One Two Decimal Place Electronic Weighing Balance Biology Essay

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The research question that was investigated was Is it cost effective to buy and use sunscreens of different SPF values as we are led to believe. The reason this topic was chosen was that the use of sunscreens- which are said to, to an extent, protect one from skin cancers- is a very current issue in the news today. To test this, Yeast, a fungus which grows under UV light, was used as a measuring tool. A constant amount of yeast, mixed in a glucose solution (to provide ATP for yeast growth) was placed under direct sunlight in Petri dishes. Five different SPF values of sunscreens were placed on these Petri dishes for one whole hour, in order to give the yeast an optimum amount of time to grow. Twenty five Petri dishes were kept in the sun in order to obtain five trials, of five different SPF values of sunscreen. The initial and final cell counts of yeast were taken, and the data was processed to reach a conclusion. By analyzing the data, it was concluded that sunscreens of SPF values until 50, are very beneficial for use, and are cost effective. However, past SPF 50, the potential of the sunscreens are quite noticeably similar. Hence, they aren't as beneficial or cost effective. They can be assumed to be mainly for marketing purposes. Ergo, it is cost effective to buy and use sunscreens up to SPF 50, but not beyond that.

The research title is "Is it worth buying and using sunscreens of different SPF values as we are led to believe?" Sunscreens are popularly known and are used to protect one's skin against sunburn caused by Ultraviolet (UV) rays from the sun. There are two different UV rays, UVA and UVB. Sunscreens tend to provide protection from mostly UVB rays. A higher SPF (Sunburn Protection Factor) number means more protection from only UVB rays. UVA rays can cause long term effects like cancer and skin aging, and sunscreens as UVA ray absorbers are being developed. At the moment, the only sunscreens of this sort that are available are still in laboratories and not for public purchase. Sunscreens work by either absorbing or reflecting ultraviolet radiation minimizing the harmful effects of UV rays on the skin, ("Lesson 2: All about sunscreens", 2012). Sunscreens are made up of organic and inorganic compounds. The organic compounds generally contain Carbon, Hydrogen, Oxygen and Nitrogen. Octyl methoxycinnamate (C18H26O3) is an example of an organic sunscreen ingredient. Inorganic compounds are usually Zinc Oxide (ZnO) and Titanium Oxide (TiO2), (Sunscreen, 2012). Each individual organic compound present in a sunscreen tends to absorb one particular wavelength of UV light. That is why more than one kind of organic molecule is used- it provides better protection of the skin. Fifteen organic molecules are approved for use in sunscreens, and thirteen of them protect against UVB rays. Inorganic compounds tend to absorb all wavelengths

of light above a certain value called a critical value. This value depends on the compound used. It can absorb both UVA and UVB rays. When the organic and inorganic compounds are mixed together, they make a good barrier from UV rays for the skin.

In order to do answer the research question, a measuring tool, for measuring the effectiveness of sunscreens is required. Yeast is a good substance for this, as yeast cells can grow under UV light, which is present in sunlight (Kohl, 2012).Then, since sunscreens block out some UV light, the same amount of yeast will be allowed to grow under sunlight with the use of different SPF's of sunscreen. Hence, this allows the effectiveness of the sunscreen to be measured in a controlled manner. The yeast cells will be mixed in a glucose solution, which will dissolve into it. This is because the glucose acts as an ATP source. The yeast and glucose solution will be placed in a Petri dish, whose lid will be covered in sunscreen, to block out UV light. Different SPF's of sunscreen will cause different amounts of UV light blocked out. Hence, when the initial and final number of yeast cells is counted, if the hypothesis is correct, a distinct pattern will be seen.

To test this, I discovered that the effect of sunlight on yeast is quite simple: the UV light in sunlight helps yeast grow. As discovered through research by the BioChem journal from 1940, UV light exposure causes the release of a growth hormone consisting of a nitrogenous material to occur from yeast cells in order to repair cell damage. Most of that material consists of an amino-N substance. When this material is released, it triggers off yeast cell growth and reproduction. These results were supported by a study held in 1923, by the University of Chicago press. This kind of growth is similar to the growth hormone that is present in

humans during tissue repair. This is also because yeast cells are eukaryotic, so do have similarities with human cells, (Highland, 2011).

Hypothesis:

If the SPF number of the sunscreen used is increased, the growth of yeast cells will decrease. This is because the stronger the sunscreen used (the bigger the SPF number) the less UVB rays will pass through the Petri dish to affect the yeast cells present in glucose solution. Since UV rays stimulate yeast cell growth proportionally, the lesser the UV rays, the lesser the yeast cell growth should occur when compared to the yeast cell growth that would occur with more UV rays. So the lower SPF values of 20 and 40 on Petri dishes will have more yeast cells present by the end of the experiment than the higher SPF values of 70 and 100.

Variables:

Dependant variable

Number of yeast cells present in glucose solution after being exposed to UV light in sunlight

Controlled variables

Independent variable

Fixed variables

Uncontrolled variables

Varying amount of sunlight over the time that the Petri dishes were kept outside, change in heat in the environment, minor losses of water and glucose solution during the experiment, slight variation of time that all Petri dishes were exposed to sunlight for

Apparatus:

One 1500ml beaker

One weighing boats

One two decimal place electronic weighing balance

26.00g Active dry yeast

130.00g Sugar

1800.0ml Distilled water

Two 250ml beakers

One 50ml beakers

One 250.0ml measuring cylinders

Two 50.0ml graduated syringes

One 1000.0ml measuring cylinders

One glass stirring rods

Twenty-five Petri dishes

One 10.0ml graduated syringe

Five plastic tray boxes

One light microscope

One glass slide for the microscope

A refrigerator

Sunscreens of the same brand of SPF 20, 40, 50, 70 and 100

One timer

One permanent marker

One pipette

Tissue paper

Method:

Take one of the weigh boats and weigh it.

Now weigh exactly 1.000g of yeast in the weighing boat and keep it aside safely. By measuring an exact amount of yeast, we make it a constant variable.

Now take a 1500ml beaker and a 1000ml measuring cylinder and first measure out 1000ml of distilled water making sure to read off the bottom of the meniscus and pour it into the beaker. Then measure out 300ml of distilled water so there is a total of 1300ml of distilled water and pour this into the beaker too. Measuring it makes it a constant variable. Now take a 50ml beaker and weigh it.

Weigh exactly 130.00g of sugar in it and keep it aside safely. By measuring this amount, we make it a constant variable.

Now, add the sugar to the water, and stir it using the glass stirring rod until all the sugar has dissolved.

Now, add the yeast to this glucose solution and stir it again until all the yeast has dissolved.

The solution needs to be diluted down until a workable number of yeast cells are present. Exactly 26.0ml of the solution is measured out using the 50ml graduated syringe and is put into a 250ml glass beaker. The reason for taking 26.0 ml of solution when only 1.0ml of each will go into 25 Petri dishes is due to possible loss of solution during the dilutions and transfer of liquids.

Then, using a 250ml measuring cylinder, exactly 234.0ml distilled water is measured and added to the glass beaker. The constant values used here ensure a constant variable.

Stir the solution using the same glass rod as used in step 6, after it has been rinsed in distilled water.

Now, the new dilution is diluted down again. Exactly 26.0ml of the solution is measured again using a new 50ml graduated syringe and is put into another 250ml glass beaker.

Using the same 250ml measuring cylinder as in step 9, measure out 234.0ml of distilled water and add it to the second 250ml glass beaker.

Stir the solution with the same glass stirring rod as in step 10, after it has been rinsed in distilled water.

Now this new dilution is workable.

Place this beaker with the useful dilution in the refrigerator so that the yeast cells don't grow before they are required to (Since enzymes can't grow in extremely cold conditions)

Now, take out twenty five new plastic Petri dishes and lay them out so there are five rows containing five Petri dishes each. Using new plastic Petri dishes of the same brand keeps it a constant variable. Having five in each row enables five trials of five varied times.

The Petri dishes need to be labelled according to varying sunscreens of the same brand. Keeping sunscreens of the same brand makes that a constant variable.

The lids of the first row need to be marked as 'SPF 20', second row as 'SPF 40', third row as 'SPF 50', fourth row as 'SPF 70', fifth row as 'SPF 100'.

Turn on the light microscope and turn to the 40x lens power and take a slide. Keep all these three the same throughout the experiment, in order to keep the variable constant.

Take a drop of the solution using a dropper, and place it on the slide.

Put it under the microscope, and count the number of cells that are seen in that one particular area and record it.

Clean the slide, and repeat this count for all the remaining Petri dishes.

Then, take the sunscreen bottle of SPF 20 and the appropriate Petri dishes.

Place sunscreen on the lids.

Repeat this process with the remaining four SPF's of sunscreen for the remaining twenty Petri dishes appropriately as labelled.

Place the Petri dishes in the plastic tray boxes keeping five of each SPF in one plastic tray.

Take one of the trays outside with the timer and find an area where there is lots of free space trays and direct sunlight. Keep all trays in the same area in order to keep this constant.

Place the tray here and start the timer.

Get the remaining four plastic tray boxes as quickly as possible.

When the timer reads 60 minutes, get the plastic tray box that was placed outside from first to last back inside.

Then, starting with the Petri dishes labelled SPF 20, using the same dropper as earlier, take a drop of solution, place it on the slide and count the number of yeast cells viewed and record results.

Repeat this with the remaining twenty-four Petri dishes.

Data processing:

Raw data:

Initial number of yeast cells in a yeast and glucose solution that were then kept for one hour in sunlight, counted using a light microscope for five different sunscreens used for five trials

Trial

Number of yeast cells counted/ cells*

20SPF

40SPF

50SPF

70SPF

1

11

15

11

13

2

12

13

16

19

3

18

15

15

14

4

14

16

17

17

5

13

19

14

12

*No uncertainty is listed for the cell counts because each cell count has its own individual uncertainty (Celeromics, 2012).

Final number of yeast cells in a yeast and glucose solution after being kept for one hour in sunlight counted using a light microscope for five different sunscreens used for five trials

Trial

Number of yeast cells counted/ cells*

20SPF

40SPF

50SPF

70SPF

1

70

61

56

48

2

78

64

53

51

3

72

68

49

50

4

73

62

54

53

5

75

63

55

52

*No uncertainty is listed for the cell counts because each cell count has its own individual uncertainty (Celeromics, 2012).

Processed data:

Average number of yeast cells in a yeast and glucose solution kept for one hour in sunlight of initial and final readings of five trials counted using a light microscope of the for five different sunscreens used

20SPF

40SPF

50SPF

70SPF

100SPF

Average number of yeast cells counted/ cells*

Initial

14

16

15

15

13

Final

74

64

53

51

44

*No uncertainty is listed for the cell counts because the individual number of cells didn't have an uncertainty (Celeromics, 2012).

Average initial count for 20SPF = (number of cells of trial 1+trial2+trial3+trial4+trial5)/5

= (11+12+18+14+13)/5= 14 cells

Percentage change of the average number of yeast cells in a yeast and glucose solution kept for one hour in sunlight of initial and final readings of five trials counted using a light microscope of the for five different sunscreens used

20SPF

40SPF

50SPF

70SPF

100SPF

Percentage difference in number of yeast cells counted/ %*

429

300

253

240

238

*No uncertainty is listed for the cell counts because the individual number of cells didn't have an uncertainty (Celeromics, 2012).

Percentage difference for 20SPF=

[(Final average number of cells-initial average number of cells)/ initial average number of cells] x 100

= [(74-14)/14] x100 =429%

Average of five trials of initial and final counts of yeast cells in glucose and yeast solution kept for one hour in sunlight counted using a light microscope for varying sunscreens

Percentage difference between the average final and initial number of yeast cells counted using a light microscope of five trials of yeast and glucose solution kept in sunlight for one hour using five different sunscreens

Analysis:

The aim of the experiment was to find out if it was beneficial in terms of UV protection to buy and use sunscreens of different SPF values. This was done through the use of yeast growth as a tool to measure the usefulness of sunscreens. The hypothesis stated that the lower SPF values of sunscreen would have the most yeast cell growth, while the higher SPF values of sunscreen would have the least yeast cell growth after being left in sunlight for one hour.

Initially, an average count of yeast cells was taken of the five trials of the five sunscreen SPF values. However, it was difficult to say whether the hypothesis was correct with this data as the initial counts of yeast cells were all different. So a percentage change was taken between the average final and average initial count of yeast cells of five sunscreen SPF values. From these numbers, it was clear that the lower sunscreen SPF values had most yeast cell growth, for example 20 SPF had 428% difference, which was the biggest change, while the higher sunscreen SPF values had the least yeast cell growth as it had a lowest percentage change, like 100 SPF had a 238% change. The increase in sunscreen SPF caused a decrease in percentage change of yeast cells present. This was more clearly seen when both the average count of yeast cells, and percentage change was graphed to be seen visually. This proves that the hypothesis was correct.

When the average yeast cell count was graphed with a trend line, it was more obvious that the hypothesis was correct. Though the average initial cell counts were not exactly equal, there is a distinctive constant decrease in the average final yeast cell count that would not be very

different even if the average initial yeast cell counts all started at the same number. In order to avoid this problem, the percentage change was graphed as well. The same result was seen there- there was a distinctive decrease in yeast cell growth with the increase of sunscreen SPF values from 20SPF to 40 SPF to 50 SPF. However, the percentage differences between 50 SPF, 70 SPF and 100SPF were quite close together. There was no significant change between them like the 20SPF and 40 SPF value. This proved that the higher the SPF value, the lesser the yeast cell growth because a lot of the UVB light rays are blocked out, so the overall amount of UV light entering the Petri dish is reduced which results in lesser nitrogenous growth hormone secretion in yeast cells, so lesser yeast cell growth. However this only applied to the 20, 40 and 50 SPF values. After that the strength of the SPF's are quite similar- so the higher SPF values have a very mild difference between them.

This then answers the research question, that yes, it is useful to buy and use sunscreens of different SPF values, however, not beyond 50 SPF. There is no drastic change beyond that. Since yeast cells are eukaryotic, like human cells, they are similar in some ways. Hence, no major difference- like the inefficient use of sunscreen- should occur if sunscreens are used on human skin either. If it has a clear, distinctive effect on the yeast cells, and is proven to be effective as a UV light blocker to an extent, then it will also work on human cells- hence protecting the human skin. Therefore, it is worth buying and using sunscreens of certain different SPF values as we are led to believe, but past 50 SPF, it is not cost effective. It costs a lot more to buy a 100SPF sunscreen than a 50SPF sunscreen. Banana Boat for example, sells 100 SPF sunscreen for $9.59 (Drugstore, 2012) and 50 SPF sunscreen for $7.99 (Drugstore,

2012). However the effect of the two are quite similar. It would certainty protect the skin from skin cancer which is proven to be caused by excess UV radiation, but not with a very drastic difference between the two.

The data in this experiment correlates with published research, as stated in the introduction, however, it cannot be said to be fully reliable for a number of reasons. Some are discussed in the evaluation, but the others are that in counting cells, each cell count has an individual uncertainty (Celeromics, 2012). However, this was only published and backed up by this particular source. There were set values for the uncertainties for each value of yeast cells that were counted. However, the numbers were not corresponding with each other in calculations and so no uncertainties were used. Also, it gives uncertainties that are so large, that the percentage uncertainty is almost 100%. Second, it was assumed, during the experiment, that the cells counted under the microscope were evenly spread everywhere. This means that the number of cells that were counted to be present would have been seen if another section of the slide was observed as well. The data in this experiment corresponds within itself but cannot be said to correspond otherwise. Hence it is not very reliable.

Evaluation:

Some problems during the experiment were that yeast might have started growing when it was taken out of the fridge and when it was returned inside from exposure to sunlight. If the yeast started to grow then, it would have continued growing when the initial and final number of yeast cells was counted too, which means that the numbers documented aren't very accurate

and could possibly cause the final results to have been different than what was seen. If the Petri dishes were kept in an ice bath when the initial and final cell counts were being taken, it would have prevented the cells from growing then since enzymes cannot grow in such cold conditions. It would have reduced the rate of yeast cell growth significantly to not make much of a difference to the data itself. Another error was that not constant UV light exposure was occurring to all twenty-five Petri dishes when they were kept outside. This means that different Petri dishes got different amounts of UV light- so the different number of final yeast cell counts could not only be because of the sunscreen used, but also because of the amount of UV light it was exposed to. If a UV lamp is used, it can ensure equal amount of controlled exposure of UV rays to all the Petri dishes. This will significantly make the data much more reliable and useful. A third error is that not the same amount of sunscreen was used for all the Petri dishes. This would have meant that some Petri dishes got more UV ray blockage than others which meant that some yeast cells would have grown less than others because of this uneven spread, rather than the effectiveness of sunscreens. The sunscreen should be weighed on the lid of the Petri dish to fix this problem. Exactly 2.00g of sunscreen should be placed on the lid and then spread on evenly for all the Petri dish lids. This ensures that exactly the same amount of sunscreen will be used throughout.

Fourthly, graduated syringes cause extremely inaccurate measurements. Some of the solution is left behind in the graduated syringe which means that some Petri dishes would have more liquid than others, so some Petri dishes would have a more initial yeast cells than others

causing the difference in the final yeast cell count to be because of this rather than the effectiveness of sunscreens in blocking UV light. Using a plastic graduated pipette is more reliable as long as no air bubbles enter it. Also, using a pipette is better because very little liquid is usually left behind after it is used unlike in a graduated syringe where lots of liquid has the potential to be left behind. Also, using one dropper for viewing the yeast solution in the slide increases the chances of error in the initial and final counts of yeast cells. Using one pipette contaminates the yeast solution as then the solution of one Petri dish would be transferred to another. It could also affect the number of yeast cells viewed in the microscope to be different from the actual number of yeast cells present. One pipette to view the yeast cells of one Petri dish should have been used. Even though the pipette was cleaned, sometimes it was not cleaned as though roughly as it should have been which could have caused an error. If twenty five pipettes are used, then this mistake is prevented. Also, when using a weighing boat to weigh yeast, some yeast could be left behind in the weigh boat. Though this is a consistent error throughout the experiment, it wouldn't cause an error to a part of the experiment but would affect all the number of yeast cells that were observed as a whole. If less yeast was used, less yeast cells would be present throughout. An initial and final weighing of the weighing boat should have been done. If it was, then the amount of yeast that was put into the yeast solution could have been calculated and it could have been ensured that exactly 26.0g was indeed put into the glucose solution. If these procedures are followed, and the method is adjusted by the improvements stated above, the data would be much more reliable and accurate.

Conclusion:

In conclusion, though the data itself has errors present, it is reliable enough that it follows the theories that were stated and proven to be true. Through the use of yeast cells, it is seen that sunscreens up to 50 SPF worth being bought and used as we are led to believe because they genuinely do block out some UV light, which on humans, is very beneficial due to skin cancer- a topic which is very much in the news today.

Bibliography

Banas, Timothy. The Effect of UV Radiation on Yeast? Retrieved on July 27 2012 from

http://www.ehow.com/list_6375291_effects-ultraviolet-radiation-yeast.html

Celeromics. Cell counting error. Retrieved on September 1 2012 from http://www.celeromics.com/en/resources/Technical%20Notes/cell-counting-error/cell-counting-error.php

Drugstore (2012). Banana Boat Ultra Defense Broad Spectrum Sunscreen, SPF 50 Lotion. Retrieved on October 13 2012 from

http://www.drugstore.com/banana-boat-ultra-defense-broad-spectrum-sunscreen-spf-50-lotion/qxp245783?catid=184136

Drugstore (2012). Banana Boat Ultra Defense Max Skin Protect Continuous Spray Sunscreen, SPF 100. Retrieved on October 13 2012 from

http://www.drugstore.com/banana-boat-ultra-defense-max-skin-protect-continuous-spray-sunscreen-spf-100/qxp338894?catid=184137 

Highland, James (2011, March 30). The Effects of Uv Light on Yeast. Retrieved on July 16 from

http://www.livestrong.com/article/252729-the-effects-of-uv-light-on-yeast/

Kohl, April. What is the Effect of UV light on Yeast? Retrieved on July 27 2012 from

http://www.ehow.com/list_6375291_effects-ultraviolet-radiation-yeast.html

Lesson 2: All about sunscreens. Retrieved on July 27 2012 from http://nanosense.org/activities/clearsunscreen/allaboutsunscreens/CS_Lesson2Teacher.pdf

Sunscreen (2012). Retrieved on July 16 2012 from http://en.wikipedia.org/wiki/Sunscreen

Wassenaar, Trudy, et al. Yeast and Temperature (2012). Retrieved on July 27 2012 from http://www.newton.dep.anl.gov/askasci/bio99/bio99693.htm

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