Exercise 2- Estimating the Number of Yeast Cells per milliliter Yeasts are single celled fungi. They are economically important in both the baking and brewing industries. The respiration of yeast cells releases carbon dioxide gas that is trapped by the gluten proteins of wheat flour to cause breads to rise. The brewing industry is also based on the respiration yeast cells. Once the brewing yeast cells use up all the dissolved oxygen, they switch to anaerobic respiration which produces alcohol as a byproduct. Brewers monitor the concentration of yeast cells in the beer as part of the brewing process. One way to estimate the number of cells in a$ml$is to use a specialized microscope slide called a hemocytometer. Number of Yeast Cells per milliliter Essay Paper

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The hemocytometer was originally designed to count blood cells (hence the name) but can be used to estimate the number of any type of cell in a liquid. The hemocytometer has a 3$mm_{2}$etched glass grid engraved into it (Figure 1). This grid is in turn subdivided into nine 1$mm_{2}$grids (Figure 1 pink area), which can be further subdivided into even smaller areas. When a coverslip is placed on the hemocytometer it creates a chamber that is$0.1mm$in depth providing areas of known volume (Figure 2). Because we know the volume of a given area, we can convert the number of cells found in that volume into the number of cells found in a milliliter of liquid. In the brewing world this would tell a brewer how much water to use to dilute the yeast solution to correct concentration! Today you will use a photograph of a$0.04mm_{2}$area to estimate the number of cells found in a$0.1%$yeast solution ($0.1$gram of yeast in$100ml$of water).

This corresponds to the area coded in yellow in the above diagram. Note that the volume of the area is$4nl=0.004μI=$$0.000004ml$. PROCEDURE 1) Observe slide 11. The total area is$0.2$by$0.2mm$area holding a volume of 4 microliters. Note that the area has been further subdivided into sixteen squares to make it easier to count the cells. 2) Count the number yeast cells in each of the sixteen areas. a. In order to get similar counts, count ONLY full sixed cells. DO NOT count small cells like those circled in blue on the diagram. b. On the outside edges, count only those cells if they are touching the middle line. c. Count cells on an interior line only once- do NOT double count them. d. You can enter the total number of cells into the grid diagram below (Figure 3). e. Sum the total number of cells from all sixteen grids to get the cell count. f. Record the total cell count on slide 12 . Fig. 3- Yeast Cell Counting Grid$0.2mm$3) Find the estimated number of cells per milliliter by dividing the number of cells using the following formula a. (# of Counted Cells/volume of the square in$ml)$b. In our case (# of Counted Cells$/0.000004ml$) c. Record the number of cells per$ml$on slide 12 . 4) We used$0.1$grams in$100ml$of water to make our solution (a$0.1%$Yeast Solution). Multiply the estimated number of cells found in step 3 to find the number of cells in$0.1$grams of yeast. Record the number of cells in$0.1$gram on slide 12 . 5)

Calculate the number of yeast cells in 1 gram of yeast by sliding the decimal 1 place. Record the number of cells in 1 gram of yeast on slide 12 . 6) One tablespoon of yeast contains about six grams of yeast. One tablespoon of yeast is usually used to make one loaf of bread. Calculate the number of yeast cells used to make a loaf of bread by multiplying by six. Record the number of cells in one loaf of bread in slide$12.$Yeast Cells per ml – Yeast Cells in Grid= – Yeast Cells per ml= – Yeast Cells in$0.1g$yeast= – Yeast Cells in$1g$yeast= – Yeast Cells in one loaf bread Number of Yeast Cells per milliliter Essay Paper