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There are two types of respiration in yeast:
Anaerobic: Glucose [IMAGE] Carbon dioxide + ethanol + energy
Respiration is controlled by enzymes, which are proteins which speed
up one or more biological reactions. Within any cell many chemical
reactions are going on at any one time. Yeast has many different types
of enzymes that speed up respiration.
I predict that as temperature increases, the rate will also increase,
until a certain optimum temperature, after which, the rate will
decrease until the rate is zero as respiration has stopped completely.
As temperature increases, rate of respiration increases, because
particles move faster and with more energy, which in turn means more
particles collide with enough energy to react. However, as temperature
increases, enzyme stability decreases, so at temperatures above the
optimum temperature, the rate will decrease, until all the enzymes
have been fully denatured and all the active sites have been lost.
Enzymes speed up reactions in organisms. Each enzyme works on a
specific substance, called its substrate.
The diagram below shows an “E” (an enzyme) catalysing the breakdown of
“S” (the substrate) into two different products (“P”). Catalysis
occurs because substance S fits precisely into surface of the enzyme
E, so this reaction and no others are speeded up.
Diagram showing an enzyme catalsying the breakdown of its substrate into two product molecules.
As can be seen from the diagram, if the enzyme changes shape, the
active site (the area where the substrate reacts) would no longer be
able to fit the substrate. This would mean the enzyme would lose its
effect; the substrate would not break down.
This happens when the temperature is too high; the process is called
“denaturing”. When an enzyme reaches a certain temperature, it will
have so much energy that it is de-shaped; it is “denatured”. This
diagram shows how a denatured enzyme will not work:
The enzymes will hardly work at very low temperatures (they wont be
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Each enzyme has an optimum temperature in which they are most active.
When the temperature is too high for that particular enzyme, the
enzymes are denatured.
In yeast, there is more than one type of enzyme involved in anaerobic
respiration. This means that the yeast enzymes do not denature at the
same time, so although yeast does have an overall optimum temperature,
the decrease in the rate of respiration that occurs after this point
is gradual, as the number of actives sites decreases.
This is the apparatus I will be using in my experiment.
This graph summarises how and why temperature affects enzymes:
I will be changing the temperature of the yeast, and I intend to use
these temperatures: 20, 30, 40, 50, 60, 70, and 80 (oC), although it
does not matter what exact temperatures I use, as long as they are at
a suitable range. I will just use the temperatures which are most
convenient when I am making the water baths.
I will measure how much water will be displaced by Carbon Dioxide in 3
minutes, in cm[IMAGE].
Temperature is my independent variable.
Volume of carbon dioxide produced in 3 minutes is my dependent
I will also control the concentration of yeast (this will be 10%), the
concentration of glucose (also 10%). I will always use 10cm[IMAGE]of
glucose, and 10cm[IMAGE] of yeast. I will not change the time taken to
collect the carbon dioxide (3 minutes) or the volume of liquid
paraffin (2 drops) or volume of diazine green (2 drops). The reason I
won’t change any of these is because they might affect the volume of
carbon dioxide produced, which would make my experiment unfair.
This is my planned method:
Glucose solution (at a concentration of 10%) will be boiled to remove
all the oxygen, and then cooled. 10cm[IMAGE] of this will be added to
10cm[IMAGE] of a 10% concentration yeast mixture. This will then be
poured in a test tube. 2 drops of diazine green will be added so I can
tell when all the oxygen has been used up in aerobic respiration. It
will be blue when oxygen is present and pink when it is not. I will
put the test tube with the other apparatus as shown in my diagram
(without the rubber bung in the test tube). I will add 2 drops of
paraffin in the test tube to prevent oxygen getting into the
suspension. When the suspension appears pink, I will start the
experiment by placing the rubber bung in the test tube and measuring
how much carbon dioxide is produced in 3 minutes. Each time I use a
new temperature, I will use a new set of yeast, glucose, paraffin and
diazine green. I will then repeat the experiment, making sure each
time that anaerobic respiration has started (i.e. mixture is pink).
These are the temperatures I plan to use:
20 oC 30 oC, 40 oC, 50 oC, 60 oC& 70 oC although as long as the
temperatures I use have a suitable range, it does not matter if the
temperatures are exact or not, for example 22 oC is just as good as 20
Diazine green can be dangerous if it gets into your eyes, so I will
wear safety goggles. I will ensure that no obstructive items such as
bags are on the floor at the time of the experiment. As I will be
using glass apparatus I will have to be careful it doesn’t break and
To ensure my results were reliable I will do 3 repeats in total of the
same temperature. If the results look similar enough I can assume they
are reliable and I will keep them. If they do not, I will redo them
until they look reliable. This will keep my results accurate.
There are many possible causes of error that could affect my results:
· If the yeast and glucose mixture is not the same temperature of the
· If the temperature of the water bath (and hence the glucose and
yeast mixture) goes down during the experiment, so will the rate.
· If the paraffin does not completely cover the mixture and oxygen can
get into the mixture, aerobic respiration would start.
· If a mixture is not completely mixed there will be less successful
collisions per second between the reactants which will decrease the
· Different volumes of water in the water bath will affect how much it
changes the temperature of the yeast and glucose mixture.
I wanted to see if I needed to make any changes to my experiment, so I
carried out the experiment as planned so far, and I got the following
CO2 collected (cm[IMAGE]) (Run1)
CO2 collected (cm[IMAGE]) (Run 2)
I did two runs, so I could see anomalies if I got any.
During this experiment I noticed that the contents of the test tube
where bubbling over into the delivery tube. Another problem was that I
could not see the colour of the diazine green in the mixture so it was
hard to tell when aerobic respiration had fully stopped. Also as the
mixture bubbled over the paraffin had no effect. These are probably
the reasons that my results don’t show a good trend. This means I am
going to have to modify my experiment:
I will carry out my experiment as I planned before, but I will use 6
drops of diazine green instead of 2, because with 2 drops I couldn’t
see the colour so I couldn’t tell when aerobic respiration had
stopped, but should definitely be able to with six. Also I will only
use 5cm[IMAGE] of the yeast mixture and 5cm[IMAGE] of the yeast
mixture so that the mixture doesn’t bubble over. I am going to also
use 6 drops of paraffin so there is no way that oxygen can get in and
trigger aerobic respiration. Also I am going to take 3 runs for each
temperature rather than 2, and work out an average.
My results show that below 20 oC, anaerobic respiration has hardly
started and is too slow to allow a measurable volume of carbon dioxide
to be evolved in three minutes. This is because the reactants rarely
collide frequently enough and with enough energy to react because the
temperature is too low. At temperatures above 20 oC, measurable
respiration starts, and rate of anaerobic respiration increases as
temperature increases, because as the temperature increases, the
reactant particles collide more frequently and with more force, so the
number of successful collisions per second increases. This increase in
rate stops when the temperature reaches an optimum temperature where
rate is at the highest. On my graph the optimum temperature appears to
be at 48 oC, but it could be anywhere between 42 oC and 52 oC. At
temperatures above 48 oC, the rate appears to decrease as temperature
decreases because the enzymes are beginning to denature. As
temperature increases, the active sites of the enzymes slowly misshape
until they completely denature and anaerobic respiration cannot take
place. At temperatures above 48 oC, as temperature increases the rate
of respiration decreases because the enzymes are losing their active
sites so the substrate particles cannot react as often. At 76 oC the
rate is zero and respiration has completely stopped because all of the
enzymes have been denatured and the active sites have been lost so the
reaction cannot take place.
My graph shows this trend clearly. At 20 oC the line starts as the
rate increases until the highest point which is at 48 oC. The highest
point on the true line of best fit however could be anywhere between
the two points either side of 42 oC and 52 oC.
To work out the average volume of carbon dioxide produced in 3 minutes
(cm[IMAGE]), I added the 3 runs from one temperature together and
divided the sum by three because:
Mean value = _Sum of values__
Number of values
I can conclude that as temperature increases, the rate of anaerobic
respiration in yeast increases, until an optimum temperature where the
rate is highest. At temperatures higher than this temperature, as
temperature increases, rate decreases, until all the enzymes have
fully denatured, all the active sites have been lost, and the rate is
This is exactly as I predicted; I predicted that as temperature
increases, the rate will also increase, until a certain optimum
temperature, after which, the rate will decrease until the rate is
zero as respiration has stopped completely. The experiment has
verified my prediction.
My experiment was successful. The results showed a good trend and the
results were accurate and repeatable. I can tell they are repeatable
because the repeats in my results table are consistent. I have also
shown this on my graph by adding error bars. This shows how they work:
I have not included errors bars at temperatures where all three
readings were the same.
I had one anomaly (the 2nd reading at 48 oC), possible reasons are
· If the yeast and glucose mixture was not the same temperature of the
· If a temperature of the water bath went down during the experiment,
this would have lowered the rate.
· If the paraffin did not completely cover the mixture, oxygen could
have got into the mixture causing aerobic respiration.
· If the mixture was not completely mixed there would be less
successful collisions per second between the reactants.
· Different volumes of water in the water bath will affect how much it
changes the temperature of the yeast and glucose mixture.
To improve the experiment I could use a gas syringe instead of a
measuring cylinder as they are more accurate. I would attach it to the
delivery tube, but I would no longer need to use a bucket to hold
water, and I could use a conical flask instead of a test tube so the
yeast and glucose mixture doesn’t bubble over, but if I did this I
would need a larger volume of liquid paraffin to cover the glucose and
yeast mixture. Also I could use a thermostatically controlled water
bath instead of water in a beaker, because this would keep the
temperature of the water bath constant so the temperature of the yeast
and glucose mixture would not change throughout the experiment and
would keep my results more accurate. This diagram shows what I would
I could repeat the experiment with the aim of find the optimum
temperature for anaerobic respiration in yeast. I would repeat the
experiment with the above improvements but use the following
41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52 (oC)
If I wanted I could also find a more accurate optimum temperature by
using a small range of results at small intervals, for example if the
highest reading was at 47 oC, I could then measure the rates of
respiration at the following temperatures:
46.5, 46.6, 46.7, 46.8, 46.9, 47.0, 47.1, 47.2, 47.3, 47.4, 47.5 (oC)
This would give me a more accurate optimum temperature, as the optimum
temperature is only as accurate as the temperature intervals are
Another experiment I could do would be to find the temperature at
which all the enzymes in yeast involved in anaerobic respiration are
fully denatured. I would repeat the experiment with the same changes
(conical flask, gas syringe and thermostatically controlled water
bath) but used the following temperatures instead:
66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76 (oC)
Then I could find this a more accurate value, for example if 70 oC was
the lowest temperature which gave all the volumes of carbon dioxide
produced in 3 minutes (cm[IMAGE]) as 0, then I would do the same but
with the temperatures:
74.1, 74.2, 74.3, 74.4, 74.5, 74.6, 74.7, 74.8, 74.9, 75.0 (oC), and
then the lowest of these values which gave which gave average volume
of carbon dioxide produced in 3 minutes as 0 would be the temperature
at which all the enzymes in yeast fully denature (to 1 decimal place).