It’s aliiiive! Well, once it touches water. Active yeast is a single-celled fungus that acts as a leavening agent by eating away at the sugars in flour and consequently releasing carbon dioxide. That release causes bread and other baked goods like cake, biscuits, rolls and doughnuts to rise at a slow and steady speed. (This is different from nutritional yeast, which is deactivated and used as a vegan seasoning.)
Gluten (if you’re using wheat flour) also helps the rising process. That's because the two proteins it’s made of fill with gas bubbles as the yeast activates. The flour’s starch releases sugar for the yeast to feed on, and fortifies those gas bubbles during baking. Then, the dough is cooked until the temperature gets so high that the yeast dies, and the stretchy, gummy gluten hardens into the bread we know and love.
Sadly, there’s no perfect replacement for yeast when it comes to kneaded bread dough. But these substitutes can do the trick for a lot of batter-based recipes in a pinch. Your finished product might have a different texture, color or height than you’re used to, but these swaps can get the job done. Just be sure to get your concoction into the oven ASAP to bake with as much captive carbon dioxide as possible.