Required Practical 6 Aqa Biology A Level

AQA A-Level Biology Required Practical 6: Investigating the effect of temperature on the activity of an enzyme

This article provides a practical guide to AQA A-Level Biology Required Practical 6, focusing on investigating the effect of temperature on enzyme activity. We'll cover the practical procedure, expected results, scientific explanations, potential sources of error, and frequently asked questions. Understanding this practical is crucial for your A-Level Biology exams and demonstrates a strong grasp of enzyme kinetics and biological methodology.

Introduction

Enzymes are biological catalysts that speed up chemical reactions within living organisms. This practical investigates the relationship between temperature and the rate of enzyme activity, using a readily available enzyme and substrate. Here's the thing — this experiment allows you to develop practical skills in experimental design, data collection, analysis, and evaluation, crucial components assessed in your A-Level Biology examinations. In real terms, the specific enzyme and substrate used might vary slightly depending on your teacher's instructions, but the underlying principles remain consistent. Their activity is highly sensitive to environmental factors, particularly temperature. We'll focus on a common example: the enzyme catalase and its action on hydrogen peroxide.

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Materials and Methods

Before embarking on the practical, ensure you have all the necessary materials and understand the safety precautions. Always wear appropriate safety goggles.

Materials:

• Potato (source of catalase enzyme)
• Hydrogen peroxide (substrate)
• Beakers
• Graduated cylinders
• Stopwatch/timer
• Thermometer
• Test tubes
• Boiling tubes
• Syringe or pipette
• Ruler or measuring cylinder (for gas collection method)
• Water bath (for temperature control)
• Ice bath (for low temperature control)

Method:

The method below describes a common approach to investigating the effect of temperature on catalase activity. Your specific instructions might vary, so always follow your teacher's guidelines. Two common methods exist: gas collection and colour change.

Method 1: Gas Collection (Oxygen Production)

1. Prepare a series of water baths set to different temperatures (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C). Ensure the water baths maintain a relatively stable temperature throughout the experiment.
2. Prepare potato samples: Use a cork borer to create identical cylinders of potato. These should be carefully washed and blotted dry to remove excess water.
3. Prepare hydrogen peroxide solution: Prepare a dilute solution of hydrogen peroxide (e.g., 3%). The exact concentration will be specified by your teacher.
4. For each temperature, place a potato cylinder into a test tube containing a set volume of hydrogen peroxide.
5. Immediately start the timer and measure the volume of oxygen gas produced over a set time period (e.g., 1 minute). You can collect the gas using an inverted measuring cylinder filled with water, capturing the oxygen gas produced.
6. Repeat steps 4-5 at least three times for each temperature to obtain reliable results. This helps account for random errors.
7. Record the volume of oxygen produced at each temperature.

Method 2: Colour Change (Hydrogen Peroxide Decomposition)

This method focuses on the disappearance of hydrogen peroxide, often using a colorimetric indicator.

1. Prepare the water baths at different temperatures (same as in Method 1).
2. Prepare potato extract by blending a section of potato with a buffer solution. This will provide a more homogenous reaction. This method may be less accurate for studying reaction rates.
3. Prepare a hydrogen peroxide solution with a colorimetric indicator (e.g., potassium permanganate). The indicator will change colour as hydrogen peroxide is consumed.
4. Add a set volume of potato extract to a test tube and place it in the water bath to reach the required temperature.
5. Add a set volume of hydrogen peroxide solution.
6. Start the timer and measure the time taken for the colour change to reach a predetermined point (e.g., the solution turns completely clear).
7. Repeat steps 4-6 at least three times for each temperature to obtain reliable results.
8. Record the time taken for the colour change at each temperature.

Data Analysis and Results

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The data collected needs to be carefully analyzed and presented. For both methods, the data will show the relationship between temperature and enzyme activity.

• Method 1 (Gas Collection): Plot a graph of oxygen production (y-axis) against temperature (x-axis). The shape of this graph will be crucial in your analysis and interpretation. Expect an initial increase in oxygen production with rising temperature, followed by a sharp decrease at higher temperatures.
• Method 2 (Colour Change): Plot a graph showing the rate of reaction (1/time) (y-axis) against temperature (x-axis). A similar trend to Method 1 is expected.

Regardless of the method used, you should calculate mean values for each temperature and include error bars to represent the variability in your data. Now, error bars will show the standard deviation or standard error of the mean. This helps to evaluate the reliability of your results. Surprisingly effective.

Scientific Explanation

The results should demonstrate that enzyme activity increases with temperature up to an optimum temperature, after which activity decreases rapidly. This is due to the effect of temperature on enzyme structure.

• Increasing Temperature: As temperature rises, enzyme and substrate molecules gain kinetic energy, leading to more frequent collisions and an increased rate of reaction. This is because the molecules move faster, increasing the chance of successful collisions with the correct orientation for the reaction to occur.
• Optimum Temperature: There is an optimum temperature at which enzyme activity is highest. This is usually around 37°C for human enzymes (but may vary for enzymes from other organisms).
• Decreasing Temperature: Above the optimum temperature, the enzyme's three-dimensional structure begins to denature. The high kinetic energy causes the bonds holding the enzyme's tertiary structure together (hydrogen bonds, disulfide bridges) to break. This changes the active site's shape, preventing the substrate from binding effectively, and thus reducing the rate of reaction. At very high temperatures, the enzyme is permanently denatured.

Evaluation of the Practical

A critical evaluation of the practical is essential. Consider the following points:

• Sources of Error: Identify potential sources of error in the experiment and suggest improvements. These could include variations in potato samples, inconsistencies in temperature control, inaccuracies in measuring volumes, or subjective assessments of colour changes in Method 2.
• Limitations: Discuss the limitations of the experimental design. Here's a good example: using only one concentration of substrate might not reveal the full picture of the enzyme's behavior.
• Improvements: Suggest improvements to the experimental procedure to increase the accuracy and reliability of the results. This could include using more precise equipment, increasing the number of replicates, or employing more sophisticated temperature control methods.
• Reliability and Validity: Discuss the reliability and validity of your results. Reliability relates to the consistency of your measurements; validity concerns whether the experiment actually measures what it intends to measure.

Frequently Asked Questions (FAQ)

• Why use potato as a source of catalase? Potato is readily available, inexpensive, and contains a sufficient amount of catalase for this experiment.
• What is the role of hydrogen peroxide? Hydrogen peroxide acts as the substrate for the enzyme catalase. Catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen.
• Why is it important to control the temperature? Temperature significantly affects enzyme activity. Consistent temperature control ensures that changes in the rate of reaction are due to temperature variations and not other factors.
• How can I improve the accuracy of my results? Increasing the number of replicates, using more precise measuring equipment, and carefully controlling the temperature are all ways to improve the accuracy and precision of your results.
• What other factors can affect enzyme activity? Besides temperature, other factors such as pH, substrate concentration, and enzyme concentration can also affect enzyme activity.

Conclusion

This practical provides a valuable opportunity to explore the principles of enzyme kinetics and develop crucial practical skills. Remember to carefully document all your procedures, data, and analysis in a clear and concise manner, as this is crucial for your A-Level Biology assessment. This thorough understanding of the practical, its intricacies, and potential limitations will put you in a strong position to succeed in your A-Level Biology exams. By carefully following the procedure, analyzing the results, and critically evaluating the experimental process, you'll gain a deeper understanding of how temperature affects enzyme activity. The ability to critically evaluate the experiment and suggest improvements is also a key skill that will be assessed. Remember to always consult your teacher's instructions and guidelines, as these might differ slightly from the general procedure outlined here.