Biology
Cellular Respiration Lab
Introduction:
Cellular respiration is often confused with the process that exchanges vital gases with the atmosphere.
Breathing, also called respiration, is the act of bringing oxygen into the body and getting rid of carbon dioxide,
which the body produces but does not use. Cellular respiration is a process which uses the oxygen which has been
inhaled to release the energy stored in food. Cellular respiration is, in many ways, similar to combustion, or
burning. You can think of food as the fuel which is being burned. In both processes, respiration and combustion,
energy is released and oxygen is required for the processes to continue. As with combustion, respiration produces
carbon dioxide, CO2 as one of its products. Because our bodies do not use the CO2, it is exhaled as a waste
product.
Most organisms constantly produce carbon dioxide, CO2, when they use energy. CO2 is easily dissolved in
water. When CO2 dissolves in water it forms a weak acid. You can test for the presence of CO2 by bubbling your
breath through water and checking for a decrease in the pH of the water. You can measure the amount of CO2 in
the water using two chemicals: phenolphthalein (FEE-NOLL-THAY-LEEN) and sodium hydroxide (NaOH).
Phenolphthalein remains colorless in the carbonic acid formed by the dissolved CO2. Phenolphthalein will indicate
the presence of a base, such as NaOH, by turning pink. By adding a known quantity of the NaOH, you can
determine the amount of CO2 dissolved in the water. In this lab you will be determining how much CO2 your body
is giving off while sitting calmly, and after mild and vigorous exercise.
Materials:
1% phenolphthalein
100 ml cylinder
clean soda straws
clock with a second hand
0.4% NaOH
1 small flask, with a stopper
1 large flask, without a stopper
white paper
Purpose: To discover if your body’s CO2 output changes with exercise.
Procedure:
1. Measure 100 ml of tap water and transfer it to the small flask.
2. Add 5 drops of phenolphthalein to the tap water and swirl the flask gently.

If the water turns pink, there is very little or no CO2 in the water. Stopper this and set it aside for your
standard of pinkness.

If the water does NOT turn pink, the tap water has some CO2 in it. To find out how much, add NaOH
drop-by-drop until the solution turns pink. It may help to hold the flask up against the white paper to see
the color change more easily. Count the number of drops of the NaOH it takes to turn the water slightly
pink, indicating that the CO2 has been removed from the water. Once the water has a slight pink color,
stopper it and set it aside for your standard of pinkness. This is your control.
3. In the following steps, try to match the color with the standard which you have set aside.
4. Prepare the large flask with phenolphthalein, like you did the first flask. (100 mL of water + 5 drops PPT.)
5. Sit quietly for one minute. Put the soda straw in the water. Blow air from your lungs into the water for 10
seconds. You may need to cover the mouth of the flask with your fingers to prevent being splashed by the
water/phenolphthalein.
6. Swirl the water gently as you add drops of NaOH. Be sure to count and record the number of drops of NaOH it
takes to turn the water the same amount of pinkness as your standard.
7. Dump the contents of the large flask down the drain and then prepare another 100 ml of water and 5 drops of
phenolphthalein. KEEP YOUR FIRST FLASK WITH THE STOPPER IN IT.
8. Walk in the hallway for one minute, then immediately blow into the flask for 10 seconds as you did in step #5.
9. Repeat step #6.
10. Repeat steps #7 and #8, but exercise more vigorously this time by running (fast) or doing chair step-ups for 1
minute. You should be out of breath by the end of the exercise.
Hypothesis:
Name _________________ Per ___
What do you think will be the effect of exercise on CO2 production, AND WHY? Please write your hypothesis in
If….then….because format!
Results:
Relative Amounts of CO2 Produced by Several
Levels of Activity
Testing Conditions
Number of Drops
Standard
After Sitting
After Mild Exercise
After Vigorous Exercise
Graph:
On a piece of graph paper, make a BAR GRAPH (histogram) of the data collected in the data table. Make sure
your graph is appropriately titled and labeled. You DO NOT have to write a summary of the graph.
Analysis: (CSIQ)
1. Using the data, how is CO2 production related to exercise?
2. Using your lab data, describe in quantitative terms (actual numbers), the effect of exercise on CO2 production.
3. In today’s lab, where does the carbon dioxide come from? (Hint – It’s not the lungs)
4. In cellular respiration, what molecule is being burned/used?
5. Cellular respiration should increase with exercise. Explain why this happens.
TEACHER NOTES
To prepare NaOH: 500 mL distilled water + 6 pellets of NaOH