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Dana Edwards 09/30/07
AP Environmental Science
Yeast Population Density and Population Growth
My group chose the environmental condition of high temperature, placing the yeast sample in an incubator.
The population begins by increasing, the first stage of the graph, reaches its peak around day five, the second stage, stays relatively constant with a small decrease, the third stage, and finally declines greatly, the fourth and final stage of the graph.
The population increases greatly during the first four days. Between day one and day two, the number of cells more than quadrubles, from an average of 15 cells per field of view to an average of 65. From day two to day three, the population encounters its largest growth rate in all the nine days of the experiment, 532 %, rising to an average of 346 cells. This high rate of growth observed in the first two days is perhaps due to density-dependent factors of environmental resistance. There was plenty of molasses (the yeast’s food source) to start, and so it did not limit growth. Between day three and day four, the growth rate drops to 226%, a significant decline. The yeast may have, at this point, encountered resistance for the first time. The amount of molasses, and perhaps an issue of crowding among the cells, limited much further growth, as the population rose only slightly from 783 on day four to 817 on day five. This was the highest population recorded, and perhaps a bit overshot of the carrying capacity. It declined to 713 on day six, probably at or near the carrying capacity for the yeast, for it remained nearly constant until day seven, at which point it dropped sharply all they way through day nine.
Our yeast culture showed some significant differences in growth pattern as compared to the control culture. It grew at a greater rate; in the first few days the population increased four or five-fold each day, while in the control it barely doubled. This is perhaps due to the incubator in which our culture was placed. The high temperatures could have expedited growth. The control’s population remained relatively constant near its carrying capacity for longer than ours did. Perhaps, because of its fast growth, our population went above its carrying capacity, and then went down to it, remaining there for one day. Ours stayed for a shorter time because of the overshoot; it peaked and then fell. Finally, ours dropped at a greater rate than did the culture of the control. This may have to do with our culture’s high rate of growth and high populations during most of the trial. The yeast used up its recourses quicker than the control’s did.
There were a few possible sources for error in the conduction of this experiment. The process of visually recording the numbers of cells, through the microscope, was difficult and imprecise, and so the density values attained could be incorrect. Another possible source for error lies in the distribution of the yeast cells in the container. Because they tended to clump on the bottom, it was necessary to shake the container prior to each recording, in order to distribute the cells uniformly. It is possible that the containers were not shaken sufficiently each time, skewing the results. By making sure to shake the container the same amount before each viewing, the results would be more accurate representations of the actual populations of the yeast.
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