Calculating rates of respiration

The rate of respiration in yeast

Yeast can be used to investigate how the rate of respiration changes when different carbohydratesubstrates are used. For example, yeast can use glucose directly in respiration, or it may first have to break down other carbohydrate substances such as starch or sucrose to release the glucose that can be used in respiration.

The rate of a reaction can be calculated by measuring how much product is made in a specific amount of time. Carbon dioxide will be produced when yeast respires. This is a gas. The carbon dioxide can be collected using the equipment in the diagram below.

Experiment showing the rate of respiration in yeast

An example method that might be followed to investigate the rate of respiration is:

  1. Set up an inverted measuring cylinder in a water bath so that the cylinder is full of water and clamp into position.
  2. Add 20 cm3 of water to 1 g yeast and 0.5 g glucose and stir. Add this mixture to the round-bottomed flask.
  3. Place a rubber bung with a delivery tube into the neck of the round-bottomed flask and place the flask in a water bath at 40°C.
  4. Wait for five minutes for the yeast to begin respiring at a constant rate.
  5. Bubbles should now be emerging from the end of the delivery tube. Place the end of the delivery tube under the open end of the measuring cylinder and start the timer.
  6. Record the volume of gas produced after 3 minutes.
  7. Repeat steps 1 to 6 twice more.
  8. Repeat steps 1 to 7 using the other carbohydrate mixtures.

The amount of carbon dioxide released by the yeast in three minutes can be compared for each of the carbohydrates. The more carbon dioxide that was released in three minutes, the faster the rate of respiration with that carbohydrate substrate.

Question

To be able to compare results we need to ensure that control variables are kept the same during the experiment. Name two control variables for the experiment above.

Two from:

  • the temperature of the water bath
  • the mass of yeast used
  • the mass of carbohydrate used
  • the volume of water used
  • the species of yeast used
  • the time taken before the measurements are taken

Calculating rates of respiration

Question

In an experiment, Sarah found that 1 g of yeast produced 20 cm3 of carbon dioxide in three minutes when using glucose as a substrate.

What was the rate of respiration in cm3 of CO2 per minute when using glucose?

The rate needs to be calculated in cm3 of CO2 per minute. We know that 20 cm3 of CO2 was produced in three minutes. To calculate the rate of CO2 produced per minute we need to divide the volume of CO2 produced by the time it took to produce that volume of CO2.

Volume of CO2 produced = 20 cm3

Time taken to produce = 3 minutes

Rate of CO2 produced per minute = 20 ÷ 3 = 6.7 of CO2 produced per minute

Question

Using the table below, which substrate was the best for the yeast in terms of releasing energy quickly? Explain your answer. [3 marks]

Carbohydrate substrateRate of respiration (cubic cm of carbon dioxide per minute)
Glucose5.3
Sucrose3.1
Starch0.5
Lactose1.2

Glucose was the best substrate for the yeast (1 mark) because it gave the fastest rate of respiration (1 mark) meaning that the most energy was released in the least amount of time for this substrate compared to the others (1 mark).

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