Gas bubbles alone do not make bread rise. Imagine yeast making CO₂ in a bowl of thin pancake batter: the gas just escapes. Bread dough holds its gas because it contains a stretchy, elastic net of gluten.

Gluten is not one molecule. It is a family of proteins in wheat flour, and two groups matter most:

• Glutenins are long protein chains that join to each other to form very big, cross-linked networks. They make dough elastic: it springs back.
• Gliadins are smaller, single chains. They act like a lubricant, making dough extensible: it stretches without snapping.

Wheat gives dough both properties at once, which is why wheat bread is so airy. Barley and rye have related proteins.