The effect of different sugar substrates on the rate of yeast respiration

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The effect of different sugar substrates on the rate of yeast respiration

AIM

In this investigation I will be using various sugars to see the effect on the rate of respiration of yeast. The sugars I will be using are glucose, fructose, sucrose, maltose and galactose. All the sugars used will be of a 2%concentration. In order to obtain my results I will use the same type of yeast i.e. Bakers yeast. I will place the 2% solution of the sugars in a waterbath at 35ºc. Together with the sugars I will place 20cm³ of yeast solution. I will measure out 25cm³ of both sugar and yeast and place them into a conical flask in the same waterbath. As soon as I do this I will place the rubber bung and delivery tube which will be attached to the gas syringe and onto the flask. I will then measure the volume of Carbon Dioxide produced by the yeast every 30 seconds until the yeast stops respiring and this will repeated for each sugar. After completing my results I will draw a graph to work out the rate of respiration of yeast and also statistically analyse my results.

Background Information

Yeast is a unicellular (eukaryotic) fungus that reproduces asexually by budding. Cells of yeast are very active metabolically and usually respire aerobically. However when they are deprived of oxygen they switch to anaerobic respiration where it produces carbon dioxide and ethanol as waste products. These waste products are important in industry. Cells of yeast have a single nucleus and are usually egg shaped. Individual cells vary in size but nevertheless contain most of the organelles of a typical eukaryote. Yeast can reproduce both sexually and asexually. In asexual reproduction, a single cell divides by budding and separates into two cells. Some buds group together to form colonies, and others separate to grow individually into new yeast. In sexual reproduction, two cells fuse to form a diploid cell which then forms haploid spores by meiosis.  

Below is the equation for the aerobic respiration of yeast using glucose as an example,

Glucose + Oxygen                           Carbon Dioxide + Water

When the yeast is mixed with sugar it soon starts to respire. The yeast uses sugar and oxygen to produce carbon dioxide, water and energy by aerobic respiration. Glucose is phosphorylated, eventually producing two molecules of pyruvate. The process is called Glycolysis and takes place in the cytoplasm of cells. The presence of oxygen enables pyruvate to enter the mitochondrial matrix where it is oxidatively decarboxylated to make acetyl CoA by the dehydrogenase. This reaction links glycolysis to the Krebs cycle, in which the acetyl CoA is completely oxidized to give two molecules of CO2 and reductive equivalents in form of NADH and FADH2. These are then transported to the electron transport chain where they are used to produce ATP energy.

Below is the equation for anaerobic respiration or sometimes know as Fermentation of yeast again, using glucose as an example.

Glucose                            Ethanol + Carbon Dioxide + Energy

During anaerobic respiration, because there is no oxygen present only glycolysis can take place and so the link reaction, Krebs cycle and the electron transfer chain cannot occur. In yeast the end products of Glycolysis is carbon dioxide and pyruvic acid which is then converted into ethanol.

Although yeast can survive during anaerobic respiration, it does not grow and multiply as it would during aerobic respiration. Anaerobic respiration releases much less energy than aerobic respiration. In anaerobic conditions most energy remains locked in the ethanol. One problem is that alcohol is poisonous in large amounts. If the concentration of alcohol gets more than 14% it kills the yeast and the fermentation stops. Thus, if the experiment was carried out in anaerobic conditions the experiment couldn't be carried out for too long. This is one factor that may influence my investigation and my results. In my investigations I will have to make sure that the yeast has plenty of oxygen to in order to carry out aerobic respiration, most of the time. A method which I could use is to keep swirling the mixture in order to provide it with the oxygen

The starting point of glycolysis during both aerobic and anaerobic respiration is the break down of the sugar substrate available to the yeast by the hydrolytic enzyme in yeast. The enzyme in yeast respiration is called Zymase.

Sugars begin the process of respiration and different sugars will affect the rate of respiration of yeast differently. This is because some sugars will be more readily broken down than others and so faster respiration will occur. The amount of carbon dioxide produced by the yeast after a specific amount of time will help determine the rate of respiration of the sugars. Theses rates can then be compared to see if different sugars do have an effect on the respiration of yeast.

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Enzymes are three dimensional proteins with a tertiary structure. They have an area on their surface called the active site which has a specific shape. The substance the enzyme acts on is the substrate. Both the substrate and active site have a specific complementary shape that allows them to bind together. When the substrate binds to the active site it forms a temporary bond and is know as an enzyme-substrate complex. The substrate reacts within the complex to form the products that then leaves the active site. This description of enzyme activity I know as the lock and key mechanism.

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This is a very good piece of planning and results. The method is fully explained, valid reasons given for each piece of apparatus and variables. There is no analysis or evaluation but what is there is very good. 4 stars.