Practical skill 13: Temperature and respiration in yeast

Aim

To investigate how temperature affects the rate of respiration in yeast by counting the bubbles of carbon dioxide released in 2 minutes.

Background

Yeast is a single-celled fungus. Yeast cells respire glucose to release energy, and carbon dioxide is one of the products. The rate at which carbon dioxide is produced shows the rate of respiration.

Respiration is controlled by enzymes. As the temperature increases the reaction becomes faster, up to the optimum temperature. Above the optimum, the enzymes are denatured, so respiration slows and then stops.

In this investigation the yeast is in a solution of glucose, which is its source of energy. The carbon dioxide is shown by bubbles from a delivery tube, and it can be identified using limewater, which turns cloudy.

Hypothesis

The rate of respiration in yeast will increase as temperature increases up to an optimum temperature, and then decrease, because the enzymes are denatured at high temperatures.

Variables

Equipment

• Yeast suspension (dried yeast mixed with water, about 1 g in 10 cm³), made up by the technician at least 15 minutes before the lesson
• 10% glucose solution
• 6 boiling tubes (large test-tubes, 150 mm × 25 mm), each with a bung and delivery tube
• 6 standard test-tubes for 5 cm³ portions of yeast suspension
• 6 test-tubes of water to collect bubbles
• 250 cm³ beakers as water baths, or thermostatically controlled water baths
• Hot water from a kettle, cold water and ice
• Thermometers, –10 °C to +110 °C
• Measuring cylinders, 10 cm³, or 10 cm³ syringes
• Stop-clock reading to 1 s or better
• Test-tube racks, and a marker pen for glassware
• Limewater (to identify the gas)
• Eye protection

Risk assessment

Method

1. Put on eye protection. Label six boiling tubes and six test-tubes with the temperatures 10, 20, 30, 40, 50 and 60 °C.
2. Use a measuring cylinder to put 10 cm³ of glucose solution into each boiling tube.
3. Stir the yeast suspension and put 5 cm³ of it into each labelled test-tube.
4. Set up a water bath in a beaker for each temperature. Use a thermometer to check it, adding hot water or ice to adjust it.
5. Stand each boiling tube of glucose and its test-tube of yeast in the matching water bath for 5 minutes, so that both reach the temperature of the bath.
6. Pour the yeast into the glucose solution in the first boiling tube. Fit the bung and delivery tube straight away and put the end of the delivery tube under the water in a test-tube.
7. Wait 2 minutes for the bubbling to become steady, then start the stop-clock.
8. Count the number of bubbles of gas that come from the delivery tube in 2 minutes. Record the number.
9. Repeat steps 6 to 8 at each of the other temperatures. Check each water bath with the thermometer and add hot water or ice to keep it at the right temperature.
10. Repeat the whole investigation twice more, using fresh solutions, so that you have three readings at each temperature.
11. Calculate the mean number of bubbles in 2 minutes for each temperature, then the rate using: rate = mean number of bubbles ÷ 2.
12. To identify the gas, lead the delivery tube from the 30 °C or 40 °C tube into limewater instead of water and observe that the limewater turns cloudy.
13. Plot a graph of rate against temperature.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

A line graph with temperature / °C on the x-axis (0 to 60) and rate / bubbles per min on the y-axis. Plot the six points as small crosses (×) and draw a single smooth curve of best fit, which should rise to a peak and then fall. Use a scale so that more than half of the grid is used in both directions. Read the optimum temperature from the highest point of the curve.