Use the muscle anaerobic word equation
Keep lactic acid separate from yeast alcohol and gas.
Section B12.1 · Respiration
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall cell processes, enzymes and the distinction between gas exchange and breathing.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which cell example forms lactic acid in this syllabus?
What you will learn
- Describe the vigorous-exercise model
- Compare organism-specific products
Use the muscle anaerobic word equation
| Feature | Meaning |
|---|---|
| Muscle model | Glucose → lactic acid |
| Yeast model | Glucose → alcohol + carbon dioxide |
| Shared feature | Energy released without oxygen use in that pathway |
| Whole body | Aerobic respiration can continue |
Describe the vigorous-exercise model
For muscle anaerobic respiration during vigorous exercise, the specified word equation is glucose → lactic acid. Energy is released without oxygen being used by that pathway. The model is used when energy demand is high relative to the oxygen supply associated with aerobic respiration; it does not say that the person has no oxygen anywhere.
The muscle equation has a different product from the yeast equation glucose → alcohol + carbon dioxide. Do not add ethanol or carbon dioxide to the specified muscle anaerobic word equation just because they occur in the yeast example. The pathway and cell type supplied in the question determine the equation you should use.
Compare organism-specific products
Aerobic respiration may continue in exercising muscle alongside the anaerobic contribution. Carbon dioxide detected in expired air during exercise can come from aerobic respiration and transport, so its presence does not require adding carbon dioxide to the muscle anaerobic equation. Keep a whole-body gas observation separate from the selected cell pathway.
Cambridge uses the school-level term lactic acid in this model. Physiological details are more complex than the short word equation, but learners should recognise and state the specified product accurately. Do not turn the equation into a claim that lactic acid alone explains every exercise symptom or that later muscle soreness is proved by observing this reaction.
Sources for this lesson
Pause and explain
Which equation belongs to the specified muscle pathway?
Worked example
Give the specified anaerobic muscle equation during vigorous activity.
Show the worked solution
- Use the muscle example.
- The product named by this syllabus is lactic acid.
Answer Glucose → lactic acid.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the muscle anaerobic word equation.
Give me a hint
Use the single specified product.
Compare my reasoning
- Start with glucose.
- Place lactic acid after the arrow.
Glucose → lactic acid.
Look for these in your work
- Name glucose
- Name lactic acid
Compare the specified anaerobic products in yeast and muscle.
Give me a hint
Do not mix the two examples.
Compare my reasoning
- Yeast forms alcohol and carbon dioxide.
- Muscles form lactic acid.
Yeast: alcohol (ethanol) and carbon dioxide. Muscle: lactic acid.
Look for these in your work
- Give both yeast products
- Keep the muscle product distinct
CO₂ is detected in expired air during vigorous exercise. Must it be added to the muscle anaerobic equation?
Give me a hint
The body can also carry out aerobic respiration.
Compare my reasoning
- Aerobic respiration forms CO₂.
- A whole-body gas observation does not change the specified muscle equation.
No. CO₂ can come from aerobic respiration; the muscle anaerobic model remains glucose → lactic acid.
Look for these in your work
- Retain the equation
- Separate whole-body and pathway evidence
Common mistakes
- Adding yeast ethanol to muscle products.
- Inferring every exercise symptom from one equation.
What can you explain now?
Which equation belongs to the specified muscle pathway?
Compare with the explanation
Glucose → lactic acid
This is the muscle anaerobic word equation.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Write the muscle equation and compare it with yeast.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.