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Investigating the effect of concentration of sugar on the respiration
rate of yeast
We did an investigation to find how different concentrations of sugar
effect the respiration rate of yeast and which type of concentration
Respiration is not breathing in and out; it is the breakdown of
glucose to make energy using oxygen. Every living cell in every living
organism uses respiration to make energy all the time. Plants respire
(as well as photosynthesise) to release energy for growth, active
uptake, etcâ€¦. They can also respire anaerobically (without oxygen) to
produce ethanol and carbon dioxide as by-products. This reaction is
shown in the equation:
Ethanol + Carbon Dioxide + Energy
2C2H5OH + 2CO2
Anaerobic respiration by yeast is generally called fermentation. Yeast
is a living organism that produces enzymes. These enzymes break down
glucose (by colliding with each other) to be able to respire
I predict that the rate of fermentation will increase proportionally
as the concentration of sugar increases but only up to a certain point
were it will begin to decrease and eventually stop.
I believe this because the more sugar added to the yeast the more
glucose broken down producing ethanol and carbon dioxide. The rate of
carbon dioxide produced in a minute will also increase because the
higher the concentration of sugar the more heat energy produced and so
the more the molecules will move around and collide. Also the higher
quantity of glucose molecules the higher chance of them colliding with
I believe the reaction will slow down and eventually stop when the
sugar reaches a certain concentration because the yeast will be killed
1. The high concentration of ethanol produced as a by-product.
2. The temperature of the reaction, as some of the energy produced
converts into heat energy. At really high temperatures the reaction
will stop because the heat will have denatured the enzymes.
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3. When there is a high sugar concentration the sugar molecules
diffuse into the yeast. But when the concentration of sugar is too low
the water in the yeast will diffuse out to equalise the concentration,
creating a net of movement of water into an area where there is less
water. The respiration goes down because more of the yeast is dying as
the sugar molecules are much larger and so enter the yeast at a much
slower rate than the water in the yeast diffuses out. Water actually
moves both ways but as not all the sugar will have dissolved in the
sugar solution the water mixes with the sugar and so can not go back
into the yeast through the tiny halls in the cell membrane as sugar
particles are much larger than water ones.
The apparatus used for this experiment is:
Â· Water bath, to keep a constant temperature of yeast through out the
tests. This will be kept at body temperature, as
Â· Stop clock, to time how many bubbles there are in a minute.
Â· Sugar solution.
Â· Test tube with bung, to put yeast and sugar solution.
Â· Long tube, to attach one end to bung and other end in water filled
Â· Water filled beaker, to see the carbon dioxide bubbles being
To do the experiment weâ€¦
Â· First need to warm the yeast to body temperature (37Â°C) in the water
bath, as this is the best temperature enzymes work in. After heating
the yeast to the right temperature we leave the test tube with the
yeast in the water bath so the temperature stays constant through the
Â· We then measure the amount of sugar needed and we prepare a beaker
Â· Once we add the sugar into the yeast we place a bung to close the
test tube with the solution and attach a tube from the bung to the
water filled beaker.
Â· Once the first bubble appears out of the tube and into the water
filled beaker, we start the stop clock and count the number of carbon
dioxide bubbles appearing. We must wait for the first bubble to appear
because the enzymes do not collide straight away with the glucose. We
do this for one minute then do the experiment again with a different
concentration of sugar.
We did a preliminary experiment that helped me choose my method.
Instead of finding the respiration rate by the number of bubbles we
tried to find the volume of carbon dioxide being produced. We used the
same apparatus but instead of leaving the end of the tube in the water
beaker we placed it in a water-filled upside down beaker. When the
bubbles came out of the tube the cylinder began filling with carbon
dioxide. So instead of counting the number of bubbles we had to
measure the difference between the starting volume of water and the
end volume to see how much carbon dioxide had been used. This
experiment resulted being very impractical, as the respiration rate
was far too small to be able to detect with the measuring cylinder.
It is necessary to:
Â· Keep the temperature the same through out all the tests because the
temperature of the yeast causes the enzymes to work faster or slower.
Â· Heat the yeast before adding the sugar because as soon as we add the
sugar the enzymes will begin to break down the glucose. This will then
not be a fair test, as the reactions will have begun at different
Â· Keep the amount of yeast used in the test constant so there is the
same amount of enzymes working each time. More enzymes would cause
more collisions with the glucose and so a higher respiration rate.
Â· Not shake or help mix the sugar solution with the yeast, as this
will help the enzymes and glucose to collide faster and so the
respiration rate to increase. We would also not be able to mix all the
solutions at the same rate causing an unfair test, as some solutions
would be more mixed than others would.
Â· Use the same sugar and yeast for all experiment as some sugars break
down more easily than others do and there are different types of
yeast's with different properties which could affect the way the
We will investigate five different concentrations of sugar to get as
many results as possible to see if my prediction is correct. We will
time each concentration for five minutes and time the number of
bubbles released each minute. This will give us five repeats of the
same concentration to be able to find the average to get as much of an
accurate result as possible.
Text Box: Amount of sugar added to yeast (g)
Number of CO2 bubbles released per min
* This number is not included to form the average, as it is obviously
The graph shows that the higher the concentration of sugar the lower
the respiration rate. I believe this is because the more amount of
sugar added the more yeast die. This is due to either:
1. The high concentration of ethanol produced as a by-product
poisoning the yeast.
2. The temperature of the reaction being too high causing the yeast to
die or the enzymes to be denatured.
3. The water in the yeast diffusing out to equalise the concentration
causing the yeast to dehydrate, contracts and eventually dies.
The results fit with the second part of my prediction were eventually
the respiration rate begins to decrease and eventually stops.
I believe the experiment was accurate. The rate of fermentation was
very slow and so the bubbles could not be miscounted. All things were
kept constant for a fair test. We used the same apparatus through out
all the experiment.
I believe the results are inaccurate as the pattern is very vague and
the range in the numbers of the repeats is very large. The average
respiration rate results for the sugar concentrations of 3g and 5g did
not follow the pattern of the rest of the results. They were only 2g
out of place so I believe a minor mistake might have been made like a
mistiming, miscounting of the bubbles, inaccurate concentration of
sugar, etcâ€¦. The first reading of the respiration rate of the sugar
concentration of 1g was very inaccurate. This was due to an early
timing, before the first bubble appeared.
If I were to repeat this experiment I would begin with a much lower
sugar concentration as these results only showed the decrease of the
respiration rate and not the increase. I would also try a range of
more varied concentrations to see if this would produce a larger
difference in the results, as the respiration rate results were very
similar and close together (apart from the average result for the
sugar concentration of 1g compared to the rest).