Practical: Investigating Anaerobic Respiration in Yeast
Enzyme Control: Respiration is a series of metabolic reactions catalyzed by intracellular enzymes. This means the rate of reaction is highly dependent on factors that influence enzyme activity, such as temperature and pH.
Kinetic Energy: As temperature increases toward the optimum, molecules move faster, increasing the frequency of successful collisions between yeast enzymes and glucose substrates. This results in a faster rate of carbon dioxide production.
Thermal Denaturation: Beyond the optimum temperature (typically around for most yeast species), the heat causes the delicate three-dimensional structure of the enzymes to vibrate excessively. This breaks the bonds holding the active site together, leading to a loss of catalytic function.
Anaerobic Environment: To ensure only anaerobic respiration occurs, a layer of liquid paraffin or oil is added on top of the yeast and sugar mixture. This creates a physical barrier that prevents oxygen from the air from dissolving into the solution.
Temperature Control: A thermostatically controlled water bath is used to maintain a constant temperature for each trial. This is more accurate than a Bunsen burner, which can cause local hot spots and temperature fluctuations.
Measuring the Rate: The rate is calculated by counting the number of carbon dioxide bubbles produced over a fixed duration (e.g., 2 minutes). For higher precision, a gas syringe could be used to measure the exact volume of gas evolved.
Limewater Indicator: Connecting the delivery tube to limewater serves as a qualitative verification step. If the gas produced turns the limewater cloudy or milky, it confirms that the yeast is indeed producing carbon dioxide.
| Feature | Aerobic Respiration | Anaerobic Respiration (Yeast) |
|---|---|---|
| Oxygen Required | Yes | No |
| Products | Carbon Dioxide + Water | Ethanol + Carbon Dioxide |
| Energy Yield | High | Low |
| Location | Mitochondria & Cytoplasm | Cytoplasm |
Rate vs. Volume: Students often confuse the total amount of gas with the rate. In this practical, we focus on the rate (), which indicates how fast the enzymes are working at that specific temperature.
Glucose Source: While glucose is the simplest substrate, complex sugars like starch must first be broken down by enzymes (like amylase) into simpler sugars before the yeast can utilize them for fermentation.
The CORMS Framework: In exam questions about designing this experiment, always state clearly that you will change the Temperature (C), use the same Yeast Species (O), perform Repeats (R) for reliability, measure the Bubble Count (M1) over a Fixed Time (M2), and keep the Sugar Concentration (S) constant.
Limiting Factors: Recognize that at very high temperatures, the rate is limited by the denaturation of enzymes. At low temperatures, the rate is limited by the low kinetic energy of the system.
Equilibration: Always allow the yeast mixture to sit in the water bath for a few minutes before you start counting bubbles. This ensures the contents of the tube have actually reached the desired temperature of the water bath.
Identifying Errors: If the limewater does not turn cloudy, check for leaks in the bungs or bungs not being air-tight. Ensure the delivery tube is submerged in the limewater.
Ignoring the Oil Layer: Without the oil layer, yeast at the surface will respire aerobically. This would change the products and the rate of gas production, making it a test of total respiration rather than specifically anaerobic fermentation.
Optimal Temperature Assumptions: Students often assume 'the higher the temperature, the better.' You must emphasize that there is a peak (optimum) and that temperatures above typically kill the yeast cells entirely.
Mixing vs. Layering: Do not shake the tube once the oil layer is added. Shaking can incorporate oxygen into the solution, defeating the purpose of the anaerobic barrier.