Respiration in Single-Celled Organisms (AQA A-Level Biology): Revision Notes
Respiration in Single-Celled Organisms
Purpose and principle
This practical investigates how temperature affects the rate of respiration in single-celled organisms, specifically yeast. The investigation demonstrates that yeast cells carry out both aerobic and anaerobic respiration, processes that transfer energy by moving electrons to produce ATP.
The underlying principle relies on redox indicator dyes such as methylene blue. During cellular respiration, electrons are released and can be accepted by the dye, causing a visible colour change from blue to colourless. The speed of this colour change indicates the rate at which respiration is occurring.
Redox indicator dyes work by accepting electrons during cellular respiration. When methylene blue accepts electrons, it undergoes reduction and loses its blue colour, becoming colourless. This visual change provides a measurable indicator of metabolic activity in the yeast cells.
Apparatus and materials
The following equipment is required for this investigation:
- Yeast and glucose in buffered solution
- Water bath with temperature control
- Thermometer or temperature probe
- Test tubes
- Timer
- Methylene blue (redox indicator dye)
Method
Independent variable: Temperature
Dependent variable: Time taken for methylene blue to become colourless
Control variables: Concentration of yeast and glucose, volume of solutions, pH of buffer
- Prepare a water bath and set the temperature to 35°C using the thermometer to monitor accuracy.
- Add 5cm³ of the yeast and glucose solution to three separate test tubes. Place these tubes in the water bath and allow approximately 10 minutes for equilibration to ensure the contents reach the same temperature as the water bath.
- Add 2cm³ of methylene blue to each test tube and immediately start timing. Shake each tube gently for 10 seconds to ensure thorough mixing, then return to the water bath.
- Record the time taken for the methylene blue to change from blue to colourless in each test tube.
- Repeat this procedure at different temperatures: 40°C, 50°C, 60°C, and 70°C.
- Calculate the mean time for each temperature and determine the rate of respiration using the formula:
The yeast and glucose solution must be buffered to maintain a constant pH throughout the experiment, as pH changes could affect enzyme activity and confound the results. Without proper buffering, temperature changes could alter pH levels, making it impossible to determine whether observed changes are due to temperature or pH effects.
Risk assessment and safety considerations
- DCPIP and methylene blue: These chemicals can irritate skin and eyes and may cause staining. Wear eye protection throughout the experiment. If contact occurs, wash immediately with cold water.
- Biological hazards: Some individuals may have allergies to yeast. Wash hands thoroughly after handling biological materials and seek assistance if allergic reactions occur.
- Glassware: Handle test tubes and thermometers carefully to prevent cuts from broken glass. Keep glassware away from the edge of the work surface.
- Hot water baths: Use tongs when removing test tubes from heated water baths to prevent scalding. Wear eye protection when working near hot liquids.
Data collection and processing
Record results in a table showing temperature, individual times for colour change, and calculated mean times. Present data with appropriate units (seconds for time, °C for temperature).
Plot a graph with temperature on the x-axis and rate of respiration on the y-axis. This will clearly show the relationship between temperature and metabolic activity in yeast cells.
When plotting your graph, you should expect to see a curve that rises to a peak (optimum temperature) and then falls as temperature increases further. This characteristic shape reflects the balance between increased enzyme activity at higher temperatures and enzyme denaturation at excessive temperatures.
Analysis and interpretation
The results typically show that yeast has an optimum temperature range for respiration. This is evident from a peak in the graph where the rate of respiration is highest.
As temperature increases from low values, the rate of respiration increases because enzyme activity increases with temperature. However, beyond the optimum temperature, the rate begins to decrease as enzymes involved in respiration start to denature.
At temperatures significantly above the optimum, enzyme denaturation may occur, substantially reducing or stopping respiration. This means the methylene blue takes much longer to become colourless, or may not change colour at all.
Worked Example: Interpreting Results
Suppose your results show:
- At 35°C: mean time = 8 minutes, rate = 1/8 = 0.125 min⁻¹
- At 45°C: mean time = 4 minutes, rate = 1/4 = 0.250 min⁻¹
- At 55°C: mean time = 12 minutes, rate = 1/12 = 0.083 min⁻¹
This data suggests the optimum temperature is around 45°C, where respiration is fastest (highest rate). At 55°C, the rate has decreased, indicating enzyme denaturation is beginning to occur.
The biological explanation centres on the fact that respiration depends on enzyme-catalysed reactions. Since enzymes have optimal temperature ranges for activity, the rate of respiration reflects this relationship directly.
Remember!
Key Points to Remember:
- Methylene blue acts as an electron acceptor, changing from blue to colourless as respiration occurs
- Temperature is the independent variable being investigated, affecting enzyme activity in yeast cells
- Rate of respiration is calculated as 1/mean time - shorter times indicate faster respiration
- Buffered solutions are essential to maintain constant pH and ensure valid results
- Optimum temperature exists for yeast respiration - too high causes enzyme denaturation, too low reduces enzyme activity