefficient is used to quantify degree of nonlinearity. We also describe a method for amplifier nonlinearity compensation.

Why Reproduce? A Demonstration of Evolution and the Origin of Life

One of the main goals of general biology programs is explain the process of evolution. Despite long discussions of the process and its consequences, I have found students often retain their view of evolution as goal oriented, with organisms almost shopping around for a set of improved characteristics. This view is so locked into their minds that presenting a simple visual demonstration of the process seemed advisable. As a countermeasure, I developed a simple model illustrate several aspects of the biochemical theory of the origin of life and the process of evolution through natural selection. This model has students predicting the outcome of evolution in populations of beads. Since the underlying principles of evolution are so important in a general biology course, and, since student resistance learning them seems so high, I spend 45-50 minutes (and an almost equivalent time in the laboratory) working through this model with the class. I want the students themselves feel that the outcomes are obvious and inevitable so that their resistance learning the principles is overcome. This course segment fits topically between mitosis and meiosis. I begin it by addressing two questions: do cells divide? and do organisms reproduce? The most frequent responses from students these questions are, respectively, some variation on to help the organism survive and to perpetuate the species. By having them think about the cells in their own hands and algal cells in a stagnant pond, I convince them (I hope) that such teleology is absurd. I then point out that I want show them why my question reproduce? is the wrong question. A Model of Evolution