How to Demonstrate Cellular Respiration

Cellular respiration occurs as cells use molecular oxygen during the extraction of energy from chemical bonds in glucose and other food molecules. The best way to demonstrate cellular respiration is to show that a fixed volume of oxygen gas will diminish as respiration occurs. This is essentially as simple as putting respiring plant matter, such as germinating peas, into vials with attached pipettes, then submerging these "respirometers" in water; as oxygen gas is consumed by the cells, water will be taken up into the pipettes. A separate apparatus with glass or metal beads replacing the peas should be used as a control.

Things You'll Need

• 10 to 20 dried peas
• water
• paper towel
• 2 small glass vials
• 2 heavy steel washers
• 15 percent potassium hydroxide (KOH) solution
• 2 cotton balls
• 2 cotton ball-sized pieces of steel wool
• 10 to 20 pea-sized glass or metal beads
• 2 rubber stoppers with holes in the center
• 2 pipettes
• sealant
• large plastic tray

Instructions

Germinate the Peas

1. Soak the peas in room-temperature water for 24 hours.

2. Drain the water from the peas and wrap them in a moist, but not dripping wet, paper towel for another 24 hours.

3. Repeatedly moisten the towel during this period.

Assemble the Respirometers

1. Soak the two cotton balls in 15 percent KOH. Be sure you are wearing protective eyewear and gloves.

2. Insert these items into one vial in the following order: 1 heavy washer, 1 KOH-soaked cotton ball, 1 piece of steel wool, 10 to 20 germinating peas or as many as will fit. This is the vial in which respiration will occur.

3. Insert these items into one vial in the following order: 1 heavy washer, 1 KOH-soaked cotton ball, 1 piece of steel wool, 10 to 20 glass or metal beads or as many as will fit. This is the control vial.

4. Insert a stopper into each vile and apply sealant for waterproofing.

5. Insert a pipette into each stopper and apply sealant at the insertion point for waterproofing.

Equilibrate the Respirometers and Perform the Demonstration

1. Fill the plastic tray with enough water to cover the laid down respirometers.

2. Prop up the ends of the pipettes on the edges of the tray, and lay the glass vial ends of the respirometers in the water. Let them sit this way for 10 minutes as the gases inside equilibrate.

3. Completely submerge the respirometers. A little bit of water is expected to enter the mouth of the pipettes, but it should stop quickly. Make sure no water is entering the respirometers from other areas.

4. Check the respirometers every 5 minutes. Notice that the respirometer with the peas in it will slowly take in more and more water through the pipette as oxygen is used up. No change should occur in the control vial.

Tips & Warnings

• The purpose of the 15 percent KOH in this demonstration is to remove carbon dioxide gas from the air in the respirometers. Recall that carbon dioxide gas is a product of cellular respiration, and unless it is removed from the air mixture it will replace the oxygen gas that was used and no change in volume will occur.

• Potassium hydroxide (KOH) is a strong base and all children and adults present during the demonstration should be wearing protective clothing, including gloves and eyewear.

References

• For more details about cellular respiration.

Resources

• For details on a more in-depth experiment on cellular respiration.