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PAG 12.1 Full write-up (A Level Biology OCR A)
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04/07/2023PAG 12.1 – Investigating the respiration rate of Saccharomyces
cerevisiae
Aim
To research and conduct an experiment to find out how the rate of respiration is
affected by the type of substrate (Glucose, fructose, lactose, and sucrose). The
way in which this will be carried out is by measuring the amount of CO 2 released
from anaerobic respiration using a gas syringe.
Introduction
To obtain energy needed to live, cells must undergo respiration. Cellular
respiration is a sequential metabolic reaction that is important to all living cells.
Respiration produces energy from sugars in the form of ATP or adenine
triphosphate which is the basic energy currency of cells. There are two types of
cellular respiration. It can be aerobic where oxygen is needed or anaerobic
where the presence of oxygen is not needed. However, some cells produce ATP
utilising both aerobic and anaerobic respiration. 1 For example, muscle cells
usually respire aerobically, but when they do not have enough oxygen, such as
during excel is, they respire anaerobically as a last resort. In yeast, anaerobic
respiration takes place. This can also be called fermentation, where ethanol and
carbon dioxide are the products.
Anaerobic respiration (fermentation) has two types. Lactic acid fermentation is
the first type, in which molecules of glucose are transformed into lactate that
speeds up the reaction by lactate dehydrogenase. Alcohol fermentation is
another type, in which molecules of pyruvate are transformed into ethanol by
first releasing carbon dioxide which is then transformed into acetaldehyde, in
which NADH reduces it, producing ethanol. 2
Yeast is a facultative anaerobe; in other words, it can do aerobic respiration when
the environment permits it to but can respire anaerobically when the
environment lacks oxygen. Yeast produces and releases enzymes into its
surroundings where sugar molecules are present. Complex sugar molecules then
break down into simpler forms (monosaccharides) before it can be absorbed by
yeast which will then produce energy and carbon dioxide as by-product.
Therefore, one factor that effects the rate of respiration in yeast is the substrate.
Monosaccharides are the simplest sugars; they are the building block of all
carbohydrates. Disaccharide are formed when two molecules of monosaccharide
bond via a glycosidic bond. And lastly polysaccharides are formed from the
linking of many monosaccharides.3 It can be hypothesised that “If the nature of
substrates makes cellular respiration in either yeast faster or slower, then the
simpler the sugar, the quicker the rate of respiration in yeast” since
1
M. Ohanyan, Cellular Respiration in Yeast
2
Khan Academy, Cellular Respiration Review
3
E. Zachariah, The basics: What are carbohydrates?
, disaccharides and polysaccharides must be broken down before energy can be
obtained from them.
Temperature is another factor that affects the rate of respiration in yeast. A
higher temperature will be closer to the optimum temperature of enzymes in
yeast, increasing enzyme activity. Since respiration is an enzyme-controlled
reaction, as the enzyme activity increases, the rate of anaerobic respiration will
also increase. If the temperature is too high, the enzymes will denature, causing
carbon dioxide production to slow down and subsequently cease altogether. 4
Another factor that may affect the rate of respiration in yeast is the
concentration of the substrate. By increasing the substrate concentration, the
enzyme activity also increases, since there are more molecules to occupy the
active sites. Therefore, if more enzyme substrate complex forms, then more
carbon dioxide will be produced. However, this occurs only for a certain period
until all the active sites are saturated with substrates. Therefore, an increase in
substrate concentration will not result in an increase in the rate of reaction.
We have chosen to investigate how the type of substrate used affects the rate of
the reaction. In previous academic years we have had the opportunity to
experiment with the effects of temperature and substrate concentration on rates
of reactions. However, this is the first time we have had the option to be able to
experiment with different substrate types. This is why we have chosen to
investigate how the type of substrate affects the rate of respiration.
Hypothesis
If the nature of substrates makes cellular respiration in either yeast faster or
slower, then the simpler the sugar, the quicker the rate of respiration in yeast.
Apparatus
Yeast suspension
20% Sugar solutions (glucose, fructose, sucrose, lactose)
10cm3 Measuring cylinder (x6)
Gas syringe
Plastic syringes fitted with tubing
Thermometer
Flask with stopper and tubing
Water bath (at 35 deg C)
Beaker
Distilled water
Stopwatch/timer
Pipette
Boiling tube rack
Boiling tubes (x5)
Method
4
M.R. Bernard, The effect of temperature on the production of carbon dioxide over time
in saccharomyces cerevisiae
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