Hunar Sandhu

Investigate a factor that affects yeast population growth:

Introduction:

Yeast (saccharomyces) is a common single-celled fungus. They are microscopic plants that exist naturally on

Earth. They are known for their ability to ferment the carbohydrates in the flour and gain energy from this

digestion. Some food products created by yeast include bread, beer, wine and cheese. Yeast is mistaken as

lifeless, dry powder used to make bread, but, it becomes active in moist environments. Yeasts secrete digestive

enzymes outside their cells and absorb digestion products back in. This process is called extracellular digestion.

Yeasts are considered to be heterotrophs, meaning that they get food from other organisms. They have a special

molecule in their cell wall called chitin in their cell wall that allows them to do so. (Biology, Pearson).

Some cells use anaerobic respiration to produce ATP, a form of chemical energy. Anaerobic respiration means

that there is no use of oxygen required for this to take place. Cell respiration refers to many different ways that

can metabolize glucose, a monosaccharide. All those different ways start off with Glycolysis; it breaks down a

glucose molecule into 2 pyruvate molecules (3-carbon molecule). In this process, there is a gain of 2 ATP

molecules. The breakdown of organic material for ATP production is called fermentation. (Biology, Pearson).

The changing temperature can have effects on many living things. Enzyme-catalysed reactions are very sensitive

to small changes in temperature. The metabolism of some organisms can depend on their internal body

temperature, which is determined by its environment and often by the surrounding temperature. Yeast is one of

those organisms. It is used to leaven bread (make it rise). Yeast does this by fermenting sugar, producing carbon

dioxide as a waste product which is lost. However, some of the carbon dioxide is trapped which makes the bread

light, small air pockets is the reason behind this. If the yeast is not warmed properly, it will not be of much use as

a leavening agent; the yeast cells will burn sugar much too slowly. (Westminster College News RSS)

Affect of temperature upon respiration of yeast will be tested in this experiment: Kinetic energy is

increased by increasing the temperature. This means that there are faster and more collisions within the

molecules. In enzyme-catalysed reactions, increasing the temperature increases the rate of colliding with

substrate which forms more product in a short amount of time. Vibration-al energy is also increased which puts

strain on the bonds that hold the enzyme together. Higher temperature will cause the enzyme to change shape

and denature because the hydrogen and ionic bonds will break and cause the active site to change. Yeast

respires because it forms a reaction with glucose. Enzymes in found in the yeast cells catalyse this reaction.

Optimum temperature for these enzymes is 40˚C. (Day, Sam Adam)

In this experiment, you will watch yeast cells respire and act upon glucose at different temperatures, 20˚C, 30˚C,

40˚C, 50˚C and 60˚C. You will observe the yeast under anaerobic conditions. With the help of Methylene blue,

you are able to observe the rate of reaction. It goes blue when in contact with oxygen and colourless in contact

with NADH which is produced in respiration. When the Methylene blue goes colourless, we can be sure that the

reaction has finished. Stopwatch will be used to measure the rate of reaction.

Variables:

Dependent variable: The rate of reaction and the time taken for the Methylene solution to turn colourless, i.e.

the yeast's original colour is displayed.

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Independent variable: Temperature of the water yeast will be placed in. Rate of reaction will change for each

temperature and will be tested. Temperatures will be 20˚C, 30˚C, 40˚C, 50˚C and 60˚C

Controlled Variables How variables will it be controlled

How will it affect the reaction

Concentration of

Glucose (linked to

substrate

concentration)

5% concentration of glucose will

be used for all the trials.

If the concentration does not stay the

same it will take different amounts of

time for the methylene blue

to decolourise.

Concentration of yeast

(linked to enzyme

concentration)

5% concentration yeast solution

will be used for all the trials.

If different concentrations are used,

they results will be unreliable because

they will reacts at different rates and

will take different times.

Amount of Methylene

blue

3 drops will be used throughout

the experiment.

If too much or too little is used, the rate

of reactions will vary and the results

won't be reliable.

Same Equipment Same size, thickness and shape

will be used throughout the

experiment.

This will ensure that this error factor is

eliminated and only the temperature is

tested.

Same environment Experiment will be carried out in

the same room

This will ensure that the climate

conditions are the same and any

external environmental factors are not

manipulated. 

Focused Question:

To investigate the effect of temperature on the growth of yeast. Yeast will act upon the glucose and carbon

dioxide will be released. The rate of reaction will be measured by the release of carbon dioxide using the logger

pro.

Hypothesis:

The higher the temperature of the water, higher the amount of carbon dioxide released and therefore higher the

growth of yeast. The higher temperature will provide energy permitting the enzymes to move around more

rapidly, resulting with more collisions and higher rate of reaction and growth.

Safety:

-Wear appropriate goggles and safety jacket.

-Work in centre of table with plenty of space. 

-Work standing up and with chairs under tables and not in the way. 

-Do not eat or drink anything in a laboratory. If you are not careful you could end up in severe trouble in case you

happen to ingest anything that causes you harm.

-Always ensure that you wear proper fitting clothes and nothing that is too loose. You should also wear your lab

coat to avoid spilling anything on your clothes.

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-Usually there are approved safety goggles that are worn while working in the lab. These safety glasses protect

anything from entering your eyes.

-Use safety gloves to carry anything hot.

Materials:-5% concentration yeast solution.

-5% concentration of glucose solution

-2 drops of methylene blue. This indicator will be helpful when judging if the yeast is respiring, as it will go colourless when the oxygen content is reduced, which will be a clear indication that respiration is taking place. 

-A stopwatch. This will be an accurate measurement of how long it takes for the yeast to stop respiring. 

-A water bath. This will be used to keep temperatures constant during my experiment so it will achieve accurate results. I will use the water bath when measuring the 20˚C, 30˚C, 40˚C, 50˚C and 60˚C experiments. 

-Pipettes

-Test tubes. These will be used when conducting my experiment to hold the solutions.

-Thermometers. I will use the same thermometer each time so my results are fair

-Test tube holder. I will make sure it is stable and can hold all my test tubes. 

-Glass rod. This will be used to mix my solutions during the experiment

-250ml beaker. This will be filled with water and heated

-Bunsen burner. This will be used to heat the water

-Heat proof mat

-Tripod

-Water will be used to keeps temperatures constant during my experiments. 

-15ml measuring cylinders will be used to accurately measure quantities

Method: 1. Using a clean pipette, measure out 3cm³ of 5% yeast. Put the solution of yeast into a measuring cylinder to check it is the right quantity. Transfer into a clean test tube. 

2. Using a clean pipette, measure out 5cm³ of 5% glucose. Put it into a measuring cylinder, check it is 5cm³ and put it into a clean test tube. 

3. Put the test tubes in to a water bath at desired temperature(20˚C, 30˚C, 40˚C, 50˚C and 60˚C) and add 2 drops methylene blue indicator to the yeast solution. 

4. Add the glucose to the yeast and methyl blue solution whilst simultaneously starting the stopwatch. 

5. Stir the solution with a glass rod and check that the temperature is correct using a thermometer of 0.1ºC accuracy. 

6. Throughout the experiment monitor the experiment carefully and occasionally stir the glass rod in the solution. Continually check the temperature throughout the experiment so the results will be accurate. 

7. When the solution changes colour from blue back to the original creamy yeast colour stop the stopwatch. 

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8. Record the results in a table and record any observations. 

9. Repeat this experiment another 5 times for each temperature in order to be certain that the results are correct.

"College News." Westminster College News RSS. N.p., n.d. Web. 06 July 2013.

<http://www.westminster.edu/>.

Day, Sam Adam. "Factors Affecting Enzyme Activity." A Level Notes. N.p., n.d. Web. 13 May 2013.

<http://alevelnotes.com/Factors-affecting-Enzyme-Activity/146>.

Damon, Alan. Biology: Higher Level (plus Standard Level Options) : Developed Specifically for the IB Diploma.

Harlow: Heinemann, 2008. Print.