Method:
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Add 200cm3 of each sugar solution to separate, appropriately labelled flasks. Include one flask containing 200cm3 of distilled water as a control.
- Add 2g of dried yeast and 1g of culture nutrients to each flask. The culture nutrients are a mixture of equal masses of ammonium phosphate and ammonium sulphate. Swirl the flasks, or stir the contents thoroughly with a glass rod, to ensure that the nutrients dissolve and that the yeast is resuspended.
- Plug each flask with cotton wool and incubate overnight at 25°C.
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Set up a burette containing 0.1mol dm-3 sodium hydroxide solution.
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After incubation, swirl each flask thoroughly to mix the contents and remove a 25cm3 sample of each culture. Place each sample in separate, labelled conical flasks. Add 2/3 drops of phenolphthalein indicator solution to each.
- Titrate each sample against the sodium hydroxide solution to find the volume of alkali needed to neutralize the acid produced by the yeast.
Hypothesis:
I think that that the yeast will be able to utilize the carbon sources in the following order:
Fructose (monosaccharide)
Glucose (monosaccharide)
Sucrose (disaccharide – fructose and glucose)
Maltose (disaccharide – glucose and glucose)
Lactose (disaccharide – glucose and galactose)
Galactose (monosaccharide)
I think that fructose will be the most useful substrate for respiration because it is commonly found in citrus fruits where yeast cells are also abundant and therefore it is readily available. Fructose is also a monosaccharide and therefore does not need to be hydrolysed by an enzyme. Glucose is also a monosaccharide and therefore no enzyme is required. Glucose is also used directly in the glycolysis pathway that is used to produce ATP, which is of course, the point of respiration. I have decided to put sucrose next because only one enzyme is needed to hydrolyse it to form glucose and fructose, which can then be used by the yeast cells for the reasons mentioned above. However the use of an enzyme to break down sucrose means that energy is required to make this enzyme and so both fructose and glucose would appear to be a more useful carbon source. Maltose is another disaccharide and therefore needs an enzyme to break it down, but the constituents of maltose are glucose and glucose that can feed directly into the glycolysis pathway. Lactose requires an enzyme to break it down but also needs another enzyme to convert galactose into glucose so it can be used for respiration. Lactose is commonly found in milk, which is not the habitat the yeast cells are mostly found. I have put galactose at the bottom of the list because although it is a monosaccharide, it still needs to be converted into glucose via an enzyme and it is not commonly found in areas where yeast cells thrive.
Results:
These are the results for the titrations carried out with each carbon source:
This is a bar chart to show the volume of alkali required to neutralize the acid produced by yeast in each sugar solution.
From the results that I obtained, I can see that fructose produced the fastest rate of respiration and lactose produced the slowest rate. These results show that my original hypothesis was almost to perfection, however the experiment carried out shows that galactose and lactose have switched places in the list.
