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Name: Pat G.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2/19/2003


Question:
WoW! I am so impressed that this sight is available to teachers, I do an experiment with my student where they pour hydrogen peroxide over yeast and it produces oxygen. I know yeast mixed with water yields carbon dioxide. Please explain why or what is happening. No student has asked as of yet because they are so excite about the resulting reaction when they stick a glowing stick into the oxygen produced.


Replies:
Pat,

Glad you like the site. We are happy to assist.

As you probably already know, the source of oxygen is the decomposition of the hydrogen peroxide into oxygen gas and water. The enzymes in the yeast assist the decomposition. However, almost any finely divided solid will accelerate peroxide's decomposition rate. Try the experiment using a small amount of finely ground up salt or powdered sugar and compare the bubbling rate with that produced when using yeast.. Be careful, do not start experimenting, Some materials can decompose peroxide with explosive violence.

Regards,
ProfHoff 571


The yeast contain an enzyme (that we have as well by the way) called catalase. Metabolism in oxygen produces substances that are toxic to the cells such as hydrogen peroxide and superoxide anion (free radicals). Organisms must have a way of breaking this down or be poisoned. The enzyme catalase turns hydrogen peroxide into water and oxygen, both harmless. So when you pour hydrogen peroxide on the yeast, you are seeing the effect of this enzyme and the chemical reaction it mediates. When yeast metabolize sugar, they produce carbon dioxide as a by product of metabolism (which we also do by the way-yet we exhale it). So the production of carbon dioxide will not be immediate, but as they begin to use the sugar, bubbles will appear. These bubbles will be carbon dioxide. They also produce ethanol (alcohol) in the balanced chemical reaction. Bakers use yeast to rise dough-the carbon dioxide escaping gets trapped as gas pockets in the dough and it begins to expand. This also gives the bubbles in fermented beverages such as beer (also alcohol). So yes, yeast do produce both gases but for very different reasons and at different times and rates during their life cycles.

Van Hoeck


I know that I speak for all of the contributors to Newton BBS that it is our small way to contribute, to give back to our children and to their teachers that spark of why?, how? and what if? that led each of us to pursue his or her path in science. On this particularly sad day we reflect upon those special scientists and engineers who have made their ultimate contribution to each ones quests for knowledge. (2.Feb.2003)

Yeast mixed with water (and for even better results with a bit of sugar) produce CO2, the reaction that causes bread to "rise". When you mix hydrogen peroxide with yeast several things can occur that cause the evolution of oxygen. It may oxidize any number of components present in a yeast culture, or the microscopic particles of yeast can serve as sites for the spontaneous decomposition of peroxide -- that is, it may just be a physical effect in contrast to some chemical reaction.

Vince Calder


Hydrogen peroxide is a toxic by-product of respiration. Organisms that obtain energy by oxidation of foods must develop mechanisms to limit the damage it causes. This is primarily accomplished by a class of enzymes called catalases, which catalyze the reaction
   2 HOOH --> 2 H2O + O2
Yeasts can operate both in the presence and the absence of oxygen. In the presence of oxygen, they break sugars down to water and carbon dioxide, consuming oxygen and in the process making some unintended hydrogen peroxide, which must be removed using catalase. So the yeast have catalase ready to go. When hydrogen peroxide is added to them, it is broken down by the catalase.

Humans, too, must de-toxify hydrogen peroxide produced in the process of our metabolism. We, too, use catalase for this purpose. That is why hydrogen peroxide foams when poured on a cut: the catalase in our tissues breaks hydrogen peroxide down to may water and oxygen.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois



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