An investigation into the effect of different sugars on respiration in yeast.

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Jennifer Lewis 13HC

An investigation into the effect of different sugars on respiration in yeast.

I am going to carry out an experiment, measuring the effect of different sugars on the respiration in yeast.

In order to make a justified prediction I have researched different aspects of scientific knowledge, including respiration, yeast, sugar structure, enzymes and the collision theory.


Glycolysis is the splitting of a monosaccharide into two molecules of pyruvate. It takes part in the cytoplasm of a cell. Glycolysis begins with a monosaccharide with six carbon atoms, and ends with two molecules of pyruvate, each with three carbon atoms. For the first steps of glycolysis, energy from ATP is needed. However, energy is released in later steps to generate ATP. For every molecule of glucose, a net gain of two molecules of ATP is produced.

The first stage of glycolysis is called phosphorylation, and results in hexose bisphosphate. This is shown in green on the above diagram. Hexose bisphosphate then breaks down into two molecules of triose phosphate. Hydrogen is removed from the triose phosphate and transferred to NAD to produce reduced NAD. These hydrogen’s can then be used in oxidative phosphorylation to produce ATP. The end products of glycolysis are pyruvates, which still contains a lot of chemical potential energy. There are two molecules of pyruvate generated for every molecule of glucose.

Glycolysis is an anaerobic process, as none of the steps involves using oxygen. Anaerobic fermentation will continue instead of aerobic cellular respiration, as I am using yeast which is deprived of oxygen.


Yeast is a fungus and needs a supply of energy for its living and growth. Sugar is a good supply for this energy. The more sugar there is, the faster the rate of respiration and the faster the yeasts will grow. Even if there is a limited supply of oxygen, yeast can still release energy from sugar. However, the by-products of this are alcohol and carbon dioxide.

Yeast respires in anaerobic conditions. This means that the pyruvate will be converted into carbon dioxide and ethanol. To do this, the enzyme pyruvate decarboxylase removes a carbon dioxide molecule from the pyruvate. The acetaldehyde that is formed is then reduced by the enzyme alcohol dehydrogenase which transfers the hydrogen from NADH to the acetaldehyde to yield NAD and ethanol.

C6H12O6 + 2 ADP + 4 H+ + 2 HPO42- → 2 C2H5OH + 2 CO2 + 2 ATP + 2 H2O

Yeasts are active in a very broad temperature range - from 0 to 50° C, with an optimum temperature range of 20° to 30° C. When I do my preliminary, I will not use temperatures over 50˚ or under 20˚.

Sugar structure

Yeast exhibits a variable preference for different sugars. It readily assimilates four sugars, namely, sucrose (after hydrolysis to glucose and fructose by yeast invertase or sucrase), glucose, fructose, and maltose (after hydrolysis to glucose by yeast maltase). I can also assume that lactose will produce some carbon dioxide, as it breaks down into glucose and galactose. Although galactose is not able to be respired by yeast, glucose is. In yeasted dough’s, an increase in maltose occurs during the first stages of fermentation, until the initial supply of glucose and fructose is exhausted, after which the maltose content gradually declines.

Sugar molecules are formed from carbon, hydrogen and oxygen, and the natural grape sugars are the materials yeast converts into ethyl alcohol and carbon dioxide. Although sugars are made from only three elements, some sugar molecules are very large and have complicated structures. Several different kinds of sugars exist, and each sugar has its own name. The name used to denote the entire family of sugar molecules is "saccharide."

The small, simple sugar molecules are called monosaccharides, and two simple sugar molecules bound together are called disaccharides.

Glucose origins/life/ch1_2.htm

Glucose is one of the most important carbohydrates as many animals and plants use it as a source of respiration. Glucose is also one of the main products of photosynthesis and begins cellular respiration. The chemical formula for glucose is C6H12O6. This means it contains six carbon atoms connected to six water molecules. Glucose can be formed in the liver and skeletal muscle by the breakdown of glycogen stores. It can also be synthesized in the liver and kidneys from intermediates by a process known as gluconeogenesis.

Fructose sugar.html

The most common place that fructose is found is in honey together with sucrose and glucose. It is also found in the disaccharide sucrose alongside glucose. The chemical formula for fructose is C6H12O6

Galactose model_project_bi.htm

Galactose is most commonly found in the disaccharide, lactose and as the monosaccharide in peas. It is also found in milk and yogurt. It enters glycolysis by its conversion to glucose-1-phosphate. The chemical formula for galactose is C6H12O6. As this sugar is generally only found in dairy products, it is unlikely that yeast will be able to break it down for respiration. This is because the yeast will not contain the specific enzyme, galactokinase. The diagram below shows the process by which galactose enters glycolysis.

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Glucose, fructose and galactose are all monosaccharides. Monosaccharides are the simplest form of carbohydrates and consist of one sugar. They cannot be hydrolysed into smaller carbohydrate units and they share the chemical formula of C6H12O6. All of these sugars have six carbon atoms, resulting in them being hexoses.

Sucrose glossary/sucrose.htm

Sucrose is a crystalline disaccharide consisting of fructose and glucose, and has the chemical formula of C12H22O11. Unlike other disaccharides, ...

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***** A detailed description of preliminary and main experiments together with relevant and reasonably concise background theory. A level biological language is used throughout and there are clear justifications and explanations for predictions, choice of apparatus and methodology.