Investigating respiration in mealworms (larvae of the beetle Tenebrio
molitor)
Introduction
Every cell is responsible for creating enough energy necessary to sustain itself, including
producing enough energy to maintain the chemical reactions. Cells accomplish this task
by breaking down nutrient molecules (glucose) to generate ATP (adenosine
triphosphate), which can then be used to run cellular processes that require
energy. This process is called cellular respiration, and requires glucose molecules as
wells as oxygen. Carbon dioxide and water are products of this reaction, see equation
below:
There are lots of different methods available to measure the rate of cellular respiration.
The two methods we are going to compare are the Respirometer and the CO2 Gas sensor.
The Respirometer:
Potassium carbonate (K2CO3) is a solid precipitate. Any CO2 produced is immediately
converted from a gas to a solid and is therefore no longer governed by gas laws. This
allows the respirometer to measure only one variable, the consumption of oxygen gas by
living cells.
During cellular respiration, two gases are changing in volume. Oxygen gas is being
consumed by the respiring cells and carbon dioxide gas is diffusing out of the cells. The
respirometer, therefore, has to be able to deal with two simultaneously changing gas
volumes. This is accomplished by introducing potassium hydroxide into the device. KOH absorbs carbon dioxide, following this equation
CO2 + 2KOH  K2CO3 + H2O
Potassium carbonate (K2CO3) is a solid precipitate. Any CO2 produced is immediately
converted from a gas to a solid and is therefore no longer governed by gas laws. This
allows the respirometer to measure only one variable, the consumption of oxygen gas by
living cells.
The Gas sensor
The detects the amount of carbon dioxide gas within the chamber. The probe can be
connected to a laptop and calibrated to record the rate of gas production over a
specified period of time.
Your task: You are going to use and compare two different techniques to try and
calculate the rate of respiration in mealworms.
Experiment 1: Respirometer
Variables:
Independent variable
Dependent variable
Control variables
1
2
3
Method:
1. Set up your equipment as shown in figure I, you will need 5 mealworms, place
these in the petri dish on your table until you are ready to use them. Do not
place any living organism into your respirometer until you have practiced
creating the seal – do this successfully three times before adding the
mealworms, you must not drown them.
Figure I: How to set up the respirometer
Things to consider
a. KOH is toxic to living tissue, wear gloves when handling it and do not let
the mealworms come in contact with any surface that has had contact
with KOH.
b. The pipette will slowly fill with water. Make sure you stop the
experiment before the water gets too close to the mealworms.
c. The mealworms will be stressed by this whole experience, try to keep
everything as smooth as possible.
2. Record the starting position of the air bubble into a table. Use figure II to help
you take readings from the air bubble.
Figure II: Diagram showing the position of the air bubble inside the pipette.
3. Record the position of the bubble, every minute for 15 – 20 mins.
4. Plot a graph of the movement of the bubble against time; you should be able to
use the slope of this line to calculate the rate of respiration.
5. List the main evaluation points for this procedure.
6. Suggest a research question that could be investigated using this technique.
Experiment 2: Gas sensor
Variables:
Independent variable
Dependent variable
Control variables
1
2
3
Method:
1. Set up the gas sensor and connect to your laptop, you will also need 5
mealworms (you may find you need to use 10 – 15 mealworms depending on the
size of the flask), place the mealworms in the petri dish on your table until you
are ready to use them. Do not place any living organism into your gas sensor
until you are ready to conduct the experiment, this will be a stressful
experience for them.
2. Use the gas sensor to record the change in CO2 levels for 15 – 20 minutes.
3. Use this information to calculate the rate of respiration.
4. List the main evaluation points for this procedure.
5. Suggest a research question that could be investigated using this technique.
Comparison of the techniques
Questions to consider:
1. In your opinion, which of the two methods is more reliable? Why?
2. Which of the two methods is more precise? Why?
3. Which method do you think, generates the most accurate data?
4. If you had been asked to process the data from this investigation, how would you
do that? What analysis would you use?
5. How many graphs would you present as part of a data analysis for this
investigation? What type would they be? What would they show?
6. Suggest a possible research question that could be used to compare the two
procedures.
7. In your opinion, would it be a ‘good’ idea to conduct an IA comparing two
different experimental techniques?