Yeast Population Lab Report

How Environmental Factors Affect a Yeast Population’s Ability to Reproduce

OBJECTIVE

The objective of this experiment is to emphasize the influence that limiting factors have on a population. This lab tests yeast, a common component in baking, against two environmental factors (changes in temperature or concentration) to see what effect these have on the population dynamics of the yeast over a period of 72 hours.

There are two sections of tests included in this experiment: biotic and abiotic factors. The abiotic factor being tested here is what effect the temperature of the yeast’s environment has on its ability or inability to reproduce efficiently.

Yeast has many uses as a common ingredient in many foods and drinks, such as alcoholic beverages like beer and wine, and acts as a leavening agent in baking cakes, bread, and other foods by converting the fermentable sugars in the food into CO2. This is what makes the dough in many foods rise while baking.

This lab closely monitors a yeast population over a period of 72 hours, with various limiting factors being applied to the yeast population. These are factors that have the potential to greatly influence a population’s dynamics, sometimes positively and sometimes negatively. They are generally categorized into two groups: biotic and abiotic. Biotic factors pertain to life or living things, and some examples in an environment include predators that can kill or injure an animal of a species, competitors that make it more difficult for one animal to access vital resources like food and water, and pathogens or parasites that can quickly kill or weaken a species.

In most cases, the existence of predators is a good thing because it helps to keep the population from becoming unbalanced with the coexisting species living around them. However, if the predator population becomes too large, or if an abundance of new predators is introduced to the area, the population of this species will quickly decrease, and possibly be endangered or, after many years, extinct. In addition, having competition in an environment is important to keeping to well-balanced between plants and animals, but can backfire when there is too much competition, and plant life becomes scarce, unable to support the animals. This is true with all limiting factors: they can have a good or bad impact on a population.

On the contrary, abiotic factors pertain to non-living things, like sunlight, climate, temperature, and varying amounts of rainfall. For example, rainfall is essential in an ecosystem to hydrate both animal and plant life, and it is necessary for survival. However, too much rainfall at one period of time, or flooding, can wash away and kill many forms of plant life, damaging the populations that rely on plants for food, in turn. In addition, temperature can impact a population like yeast (which is tested in this experiment) positively or negatively.

For example, if the yeast’s environment is very warm, the yeast will be able to thrive in it. This is due to its activation in warmer temperatures. It is able to reproduce faster and more efficiently in a warm environment, thus producing more CO2. However, when placed in colder temperatures, it deactivates and, although it does not die or stop producing CO2, this drastically slows down the rate of reproduction and production of CO2. Therefore, these factors (both biotic and abiotic) can potentially, given the circumstances, greatly impact the dynamics of a population. This lab’s goal is to demonstrate these effects on yeast populations.

Yeast is the most efficient model to demonstrate these population dynamics because it can easily be closely watched and it is a simple organism. Testing multicellular organisms in a lab can be more challenging because many of them possess some sort of rational thinking method that can impact the results in a lab. Yeast is a simple, unicellular organism that has only two intentions in its life: survive and reproduce. This eliminates any impact that yeast’s intelligence could have on the experiment. This is one less control that needs to be worried about. Also, yeast’s reproduction is rapid.

If the lab were to test a multicellular organism, it would take weeks or even months to get a proper result. Using yeast, the lab only took 72 hours. Furthermore, collecting CO2 from the yeast is a viable method for determining the population growth of yeast because CO2 is a byproduct of the yeast’s cellular respiration process. As more yeast cells are produced, more CO2 will be produced because there will be more cells to produce the gas in the enclosed environment as they respire, as measured through the volume displacement method.

A carrying capacity in a population is “the maximum number of individuals of a given species that an area’s resources can sustain indefinitely without significantly depleting or degrading those resources” (“Population Size”). Once the carrying capacity is reached or exceeded, this will take a toll on the environment as a whole. The resources that support this species would quickly diminish, and any other animals that this species preys on would become endangered because there are too many predators killing them. This is why a good balance of limiting factors needs to be maintained to evenly support an ecosystem as a whole.

This lab investigates these factors, applies them to a yeast population, and analyzes the results.

EQUIPMENT AND PROCEDURE

Materials
Safety goggles, lab aprons
Latex-free gloves
Yeast suspension
Molasses solution
Several 1 mL graduated dropping pipettes
Plastic conical tubes
Larger glass test tubes
Glass beakers
Access to incubator and refrigerator

Procedure A (The Effect of Temperature on CO2 Production)

1.) Gather materials. 2.) Place 0.5 mL of the yeast concentration into the small conical tube. 3.) Add 15 mL of the molasses solution into the same conical tube. This tube should almost be filled to the very top. If needed, add more molasses to fill the volume. 4.) Wearing gloves, carefully cover the end of the test tube with the index finger and invert it five times to mix the solutions thoroughly. 5.) Carefully slide the larger glass test tube over the smaller tube. Try to apply pressure on both tubes with the thumb and third fingers. 6.) Quickly invert the tubes so that the bottom of the conical tube is now facing up. A small amount of the fluid may come out of the conical tube; this is okay. 7.) Repeat steps 2-6 for two more test tubes of yeast. Label each test tube with “cold”, “room”, or “hot” to indicate the environment they will be placed in.

8.) Using the measurements of the inverted tube, measure the volume of the liquid in the conical tubes and record the data. This is the initial reading. 9.) Place test tubes in three separate glass beakers. Then, place the beakers in the appropriate locations (i.e. the “hot” beaker in an incubator set at 30ᵒC, “cold” in a refrigerator at 4ᵒC, and “room” in an average environment at 22ᵒC). 10.) After 24 hours, measure the size of the bubbles with the measurements on the smaller tube. Record the data. 11.) Determine the total amount of carbon dioxide produced by subtracting the initial bubble size from the 24 hour bubble measurement. Record in data table. 12.) Return the tubes back to the appropriate locations.

13.) Repeat steps 10-12 at 48 and 72 hour intervals.
14.) When instructed, clean up the tubes and equipment.

Procedure B (The Effect of Yeast Concentration on CO2 Production) 1.) Place 0.25 mL, 0.5 mL, and 1 mL of the yeast concentration into three separate
conical tubes. Use glass beakers to hold each test tube to prevent spilling. 2.) Add 15 mL of the molasses solution into the same conical tube. This tube should be almost filled to the very top. If needed, add more molasses to fill the volume. 3.) Wearing gloves, carefully cover the end of the test tube with the index finger and invert it five times to mix the solutions thoroughly. 4.) Carefully slide the larger glass test tube over the smaller tube. Try to apply pressure on both tubes with the thumb and third fingers 5.) Quickly invert the tubes so that the bottom of the conical tube is now facing up. A small amount of the fluid may come out; this is okay. 6.) Repeat steps 2-5 for the two other test tubes.

7.) Using the measurements of the inverted tube, measure the volume of the liquid in the conical tube and record the data. This is the initial reading. 8.) After 24 hours, measure the size of the bubbles with the measurements on the smaller tube. Record the data. 9.) Determine the total amount of carbon dioxide produced by subtracting the initial bubble size from the 24 hour bubble measurement. Record data. 10.) Repeat steps 8-9 at 48 and 72 hour intervals.