Experiment to investigate the the rate of carbohydrate fermentation by Yeast

Aim: To investigate the effect yeast on glucose and other respiratory substrates such as sucrose and starch. I will measure the amount of carbon dioxide bubbles produced using a respirometer. The more bubbles produced in a given time, the higher the rate of respiration. I will achieve this by reacting the yeast (Saccharomyces cerevisiae) with glucose, sucrose and starch at constant temperature, PH, mass and volume. I will keep the temperature constant by using an electronic water bath. This will ensure a fair test because varying temperatures cause varying rates of reaction. I will keep the PH constant by using a buffer solution. This will also ensure a fair test because different PH values alter the active site structure on the yeast enzyme. This will produce inconsistent reaction rates. The mass and volumes will be taken accurately and kept constant by using an electronic weighing scale (for measuring mass) and graduated apparatus e.g. measuring cylinder, graduates pipette, syringe etc (for measuring volume).

Hypothesis:

Glucose is a monosaccharide, Sucrose is a disaccharide and Starch is a polysaccharide. All three respiratory substrate sugars have different chemical structure and therefore their reaction with yeast will be at different rate.

Glucose (C6H12O6) contains six carbon atoms and a carbonyl group, as well as a hydroxyl group. It is a simple cyclic molecule that is easy to break. Glucose is a simple cyclic structure molecule; it is easy to break because it is not complex. It requires less energy and time to break down and therefore, the rate of respiration will be higher when glucose ...

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Sucrose (C12H22O11) is a disaccharide made from glucose and fructose, linked by a glycosidic bond.

This is a disaccharide which consists of two monosaccharide linked by a glycosidic bond. It is not a simple molecular so it takes longer than glucose to breakdown and therefore it will have a lower rate of respiration than glucose.

Starch is a polysaccharide which consists of many units of glucose linked together by a glycosidic bond. It is a long chain of molecules and branched molecules.

This is a polysaccharide which has many units of glucose linked together. It is a very compact structure therefore hard to breakdown. It is difficult to break down all these glycosidic bonds and the structure is very compact. Therefore starch will have the lowest rate of respiration and will produce least amount of bubbles.

Prediction

I predict that when glucose reacts with yeast it will produce most amount of bubbling than sucrose and starch and therefore the rate of respiration of glucose with yeast will be higher than with sucrose and glucose.

Null hypothesis: Increasing the substrate will not affect the rate at which yeast (Saccharomyces cerevisiae) is broken down (fermented) to alcohol.

Method

To be able to test the rate of respiration within the yeast I will need an straightforward but accurate way of monitoring the yeast and the best approach to do this is to measure the amount of carbon dioxide produced, as a result I am going to use a method that will enable me to unmistakably see how quickly the carbon

dioxide is produced. I will make “balls” of immobilised yeast; immobilised yeast are cells, which literally that have been attached to a support that takes the form of small beads. The techniques for immobilisation vary, but in this experiment I will be using is gel entrapment: the cell will be mixed with sodium alginate and distilled water to form the gel the enzyme remains "trapped" in the gel matrix. The pores will be large enough to let the substrate in, but not the cell out.

Fair test

To ensure that the experiment is fair there are certain variables that must be kept constant. Temperature and pH must be constant in order to obtain accurate results. To keep the temperature constant I will use the water bath to maintain the temperature of 37ºC as this is the optimum temperature for the enzymes to work at its best. I will use the buffer solution to monitor the pH value. I will ensure that I use the same measuring cylinder and same volume/mass of yeast and sugars to maintain the accuracy. I will repeat the experiment three times for each sugar to increase the degree of accuracy.

Apparatus

3 Test tubes with rubber bungs
Glass rod – for mixing the substances
Pipettes/syringes
Beakers
Thermostatically controlled water bath at 40°C – for a constant temperature
Test-tube rack
Gloves
Measuring cylinder
Weighing scale
Labels for test tubes
weighing scale
weighing boat

Substances

5 grams of baking yeast (Saccharomyces cerevisiae)
Distilled water
Glucose, sucrose and starch solutions at 5%

Safety measures

My investigation will be conducted safely by wearing a laboratory coat and goggles when handling chemicals and yeast. I will make sure that the bench is sterilized before I start so that the yeast is not contaminated. I will have to take care when using calcium chloride, as it irritates when is in prolonged contact with the skin so I will be using gloves when handling this substance and be gentle when handling the glassware, I will be using Vaseline so I will have to make sure that I do not handle glass when there is Vaseline on my glove as I can drop the glassware and break it.

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