Materials

■ safety goggles

■ yeast cell culture

■ 1% methylene blue solution

■ compound microscope

Background

1. What is a limiting factor and how does it affect population size?

2. How are the terms population growth, birth rate and death rate all interrelated?

Procedure Counting Yeast Cells

1" CAUTION Always wear safety

^^ ^^ goggles and lab apron to protect your eyes and clothing. Put on safety goggles and lab apron.

^^ <lftV CAUTION Do not touch or taste any ^^ ^ chemicals. Know the location of the emergency shower and eyewash station and know how to use them. Methylene blue will stain your skin and clothing. If you get a chemical on your skin or clothing wash it off at the sink while calling to the teacher. Notify the teacher immediately of any spills. Spills should be cleaned up promptly, according to your teacher's directions. Glassware is fragile. Notify the teacher of broken glass or cuts. Do not clean up broken glass or spills with broken glass unless the teacher tells you to do so.

2. Transfer 1 mL of yeast culture to a test tube. Add

2 drops of methylene blue to the tube. The methylene blue will remain blue in dead cells but will turn colorless in living cells.

3. Make a wet mount by placing 0.1 mL, or about

1 drop, of the yeast culture and methylene blue mixture on a ruled microscope slide. Cover the slide with a coverslip.

These yeast cells have been stained with methylene blue and magnified with a high-power microscope. Methylene blue gives a deep blue color to dead yeast cells, but live yeast cells will actively remove the stain.

4. Observe the wet mount under the low power of a compound microscope. Notice the squares on the slide. Then switch to the high power. Note: Adjust the light so that you can clearly see both stained and unstained cells. Move the slide so that the top left-hand corner of one square is in the center of your field of view. This will be area 1, as shown in the diagram below.

5. Count the live (unstained) cells and the dead (stained) cells in the four corners of a square using the pattern shown in the diagram above. In a data table similar to the one below, record the number of live cells and dead cells that you counted in the entire square.

6. Repeat step 5 until you have counted all 6 squares on the slide.

7. Jlt^ Dispose of solutions and broken glass in the ^^ designated waste containers. Do not pour chemicals down the drain or put lab materials in the trash unless your teacher tells you to do so.

8. Clean up you work area and all lab equipment. ^^ Return lab equipment to its proper place. Wash your hands thoroughly with soap and water before you leave the lab and after you finish all work.

9. Refer back to your data table. Find the total number of live cells in the 6 squares. Divide this total by 6 to find the average number of live cells per square. Record this number in your data table. Repeat this procedure for the dead cells.

10. Estimate the population of live cells in 1 mL (the amount in the test tube) by multiplying the average number of cells per square by 2,500. Record this number in your data table. Repeat this procedure for dead cells.

11. Repeat steps 1 through 8 each day for 4 more days.

Analysis and Conclusions

1. Why were several areas and squares counted and then averaged each day?

2. Graph the changes in the numbers of live yeast cells and dead yeast cells over time. Plot the number of cells in 1 mL of yeast culture on the y-axis and the time (in hours) on the x-axis.

3. What limiting factors probably caused the yeast population to decline?

Further Inquiry

1. Write a question about population growth that could be explored in another investigation.

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