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The Effect of Different Substrates on the Rate of Respiration on Yeast (Saccharomyces cerevisiae).

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Introduction

The Effect of Different Substrates on the Rate of Respiration on Yeast (Saccharomyces cerevisiae) What are the substrates being used? Glucose Glucose is a monosaccharide sugar, which is a 'simple sugar' that have between 3 and 10 carbon atoms per molecule. They are sweet and all soluble in H2O. It has the chemical composition C6H12O6. Glucose is a white crystalline solid but is less sweet then ordinary table sugar. Powdered dry glucose exists mainly in straight chain form. However, when glucose molecules are dissolved in water, two different ring structures are formed. See picture. Fig 1 These ring structures are more stable in solution, so that, at equilibrium, almost all of the molecules are present as rings, with the straight chain form being a relatively short-lived intermediate. The structures of ?-glucose and �-glucose differ only in the position of the -OH and -H groups attached to carbon atom number 1. Lactose It is formed by condensation reactions (where water is removed) between two monosaccarides, glucose and galactose. This allows an O2 bridge to form between the two molecules, holding them together forming a disaccharide. This is called a gycosidic bond. It consists of galactose and glucose molecules joined by a 1�-4 glycoside linkage . Fig 2 Lactose is the only common sugar that is of animal origin. Other sugars, such as sucrose and fructose, can be found only in plants. In nature, the only place you can find lactose is in the milk of mammals. Lactose is the principal carbohydrate found in milk, and composes about 2 to 8 percent of milk in all mammals Although milk and other dairy products are the only natural sources of lactose, this sugar can also be found in many prepared foods. ...read more.

Middle

Galactose is a 'non-conventional' nutrient for yeast found in sucrose, which can be used as a sole carbon source when glucose is absent from the medium. This is one of the few pathways in yeast, which is regulated in a nearly 'all-or-nothing' mode. Fig 5 outlines what happens Fig 5 In addition to hexose sugars, yeasts can utilize a number of 'non-conventional' carbon sources. Free glucose is scarce in natural environments or in natural products used to feed yeast cells. For example, disaccharides, such as maltose, sucrose or lactose can easily be accepted as nutrients by the action of corresponding hydrolases which breaks these disaccharides down into their constituent monosaccharides. However, as shown in figure 6, S. cerevisiae cannot hydrolyse lactose. Fig 6 Variables In order to make sure that the variable being manipulated is the one making the difference, other possible variables need to be controlled Temperature affects both the time required to attain maximum activity and the maximum rate of gas production. Generally, the gas production rate doubles over a range of 0-40�C. this is the Q10 rule. Using a thermostatically controlled water bath can control the temperature and part of the trial experiments will be to find a suitable temperature for the yeast to respire at pH values between 4.0 and 6.0 are optimum for bakers' yeast that gives a constant and maximum gas production rate within this pH range. Yeast is slowly inactivated at values below pH 4.0 and above pH 6.0. To prevent a drop in gas production, buffering salts such as CaCO3 are added to water brews. In this particular experiment though it is not necessary to use buffers, as the initial rate will be measured. ...read more.

Conclusion

Conclusions from trial experiments * One problem was that when the syringe was pushed down into the conical flask air was pushed out into the measuring cylinder. This was resolved by simply discounting this first wave of air bubbles and as it was a constant for each experiment did not matter. * An airtight container was achievable but the air that was already in the container could not be remover practically. However, as this would be the same for each experiment it would not make a considerable difference between the results. * In the main experiment the reaction will be recorded for 10 minutes. This is because in the sucrose experiment after this time no more CO2 was released. Main Method 5g of dried yeast was measured out using a top pan balance. This was placed into a 250cm3 side arm conical flask. A bung was then placed over the top and a thermometer inserted through into the flask. 30cm3 of the substrate (either glucose, sucrose or lactose) were measured out using a syringe and placed in a thermostatically controlled water bath at 500C. A gas syringe was attached to the side arm conical flask and clamped horizontally. The side arm conical flask was then placed into the water bath at 500C and the substrate which had been placed in the water bath previously, which was now at 500C was attached to the bung in to conical flask. Then the substrate was released into the conical flask via the syringe and the stop clock started. Readings were taken every 30 seconds from the gas syringe, on the volume of gas that had been produced. / The Effect of Different Substrates on the Rate of Respiration on Yeast 27/04/2007 ...read more.

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