Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process and practical evidence.
Yeast pathway
In yeast, anaerobic respiration is glucose → ethanol + carbon dioxide, with a relatively small energy release. This process is often called alcoholic fermentation.
Bread making
Carbon dioxide bubbles become trapped in dough, causing it to rise. During baking, ethanol and much of the carbon dioxide escape, and heat kills the yeast. The useful bread-making product is mainly carbon dioxide.
Ethanol production
In brewing and bioethanol production, ethanol is the desired product. Conditions are managed to support yeast growth initially and fermentation later. Excess ethanol eventually inhibits yeast.
Measuring rate
Carbon dioxide production can be measured using a gas syringe, bubble count or mass loss. Gas volume is more quantitative than bubble count because bubble size varies. Temperature, yeast amount, glucose concentration, pH and time must be controlled.
Aerobic versus anaerobic yeast
With oxygen, yeast can respire aerobically and release more energy, supporting rapid growth. In oxygen-limited conditions it produces ethanol and carbon dioxide. Experimental setups must therefore control oxygen availability.
Practical or data skill
Set up yeast and glucose at a safe warm temperature with a gas-collection method. Include a control without viable yeast or without glucose. Never seal a system in a way that dangerous pressure can build.
Examination tip
Yeast produces ethanol and carbon dioxide; human muscle produces lactic acid.
Review questions and suggested answers
Question 1
State the word equation for anaerobic respiration in yeast.
Suggested answer
Glucose → ethanol + carbon dioxide.
Question 2
Which product makes bread dough rise?
Suggested answer
Carbon dioxide.
Question 3
Why is gas volume better than bubble count?
Suggested answer
Bubble sizes vary, so volume is a more quantitative measure.
