Unit 1: Microscopy

Method

   First, I was provided with a culture of yeast cells, suspended in liquid. The yeast itself, stained with methylene blue. I then placed a drop of the yeast culture onto a slide, using a pipette. I then placed the cover slip onto the yeast.

     I then placed this slide under the microscope, on the stage. Once under the microscope, turn the coarse focusing wheel so  that the lens is as close to the slide as possible. Then use the fine knob to display the yeast in the best view.

     Once the yeast cells were in full detail, I counted the yeast cells coloured blue. Once, I documented this, I counted the clear cells and documented it.

Results

Sample A- 25°C

Sample B- 37°C

Sample C- 45°C

Conclusion

   When I had documented my results, I decided that I would use the mean of each set of results collected. This was to make the results more fair and accurate, as doing this would reduce the effect of any outliers in the results.

The following results are that of the percentage of viable yeast cells using the mean results for each sample:

The equation used to work out each percentage was:

Mean number of clear x 100 = --%

Mean number of total

Sample A

2.7 x 100= 23%

13.1

Sample B

10.8 x 100= 85%

13

Sample C

3 x 100= 23%

13

     A brewer would require a yeast sample that had 90% or over living yeast cells for it to be any use to them.  Unfortunately, none of the sample that I collected and evaluated did not contain a living yeast cell percentage above 90.  However, the most useful for a brewer would most likely be sample B. This is due to the fact that at 84% living cells, a measurable amount of fermentation would take place.

     The brewer adds yeast to beer to cause fermentation. They also use a fermentation lock to prevent oxygen from reacting with the yeast. This lack of oxygen will cause the yeast to produce energy through the process of anaerobic respiration. This process of anaerobic respiration will produce ethanol which is an alcohol. The ethanol produced will kill the yeast cells as it causes proteins to denature and cell membrane damage. The temperature must be maintained throughout the process to keep the yeast alive long enough to produce a substantial amount of ethanol. The equation for anaerobic respiration is:

glucose         ethanol + carbon dioxide  

The balance equation is:

glucose         2 carbon dioxide + 2 ethanol

And the symbol equation:

C6H12O6            2CO2 + 2CH3-CH2-OH

Evaluation

   Several problems occurred when preparing the slide and counting the cells. First of all, I added too much of the dye, methylene blue, to the yeast culture. This made the yeast cells difficult to count due to the fact that, when it was viewed through the microscope, the whole slide appeared very blue. Also, when the cover slide was placed onto the slide, a large amount of the culture was pushed to the sides of the slide. This may have affected my counting and results, due to the fact the cell location was different and unviewable.

     If I was to do the whole investigation again, I would make changes to the experiment. I would make sure less dye was added to the culture. This will prevent the cover slide from sliding off the slide and making the culture easier to view. Also, with less dye, no cells will be pushed out of the cover slide, so there will be more cells to count.