Practical 14: Anaerobic respiration in yeast

Aim

To investigate how temperature affects the rate of anaerobic respiration in yeast, by measuring the volume of carbon dioxide produced.

Background

Yeast is a single-celled fungus. When it has no oxygen it respires anaerobically, releasing energy from glucose without using oxygen: glucose → ethanol + carbon dioxide. This process is called fermentation. Yeast is used in baking and brewing.

The rate of anaerobic respiration can be found from the volume of carbon dioxide produced in a set time. Carbon dioxide turns limewater cloudy, which confirms what the gas is.

The reactions of respiration are controlled by enzymes. As the temperature rises the enzymes work faster, until the optimum temperature, which is usually about 35–40 °C for yeast. At higher temperatures the enzymes are denatured, because their shape changes, and the rate falls.

To make the yeast respire anaerobically, the sugar solution is boiled and cooled to remove dissolved oxygen, and a layer of oil on top stops oxygen getting back in. The same method can be used with different sugars or sugar concentrations as the independent variable.

Hypothesis

The volume of carbon dioxide produced in 5 minutes will increase as the temperature rises to about 40 °C and then fall at higher temperatures, because enzymes work faster up to their optimum and are denatured above it.

Variables

Equipment

• Dried yeast (1 g for each run)
• 5% glucose solution (20 cm³ for each run)
• Boiling tubes (one for each run) and bungs with delivery tubes
• 50 cm³ gas syringe
• Vegetable oil
• 250 cm³ beaker for a water bath, kettle and cold water (or a thermostatically controlled water bath)
• Thermometer
• Bunsen burner, tripod, gauze and heatproof mat, to boil the glucose solution
• Stopwatch
• Measuring cylinder, 25 cm³
• Stirring rod, balance and spatula
• Eye protection
• Limewater and a test tube (to confirm the gas)

Risk assessment

Method

1. Boil some 5% glucose solution to remove dissolved oxygen and allow it to cool in a covered container.
2. Set up a water bath in a 250 cm³ beaker at 20 °C using cold water and hot water from the kettle, and check it with a thermometer.
3. Measure 20 cm³ of the cooled glucose solution into a boiling tube and stand it in the water bath for 5 minutes so that it reaches the temperature of the bath.
4. Weigh 1 g of dried yeast and add it to the glucose solution. Stir to mix.
5. Gently pour a 1 cm layer of vegetable oil on top of the mixture. This stops oxygen getting in, so the yeast respires anaerobically.
6. Put the bung and delivery tube in the boiling tube and leave it in the water bath for 5 minutes, so that the dried yeast becomes active and the mixture is at the bath temperature. Then connect the delivery tube to the gas syringe, with the plunger pushed fully in, and start the stopwatch.
7. Keep the water bath at the same temperature, adding hot or cold water if needed.
8. After 5 minutes, read the volume of gas in the syringe, in cm³, and record it.
9. Repeat steps 3 to 8 with a new tube of yeast at 30, 40, 50 and 60 °C.
10. Repeat the whole investigation twice more, so that each temperature has three runs.
11. Calculate the mean volume at each temperature, and the rate in cm³/min (mean volume ÷ 5).
12. To confirm the gas, connect the delivery tube from a fresh mixture to a test tube of limewater and observe the limewater turning cloudy. Wash your hands.

Results

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

Drawing the graph

Line graph with temperature (°C) on the x-axis and rate of gas production (cm³/min) on the y-axis, starting at 0. Plot the points and join them with a smooth curve (or straight lines between points). Identify the optimum temperature at the peak.