Use redox indicators to investigate yeast respiration
Plan a supervised DCPIP or methylene-blue investigation into how temperature or substrate concentration affects yeast respiration.
Section 12.2 · Respiration
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does reduction of DCPIP or methylene blue indicate in this investigation?
What you will learn
- Describe the reduction and colour change of redox indicators
- Plan a controlled yeast investigation with repeats and safe handling
Build the explanation
DCPIP and methylene blue are redox indicators: their colour changes when they are reduced as electrons or hydrogen are transferred during respiration. A yeast suspension can be mixed with an indicator and substrate, then the time to a defined colour endpoint measured. To investigate temperature, equilibrate the mixtures to set temperatures; to investigate substrate concentration, vary that concentration while holding temperature constant. Keep yeast amount, pH, total volume and timing consistent, and repeat each condition.
Apply it carefully
A faster colour change can indicate a faster rate under the test conditions, but colour is an indirect endpoint, not a direct ATP measurement. Exclude oxygen consistently if the chosen protocol requires anaerobic conditions, use a clear endpoint and teacher-approved handling, and do not compare trials with different yeast amounts or indicator volumes.
Pause and explain
If substrate concentration is the independent variable, which variable should be held constant among temperature, yeast amount and substrate concentration?
Worked example
Why should yeast amount be kept constant when testing substrate concentration?
Show the worked solution
- Yeast amount affects the number of respiring cells.
- Keeping it constant helps attribute a rate difference to substrate concentration.
Answer Changing yeast amount would also change respiration rate, so it should be controlled while substrate concentration is tested.
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.
Outline a controlled plan to test how temperature affects the rate of yeast respiration using a redox indicator.
Give me a hint
Name the variable, endpoint, controls and repeats.
Compare my reasoning
- Use equal yeast, substrate, indicator, pH and volume at several measured temperatures.
- Record time to a defined colour change, calculate a rate such as 1/time, and repeat each temperature.
Prepare equal yeast suspensions with the same substrate concentration, indicator volume, pH and total volume. Equilibrate samples at measured temperatures, start the reaction consistently and time the indicator's defined colour change. Compare rate as 1/time, repeat trials and keep oxygen conditions consistent. Use teacher-approved safe handling.
Look for these in your work
- Temperature as independent variable
- Colour endpoint and time measured
- Controls and repeats included
- Rate calculation stated
A learner compares 0.5% and 2.0% glucose but uses twice as much yeast in the second tube. Explain why the result is confounded.
Give me a hint
Identify the extra variable that changes cell number.
Compare my reasoning
- Yeast amount changes alongside glucose concentration.
- A faster endpoint could be due to more yeast, not the higher substrate concentration alone.
The yeast amount is also changed, so the comparison is confounded. More yeast may reduce the indicator faster even if glucose concentration has no independent effect; keep yeast amount constant and repeat each concentration.
Look for these in your work
- Confounding variable identified
- Effect on interpretation explained
- Control proposed
The indicator changes colour faster at 30 °C than 20 °C. State a justified conclusion and one limit.
Give me a hint
Use 'under these conditions' and distinguish indicator time from energy yield.
Compare my reasoning
- The measured indicator-reduction rate was higher at 30 °C in this setup.
- This does not establish a universal optimum or directly measure ATP production.
Under the tested conditions, yeast reduced the indicator faster at 30 °C than at 20 °C, consistent with a higher measured respiration rate. The result does not establish a universal temperature optimum and the colour endpoint is an indirect measure, not a direct ATP measurement.
Look for these in your work
- Evidence-bounded conclusion
- Condition specified
- Method limit stated
Common mistakes
- Calling indicator colour a direct ATP measurement
- Changing yeast amount with the independent variable
- Assuming one measured temperature is the universal optimum
What can you explain now?
What does reduction of DCPIP or methylene blue indicate in this investigation?
Compare with the explanation
Electron or hydrogen transfer during respiration
The indicator's colour change is an indirect redox measure.
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
How can indicator decolourisation be used to compare yeast respiration rates fairly?
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.