Anaerobic Respiration: Watching Yeast 'Breathe' Without Air

Applies to: OCR Gateway (covered under the physiology, responses and respiration practical, both combined and separate award) and Edexcel International GCSE, which names both respiration and anaerobic respiration specifically as core practicals. AQA doesn't set this as one of its own named required practicals, so if you're on AQA, treat this one as useful exam context rather than a practical you'll be asked about by name. Edexcel UK GCSE includes respiration for separate science students specifically, not combined.

Yeast is alive, and it's respiring right now in whatever bread or beer it's part of, even without any oxygen around. You can't watch a single cell do that. You can watch a balloon inflate because of it, though, which is close enough to feel like magic the first time you see it.

What it tests: Whether yeast can respire without oxygen, and what it produces when it does.

What you need: A packet of dried yeast, sugar, warm water, a small bottle or jar, and a balloon.

How to do it: Add a teaspoon of sugar and a teaspoon of dried yeast to about 100ml of warm water in the bottle, warm but not hot, since heat that's too high will kill the yeast rather than activate it. Give it a gentle swirl, then stretch the neck of a balloon over the top of the bottle. Leave it somewhere warm and check back every five minutes. Over twenty to thirty minutes, the balloon will slowly inflate on its own.

Safety note: Use warm water, not boiling, and supervise young children around the balloon itself, since balloons are a choking risk for little ones. Don't drink the mixture.

Why it matters for the exam: That inflating balloon is carbon dioxide, produced because yeast respires anaerobically, without oxygen, breaking down sugar into ethanol and carbon dioxide instead of into carbon dioxide and water the way aerobic respiration does. Questions in this area usually ask you to compare the two processes, including why anaerobic respiration releases less energy than aerobic respiration does. Having watched the gas actually being produced makes that comparison easier to hold onto than the word equation alone.

On motivation: Nothing visibly happens for the first several minutes, and that's worth sitting with rather than rushing past. The yeast is working the whole time, you just can't see it yet. A lot of learning looks exactly like that too: no visible progress for a while, then something you can suddenly point to. Slow and invisible isn't the same as not happening.