In Science, we investigated which of the 7 key life processes we could demonstrate with yeast - a most surprising living thing! We made basic bread doughs - some with sugar, some with salt, most with yeast (except for the control!) and observed how much they rose in comparison with each other.
We discovered that without yeast, nothing happens. The yeast respires, releasing pockets/bubbles of carbon dioxide into the mixture which make the dough rise!
With sugar and yeast, it rises much more rapidly! With salt and yeast, there is very little rising. We concluded that the yeast was using sugar for nutrition and that salt was harming the yeast's ability to grow.