What effect does substrate have on respiration in yeast?

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Elakkiya Sunthararajan

What effect does substrate have on respiration in yeast?

AIM

The aim of this investigation is to find out how the rate of respiration in yeast is affected when different respiratory substrates are used. Five different respiratory substrates will be used and the amount of carbon dioxide produced will be measured for each substrate. The five substrates that will be used are glucose, fructose, maltose, sucrose and lactose.

HYPOTHESIS

Null hypothesis, H0:

There is no significant difference between the amounts of carbon dioxide produced by yeast during respiration, regardless of the respiratory substrate used.  

Alternate hypothesis, H1:

There is a significant difference between the amounts of carbon dioxide produced by yeast during respiration, depending on the respiratory substrate used.  

PREDICTION

Natural habitat of yeast is the skin of fruit, which usually contains fructose and glucose and in some cases sucrose. Yeast is also found on malt so it is familiar with maltose as well. Therefore it will be able to secrete the enzymes needed to break down glucose, fructose, sucrose and maltose. However, lactose is present in milk and other dairy products, where yeast does not live. As a result, it is likely that yeast would not have the enzyme, lactase, needed to break down lactose.

The table below shows each substrate, the enzyme needed to hydrolyse this substrate and what the substrate is broken down into.

Although glucose and fructose have the same molecular formula, C6H12O6, the arrangement of their carbon chain is different, these types of molecules are said to be isomers. As their structure and their shapes will not be the same, they will require different enzymes in order to be broken down. This is because every enzyme has a unique active site and only one kind of substance can bind to this active site and be broken down. Enzymes are specific. Since the glucose and fructose are in different forms, they will not be able to join to the active site on the same enzyme. Likewise, maltose, sucrose and lactose have the same molecular formula, C12H22O11, but they also need different enzymes in order to be broken down. As enzymes are proteins, changes in temperature, pH and concentration of substrate can affect the rate of reaction, in this case, the rate of respiration. So these factors would have to be kept constant during the experiment, in an attempt to obtain consistent results. In addition, high temperatures above the optimum can change the active site of the enzymes, as they are just proteins in their tertiary structure. When the temperature is high the ionic bonds, sulphur bridges and hydrogen bonds can be broken altering the shape of the active site and preventing the formation of enzyme-substrate complexes. Therefore, great care must be taken to ensure that the temperature is kept at its optimum.

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Glucose and fructose are monosaccharides so an enzyme is not always required to carry out respiration using these two substrates. They can simply enter the yeast through their specific carrier proteins, found on the cell membrane of the yeast, by facilitated diffusion and they will undergo glycolysis at once. On the other hand, maltose, sucrose and lactose are disaccharides, so enzymes are needed to hydrolyse them into monosaccharides first, so that they can then be broken down during glycolysis.

Glycolysis is the first process in aerobic and anaerobic respiration. Aerobic respiration only occurs when oxygen is present and in yeast ...

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