The Effect Of Temperature on the Respiration Of Yeast.

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The Effect Of Temperature on the Respiration Of Yeast

Introduction: -

        I am going to find out how temperature effects the respitory action of yeast. I am going to do this by using six different temperatures, 25oC, 30oC, 35oC, 40oC, 45oC and 50oC and a solution called TTC which is absorbed by the yeast cells turning them pink when hydrogen is removed from the metabolic pathway by the dehydrogenase enzyme.

Background Information: -

Yeast: -

        Yeast is unicellular fungus that buds profusely under favourable conditions. They are classified as sac-fungi (Ascomyctes) even though they are unicellular. Yeast is common in areas where there is sugar because of this they are given the name saccharomyces (Sugar fungi). They grow on the surface of fruits, in the nectar of flowers, in sap excluded from trees, in the soil and in fresh water. Yeasts are used in all sorts of arrears such as alcoholic fermentation (fermentation is the name given only to the anaerobic respiration of yeast) to baking bread. Yeasts are of a great economic importance and are yeast for biochemical research for example in respiration and enzymes. Yeast exists as Diploid or Haploid cells and divides by mitosis and meiosis. 

Yeasts are not plant or animal because they do not photosynthesise and they do not have any chloroplast. Yeast is a saprophyte. This is an organism, which obtains its nutrients from dead or decaying organic matter. Many bacteria and Fungi are saprophytes.  They secrete digestive enzymes onto the surface of their food. The enzymes break down complex molecules such as those of starch and protein into smaller, soluble ones, which are then absorbed. 

Yeasts can both sexually and asexually reproduce. When the yeast asexually reproduces they multiply by repeated budding and separation. This continues whilst conditions are favourable and enables the yeast to exploit the food source quickly, whilst it is available. Sexual reproduction of yeast only occurs in the yeast colonies occasionally. The genetic mixing that occurs in meiosis and the random fusion of haploid cells, results in genetic variation in the offspring.

Respiration: -

        Yeast respires aerobically and anerobically. In this experiment the yeast is bubbled to make it oxygenated therefore the experiment will be aerobic for about 20 minutes before it will switch to anerobic respiration.

Aerobic respiration: -

C16H12O6 + 6O2                6CO2 + 6H2O

                                     Glucose + Oxygen      Carbon + Water

      Dioxide

Anerobic respiration: -

                                                     C16H12O6                  Alcohol + 6CO2

                                                         Glucose                                  Carbon

                                                          Dioxide

                       

Enzymes and TTC: -

Enzymes have a big role in the anaerobic respiration of yeast. The enzyme that is very important to this experiment is the dehydrogenase enzyme. This enzyme is important because it can donate the hydrogen that it has removed from the respiratory substrate to the colourless solution, 2,3,5 Triphhenyl Tetrazolium Chloride (TTC) causing a colour change. When the colourless chemicl TTC diffuses into actively respiring tissues it accepts electrons from the mitochondrial electron transport chain, reducing it to a pink compound. The intensity of the Pink colour is proportional to the rate of respiration. 

In glycolosis ATP donates one phosphate group to glucose to change ATP to ADP. The glucose phosphate is reorganised to a fructose phosphate molecule. The fructose phosphate is activated by the further donation of the second phosphate group by an ATP molecule. The 6-carbon fructose is then split in to 3-carbon triose phosphate molecules. Hydrogen atoms are removed from triose phosphate molecules and taken up by NAD. Inorganic phosphate is then added to further activate the triose phosphate. The phosphate molecule is then lost and ATP is regenerated and a water molecule is also lost, for each triose phosphate. The final products are two molecules of pyruvate. During the final stages of glycolosis, four molecules of ATP are synthesised from ADP using the phosphate attached to the triose phosphate and inorganic phosphate in the cytoplasm.

NAD (niculotinamide adenine dinucleotide) is made of two linked nucleotides that are linked together. Both nucleotides contain ribose. One nucleotide contains the nitrogenous base adenine. The other has a nicotidinamide ring, which can accept a hydrogen ion and two electrons, thereby becoming reduced.

             

  NAD + 2H  reduced NAD

NAD+ + 2H  NADH+ + H+

The dehydrogenase enzyme removes the hydrogen from the triose phosphate and passes it on to a hydrogen acceptor called NAD. Oxidation has taken place and the NAD is then said to be reduced to NAD + H+. This reduced NAD changes the colour of the TTC solution from colourless to pink. NAD is called a coenzyme, a non-protein organic compound essential for the functioning of the association enzyme.

At the end of glycolosis the product made, pyruvate still has a lot of chemical potential energy left over. When there is free oxygen available, some of this energy can be released via Krebs cycle and oxidative phosphorylation.

If under anaerobic respiration, pyruvate gives off CO2, ethanal, which is then, reduced to ethanol.

If under aerobic, pyruvate first enters the Link reaction that takes place in the mitochondria. The glucose gets broken down in to pyruvate and then lactate.

The Link Reaction: -

After glycolosis, if oxygen is available the pyruvate enters the mitochondrial matrix and is then decarboxylated (here the carbon dioxide is removed).  After the pyruvate is decarboxyilated it is then dehydrogenated and then combined with Coenzyme a to form acetylcoenzyme A (AcoA).

pyruvate (3C) + coenzyme A     Carbon Dioxide            acetyl (2C) coenzyme A   

                                                           

                                                    NAD    NADred

Coenzyme A consists of adenine, ribose and pantothenic acid. Coenzyme A transfers an acetyl group (witch 2 Carbon atoms) from pyruvate in to Krebs cycle.

Krebs cycle: -

        The Krebs cycle also takes place in the mitochondrial matrix. First a 6-Carbon compound is made when the acetyl (2 C) from coA is combined with a 4 C compound called oxaloacetate to give a 6-carbon compound. The new compound citrate is converted back into oxaloacetate when process such as decarboxylation and dehydrogenation takes place and carbon dioxide and hydrogen are given off as a waste product. The hydrogen’s that are removed are accepted by NAD or FAD (Flavin adenine dinucliotide). One FAD and three NAD molecules are reduced to NADred and FADred. The main role of the Krebs cycle is to produce a pool of hydrogen carriers to be passed on to the next stage. The regenerated oxaloacetate can combine witch another AcoA.

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Enzymes: -

        There are two main enzymes that are used in anaerobic respiration, decarboxylase and dehydrogenase. Enzymes and substrate molecules are constantly moving and often collide. When the enzymes collide the substrate may fit in to the enzymes active site for a brief moment, forming an enzyme substrate complex. Once the substrate enters the active site of the enzyme, the enzyme changes shape to mould itself around the substrate. This is called induced-fit. While this is happening the R group on the polypeptide forming the active site are brought in to a position which combines with the substrate, forming temporary ...

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***** This is a very detailed and comprehensive plan of an investigation. The author demonstrates a clear understanding of all relevant theory and considerable attention to detail is evident throughout. The writing is of a very high standard.