The factors that affect the respiration of immobilised yeast

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The factors that affect the respiration of immobilised yeast;

Plan:

Hypothesis;

The time taken for the yeast beads to rise will decrease with increasing temperature until it reaches a minimum point, known as the optimum temperature, after which the time taken for the yeast beads to rise will then increase with increasing temperature.

Null Hypothesis:

Temperature will have no effect on the time taken for the yeast beads to rise.

Prediction:

I predict that a rise in temperature will produce a faster rate of reaction due to higher temperatures causing an increase in the kinetic energy of the enzyme and substrate molecules. This means that having solutions at higher temperatures will effectively cause more collisions between the substrate and active sites of the yeast (saccharomyces cerevisiae). However, an increase in temperature will also affect the stability of the enzyme molecules. Above the optimum temperature, the enzyme molecules begin to vibrate so violently that their tertiary and quaternary protein structures, causing a globular shape, break apart their hydrogen bonds and ionic forces which hold them together. This in turn alters the precise shapes of their active sites, which are essential for catalytic activity. With an altered active site, the enzyme loses its specificity and therefore cannot bind to its specific substrate and cannot function. The enzyme is therefore said to be denatured.

The higher the temperature, the more kinetic energy the hexokinase enzymes have. This means that there is a greater chance of the substrate binding with the hexokinase active site in the first stage of glycolysis. For this reason, more pyruvate is produced which can then be converted to acetaldehyde by a decarboxylase enzyme. This is done by removing carbon dioxide from pyruvate. Therefore, the more collisions that occur, the more carbon dioxide that builds up and becomes trapped inside the yeast balls. For this reason, the balls get less dense and begin to float up to the top of the surface. However, too much heat may break the hydrogen bonds and ionic forces holding the two enzymes, hexokinase and decarboxylase, together. This means that with high temperatures less carbon dioxide would be produced and therefore the yeast beads would not float up to the surface.

A predicted graph for the rate of reaction against temperature;

Background information:

Anaerobic respiration;

Anaerobic respiration in yeast is also known as alcoholic fermentation, a process that was discovered in ancient times by accident. Although people in ancient times used alcoholic fermentation to produce drinks such as wine and beer for years, they had no idea about the science behind it. For fermentation of occur, yeast requires a source of carbohydrates, anaerobic conditions and a suitable temperature.

        Fermentation stops when the yeast becomes poisoned by its own waste, the alcohol, which acts as a non competitive inhibitor to the enzymes involved in the process. This means that ethanol molecules bind to the allosteric site of the enzyme rather than the active site and in effect breaks the hydrogen bonds and ionic forces that hold the globular structure of the enzyme together. By doing this, the non competitive inhibitor changes the active site of the enzyme, meaning that it can no longer function and is said to be denatured. This is the reason why most alcoholic drinks made by fermentation are no stronger than 14% alcohol by volume. Alcoholic drinks that are more than 14% alcohol by volume such as whisky and gin need to be produced by distilling them from a fermented mixture.

(Source- Collins Educational Human Biology).

In order for respiration to get started, pyruvate needs to be formed from glucose.

Glycolysis is the first stage of respiration (aerobic and anaerobic), which occurs in the cytoplasm of cells. It consists of a series of enzyme catalysed reactions in which each molecule of glucose is converted step by step into two molecules of pyruvate.

Pyruvate is a compound that contains three carbon atoms and is very important in respiration since it can be converted into either acetate for aerobic respiration or acetaldyhyde for anaerobic respiration.

The initial reaction in glycolysis involves glucose being phosphorylated by ATP to form glucose 6-phosphate:

                                 Hexokinase

Glucose + ATP                                               Glucose 6 Phosphate + ADP + H+ 

This phosphorylation of glucose serves two purposes;

  1. It prevents glucose from leaving the cell, because the membrane is impermeable to sugar phosphates.
  2. Phosphorylation makes glucose more reactive so that it can be readily converted into phosphorylated three- carbon compounds.

Sucrose

                                         Glucose [C6]                                                    Fructose                                                                                                            

                                                               ATP        

                                                               ADP

                                 Glucose 6-phospahte [C6]

                     

                                Fructose 6-phosphate [C6]

                                                               ATP        

                                                               ADP

                            Fructose 1,6– biphosphate [C6]

Dihydroxyacetone phosphate [C3]                                  Glyceraldehyde 3-phosphate [C3]

                                                                                                               NAD+ 

                                                                                                              NADH

                          1,3 biphosphoglycerate [C3]                                                    

                                                              ADP        

                                                              ATP

                     Glycerate 3-phosphate (GP) [C3]

                                                   

                                               

                          2- phosphoglycerate [C3]

                       

       

                          Phosphoenolpyruvate [C3]

                                                              ADP        

                                                              ATP

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                                    Pyruvate [C3]

(The stages of glycolysis with regards to our experiment)

Overall, per glucose molecule, glycolysis produces;

  • 2 molecules of ATP (4 are produced but 2 are used up in glycolysis)
  • 2 molecules of reduced NAD, which later feeds electrons into the electron transport chain.
  • 2 molecules of pyruvate, which enters the ‘link’ reaction of oxygen is available.

In the presence of oxygen, yeast is able to go through aerobic respiration in which the pyruvate ...

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