(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

C₁₂H₂₂O₁₁  +  H₂O  ===> C₆H₁₂O₆  +  C₆H₁₂O₆ 

The actual anaerobic fermentation reaction is ...

glucose/fructose (sugar) == enzyme zymase ==> ethanol + carbon dioxide

C₆H₁₂O_6(aq) ===> 2C₂H₅OH_(aq) + 2CO_2(g) 

You can following the speed of the reaction by measuring the volume of carbon dioxide formed.

Experimental procedure and analysis of results

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 20^oC and repeat the experiments several times for each temperature, and then raise the temperature by 5^oC 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 CO₂/min.

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.

(i) with 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 concentration,

(iii) for a specific yeast, the effect of changing its concentration at constant temperature and constant sugar concentration.

Note:

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 anaerobic respiration..

You get exactly the same result if you blow some of your expelled breath through limewater - the same carbon dioxide from your aerobic respiration.

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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 CO₂/minute.

 

See also ENZYMES - structure, function, optimum conditions, investigation experiments 

See also Enzymes and Biotechnology (gcse chemistry notes)

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