The puffed-up appearance of dough is due to production of

Answer

**Step-by-Step Solution:**

1. **Understanding the Context**: The question refers to the "puffed-up appearance of dough," which is typically associated with bread-making. The dough rises due to certain biological processes.

2. **Identifying the Microorganism**: In bread-making, yeast is the primary microorganism responsible for fermentation. Yeast is a type of anaerobic bacteria that thrives in environments without oxygen.

3. **Anaerobic Respiration**: When yeast ferments, it undergoes anaerobic respiration. This process begins with glycolysis, where glucose is broken down into pyruvate.

4. **Conversion of Pyruvate**: In anaerobic conditions, instead of entering the Krebs cycle, the pyruvate produced from glycolysis is converted into two main byproducts: carbon dioxide and ethanol.

5. **Role of Carbon Dioxide**: The carbon dioxide produced during fermentation is a gas. As it forms, it creates bubbles in the dough, leading to the characteristic puffed-up appearance.

6. **Evaporation of Ethanol**: While ethanol is also produced, it evaporates during the baking process due to the high temperatures in the oven. This means that the primary contributor to the dough's rise is carbon dioxide.

7. **Conclusion**: The puffed-up appearance of dough is primarily due to the production of carbon dioxide during the fermentation process facilitated by yeast.

**Final Answer**: The puffed-up appearance of dough is due to the production of carbon dioxide.

---