Yeast Experiment Homework Assignments

What is going on?

Your experiment was testing the effect of water temperature on the growth of yeast. Yeast are egg-shaped microscopic cells of fungi which are dormant whilst kept in dry and cool conditions. However, yeast will rapidly divide once exposed to water and a sugar in ideal temperatures. In the right temperature, yeast cells will change the sugar into glucose by using the water plus as enzyme catalyst (invertase). Once the yeast has converted the sugar to glucose fermentation can then occur to produce carbon dioxide and ethanol as per the equation below;

Glucose ⟶ Ethanol + Carbon dioxide

which can be written as...

C6H12O6(aq)  ⟶ 2C2H5OH(aq) + 2CO2(g)

In your experiment you were trapping the carbon dioxide released during fermentation process. The more active the yeast, the more carbon dioxide the yeast produced! In your experiment the different water temperatures will have produced different results as some water may been too hot for the yeast to survive where as other bottle may been too cold. By introducing a variable to test in your experiment, you're doing real science! The following list of temperatures is worth keeping in mind when assessing your results:

  • 55° C – 60° C
    Yeast cells die (also known as the thermal death point).
  • 41° C – 46° C
    Ideal temperature of water for dry yeast being reconstituted with water and sugar.
  • 4° C
    The temperature of a fridge - yeast will be too cold to work properly.

Yeast is used to make bread rise and to ferment beer. There are many different species of yeast, but the one most commonly used in cooking and baking is called Saccharomyces cerevisiae, which is also known as brewer's yeast.

Yeast we used for our science experiment.

Yeast can break down many types of simple carbohydrates (monosaccharides) however they cannot break down complex carbohydrates such as starch. This means that extra enzymes are needed to break down starch into sugars that the yeast can use, for example during beer production we use enzymes from germinating barley to do this.

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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).


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 breaks down in yeast fermentation and more amount of bubbling will be visible as more energy is released in a short time.

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.


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.


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 37�C 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.


1. 3 Test tubes with rubber bungs

2. Glass rod – for mixing the substances

3. Pipettes/syringes

4. Beakers

5. Thermostatically controlled water bath at 40?C – for a constant temperature

6. Test-tube rack

7. Gloves

8. Measuring cylinder

9. Weighing scale

10. Labels for test tubes

11. weighing scale

12. weighing boat


1. 5 grams of baking yeast (Saccharomyces cerevisiae)

2. Distilled water

3. 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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