Blow up a balloon using yeast

The yeast digests the sugar and creates a gas as waste product. This gas is called carbon dioxide (CO2) - it’s the same gas that is responsible for the fizzy bubbles in your lemonade. Since the gas carbon dioxide takes up more space in the bottle than the solid sugar, the balloon expands.

You might be wondering where this carbon dioxide is coming from. The gas originated from the sugar and parts of the oxygen. Oxygen is in the air around you and every human needs it to breathe.

However, since there was only little oxygen in the bottle, the yeast formed carbon dioxide mainly by fermentation instead of forming it by respiration. Yeast is pretty good at fermentation - much better than us humans. This special ability of yeast is used in the process of brewing beer, as alcohol is a waste product of fermentation.

Yeast has two survival strategies: either it lives with oxygen (aerobic) or without oxygen through fermentation (anaerobic). Carbon dioxide is produced during both processes.

Plants need carbon dioxide, water, nutrients and sunlight (energy) to grow. So they use the gas, which humans, animals and also fungi constantly emit, as food. This creates a cycle consisting of plants producing oxygen and animals and fungi producing carbon dioxide. Therefore, the diet of fungi is more comparable to that of animals. Fungi  are neither animals nor plants and form their own incredibly diverse kingdom in nature.

Perhaps it’s easier to realize if you think of the following: Plants are mostly green as they harvest solar energy with the help of the green pigment chlorophyll. Green fungi are rare as fungi don't have the ability to use sunlight as a source of energy. Plants have to do this job for them - just like for animals, too. However, one should not exclusively  rely on the characteristic “green color” as a criteria for the ability of using sunlight as a source of energy. There are for example green animals (tree frogs, grasshoppers ...), which -despite of their green color -  do not belong to the kingdom of plants.

• Under which conditions is baker’s yeast particularly active?
• Take a look at a recipe for dough made with yeast and think about the purpose of the single steps. What’s the balloon envelope in a yeast dough? Why is it important to knead the dough for quite a while and why should it become elastic? Why do you always add a little sugar to the dough? And why is it important to place the kneaded dough at a warm place?
• Is the yeast still alive after he baking process?
• How could you prove that carbon dioxide was produced  during the experimet- and not for example oxygen?A little hint: not only organisms need oxygen ...