Like all batteries, eventually it gets depleted and needs to be replaced; this can be expected to happen after 5-10 years, depending on system usage. CMOS checksum error (and similar messages) or the system clock being reset are classic signs of the battery having been depleted. The BIOS settings contain several system-critical values.
In order to grow, yeast need glucose (C 6 H 12 O 6) or sucrose (table sugar, C 12 H 22 O 11), water (H 2 O) and warm temperatures. As yeast digest sugar, they turn into alcohol. In the process they give off carbon dioxide (CO 2) gas. The alcohol dissolves in the water and the gas escapes into the air. Yeast is used to make bread.
After the ingredients for bread are mixed together, fermentation occurs when the yeast cells break down large starch molecules into sugars for energy. They use this energy for survival and reproduction. The sugars digested by the yeast burp out carbon dioxide and ethyl alcohol into existing air bubbles in the dough. And this causes the dough to rise. In the meantime, while you're working with the dough these bubbles of carbon dioxide and alcohol burst, allowing for two proteins in the flour, glutenin and gliadin, to glom onto water particles.