Apply aerobic equation amount ratios
Use proportional amounts without inventing a rate.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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
What does a coefficient show here?
What you will learn
- Read the reaction ratio
- Separate amount and elapsed time
Apply aerobic equation amount ratios
| Feature | Meaning |
|---|---|
| 1 glucose | Uses 6 O₂; forms 6 CO₂ + 6 H₂O |
| 2 glucose | Uses 12 O₂; forms 12 CO₂ + 12 H₂O |
| 3 glucose | Uses 18 O₂; forms 18 CO₂ + 18 H₂O |
| Rate limit | A time measurement is still needed |
Read the reaction ratio
The balanced equation gives relative molecule amounts: one glucose uses six oxygen and forms six carbon dioxide and six water. Doubling the glucose amount doubles each corresponding amount under the same complete reaction model. This proportional relationship is read from coefficients, not from the sizes of symbols printed on the page.
If twelve oxygen molecules are used in the specified model, two glucose molecules correspond and twelve carbon-dioxide molecules form. These are deliberately small model counts for learning ratios, not measured counts inside a living cell. The same coefficient ratio can apply to proportional chemical amounts, without requiring a molar calculation for this Biology requirement.
Separate amount and elapsed time
An amount is different from a rate. Eighteen carbon-dioxide molecules in the model correspond to three glucose molecules, but no per-second production can be calculated unless a measurement time is supplied. Complete-reaction ratios do not prove all substrate in a live sample was consumed; that depends on the stated conditions and measurements.
In real respiratory measurements, carbon-dioxide production and oxygen use may be influenced by the nutrients being used and other processes. Apply the one-glucose equation when that is the specified model, and avoid treating its ratio as a universal diagnostic of every organism. The task here is a proportional consequence of the balanced syllabus equation.
Sources for this lesson
Pause and explain
How much CO₂ corresponds to three glucose molecules in this complete model?
Worked example
Two glucose molecules react completely in the model. Find the oxygen and water amounts.
Show the worked solution
- Multiply both coefficient-six quantities by two.
- 2 × 6 = 12.
Answer Twelve O₂ molecules used and twelve H₂O molecules formed.
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.
Four glucose molecules react completely. Find O₂ used and CO₂ formed.
Give me a hint
Use the coefficient-six ratio.
Compare my reasoning
- 4 × 6 gives oxygen molecules.
- The CO₂ coefficient is also six.
Twenty-four O₂ molecules used and twenty-four CO₂ molecules formed.
Look for these in your work
- Use proportional coefficients
- Give both amounts
Eighteen O₂ molecules are used in this complete glucose model. Find glucose used and water formed.
Give me a hint
Divide oxygen by six, then multiply glucose by six.
Compare my reasoning
- 18 ÷ 6 = 3 glucose molecules.
- 3 × 6 = 18 water molecules.
Three glucose molecules used; eighteen H₂O molecules formed.
Look for these in your work
- Reverse the ratio correctly
- Retain formula identity
A model gives twelve CO₂ molecules but no time. Can its CO₂ production rate be calculated?
Give me a hint
Amount per time needs a time.
Compare my reasoning
- The equation gives a corresponding amount of two glucose molecules.
- A rate requires the elapsed time.
No rate is established without time; the amount ratio corresponds to two glucose molecules in this specified model.
Look for these in your work
- Distinguish amount and rate
- Apply the amount ratio
Common mistakes
- Reading coefficients as seconds.
- Extending a glucose-specific model to all fuels without evidence.
What can you explain now?
How much CO₂ corresponds to three glucose molecules in this complete model?
Compare with the explanation
Eighteen molecules
3 × 6 = 18.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use the 1 : 6 : 6 : 6 ratio and keep amount distinct from rate.
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.