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13 Population Dynamics

# 13 Population Dynamics - Experiment Population Dynamics 13...

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Biology with Calculators 13 - 1 Experiment 13 Population Dynamics As organisms reproduce, die, and move in and out of an area, their populations fluctuate. In a closed population, organisms do not move in and out of an area, so their populations change only through natality , or births, and mortality , or deaths of individuals. Yeast are very small, rapidly reproducing organisms. They can experience dramatic population changes over a relatively short time period. In this experiment, a small population of yeast will be given a small amount of food and placed in a closed environment. Organic materials will not enter or leave the environment—only inorganic gases will be exchanged. The population of yeast can be monitored by measuring the turbidity , or cloudiness, of the culture that contains the yeast. To measure the yeast population, you will be using the Colorimeter shown in Figure 1. In this device, light from an LED light source will pass through the culture containing yeast and strike a photocell. Photons of light that strike a yeast cell will be reflected away from the photocell and will make the culture appear more turbid. The Colorimeter monitors the light received by the photocell as either an absorbance or a percentage transmittance value. The absorbance value is proportional to the population of yeast present in the culture. Figure 1 In this experiment, a culture of yeast in a broth is divided into a number of test tubes. Every day for two weeks, a test tube is removed from the incubator and the amount of yeast is determined by microscopic count and by measuring its turbidity. After nine days, you will plot the population of yeast as a function of time. MATERIALS LabPro or CBL 2 interface dropper pipet or Beral pipet TI Graphing Calculator glass-marking pencil DataMate program graph paper Vernier Colorimeter microscope 18 X 150-mm test tubes microscope slide and cover slip cuvette and lid test-tube rack two 5-mL pipettes or 10-mL graduate cylinders cotton swabs Graphical Analysis (optional) PRE-LAB ACTIVITY Your instructor will be placing a small amount of yeast in a closed environment containing food at the start of the experiment. Predict how the yeast population might change over a long time period in Table 1. Sketch a plot of the amount of yeast vs. time in Table 1.

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13 - 2 Biology with Calculators Experiment 13 PROCEDURE Prepare the calculator and CBL If instructed to do so by your teacher, one team should prepare the TI Graphing Calculator and LabPro or CBL 2 interface for use. The TI Graphing Calculator and interface will be located on a resource table for use by all of the class teams.
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