Topic 21.2 · Core
Yeast in Biofuel and Bread-Making
One anaerobic reaction in yeast produces two separate products, and two completely different industries each want a different one of them. Naming the word equation on its own doesn’t earn the mark here — matching each product to the application that actually uses it does.
Yeast is a microorganism that can respire anaerobically when oxygen isn’t available. In a biotechnology context, this anaerobic breakdown of sugar by yeast is called fermentation, and it follows one word equation that’s worth having exact:
glucose → ethanol + carbon dioxide
Everything else on this page is really just asking what happens to each of those two products once they’re made.
Ethanol: yeast and biofuel
When yeast is supplied with a source of sugar under conditions where it respires anaerobically, the ethanol it produces can be collected and used as a biofuel. The important chain to hold onto is: sugar → anaerobic respiration by yeast → ethanol → biofuel. The ethanol isn’t added by anyone afterwards — it’s a direct product of yeast’s own metabolism, collected rather than manufactured separately.
Carbon dioxide: yeast in bread-making
Yeast mixed into bread dough has access to carbohydrates that provide sugars, and as it respires anaerobically it produces the same two products — but this time it’s the carbon dioxidethat matters. Bubbles of carbon dioxide become trapped in the dough, causing it to expand and rise. During baking, the structure of the bread becomes fixed around those trapped bubbles, and the ethanol produced alongside the carbon dioxide plays no part in the rising itself. The chain here is: anaerobic respiration → carbon dioxide production → gas trapped in dough → dough rises.
Why the same reaction serves two industries
The fuel industry wants the ethanol and has no particular interest in the carbon dioxide; the baking industry wants the carbon dioxide bubbles that leaven the dough and isn’t trying to collect the ethanol, most of which is driven off during baking. Neither industry depends on the other’s product — they’re simply drawing on different halves of the same anaerobic reaction. That’s the actual explanatory point Cambridge is testing: not that yeast respires anaerobically in general, but that naming both products and matching each to its specific application is what a complete answer looks like.
Don’t confuse
This is anaerobic respiration — no oxygen is involved, so don’t reach for aerobic respiration terminology here. And keep the two applications apart: biofuel production uses the ethanol; bread-making uses the carbon dioxide. Stating the word equation without saying which product does which job leaves the answer incomplete even when the chemistry is correct.