Higher Biology Assignment

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Investigating respiration rate in different concentrations of yeast

Aim

To investigate the effect of different concentrations of yeast on the rate of respiration

Underlying biology (P1)

• The first stage of respiration is glycolysis.

• Glycolysis occurs in the cytoplasm and is the breakdown of glucose into pyruvate.

• In the energy investment phase of glycolysis ATP is required for intermediate products and glucose to be phosphorylated.

• The phosphorylation in the energy investment phase results in the generation of more ATP, this leads to a net gain of ATP.

• When oxygen is present, an acetyl group is formed when pyruvate is broken down. The acetyl group then combines with coenzyme A to form acetyl coenzyme A.

• Dehydrogenase enzymes remove electrons and hydrogen ions from molecules. The electrons and hydrogen ions are passed onto coenzyme NAD to form NADH.

Underlying Biology (P2)

• The second stage of respiration is the citric acid cycle. The citric acid cycle occurs in the matrix of the mitochondria when oxygen is present.

• In the citric acid cycle citrate is formed when oxaloacetate combines with the acetyl group from acetyl coenzyme A.

• Gradually, citrate is converted back into oxaloacetate in a series of enzyme controlled reactions. The conversion of citrate into oxaloacetate generates ATP and releases carbon dioxide.

• Dehydrogenase enzymes remove hydrogen ions and electrons from molecules. These hydrogen ions and electrons are passed onto coenzyme NAD to form NADH.

Underlying biology (P3)

• Electrons and hydrogen ions from NADH are passed onto the electron transport chain in the inner mitochondrial membrane. Electrons from NADH are then passed along the electron transport chain. Through this process energy is released.

• The energy produced is used to pump hydrogen ions across the inner mitochondrial membrane. Hydrogen ions flow through ATP synthase, an enzyme found in the membrane, generating ATP.

• After hydrogen ions flow through ATP synthase they form water by combining with electrons from NADH and oxygen. Oxygen is the final hydrogen acceptor.

Underlying biology (P4)

Rezasurin is a substance thats colour changes depending on p.H. Rezasurin has a purple colour at high p.Hs and a paler beige colour at lower p.Hs. Since p.H is a measure of the concentration of hydrogen ions and hydroxide ions in a solution, rezasurin can be used to measure the concentration of hydrogen ions in a solution produced by respiration.

Description of experiment

Three different concentrations of yeast in the form of sodium alginate yeast balls were added to test tubes with glucose and rezasurin. The test tubes were then shaken gently and placed in a water bath at 35⁰C. The colour of solution was recorded at regular intervals, beginning when the stopclock started . Time was measured using a stop clock and colour was measured according to a colour chart, using sight.

Analysis

• My results show that as concentration of yeast solution increases from 1% to 4% average number on colour chart decreases. At 1% yeast concentration the average number on colour chart decreases by 25%, at 2% yeast concentration the average number on colour chart decreases by 37.5%, and at 4% yeast concentration the average number on colour chart decreases by 50%. A decrease in average number on colour chart indicates an increase in rate of respiration because as hydrogen ion concentration from respiration increases, the average number on colour chart decreases.

• My source data shows the same trend. At 2.5% yeast concentration, the average light absorbed by the solution decreases by 26.68%. At 5% yeast concentration the average light absorbed by the solution decreases by 50%. At 10% yeast concentration, the average light absorbed by the solution decreases by 54.37%. As yeast concentration increases, light absorbed by the solution decreases. This indicates an increase in rate of respiration because as hydrogen ion concentration from respiration increases, light absorbed decreases.

Evaluation statement (Colorimeter)

One way accuracy of my results could have been increased is by using a colorimeter to determine the colour of the solution instead of just using my sight. The colour of the solution, according to the colour chart, is subjective. Using a colorimeter would have allowed me to more accurately determine the colour changes occurring through the reaction.

Evaluation statement (Range of values)

Another way the validity of my results could have been increased is by increasing my range of values for yeast concentration. Increasing my range of yeast concentration values would have allowed me to observe the trend in greater detail. It would also allow me to determine if there was a concentration of yeast solution where rate of respiration reached it’s maximum.

Evaluation statement (Control)

Another way the experiment could have been improved is by using a control. No control was used, but using one would have allowed me to ensure it was yeast concentration having an effect upon number on colour chart

Evaluation statement (Reliability - My Data)

My results are reliable. I know they are reliable because the number on colour chart at the end of the experiment in each concentration was quite similar for test 1, test 2, and test 3.

Evaluation statement (Reliability - Source Data)

My source data was reliable as it was repeated twice for each concentration. Three tests were carried out for each concentration.

Evaluation statement (p.H)

Another way my experiment, and therefore my results, could have been improved was by using a p.H buffer. It is possible p.H varied across test tubes which could have had an effect on results because dehydrogenase enzymes can be affected by pH. A p.H buffer would have maintained a constant pH across all test tubes.

Conclusion

In conclusion, different yeast concentrations do have an effect upon rate of respiration. As yeast concentration increases, rate of respiration increases too. This trend is shown for my data and also my source data through a decrease in average number in colour chart for my data and a decrease in light absorbed by the solution in my source data. This colour change and decrease in light absorption was caused by an increased concentration of hydrogen ions produced through respiration.