Respiration in Single-Celled Organisms

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This practical investigates the effect of a named variable (e.g., temperature) on the rate of respiration in cultures of single-celled organisms, such as yeast. Respiration is measured using a redox indicator dye like methylene blue, which changes from blue to colourless as it accepts electrons during respiration.

Equipment List

Yeast and glucose in buffered solution
Water bath
Thermometer/temperature probe
Test tubes
Timer

Method

In this method, the named variable is temperature:

Set up a water bath at 35°C.
Add 5 cm³ of yeast and glucose solution to three test tubes. Place the test tubes in the water bath and leave for 10 minutes to equilibrate.
Add 2 cm³ of methylene blue to the test tubes and start the timer. Shake each tube for 10 seconds, then return to the water bath. Record the time taken for the methylene blue to turn colourless for each test tube.
Repeat the experiment at different temperatures (e.g., 40°C, 50°C, 60°C, and 70°C).
Calculate the mean time for each temperature, and use this to determine the average rate of respiration using the formula:

\text{Rate of respiration} = \frac{1}{\text{mean time}}

NB: Ensure the yeast and glucose solution is buffered to maintain a constant pH.

Risk Assessment

Hazard	Risk	Safety Precaution	In Emergency	Risk Level
Methylene blue	Irritant to skin and eyes; may cause staining	Wear eye protection; avoid skin contact	Wash eyes/skin with cold water immediately	Low
Broken glass	Cuts from sharp objects	Handle test tubes carefully; keep away from desk edges	Elevate cuts, apply pressure, seek medical aid	Low
Hot liquids	Scalding	Use tongs to handle test tubes; wear eye protection	Run burn under cold water; seek medical aid	Low

Analysis

Graph: Plot a graph of rate of respiration against temperature.
Identify the optimum temperature for respiration, shown by the highest rate of reaction.

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Conclusion

Yeast has an optimum temperature range for respiration, typically shown as a peak on the graph. At this temperature, enzyme activity is at its highest, and respiration occurs most efficiently.
As the temperature moves away from the optimum:

Below optimum: The rate of respiration decreases due to insufficient enzyme-substrate collisions.
Above optimum: The enzymes begin to denature, altering the shape of the active site and reducing respiration rates. This makes methylene blue take longer to turn colourless.

At high temperatures, denaturation leads to irreversible loss of enzyme function.

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Summary for Exam:

Ensure clear understanding of enzyme denaturation and how temperature affects enzyme activity.
Focus on accurate time measurements and maintaining consistent experimental conditions for reliable results.

Respiration in Single-Celled Organisms Simplified Revision Notes for A-Level AQA Biology

Respiration in Single-Celled Organisms

Equipment List

Method

Risk Assessment

Analysis

Conclusion

Summary for Exam:

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