Manal Raza
This experiment is guided by the following research question:"How does the concentration of glucose (0, 0.25, 0.50, 0.75, 1 mol dm-3) affect the rate of CO2 production by yeast during fermentation, as measured using a gas pressure sensor?".
Background Information
Glucose is a simple sugar that serves as a fundamental source of energy in various chemical reactions. Its molecular structure is composed of a six-carbon ring, with each carbon atom bonded to a hydrogen atom and a hydroxyl group (-OH), and one carbon atom as part of a carbonyl group (C=O).
Alcohol fermentation involves the conversion of glucose into ethanol and carbon dioxide in the absence of oxygen. This biological process takes place through a series of enzymatic steps in yeast cells which start with the concept of collision theory. First, glucose molecules must collide with the yeast with a sufficient amount of activation energy and correct orientation for the process of fermentation to commence, as stated by collision theory. This will allow glucose to be broken down into two molecules of pyruvate through the process of glycolysis. Then pyruvate undergoes decarboxylation where it loses a carbon atom to form carbon dioxide as a byproduct, leaving acetaldehyde as an intermediate product. Acetaldehyde is then reduced by NADH, a coenzyme produced during glycolysis. This reduction reaction is catalyzed by alcohol dehydrogenase and results in the regeneration of NAD+ and the production of ethanol as shown by Figure 1.
The reaction equation is represented as follows:
C6H12O6(aq) → 2C2H5OH (aq) + 2CO2(g) + 2ATP

Figure 1: Steps of alcoholic fermentation in yeast cell
In the process of fermentation the rate-determining step is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by the enzyme phosphofructokinase (PFK). The rate-determining step is the slowest step in a reaction and controls the overall rate of the entire reaction. In this context, PFK is important due to it being regulated by molecules like ATP (which slows it down) and AMP (which speeds it up), allowing the cell to adjust the rate of glycolysis according to how much energy is needed. This step ensures that the glucose molecule will continue through a series of reactions to produce ATP and CO2. Therefore, the rate at which ATP and CO2 are produced is not only determined by how many collisions occur between reactant particles in the correct orientation with a sufficient amount of activation energy but also by how fast fructose-6-phosphate is converted to fructose-1,6-bisphosphate by PFK.
If the concentration of the reactant (glucose) were to be increased in this reaction the rate of reactions would increase due to more particles being available per unit of volume. This increases the likelihood of successful collisions between reactants with a sufficient amount of activation energy, ultimately increasing the rate of reaction as shown in Figure 2. When the concentration of glucose is increased, there are more sugar molecules for the enzymes involved in fermentation to act upon which allows the reaction to occur faster. This will result in a faster production of carbon dioxide which can be measured by change in gas pressure over time. However, as depicted in Figure 2 the graph will eventually reach a plateau due to glucose being a limiting reactant. This means its availability will determine the rate and extent of the reaction. As the reaction progresses, glucose concentration will decrease and the rate will reach a plateau, indicating that the reaction can no longer proceed at the same rate due to the lack of glucose, resulting in a constant product yield.

Figure 2: Concentration impact on rate of reaction
Variables
Independent variable: Concentration of glucose (0, 0.25, 0.50, 0.75, 1 mol dm-3)
Dependent variable: Change in gas pressure per second (kPa/s)
Control variables:
Temperature at 38ºC
The temperature of the glucose and yeast solution must remain constant throughout the experiment as it can directly impact the rate of reaction based on what is known of enzyme activity and collision theory. As we already know the process of fermentation is catalyzed by enzymes which in order to function at an optimal level must be kept at a specific temperature (38ºC), if not kept at this temperature they will denature and fail to catalyze the reaction. Denaturation occurs when proteins lose their structure due to external stress, such as heat or pH changes, resulting in a loss of function. High temperatures increase kinetic energy, disrupting chemical interactions and breaking hydrogen bonds and hydrophobic interactions. This causes the protein to lose its shape and become nonfunctional. So, if the glucose and yeast solution reaches over 38ºC then the enzymes in it will denature and the rate of reaction will decrease, ultimately reducing the volume of CO2 produced in a fixed period of time. Moreover, according to collision theory, a decrease in the temperature of a solution can decrease the average kinetic energy within the reactant particles. This means that when the solution is below 38ºC, there will be less kinetic energy, and the likelihood of particles colliding with the correct orientation and with energy greater than or equal to the activation energy will decrease. Consequently, this will reduce the rate of reaction and the volume of CO2 produced in a fixed period of time.
Concentration of yeast
The concentration of the yeast must be kept constant for each trial since it can impact the rate of reaction according to collision theory. Based on collision theory as the concentration of the catalyst increases there will be more of the catalyst available to interact with the reactant molecules. This higher concentration increases the likelihood of successful collisions between the reactants and the catalyst ultimately leading to a greater rate of reaction. If the concentration of one trial was greater than another trial then it would increase the rate of reaction eventually causing there to be a larger volume of CO2 produced in a fixed time.
Time taken for the reaction
The time taken for the reaction would affect the overall volume of CO2 produced by the chemical reaction. The time taken for the reaction to occur must be kept constant throughout the trials, which is 5 minutes, so that the reaction has enough time to fully complete. If one trial took less than 5 minutes then it would lead to varying amounts of CO2 produced, causing variability and inconsistency. This inconsistency would make it difficult to isolate the effect of a specific variable, such as the concentration of glucose, in the investigation.
Stirring glucose and yeast to completion
The distribution of the yeast and glucose in a solution must be even throughout the trials since it can impact the rate of reaction based on what is known of collision theory. Collision theory states that in order for a reaction to occur reactant particles must collide in the correct orientation and with energy greater than or equal to the activation energy. This means that to ensure the reaction occurs evenly, there must be an equal distribution of yeast and glucose in the solution, creating a homogeneous mixture. This ensures that there are enough reactants per unit of volume for the reaction to proceed. If solid glucose and yeast remain at the bottom of the test tube, the reaction will take longer and will not reach completion because there are not enough collisions between reactants due to their uneven distribution. This ultimately reduces the reaction rate and the volume of CO2 produced within a fixed period of time.
Materials
1 x Computer with Logger Pro
1 x Electric Kettle
1 x Water bathtub
1 x Gas pressure sensor (± 0.0000001 kPa)
1 Tube
1 Rubber Stopper
5 x 10cm3 Test Tube
1 x 100cm3 Beaker
5 x 50cm3 Beaker
1 x 100 cm3 Volumetric flask (± 0.10)
1 x 40 cm3 Volumetric flask (± 0.04)
1 x 50cm3 Graduated Cylinder (± 0.5)
1 x Glass Stirring Rod
1 x Digital Weighing Scale (± 0.001)
30g Instant Dry Yeast Powder
18.016g of Glucose Powder
500cm3 of Distilled Water
Safety
Goggles and gloves should be worn for this procedure as they will act as a barrier against exposure to yeast and glucose. Although glucose and yeast are generally harmless substances to humans they may irritate the eyes or skin. If skin comes in contact with any of these substances it is advised to rinse the affected area with water to avoid any future irritation. If any of these substances were to come in contact with someone's eyes then immediately use an eye wash to rinse out the substances for at least 10 minutes. While glucose may cause redness and irritation to the eyes, yeast has the potential to cause an eye infection if not dealt with immediately. Hands should also be thoroughly washed before and after the experiment to ensure no substance transfer. Regarding disposal, since glucose is a sugar it is fine to dispose of it down the drain however since yeast is a living organism it can be hazardous to the environment so it is best to dispose of it properly in a designated waste beaker.
Procedure
Preparing different concentrations of glucose:
Heat up water using electric kettle until 38ºC is reached
Use digital scale to measure 18.016g of glucose powder
Pour 50cm3 of heated water into a 100cm3 beaker using a 50cm3 graduated cylinder
Add 18.016g of glucose powder
Use a stirring rod to mix until the glucose powder is fully dissolved
Once fully dissolved pour this solution into a 100cm3 volumetric flask
Pour heated water into the flask until the 100cm3 mark
Gently swirl the flask to make sure the glucose powder is fully dissolved, this will be the stock solution
Set aside 40cm3 of this solution in a 50cm3 beaker using a 50cm3 graduated cylinder, this will be the 1 mol dm-3 solution used in the reaction
For the 0.75 mol dm-3 solution of glucose measure out 30 cm3 of the stock solution using a 50cm3 graduated cylinder into a 40cm3 volumetric flask
Fill the volumetric flask with heated water until the 40cm3 mark
Gently swirl the flask until the glucose powder is fully dissolved
Repeat steps 9-13 for the remaining 3 concentrations as follows:
0 mol dm-3: 0 cm3 stock solution
0.25 mol dm-3: 10 cm3 stock solution
0.5 mol dm-3: 20cm3 stock solution
Reaction:
Open Logger Pro on your computer. Plug in the Vernier Logger Pro to your USB port.
Set up the water bath with enough water to submerge the test tube three fourths of the way and make sure the water stays between 35-38ºC.
Measure 5cm3 of 1 mol dm-3 glucose solution using a 5cm3 graduated cylinder.
Add 5cm3 of 1 mol dm-3 glucose solution into a 10cm3 test tube.
Place test tube into water bath.
Measure 1.000g of instant yeast using a digital scale.
Take test tube out of the water bath and add 1.000g of instant yeast powder into to test tube.
Gently and briefly swirl the solution in the test tube to ensure than no solid reactants remain on the bottom.
Close with rubber stopper immediately after and place bath into the water bath.
Record the change in gas pressure (kPa) over a span of 5 minutes on Logger Pro while making sure the temperature of the water bath stays between 35-38ºC.
Repeat steps 3-10 for each concentration of glucose (0, 0.25, 0.50, 0.75, 1 mol dm-3) 5 times each to have a total of 25 trials (Using the same test tube for each concentration).
After each trial dispose of the solution in a waste beaker and rinse out the test tube.
Results
Concentration (mol dm⁻³) | Trial 1 (kPa/s) | Trial 2 (kPa/s) | Trial 3 (kPa/s) | Trial 4 (kPa/s) | Trial 5 (kPa/s) |
0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
0.25 | 0.10 | 0.12 | 0.15 | 0.11 | 0.14 |
0.50 | 0.20 | 0.23 | 0.25 | 0.21 | 0.22 |
0.75 | 0.28 | 0.30 | 0.29 | 0.27 | 0.31 |
1.00 | 0.33 | 0.34 | 0.35 | 0.32 | 0.33 |
The data collected shows the rate of CO₂ production by yeast at varying glucose concentrations (0, 0.25, 0.50, 0.75, and 1 mol dm⁻³), measured through changes in gas pressure (kPa) per second. The following trends were observed:
0 mol dm⁻³ glucose: No measurable increase in gas pressure, indicating minimal or no CO₂ production, as expected due to the absence of glucose for yeast fermentation.
0.25 mol dm⁻³ glucose: Small increase in gas pressure, with a modest CO₂ production rate, reflecting a limited substrate availability for yeast metabolism.
0.50 mol dm⁻³ glucose: Noticeably higher gas pressure change rate compared to 0.25 mol dm⁻³, indicating increased CO₂ production as more glucose became available.
0.75 mol dm⁻³ glucose: Further increase in CO₂ production rate, though not as dramatic as from 0.25 to 0.50 mol dm⁻³, suggesting diminishing returns in the rate of CO₂ production.
1 mol dm⁻³ glucose: The highest rate of CO₂ production was observed, though the increase in the rate of gas pressure change was smaller than the previous jumps, suggesting a near-saturation effect in the yeast's enzymatic activity at this concentration.
The results indicated an increasing trend in CO₂ production rate with higher glucose concentrations, with a clear plateau starting at around 0.75 mol dm⁻³ and continuing at 1 mol dm⁻³, as seen in the average rates of gas pressure change across trials.
Conclusion
The experiment demonstrated that glucose concentration positively affects the rate of CO₂ production by yeast during fermentation up to a certain threshold. Initially, as glucose concentration increased, the availability of substrate allowed yeast enzymes to catalyze the reaction more frequently, thus increasing CO₂ production. However, the rate plateaued around 0.75-1 mol dm⁻³, suggesting that yeast reached its enzymatic capacity for glucose metabolism at this concentration, aligning with the principles of enzyme kinetics and collision theory. These findings support the hypothesis that increased glucose concentration initially boosts fermentation rates but that saturation effects eventually limit the reaction rate, leading to a plateau in CO₂ production.
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