Investigation - What Factors Affect Cellular Respiration
Name:____________________________________________________ Date:__________________

Investigation - What Factors Affect Cellular Respiration? 


This investigation uses respirometry techniques to calculate the rate of oxygen consumption (cellular respiration) in germinating pea seeds.   The effect of temperature and whether a seed has broken dormancy are quantified and graphed.  The ideal gas law and its concepts are reviewed and applied.

Essential Questions:


Each individual cell is responsible for the energy exchanges necessary to sustain its ordered structure.  Cells accomplish this task by breaking down nutrient molecules to generate ATP (adenosine triphosphate), which can then be used to run cellular processes that require energy.  This process is called cellular respiration which requires nutrient molecules and oxygen.   Carbon dioxide and water are products of the series of reactions involved in cellular respiration.


Measuring the rate of cellular respiration can either rely on measuring the amount of oxygen taken in, or the amount of carbon dioxide being released.   Respirometers are devices that measure these types of gas volume changes, and therefore provide information about the rate of cellular respiration.

The ideal gas law, describes the relationship between temperature, pressure and volume. (PV = nrT)

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.

Assembling the Respirometers

You will assemble two sets of respirometers, one that contains dry seeds and the other that contains germinating seeds that have been soaked in water overnight. You will then measure the rate of respiration for your sample at an assigned temperature. In order to ensure the volumes of the vial are equal, beads will be added to the dry seeds. KOH will be placed on cotton at the bottom of the chamber to absorb the carbon dioxide being released during respiration. The respirometers will be submerged in a pan of water; as the peas take in oxygen;   as oxygen is used by the respiring seeds, the gas pressure inside the respirometer will decrease and the water will flow into the pipet down its pressure gradient.  .

Lab Materials:   20 Germinating pea seeds,  20 dry seeds,  plastic beads,  2 respirometer vials, Weights for vials, 2 stoppers,  2 ml graduated pipets, sealant (Vaseline), absorbent cotton, nonabsorbent cotton, 1 round wood stick,  water bath, ice, 100 ml graduated cylinder, stopwatch or clock, water.    Dropper Bottle of 15% KOH

Safety – wear safety goggles.  KOH is caustic, avoid direct skin contact. 

*Respirometers may have already been assembled for you, with pipets permanently sealed with superglue.


1. Fill a graduated cylinder with 20 ml of water.  Count out 20 germinating seeds and place them into the graduated cylinder. Record the volume of your 20 seeds by subtracting 20 from the cylinder's final reading. Remove your seeds from the cylinder and place on a paper towel.checkbox

Volume of germinating seeds: ______

2.  Fill the graduated cylinder with 20ml of water.  Count out 20 dry (nongerminat ing) seeds and place them into the water.  Drop plastic beads into the cylinder until the final volume is the same as from step 1. Remove the seeds/beads from the chamber and place on a paper towel. checkbox

Volume of dry seeds + beads ______

3. Obtain the glass respirometer vials. Place an absorbant cotton ball in the bottom and add a few drops of 15% KOH to saturate the cotton.. Place nonabsorbant cotton on top of the KOH soaked cotten to prevent this caustic substance from touching your living specimens. checkbox

4.  Add the peas, peas/beads, and beads to the appropriate respirometer.  Place the stoppers on each of the vials and ensure they are secured tightly.  You will be assigned a temperature to measure the respiration rates (cold or room temperature).checkbox

5. Create your water bath with your assigned temperature. Lean your respirometers on the edge of the bath so that the temperature inside the chamber equals the temperature of the bath. Do no submerge them yet! Let them equalize for about 5 minutes. checkbox

6. If your water bath is in a dark plastic bin, place white paper towels at the bottom to make it easier to read the pipet. Alternately, a drop of food coloring can be used to see the movement of the gas in the pipet.checkbox

7. Carefully submerge your respirometers, a little bit of water will enter the tips at first. You may need to rotate the pipet in order to take readings, but once you have them submerged and situated, limit movement as this will effect your results. checkbox

8.  If your respirometers float, you may need to weight them.  Some come with weights inside and some do not. You can improvise here, stainless steel dissection scissors; for instance, can serve to weight the tubes. checkbox

9. Collect data on the data table for your assigned temperature. You will take readings at 4 minute intervals. checkbox

10. Note: Read the pipet as the water bubble moves down the tube toward the respirometers. You are using a 1 ml pipet, so each of the units on the reading are .9, .8, .7, etc. If you are having trouble finding the bubble, place a drop of food coloring near the tip of the pipet. checkbox


Time interval (min): 0 min 4 min 8 min 12 min 16 min
Room Temperature, Dry Peas  
O2 Consumed (Final reading - Initial Reading) 0
Room Temperature, Germinating Peas  
O2 Consumed (Final reading - Initial Reading) 0
Cold, Dry Peas  
O2 Consumed (Final reading - Initial Reading) 0        
Cold, Germinating Peas            
O2 Consumed(Final reading - Initial Reading) 0        

Graph: Graph your data below. There will be 4 lines on this graph, be sure to label and color code them so that the graph is readable. You should use a LINE OF BEST FIT, so that slope can be calculated. 

Graph: Respiration of Pea Seeds

Calculate Slope

The slope of your lines will represent the respiration rate of each of your samples:

Respiration Rates for each sample:






1.   State a hypothesis that relates to temperature that is being tested by this lab exercise.



2.   State a hypothesis that relates to the state of seed germination that is being tested by this lab exercise.



3.  In this lab exercise, what is the purpose of the ….

a) Beads


b) KOH


c) Respirometer


4. Use the Ideal Gas Law to explain why water moved into the pipet. 





5. Write a summary (conclusion) for this experiment. Consider all of the variables that were tested and determine which factors had the greatest impact on the rate of respiration. Your conclusion should use EVIDENCE BASED REASONING.