(6) Experiment to investigate the anaerobic
respiration rate of yeast
You can investigate the rate of anaerobic
respiration of yeast cells using a sugar substrate.
The variables for a given yeast are: temperature,
concentrations of enzyme, concentration of substrate sugar molecules
Investigating the chemistry of the anaerobic
respiration of yeast cells
You can also do the experiment with
germinating seeds, but
yeast produces more consistent results.
If you start with sucrose, the enzyme
invertase hydrolyses sucrose and breaks it down into glucose and fructose.
sucrose + water ==
enzyme invertase ==> glucose + fructose
+ H2O ===> C6H12O6
The actual anaerobic fermentation reaction
glucose/fructose (sugar) == enzyme zymase ==> ethanol + carbon
===> 2C2H5OH(aq) + 2CO2(g)
You can following the speed of the
reaction by measuring the volume of carbon dioxide formed.
Experimental procedure and analysis of
Using a thermostated bath you can
investigate the effect of temperature on the rate of fermentation.
You must keep the concentration of
the sugar plus yeast mixture constant - fixed volumes of previously
prepared stock solutions of the sugar or yeast suspension.
You can start at 20oC and
repeat the experiments several times for each temperature, and then
raise the temperature by 5oC at a time to see the effect.
You can measure the rate of
respiration in terms of the rate of evolution of gas e.g. cm3
Using the above apparatus, or that
described below (gas syringe) you can measure the rate of respiration
under varying conditions, BUT make sure you only vary one factor, to
measure its quantitative effect on the rate of respiration.
different substrate sugars of similar concentration, same enzyme
concentration, same temperature
(ii) for a fixed substrate
sugar, the effect of changing
its concentration at constant temperature and constant enzyme
(iii) for a specific yeast, the effect of
its concentration at constant temperature and constant sugar
If you bubble the gas from the
reaction mixture through a limewater you get a white precipitate
('milkyness'), a positive test for carbon dioxide from the yeast
You get exactly the same result if
you blow some of your expelled breath through limewater - the same
carbon dioxide from your aerobic respiration.
You can use a gas syringe system to
make more accurate experiments.
Doing the experiments at constant
room temperature, you can keep the yeast concentration constant and vary
the concentration of the sugar OR you can vary the substrate sugar (but
keeping the sugar concentration constant).
Typical graphical results you might
obtain based on a rate of evolution of carbon dioxide e.g. cm3
ENZYMES - structure, function, optimum conditions,
Enzymes and Biotechnology
(gcse chemistry notes)
Keywords, phrases and learning objectives for this section on respiration
Be able to describe an experimental method to
investigate the rate of anaerobic
respiration of yeast in the fermentation of sugar.
Know the apparatus required from a diagram describe
an outline of the procedure.
Know how to record the
results and from the data graphs to show the variation of the rate
of aerobic respiration with temperature or varying the concentration
of the sugar (or yeast).
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