HERE IS THE THEORY OF CHEMICAL ACTIVITY OF YEAST CELLS (BALLOON EXPERIMENT)

Have you ever wondered why breads are porous? 
The pores in breads we see arise from the chemical and physiological activities of yeasts. The yeast used in baking, contain cells! (hope you didn't know this). The cells can grow and reproduce like other cells. And to do so they must eat to obtain energy for their growth and reproduction among other physiological processes. When you are preparing a bread at home, you mix the flour, sugar and the yeast. The sugar in this mixture is the food for the yeast.

As a living organism, to obtain energy from it's food, the yeast cell ferments the sugar (anaerobic respiration) in a process which carbon dioxide gas is release as a by-product. After a couple of hours, the gas so released, creates bubbles around every yeast cell in the bread mixture which is the reason why a bread rises during baking. "The bread rises in baking due to the accumulation of carbon dioxide gas produced after fermentation of sugar by the yeast cells" Like all other cells, The yeast cells can also be affected by temperature. Thus, at lower temperatures, the activity of yeast cells is undermined while higher temperatures kill them. Once the bread has risen to the required amount and placed in an oven, the yeast cells gets killed and the carbon dioxide gas gets removed from the bread which leave the pores you always see in breads.This is the theory behind all bakery products.

In an more accurate way, the chemical activities of yeasts can be investigated in the laboratory by using balloons. I have provided you with simple experimental descriptions which will help you understand the impact of yeast's respiration on the balloons.
                                     
                                       Click here to view more details on the balloon test with yeasts.
                                     
                                       You can also follow this link for more adventurous yeast findings.