Required practical: effect of temperature on respiration in yeast

What you'll learn

This required practical investigates how temperature affects the rate of anaerobic respiration in yeast cells. You need to understand the experimental method, identify variables, analyse results using gas production as a measure of respiration rate, and evaluate the procedure. This practical appears frequently in exam questions worth 4-6 marks, so thorough understanding is essential.

Key terms and definitions

Anaerobic respiration — respiration without oxygen, producing ethanol and carbon dioxide in yeast (glucose → ethanol + carbon dioxide)

Independent variable — the factor you deliberately change in an investigation; in this practical, it is temperature

Dependent variable — the factor you measure in an investigation; here, it is the volume of carbon dioxide produced or time taken for a set volume

Control variables — factors kept constant to ensure a fair test, such as yeast concentration, glucose concentration, and volume of solution

Respiration rate — the speed at which respiration occurs, measured by volume of CO₂ produced per unit time

Optimum temperature — the temperature at which enzymes work most efficiently, giving the fastest respiration rate (typically 30-40°C for yeast)

Denaturation — permanent change to enzyme shape caused by high temperatures, preventing substrate binding and stopping respiration

Water bath — equipment used to maintain constant temperature during the investigation

Core concepts

The science behind yeast respiration

Yeast is a single-celled fungus used in baking and brewing because it respires anaerobically when oxygen is limited. The word equation for anaerobic respiration in yeast is:

glucose → ethanol + carbon dioxide (+ energy)

The carbon dioxide produced makes bread rise and creates bubbles in beer. In this practical, we measure CO₂ production to determine respiration rate.

Temperature affects respiration because enzymes control the reactions. Like all proteins, enzymes work faster as temperature increases (more kinetic energy means more successful collisions between enzyme and substrate). However, above approximately 45°C, enzymes denature and the active site changes shape permanently, stopping respiration.

Method for the investigation

Standard apparatus setup:

1. Add a known volume (typically 10 cm³) of yeast suspension to a boiling tube
2. Add a known volume (typically 10 cm³) of glucose solution
3. Add a layer of oil on top to prevent oxygen entering (ensuring anaerobic conditions)
4. Attach a bung with delivery tube leading to an inverted measuring cylinder filled with water in a trough
5. Place the boiling tube in a water bath at a specific temperature (e.g., 20°C, 30°C, 40°C, 50°C, 60°C)
6. Allow 5 minutes for the mixture to reach water bath temperature
7. Measure the volume of CO₂ collected in the measuring cylinder over a fixed time period (e.g., 10 minutes)
8. Repeat at different temperatures

Alternative method using a gas syringe:

Instead of collecting gas in water, attach a gas syringe to the bung. The plunger moves outward as CO₂ is produced, allowing direct volume measurement. This method is more accurate as CO₂ is slightly soluble in water.

Alternative method measuring time:

Keep everything else constant but measure the time taken to produce a fixed volume of CO₂ (e.g., 5 cm³). Calculate rate as: rate = volume ÷ time.

Variables in this practical

Independent variable: Temperature (°C)

• Use at least 5 different temperatures between 10°C and 70°C
• Control temperature using water baths
• Check actual temperature with a thermometer, not just the water bath setting

Dependent variable: Volume of carbon dioxide produced (cm³)

• Read the measuring cylinder at eye level to avoid parallax error
• Alternatively, measure time taken for a set volume of gas

Control variables to keep constant:

• Volume and concentration of yeast suspension (affects number of respiring cells)
• Volume and concentration of glucose solution (affects substrate availability)
• pH of solutions (affects enzyme activity)
• Time period for gas collection (if measuring volume)
• Volume of gas collected (if measuring time)

Failing to control these variables makes the test unfair and results unreliable.

Expected results and analysis

Typical pattern of results:

• 10-25°C: Low respiration rate, small volume of CO₂ produced (enzymes have low kinetic energy)
• 30-40°C: Maximum respiration rate at optimum temperature (fastest enzyme activity)
• 45-50°C: Rapidly decreasing rate (enzymes beginning to denature)
• 60°C and above: No CO₂ produced (enzymes completely denatured, respiration stopped)

Drawing graphs:

Plot a line graph with:

• Temperature (°C) on the x-axis (independent variable)
• Volume of CO₂ (cm³) or rate (cm³/min) on the y-axis (dependent variable)
• Draw a smooth curve through the points

The graph should show:

1. An increasing trend up to the optimum (approximately 30-40°C)
2. A peak at the optimum temperature
3. A sharp decrease after the optimum
4. Zero or near-zero values above 60°C

Calculating rate:

If you measured volume over time: rate = volume of CO₂ ÷ time

If you measured time for a fixed volume: rate = fixed volume ÷ time

Units must be consistent (e.g., cm³/min or cm³/s).

Safety considerations and practical issues

Safety points:

• Wear eye protection when handling glucose solutions
• Take care with boiling water in water baths
• Glass apparatus can break; handle carefully
• Dispose of yeast solution safely down the sink with running water

Common practical problems:

1. Gas leaks: Check all connections are secure; use petroleum jelly on bung if needed
2. Temperature fluctuation: Water baths may not maintain exact temperature; stir water and check regularly with thermometer
3. Yeast settling: Shake suspension gently before use to ensure even distribution
4. CO₂ dissolving in water: Use a gas syringe or work quickly to minimise loss
5. Contamination: Use fresh solutions; old glucose or yeast gives unreliable results
6. Insufficient mixing: Swirl the tube gently after adding glucose to ensure yeast contacts substrate

Evaluation and improvements

Reliability:

• Repeat each temperature at least three times
• Calculate mean values, excluding anomalies
• Anomalies appear as points far from the pattern; they may result from leaks, contamination, or measurement errors

Accuracy improvements:

• Use a data logger and temperature probe for more precise temperature control
• Use a gas syringe instead of collecting over water (reduces CO₂ loss through dissolution)
• Use a pH buffer solution to control pH more effectively
• Increase the number of temperature intervals for more detailed results (e.g., 25°C, 30°C, 35°C, 40°C, 45°C)

Validity:

• Ensure all control variables are kept constant
• Use freshly prepared solutions
• Allow sufficient equilibration time before starting measurements

Precision:

• Use more accurate measuring equipment (e.g., burette for volumes, digital thermometer)
• Take readings at smaller time intervals
• Use graduated equipment with finer divisions

Worked examples

Example 1: Describing the method (6 marks)

Question: Describe how you would investigate the effect of temperature on the rate of respiration in yeast.

Mark scheme answer:

1. Add 10 cm³ of yeast suspension to a boiling tube (1 mark)
2. Add 10 cm³ of glucose solution (1 mark)
3. Attach a bung and delivery tube leading to an inverted measuring cylinder filled with water (1 mark)
4. Place in a water bath at a specific temperature, e.g., 20°C (1 mark)
5. Measure the volume of carbon dioxide collected over a fixed time period, e.g., 10 minutes (1 mark)
6. Repeat at different temperatures, e.g., 30°C, 40°C, 50°C, 60°C (1 mark)

Examiner note: You must include specific volumes, time periods, and temperatures to gain full marks. Vague statements like "add some yeast" or "change the temperature" score zero.

Example 2: Identifying variables (3 marks)

Question: A student investigates the effect of temperature on respiration rate in yeast. (a) State the independent variable. (1 mark) (b) State the dependent variable. (1 mark) (c) Give one control variable. (1 mark)

Mark scheme answer:

(a) Temperature / temperature of water bath (1 mark) (b) Volume of carbon dioxide produced / rate of CO₂ production / time taken to produce a set volume (1 mark) (c) Volume or concentration of yeast suspension / volume or concentration of glucose solution / pH / time period (1 mark)

Examiner note: For control variables, you must be specific. "Amount of yeast" is too vague; state "volume of yeast suspension" or "concentration of yeast suspension."

Example 3: Analysing results (4 marks)

Question: The table shows a student's results:

(a) Calculate the rate of respiration at 30°C in cm³/min. Show your working. (2 marks) (b) Explain the result at 60°C. (2 marks)

Mark scheme answer:

(a) Rate = volume ÷ time (1 mark) Rate = 18 ÷ 10 = 1.8 cm³/min (1 mark)

(b) At 60°C, enzymes controlling respiration are denatured (1 mark) The active site changes shape / enzymes no longer catalyse the reaction, so respiration stops (1 mark)

Examiner note: Always show working for calculations. For explanation questions, link the temperature effect specifically to enzymes and their denaturation.

Common mistakes and how to avoid them

• Mistake: Writing "the yeast dies at high temperature" Correction: State that "enzymes denature" — yeast cells may not be dead, but respiration stops because enzymes controlling the reaction no longer function

• Mistake: Forgetting to add oil or creating anaerobic conditions in the method Correction: Explicitly mention adding a layer of oil to prevent oxygen entry, ensuring anaerobic respiration occurs; without this, results will be invalid

• Mistake: Not calculating rate correctly when asked — simply stating the volume Correction: Rate = volume ÷ time or rate = 1 ÷ time (if measuring time for fixed volume); always include units (cm³/min or cm³/s)

• Mistake: Plotting bar charts instead of line graphs Correction: Temperature is a continuous variable, so use a line graph with a smooth curve; bar charts are only for categoric data

• Mistake: Failing to control variables in the method description Correction: State specific volumes, concentrations, and time periods; mention keeping glucose concentration, yeast volume, and pH constant

• Mistake: Not explaining why we measure CO₂ production Correction: Carbon dioxide is a product of anaerobic respiration in yeast; measuring its volume or rate of production indicates respiration rate

Exam technique for "Required practical: effect of temperature on respiration in yeast"

• Method questions (typically 4-6 marks): Include all key steps in logical order with specific measurements (volumes in cm³, times in minutes, temperatures in °C). Always mention repeating the experiment and calculating means. State how you would make it a fair test by controlling variables.

• "Explain" questions about results: Link temperature to enzyme activity explicitly. Below optimum: lower kinetic energy means fewer enzyme-substrate collisions. Above optimum: enzymes denature, active site changes shape, substrate cannot bind. Use the word "denature" not "die" when discussing high temperatures.

• Calculation questions: Show all working clearly. Write the formula first (rate = volume ÷ time), substitute numbers, then give the answer with correct units. Even if your final answer is wrong, you can gain method marks.

• Evaluation questions: Mention repeating measurements to identify anomalies and calculate reliable means. Suggest specific improvements like using a gas syringe (more accurate than collecting over water) or a data logger for temperature control. State how each improvement makes results more accurate, precise, or valid.

Quick revision summary

The yeast respiration practical investigates how temperature affects anaerobic respiration rate by measuring CO₂ production. Set up yeast and glucose in a water bath at different temperatures, collecting gas in a measuring cylinder or gas syringe. Respiration rate increases to an optimum (30-40°C) where enzymes work fastest, then decreases as enzymes denature above 45°C. Control variables include yeast concentration, glucose concentration, volume, and pH. Calculate rate as volume ÷ time. Repeat experiments and calculate means for reliability. Common improvements include using gas syringes and data loggers for more accurate measurements.