'An investigation into the ability of two strains of the yeast Saccharomyces cerevisiae to utilise different carbon sources as substrates for cellular respiration'.

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‘An investigation into the ability of two strains of the yeast Saccharomyces cerevisiae to utilise different carbon sources as substrates for cellular respiration' 


The purpose of this investigation is to compare the ability of two different strains of yeast to respire, when using different sugars as respiration substrates.

Considering the lengths that have been reached to develop varieties of yeast with greater suitability and effectiveness for very particular fermentation purposes, it seems reasonable to suppose that two different strains of the same species of yeast, selected for their different fermentation properties, have developed requirements that are not uniform.

As a result of the selection and development process, yeast best suited to ferment in a given application, possess a range of different characteristics. One such characteristic may be the ability to metabolise different carbon sources at different rates. This quality is important because in each application where different respiration substrates are available, a specific strain of yeast may be required. Yeast unable to utilise the available carbon sources will have undesirable fermentation rates, and therefore may not be selected for use in that application.

The two yeast here compared, have two such different applications. The first yeast from the Saccharomyces cerevisiae variety is of the Hansen strain and is used in the baking industry, whereas the second also of the Saccharomyces cerevisiae family is the Narbonne strain, utilised in the brewing of wine.


The optimum ability of a variety of yeast to use a carbon source for respiration will depend upon the presence of certain enzymes within the yeast cells.

An enzyme is a biological catalyst, which speeds up metabolic reactions, and in the case of yeast is required in order to hydrolyse a particular carbon source.

Enzymes are globular proteins, which possess complex tertiary structures. The ‘lock and key’ mechanism is a theory, which explains enzyme-substrate complex formation. The globular nature of an enzyme’s structure gives rise to an area on the enzyme known as an active site. Due to the substrate being complimentary in shape to that of the enzyme’s active site, they fit together as a key would in a lock.

The binding together of the enzyme and substrate results in the substrate being hydrolysed. In the case of yeast respiration, this degradation turns the substrate into a form more usable in glycolysis, the first stage of cellular respiration.


         Enzyme + substrate enzyme-substrate complex enzyme + product

Respiration of yeast will be most rapid when glucose is available as a substrate, because it is the simplest usable carbon source, and will be used preferentially in its role as a precursor of glycolysis. Other simple sugar molecules will also facilitate respiration but at slower rates, because they will not form complexes with the yeast’s enzymes as readily as glucose.

During the bread-making process, yeast is added to the dough mixture to facilitate fermentation. The evolution of carbon dioxide as a result of this process is required in order to leaven (raise) the dough.

The substrates available for respiration during this process are largely derived from the complex sugar, starch. Two types of starch exist in flour, amylose and amylopectin. Although the Hansen strain of baking yeast contains neither the enzyme α-amylase nor β-amylase, required to degrade the carbohydrate chains which form these two types of starch, they are contained in wheat flour. Additionally, the flour milling process causes damage to these lengthy chains. This damage allows the yeast to start metabolising the starch, and is achievable through the presence of the enzymes, maltase and zymase, which degrade the sugars maltose and glucose respectively.

Generally, grapes used in wine production are comprised of sugar levels between 15% and 25% of their total mass. These sugars are mainly glucose and fructose, both simple saccharides. The Narbonne strain of wine yeast is best suited to the production of wine from grapes, rather than from other fruits, vegetables or grains such as elderberry or rice.

Another important constituent of grape juice is the acid content, consisting mainly of tartaric and malic acids. As a result of the environmental demands placed upon the Narbonne strain, a higher tolerance to acidic conditions is likely to be a characteristic which differentiates it from strains not specialised in the fermentation of wine.

The carbon source provided will ultimately be used within the cytoplasm of the yeast cells, during glycolysis. As the substrate is fermented, there is a build up of reduced electron carriers (NADH+H+). In order to prevent this build up causing glycolysis and therefore metabolism to stop, the reduced electron carriers must be re-oxidised. To achieve this, pyruvate is first converted to ethanal, through the removal of a molecule of carbon dioxide:

          CH3COCOOH                                  CH3CHO     +     CO2

              (Pyruvate)                                        (Ethanal)    (Carbon dioxide)

The newly formed ethanal molecules combine with the hydrogen ions (H+) being transported by the reduced NAD+, thus re-oxidising the reduced carriers and producing the alcohol ethanol:

                      NADH+H+     NAD+


          CH3CHO                                  CH3CH2OH

           (Ethanal)                                      (Ethanol)

The re-oxidised co-enzymes are now available to accept and transport more H+ and glycolysis can continue in its cyclical nature.

This process is expressed diagrammatically below:



           2ADP+2Pi                      NAD+


                   2ATP                      NADH+H+ 

                                                                    NADH+H+       NAD+

                                 Pyruvate               Ethanal                         Ethanol

                                                         Carbon dioxide

During yeast fermentation in the brewing of wine, the desired effect is the production of ethanol (ethyl alcohol), rather than carbon dioxide as is the case in bread making. The process of winemaking takes a significantly longer time to reach completion than bread making.


The rate of respiration in baking yeast is faster than in wine yeast, when glucose is used as a respiratory substrate.

To test this hypothesis, the rate of respiration in baking yeast and wine yeast will be compared using four different carbon sources as respiration substrates.

During the respiration of the yeast cells, carbon dioxide will be produced. This carbon dioxide will dissolve within the liquid medium in which the yeast is growing, and carbonic acid will be formed. The volume of carbonic acid produced is influenced by the rate of respiration, therefore, the greater the rate of respiration, the greater the volume of carbonic acid produced.


After an eight-hour fermentation period at 25oC, the extent of anaerobic respiration that has occurred will be estimated using a titration against the alkaline 0.1 mol dm-3 sodium hydroxide solution. The volume of the sodium hydroxide solution required to neutralise the mixture will indicate the rate of respiration that has occurred. This will be indicative of the degree to which the yeast is able to use the particular carbon source as a substrate for respiration.

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Equipment and materials

  • Ten 500cm3 flasks
  • Permanent marker
  • Digital scales
  • Measuring cylinder
  • Glass rod
  • Cotton wool
  • Thermostatically controlled incubator
  • Clamp stand
  • Burette
  • Funnel
  • Five beakers
  • 25cm3 volumetric pipette and filler
  • Pink colour chart

  • Carbon sources:
  • Glucose
  • Maltose
  • Galactose
  • Lactose
  • Culture nutrients:
  • Ammonium phosphate
  • Ammonium sulphate
  • Distilled water
  • Dried yeast granules:
  • Saccharomyces cerevisiae - Hansen strain (Baking yeast)
  • Saccharomyces cerevisiae - Narbonne strain (Wine yeast)
  • Phenolphthalein indicator solution
  • 0.1 mol dm-3 sodium hydroxide solution


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