All cells carry out cellular respiration in order to meet their energy requirements and to form metabolic intermediates that can be used to form secondary products. Amongst those products formed as a consequence of cellular respiration, are ATP and CO2. The formation of ATP in the cells occurs through the use of organic molecules available in the form of food (Lehninger, Nelson, & Cox, 2008). When oxygen is not available, ATP formation goes through a fermentation process, usually, referred to as alcoholic fermentation. The process utilizes a number of different sugars as the starting material. These sugars include fructose, glucose, and sucrose. The sugars undergo a conversion process to produce ATP as the main product while ethanol and CO2 are produced as waste products (Seeley, Stephens, & Tate, 2004).
Production of ATP in the presence of oxygen occurs via a process known as the aerobic cellular respiration. The process results in the transfer of food energy to ATP. Several processes make up the aerobic cellular respiration resulting in a complex series of reactions. The processes include glycolysis taking place in the cytoplasm and leads to the production of pyruvate molecules and two molecules of ATP. The other process is the Krebs cycle, which uses the pyruvate produced during glycolysis to produce hydrogen ions, CO2 and electrons. The produced electrons and hydrogen ions go through the NAD+ and FAD electron carriers where the final aerobic respiration process takes place (Campbell & Farrell, 2011).
The ATP production process is, usually, affected by different factors since enzymes are involved in most of the processes. These factors are essential in a successful ATP production and include substrate concentration, enzyme concentration, the substrate present, presence of inhibitors, reaction temperature. Proper concentration of the substrate and enzyme ensures a speedy reaction. Presence of inhibitors results in a reduced rate of the reaction while presence of wrong substrate results in no reaction taking place. Proper temperature is also needed for optimal enzyme activity.
This experiment aimed to evaluate the effects substrate (sucrose) concentration, yeast concentration, substrate type, and the presence of inhibitors on respiration in baker’s yeast (Saccharomyces cerevisiae). The evaluation was done by monitoring the rate at which CO2 bubbles were produced.
Preparing Yeast Culture
A liter of room water tap water was made, and the temperature of the water read using a thermometer. Hot water was added and stirred completely to bring the water temperature to room temperature. One hundred mL of the water was measured and poured into a 250 mL beaker. Thirty grams of the dried yeast were weighed and added to the beaker and swirled until suspension was homogeneous.
The flexible tubing was assembled to the glass tube and the glass tube inserted through the stopper. The tube was sealed, and the stopper inserted in the flask. The pen end of the flexible tubing was placed into the gas collection vessel that was half full of water to enable bubble counting.
Into a 125 mL flask, 100 mL of water was poured, and 5 g of sucrose added. The content was swirled to dissolve completely, and 20 mL of the culture added. Swirling was done to achieve homogeneity, and the stopper inserted, the plastic tubing was placed down into the bubble beaker, and CO2 produced counted after attaining a steady rate in about 3 minutes. Bubbles were counted 5 times each for 1 minute.
Substrate Concentration Sucrose
Five grams of sucrose were dissolved into 100 mL room temperature water with 20 ml yeast culture in 125 ml Erlenmeyer flask and monitored. Different concentrations of sucrose, 0.625 g, 1.25 g, 7.5 g and 10 g were made, and CO2 bubbles produced counted.
Five runs using 2.5 mL, 5.0 mL, 10 mL, 20 mL (CONTROL), and 25 mL of the yeast culture were made, and CO2 bubbles produced counted.
Five grams of glucose, galactose, fructose, starch, artificial sweetener (Sweet'n'Low) and sucrose (control) were added in 20 ml source and 100 ml water and CO2 bubbles produced counted.
Different inhibitors, 2.5 mL ethanol, 5 mL ethanol, 10 mL ethanol, and 15 mL ethanol were employed to test the effect of inhibitors. No inhibitor was added in the control experiment. The amount of CO2 bubbles produced counted and recorded.
Substrate Concentration Sucrose
The average number of bubble observed at different concentrations of sucrose was recorded in Table 1 and used to plot a graph of the amount of bubbles against sucrose concentration.
The rate of reaction was highest at 5 g of sucrose and lowest at 1.25 g with no clear trend (Figure 1).
Figure 1: Average number of bubbles at different sucrose concentration
The average number of bubble observed at varying yeast culture was recorded (Table 2) and used to plot a graph of the number of bubbles against volume of yeast culture.
The rate of reaction increased as the volume of the yeast increased up to 20 mL of yeast (Figure 2).
Figure 2: Average number of bubbles at varying volumes of yeast culture
The average number of bubble observed at different substrates was recorded (Table 3) and used to plot a graph of number of bubbles against different substrates.
The rate of reaction was greatest in the sweetener, followed by fructose and lowest in sucrose. Glucose and starch did not give show any activity (Figure 3).
Figure 3: Average number of bubbles at different types of the substrate
The average number of bubble observed at different concentrations of ethanol was recorded (Table 4) and used to plot a graph of the bubbles against different levels of ethanol.
The rate of reaction increased as the volume of the ethanol increased (Figure 4).
Figure 4: Average number of bubbles at different types of inhibition
Increase in yeast volume resulted in an increased number of enzymes in the reaction and hence the high increase in reaction rate as yeast amount was increased. The rate of reaction was greatest in the sweetener making it the most preferred substrate by the yeast. Glucose and starch did not give show any activity due to lack of enzymes for metabolizing the substrates. Ethanol did not work as an inhibitor and increase in alcohol volume increased the rate of reaction.
This experiment was thus successful in evaluating the effects substrate (sucrose) concentration, yeast concentration, substrate type, and presence of inhibitors on respiration in baker’s yeast (Saccharomyces cerevisiae).
Campbell, M., & Farrell, S. (2011). Biochemistry. Stamford: Cengage Learning.
Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2008). Lehninger principles of biochemistry (4th ed.). New York: WH Freeman.
Seeley, R., Stephens, T. D., & Tate, P. (2004). Anatomy and Physiology (6th ed.). New York: The McGraw Hill Companies.