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Unformatted text preview: 7.1 Fermentation When you study the ability of yeast to ferment sugar, you will need to use a respirometer to measure the
amount of carbon dioxide given off. Your ﬁrst experimental procedure shows you how to prepare your respirometer. Experimental Procedure: Respirometer Practice 1. Completely fill a small tube (15 X 125 mm) with water (Fig. 7.2).
2. Invert a large tube (20 X 150 mm) over the small tube, and With your ﬁnger or a pencil, push the
small tube up into the large tube until the upper lip of the small tube is in contact with the bottom of the large tube. 3. Quickly invert both tubes. Do not permit the small tube to slip away from the bottom of the large
tube. A little water will leak out of the small tube and be replaced by an air bubble.
4. Practice this inversion until the bubble in the small tube is as small as you can make it. Figure 7.2 Respirometer for fermentation. Place a small tube inside a large tube. Hold the small tube
in place as you rotate the entire apparatus, and an air
bubble will form in the small tube. ' 76 Laboratory 7 Cellular Respiration yeast air bubble
(height to be
measured and recorded
for each tube) filled and
- ~ inverted
little finger (Now, quickly respirometer
holding invert tubes ready for use
small tube while tightly
against holding them
large tube together.) 2
3 mark small tube (tube 1) 7—2 Ethanol Fermentation Yeast fermentation to produce ethanol and carbon dioxide has
long been utilized by humans to produce wine and bread.
During the production of wine, it is the ethanol that is desired
while bread and other baked goods rise when yeast gives off
carbon dioxide (Fig. 7.3). Recently, there has been a great deal
of interest in using ethanol produced by yeast fermentation of
corn as a substitute for gasoline in cars. Testing Sugars In the Experimental Procedure that follows, we are going to
test which of several sugars is a better food source for yeast
when they ferment. It will be assumed that the ease of sugar
fermentation correlates with the amount of carbon dioxide Figure 7.3 Products of
given off within a certain time limit. As background data, fermentation. observe the structure of the three sugars involved: H OH OH H glucose fructose sucrose State a hypothesis for your experiment in which you sequence the sugars according to how well you
expect yeast to ferment them: Hypothesis: Experimental Procedure: Yeast Fermentation Have four large test tubes ready. With a wax pencil, label and mark off a small test tube at the 2/ 3-full
level. Use this tube to mark off three other small tubes at the same level. 1. Label and fill the small tubes as directed, and record the contents in Table 7.1. Tube 1 Fill to the mark with glucose solution.
Tube 2 Fill to the mark with fructose solution.
Tube 3 Fill to the mark with sucrose solution.
Tube 4 Fill to the mark with distilled water. 7—3 Laboratory 7 Cellular Respiration 77 2. Re-suspend a yeast solution each time, and ﬁll all four tubes to the top with yeast suspension
(Fig. 7.2). 3. Slide the large tubes over the small tubes, and invert them in the way you practiced. This will mix
the yeast and sugar solutions. 4. Place the respirometers in a tube rack, and measure the initial height of the air space in the
rounded bottom of the small tube. Record the height in Table 7.1. Q 5. Place the respirometers in an incubator or in a warm water bath maintained at 37°C. Note the time,
and allow the respirometers to incubate about 20 minutes (incubator) or one hour (water bath).
However, watch your respirometers and if they appear to be ﬁlling with gas quite rapidly, stop the
incubation when appropriate. During the incubation period, begin the Experimental Procedure on
cellular respiration (see page 79). 6. At the end of the incubation period, measure the ﬁnal height of the gas bubble, and record it in
Table 7.1. Calculate the net change, and record it in Table 7.1. Table 7.1 Fermentation by Yeast Initial Final
Tube Sugar Gas Height Gas Height Net Change Ease of Fermentation ____—_____———_—————————-— 1 Conclusions: Yeast Fermentation 0 From your results, evaluate how the sugars tested compare as a food source for yeast fermentation.
Enter your evaluation in Table 7.1. 0 Do your data support or fail to support your hypothesis? Explain. 0 Can your results be correlated with the comparative structure of the sugars? Explain. 0 Which respirometer was the control? 78 Laboratory 7 Cellular Respiration 7—4 ...
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