Jessie -
When active (live) yeast has both sugar and oxygen available to it, it 'breathes' by a process called
aerobic respiration.
In this reaction, yeast cells use glucose (sugar) and oxygen (from the
air) to produce energy. They also produce water and carbon dioxide (a
gas). This is the same chemical process used by humans.
If no oxygen is available, yeast will switch over to a process called
anaerobic respiration
- in this process, glucose (sugar) is fermented to produce energy,
carbon dioxide, and ethanol. Since ethanol is a type of alcohol, which
is toxic for yeast cells, anaerobic respiration is a poor second choice
to aerobic respiration. (This is, however, the process used to make
wine, so sometimes it's not an all bad thing.)
There is a very nice site all about these processes at
http://www.purchon.com/chemistry/chemical.htm
As for your experiment, I'm not sure what to say. The thing about
respiration is that the yeast cells only do it for the sake of
producing energy. And they can only produce energy when the have
glucose (sugar). If you are using only water and yeast without adding
any sugar, I don't see what could be happening. My best guess is that
there is a small amount of sugar naturally dissolved in the water or
that the yeast have a small amount of sugar stored in their cells. If
this is the case then the gas you are noticing is carbon dioxide,
although I can't imagine that there is very much of it.
My suggestion is that you try your experiment again using sugar
water in place of regular tap water. I expect you will see much more
dramatic results.
-Tamara
(published on 10/22/2007)
