Re: do yeast respire when they are placed in water without any sugar

Hi Leow,

In order to produce carbon dioxide, any cell needs a carbon source, such as sugar, protein, or some kind of lipid. As they respire, they are oxidizing the sugar, which provides energy for their cellular processes, producing carbon dioxide as a waste product. If yeast are placed in a beaker of plain water, there is no source of carbon for them to use. They will quickly use up what little sugar they are able to store in their cells, and will then begin to break down their own components to use as a source of energy.

If you or I stop eating, we can continue to produce carbon dioxide for several days, as our bodies use up stored food in the form of glycogen (a kind of starch), body fat, and protein from our muscles and other tissues. A typical human can live for several days on those stored reserves. On the other hand, a yeast cell cannot store much starch or fat. They can, however, slow down their metabolism when there is no food available. This is known as the "starvation response". During this time, they don't do much of anything, except wait for food to become available. The only source of energy they have is their own structure. As the cell consumes itself, carbon dioxide will be produced, but this is obviously not a sustainable situation. It is analogous to heating a house by burning the wood it is made of ... you may be able to heat it for awhile by using the wood from unnecessary walls and doorways and closets, but eventually, you won't have a house left. Nevertheless, if they are in water, a small percentage of the cells may survive for at least several months. They are respiring, but at a very slow rate, consuming themselves. They are producing some carbon dioxide, but such a small amount that it would not be easily detectable. Under some conditions, yeast can form spores that have a very low metabolic rate, but are considered dormant. The spores will not grow until food is available, but can remain viable for years.

Dr. Alex Brands
Lehigh University