Potatoes and Peroxide (Nicole FitzGerald)
Potatoes and Peroxide: An investigation of how enzymes affect the rate reactions.
Principle(s) Investigated:
- chemical decomposition
- catalysis
Standards
CA Content Standards
8. Chemical reaction rates depend on factors that influence the frequency of collision of reactant molecules. As a basis for understanding this concept:
8.C. Students know the role a catalyst plays in increasing the reaction rate.
NGSS
4. Analyzing and interpreting data
8. Obtaining, evaluating, and communicating information
Materials:
You’ll need one of the following for each group.
· Potatoes ( 1 whole potato for each group plus some extra for slicing)
· Hydrogen peroxide (3%)
· Petri dishes
· Beakers
· Plastic spoons
· Plastic gloves
Procedure:
Each group should be given a whole (uncut) potato, and a potato sliced into 1cm thick discs
Teacher Prep: Set aside one whole potato for each group. Pre-slice the remainder of the potatoes into 2 cm thick discs. (Do not cut wedges. The surface needs to be flat.) Pour about 100mL of hydrogen peroxide into each beaker.
Note: this experiment works best for potatoes that are not damaged or cut up on the outside
Student Activity:
1. Put on plastic gloves
2. Using a spoon measure 1 spoonful of hydrogen peroxide.
3. Pour the peroxide onto the uncut potato. Wait about 2 minutes. Did you see any gas released? Record your observations in the quickwrite.
4. Pour one spoonful of peroxide onto the sliced potato. Wait 2 minutes. Was any gas released? Record your observations in the quickwrite.
(add photo 1,2,3)
Students’ prior knowledge:
· Students know that molecules are held together by chemical bonds. When these bonds are broken, new substances can be formed.
· Students should know that activation energy is the least amount of energy needed to allow molecules to change into another substance
Explanation:
In this experiment, Hydrogen peroxide is undergoing chemical decomposition- the breakdown of a compound into simpler compounds. When H2O2 decomposes it breaks down into water (H2Ol) and oxygen (O2g). However, in this experiment, the rate of decomposition is dependent upon the presence of a catalyst. Remember a catalyst speeds up the rate of a reaction by lowering the amount of activation needed.
The catalyst in our experiment is peroxidase. Peroxidase is an enzyme that can be found inside potatoes and in many living organisms. It is crucial in metabolic reactions and helps prevent damage to cells.
peroxidase
2H2O2 ------> 2H2O +O2
So, why do we see bubbles on the sliced potato?
The bubbles indicate the release of oxygen gas. When the Hydrogen peroxide (H2O2) comes into contact with the enzyme peroxidase, it quickly breaks down into H2O (water) and O2 (Oxygen gas). The release of these free oxygen radicals creates oxidation.
(diagram #1 to be added)
It is important to note that the peroxidase, itself, is not being consumed in the reaction, it is merely acting as a catalyst. When the catalyst is introduced, the rate of decomposition occurs more rapidly than had there been no catalyst. This immediate decomposition is visible when you observed the bubbles fizzling on the sliced potato.
Similarly, peroxidase enzymes exist in the blood and tissue inside the human body. Normally the skins acts as a barrier against oxidation. But, if you apply hydrogen peroxide to a cut or scrape on your skin, the enzymes in your blood are exposed, so they quickly speed up the oxidation process. That is why you can see bubbles on your wound.
(add photo #4)
Why doesn’t it work on the skin of the potato?
The outer skin of the potato does not contain the enzyme peroxidase. Peroxidase can only act on specific substrates. Each substrate on the Hydrogen peroxide, must fit onto the active site of the enzyme -peroxidase. Without this specific enzyme, the reaction is not accelerated and does not rapidly release oxygen gas. Since the surface of the potato skin does not have the right catalyst, then the oxidation process is much slower, and all you’re left with is a wet potato. That is why there are no bubbles.
(photos to be added later)
Similarly, if you pour hydrogen peroxide on unbroken skin you will not see any rapid fizzing action. That is because these specific enzymes only exist within the blood and tissue cells inside the body, but not on the outer skin layer.
What is the significance of this experiment?
Enzymes, like peroxidase, serve as important catalysts in the day to day metabolic reactions of our bodies. Consider the following. In most of our body’s metabolic reactions the compound hydrogen peroxide is produced. However, hydrogen peroxide can damage cells, so it is important to have enzymes that break down H2O2 quickly, as it is formed, before it can cause damage to cells. If catalysts like peroxidase were not present in our body, then compounds like hydrogen peroxide would damage our inner organs and tissues.
Questions & Answers:
1. Hydrogen peroxide, even without enzymes, would break down into water an O2 gas. Then why do we need enzymes like peroxidase in our body?
Peroxidase is accelerating the decomposition rate. If H2O2 was not broken down quickly and was left in the H2O2 form too long in our body’s it would eventually begin damaging cells.
2. Michael scrapes his knee whiling playing basketball. He decides to use hydrogen peroxide to clean his wound. Michael notices the wound starts to foam after applying the peroxide, he infers the bubbles are a result of the peroxide killing the germs. Evaluate Michael’s inference. Do you agree or disagree? Why or why not?
If it is true that that the O2 gas is a result of germs dying then the same O2 gas should be visible if applied to unbroken skin. After all, there are germs on the surface of Michael’s skin too. If he were to test his idea, he would discover that the peroxide does not fizz when applied to unbroken skin. This is because the hydrogen peroxide is in fact undergoing oxidation, not because of the germs, but as a result of the peroxide reacting with the catalyst peroxidase.
3. Given what you about how H2O2 reacts with tissue and cells. Do you think it should be used as an antiseptic on small cuts and abrasions?
On the one hand, Hydrogen Peroxide does sterilize the area to help protect against infection. However, H2O2, even in diluted concentration, can cause tissue damage and slow the process of healing. It would be more advantageous to use another type of antiseptic that did not cause damage.
Applications to Everyday Life:
- Water Treatment- Hydrogen peroxide can be used to eliminate odor and ozone in drinking water. It reacts with the hydrogen sulfide, which . Since the byproducts of H2O2 are water and oxygen, it is considered environmentally friendly.
- Bleaching wood-pulp: Hydrogen peroxide is used in the manufacturing industry to make paper whiter. It helps break down complex chemicals in the cells walls of plants.