Do X-rays Affect the Growth of Yeast?
Researched by Jamie H. 2000-01	PURPOSE  HYPOTHESIS  EXPERIMENT DESIGN  MATERIALS  PROCEDURES  RESULTS  CONCLUSION  RESEARCH REPORT  BIBLIOGRAPHY ACKNOWLEDGEMENTS ABOUT  THE AUTHOR

The purpose of this experiment was to determine if X-rays affected the growth of yeast cells. 

I became interested in this idea while researching cells in the school library. I read about how cells, when exposed to large doses of x-rays or radiation could be mutated and destroyed, which lead me to wonder if x-rays would also lessen or stop a cell’s growth rate. 

The information gained from this experiment will give information on the effect of x-rays on yeast and on other organisms. 

My hypothesis was that the growth rate of the yeast sample exposed to the greatest amount of x-rays would significantly decrease or stop compared to the other yeast samples exposed for lesser amounts of time.
 

I base my hypothesis on the World Book Encyclopedia, which states "When absorbed, x-rays can destroy or ‘burn’ living tissues." Thus it can be very harmful to living cells.

The constants in this study were:

ß The intensity of the x-rays
ß The type of x-ray machine 
ß The type of yeast
ß The size and type of the container holding the yeast
ß The quantity of sugar added to the yeast samples 
ß The quantity of water added to the yeast samples
ß The quantity of yeast in each study group
ß The quantity of time the yeast is allowed to grow before being measured
ß The temperature of the water added to the yeast ( 100 degrees Celsius) 
ß The type of sugar added to the yeast samples
ß The temperature of the room that the yeast is allowed to grow (70 degrees Celsius) 
 
 

The manipulated variable was the amount of time the yeast samples where exposed to the x-rays. The responding variable was the growth rate of the yeast samples.  To measure the responding variable I use a turbidimeter. I recorded how much light was scattered, and then compared each reading to the reading of the unexposed samples in study group one to determine the growth rate.

Prepare Yeast Samples for X-raying:
1. Gather and buy materials for experiment.
2. Sanitize equipment and surfaces
3. Place a weigh boat on the electric balance.
4. Push the tear button on the balance bringing the scale back to zero. 
5. Measure eleven grams of active dry baking yeast using electric balance.
6. Place the measured yeast into a 500-ml glass jar (This will allow all of the samples in a single group to be exposed to the x-rays at the same time).
7. Repeat steps 3 through 6, five times.
8. Number the jars to indicate which samples are in each group (in case of mix up). 
9. Tightly secure lid on each jar.
X-ray Yeast Samples:
10. Lay jar carefully on its side in the center of the target area directly under x-ray machine. 
11. X-ray the group once for 1 seconds at a strength of 81 kvp. at a flow rate of 40 milibars per second.
12. Place group 3 directly under the x-ray machine.
13. X-ray the group twice for 2 seconds at a strength of 81 kvp at a flow rate of 40 millibars pre second.
14. Place group 4 directly under the x-ray machine.
15. X-ray the group 3 times at 3 seconds at a strength of 81 kvp at a flow rate of 40 millibars per second.
16. Place group 5 directly under the x-ray machine.
17. X-ray the group four times at 4 seconds at a strength 81 kvp at a flow rate of 40 millibars per second. (Group one will remain unexposed as the control group)
Divide X-rayed Yeast into 15 Test Samples:
18. Place a new weigh boat on the electric balance.
19. Measure 1.5 grams of yeast from each group and place in a 500-ml glass jar. 
20. Repeat this process three times per jar using a new weigh boat each time. (When this process is complete there will be more yeast than is needed, in case of a spill.)
21. Label each sample according to the number on the jar in which it came from with it’s group number then an a, b, or c. (In group number one, the sample would be labeled 1a, 1b, 1c. When you read the turbidity this will allow you to track each sample's growth.)
22. Dispose of each jar containing the excess amount of the yeast. 
Add Water and Sugar to Yeast:
23. Then place a new weighing boat on electric balance.
24. Press the tear button on balance bringing the scale back to zero. 
25. Measure 5 g. of sugar, using a long v shaped spatula to place sugar in and out of weigh boat.
26. Empty sugar into 250 ml flask using funnel. 
27. Then measure 100 ml of water, using 100 ml graduated cylinder.
28. Empty into flask with sugar using a funnel.
29. Stir the sugar and water solution with glass-stirring rod until completely dissolved.
30. Rinse, and dry glass-stirring rod.
31. Using a different funnel, poor yeast into the solution.
32. Swirl together for 180 seconds. 
33. Repeat steps 25-33, fifteen times, each time with a different yeast sample.
34. Label each flask according to the number and letter on the jar from which the yeast in the solution came from. 
35. Allow solution to set for 1200 seconds (20 minutes), allowing yeast to grow and multiply.
Dilute Yeast Solutions:
36. While the yeast solution is growing, measure 200ml of water into 500ml flask.
37. Place a new weigh boat on balance.
38. Press the tear button bringing the balance back to zero.
39. Then measure 10 g of sugar using long v shaped spatula to place sugar into and out of weigh boat.
40. Poor sugar in to flask with water.
41. Using glass-stirring rod, stir the sugar and water together until completely dissolved.
42. Rinse and dry stirring rod. 
43. After the yeast is done growing, swirl until the foam on the top of the solution is dissolved.
44. Then set pipet-man for 1/2 milliliters.
45. Firmly fit pipet tip onto end of pipet-man.
46. Press down on the top of pipet-man pushing the air out of the end.
47. Keeping the top of the pipet-man held down, submerge the end of the pipet-man approximately. 3 cm into solution.
48. Release top on pipet-man, allowing 1/2 ml of the solution to flow into the tip of the pipet.
49. Lift out of solution and into the 500 ml flask with the sugar and water mixture.
50. Again press down on the top of the pipet-man emptying solution out of tip and into the sugar and water mixture.
51. Stir the solution, sugar, and water mixture together with stirring rod for 20 seconds.
52. Rinse and dry stirring rod.
53. Label each 500-ml flask with the number and letter on 200 ml from which the 1/2 ml yeast solution was taken from.
Test Yeast Samples for Turbidity:
54. Fill the metal pan of ultrasonic cleaner with water 3 cm from top. 
55. Poor the mixture into your sample cell container slightly past the fill ring mark.
56. Place test tube upright in the bottom of pan of ultrasonic cleaner.
57. Turn power switch to ON.
58. Allow test tube to sit in ultrasonic cleaner for 15 seconds. Then turn power switch to OFF.
59. Remove sample from ultrasonic cleaner.
60. Remove fingerprints and water droplets form sample cell using Kim wipes.
61. Set power switch of Turbidimeter to ON.
62. Allow instrument to warm up for 1200 seconds (15 minutes). 
63. Select operation range 0-20 NTU (Nephelometric Turbidity Units).
64. Remove light shield. 
65. Place sample in cell holder.
66. Align dot on fill ring of sample cell with raised mark on the spill ring of cell holder.
67. Cover sample with light shield.
68. Read and record the turbidity of sample and from digital display.
69. Remove light shield.
70. Remove the test sample.
71. Recover cell holder with light shield.
72. Empty contents of sample cell into 1500-ml beaker this being your waste container.
73. Fill squeeze bottle to fill line with water.
74. Rinse sample cell container with squeeze bottle and empty into waste container.
75. Repeat steps 35-69 with each of the 15 samples.
76. Repeat steps 52-71 again in 24 hours, and again 24 hours following the last testing. 

The original purpose of this experiment was to determine if X-rays affect the growth rate of yeast cells. 

There was not a direct relationship between the duration of x-rays exposure and the yeast growth rate. The largest over all growth was shown by yeast exposed for 3 seconds. The least overall growth was shown by yeast exposed for 1 second.   Yeast exposed for no time whatsoever, had a growth rate that was near the midpoint of all groups.

See the table and graphs!

My hypothesis was that the growth rate of yeast samples exposed to the greatest amount of x-rays would be significantly less compared to other yeast samples exposed to less x-ray duration.

The results indicate that this hypothesis should be rejected. There was no relationship between the amount of x-ray exposure and the amount of growth.

Because of the results of this experiment, I wonder if the duration of the x-rays was not great enough to affect the growth of the yeast. If the yeast samples had been allowed to grow the before they had been x-rayed, they not have been dormant. This could have possibly caused them to have been more susceptible to x-ray damage. 

If I were to conduct this project again I would change many things. I would expose the yeast to the x-rays for longer durations and allow the samples to grow before they were x-rayed. I would also find a mixing device to stir the yeast solution more evenly before reading its turbidity. When measuring the sugar and water solution for the yeast samples in my experiment. I also should have measured and stirred the solution in one large flask. This would have prevented any sample from getting more sugar than anothe
 
 
 

Research Report Introduction  This report will explain information related to understanding what yeast is, its uses, and how it reproduces. It will explore x-rays, from their important uses to their many hazardous effects.      Yeast Reproduction  Yeast is a one type of tiny, microscopic, single-cell plant. Yeast reproduces a-sexually by budding, or by fission. They belong to a group of plant called fungi. These plants reproduce extremely rapidly. Some yeast reproduce by a process called budding where part of the cell wall becomes enlarged. Then after a short amount of time a wall of cellulose stops the growth. The growth then becomes an individual cell and begins the budding process all over again. Baker’s Yeast uses this form of reproduction. Some yeast simply divide in two. This process is called fission.       Characteristics of Yeast  Yeast is found in soil, water, on skins of fruit, in honey, in saltwater, on leaves, trees and in the air. Yeast is also found on and in the intestinal trackt of warm-blooded animals. Though yeast is a plant, it lacks chlorophyll, which enables most plants it to produce their own food. For this reason yeast can only grow and reproduce if there is water and sugar provided for it to feed on. When the water and sugar supply has run out the yeast becomes dormant. Yeast requires a certain temperature range and amount of moisture to grow. They require the temperature to be within 20-30 degrees Celsius and can not withstand temperatures above 65-70 degrees Celsius. Yeast also cannot withstand temperatures below 0 degrees Celsius. Yeast feed on many natural sugars and resources such as fruit, grain, and nectar. Yeast produces enzymes that break down their food, and convert the sugars in the food into alcohol and carbon dioxide.      Industrial Uses of Yeast   Yeast is very important in the making of many products. It is used to make industrial alcohol, B-complex vitamins, Thiamin (a stage in the production of various antibiotics), and steroid hormones. One of the more common uses of yeast is in the making of bread. As the yeast breaks down the sugar in the bread dough it creates carbon dioxide and alcohol. The carbon dioxide creates tiny bubbles in the dough causing it to rise. As the bread is baked, the alcohol is evaporated and the yeast dies.  Another very important use of yeast is in the making of wine, beer, spirits, and other alcoholic beverages. For hundreds of years people have made these beverages unknowing that yeast (which is found on the skins of many fruits) was responsible for the fermentation process which created them. Chemical changes in organic matter caused by the action of enzymes causes yeast to ferment. These enzymes are called ferments and are produced by the yeast itself. The ferments cause the sugars in the organic matter (the fruit), like glucose and fructose, to turn into alcohol and carbon dioxide, which creates wine.      Turbidity  Turbidity is the measuring of how clear water is. Turbidity is caused by suspended soil and other particles in water. This is sometimes caused by sediment and disturbed soil from banks and from some bottom feeding fish. When light passes through the water it is scattered. The scattered light can be measured by a turbidity meter.      X-rays   X-rays are a type of invisible, electromagnetic radiation that has a shorter wavelength than visible light. They are produced by the concentration of high-speed electrons on a target. German Physicist, William Conrad Roentgen, discovered the x-ray in 1895. He discovered them accidentally while studying cathode rays at high voltage. He named his discovery x-rays because of its unknown nature. X-rays are also known as Roentgen rays, named after him. The wavelength of x-rays ranges from 100 A to 0.01 A (one A equivalent to about 4 billionths of an inch). X-rays can also be measured by a Geiger-Muller or Scintillation counter, which measures the energy by its ionization. When the wavelength of x-rays is short, it has greater energy and penetration power. These types of x-ray are known as "hard x-rays". When the wavelength of the x-ray is long they are known as "soft x-rays", and have less energy and penetrating power. If the x-rays produce a mixture of wavelengths, they are known as "white" x-rays.        Hazards of X-rays X-rays can be dangerous if special precautions are not taken. Dentists, radiologists, and doctors must make sure not to over expose their patients or themselves to the x-rays. If this happens they can destroy living tissues and can be very harmful.  X-rays can cause biological, chemical, and physical changes in substances. They can also cause cancer, skin burns, reduction of blood supply and other serious conditions.  Uses of X-rays   X-rays are widely used to diagnose tumors, cancer, broken bones, certain diseases, and to locate other unhealthy things, such as a bullet. Dentists also use x-rays to detect cavities and faults in teeth. They diagnose these things by passing a beam of x-rays through a patient’s body and onto photographic film. Bones and other objects that are dense and have greater atomic weight, absorb more x-rays than muscles and other tissue with less, and leave a lighter shadow on the dark photographic film. This creates a black and white picture, which is called a radiograph. Some substances, such as barium sulfate, cause certain organs in the body to stand out on a radiograph when injected. Since x-ray techniques have been improved, small differences in tissue can now be seen.  A device called the Computerized Axial Tomograph (CAT scan) also uses x-rays to detect tumors, cancer, and diseases. The device rotates 180 degrees around the patient’s body and sends pencil thin beams of x-rays at 160 different points. Crystals positioned at opposite points record the absorption rate. Then the information is sent to a computer that turns it into a picture on a screen. The picture is 100 times clearer than a normal radiograph. An electronics engineer named Godfrey N. Hounsfield invented the CAT scan in 1972.  X-rays are also commonly used to treat cancer. The patient is exposed to a limited dose of x-rays and eventually the rays destroy the tumor. The x-rays kill the unhealthy cells and tissue more effectively than the healthy.  X-rays also have many other uses. X-rays are often used in industry as a tool for testing. They are used to reveal flaws in metal castings without damaging them. They are used to inspect products to detect if they are fake. They often do this to test jewels. They also use x-rays to detect smuggled goods when examined in costumes at air ports.      Production of X-rays   X-rays are produced when an electric current passes through an x-ray tube. An x-ray tube is a glass tube containing two electrodes. When an electric current passes through the x-ray tube the residual gas is ionized, and when the positive ions strike the Cathode they produce electrons. The electrons then form a beam called a Cathode ray and the electrons are focused on a metal target called an anticathode, or anode.      Summary  X-rays are important for doctors tying to make good medical decisions. Unfortunately x-rays are dangerous to most living organisms. Using yeast as a test organism is a safe way to study their effects.

Bibliography "Fermentation", Encarta, 1994 Goaz, Paul W.   Oral Radiology, St. Louis, Washington D.C., Teronto, C.V. Mosby Com. 21- 38, 2000. Gwendolyn R. W. Burton, Microbiology for the Health Sciences, London, J.B. Lippincott Company, 1988 John D. Weaver, "Yeast", http://www.zonehome.com/zt33/_disc/00000125.htm, 12/19/00 John T. Suchy; Earnest C. Watson, "X-ray", Encarta, 1994  Sr. John W Poston; Douglas John Crawford Brown, "Radiation", The World Book Encyclopedia, 1998,16  "Turbidity", http://tier.net/riverwatch/turbid,hmt, 10/03/01 "Uses of X-rays", The World Book Encyclopedia, 1999, 21, 542-543  "Yeast", Encarta, 1994

ACKNOWLEDGEMENTS
Iwould like to thank the fallowing people for their help in making my science project possible.
 

• Iwould like to thank ron raddas for taking time to help me use an X-ray machinefor my experimentat providence hospital
• Net i would liketo thank Melinda Simone, a microbiologest at Tree Top Inc.for her help and for allowing me to use 

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