These small molecules act as singular units or monomers, which form the building blocks of larger molecules, or polymers. When polymers are in the form of sugars they are given the name polysaccharides, meaning many sugars. Two monosaccharides may join together to form a disaccharide, and they do this through dehydration, forming a covalent, glycosidic linkage. During this reaction, aided by enzymes in the cytoplasm of cells, one glucose molecule, for example, donates a hydroxyl group from its carbon skeleton, and another glucose donates a hydrogen atom. Together this creates a loss of H2O, and a covalent bond is formed joining the two carbon skeletons through the remaining oxygen atom.
In the disaccharide maltose there is a 1α 4 glycosidic linkage, as both monomers are α-glucose and carbon numbers 1 and 4 are the carbon atoms which actually link together. Cellulose, however, is a polysaccharide found in plant cell walls that can have hundreds of thousands of 1 β 4 glycosidic linkages, as it is made entirely of β-glucose molecules with carbons 1 and 4 being covalently bonded. Starch is another sugar, used for storage, and is made up of two others: amylose and amylopectin. The difference between these two polysaccharides of glucose is that amylose consists of 1-4 linkages, and is a continuous helix shape, whereas amylopectin has some 1-6 linkages as well as 1-4, meaning that it’s helix shape is branched in places. The sugar more commonly known as table sugar is called sucrose, and is a disaccharide of glucose and fructose monomers.
Because of the aldehyde groups within most sugars, they are said to be reducing sugars or reducing agents. An aldehyde group can be recognised by a carbon atom having a double-covalent bond with an oxygen atom, and a single covalent bond with a hydrogen atom. Reducing sugars undergo oxidation in order to reduce a surrounding solution by removing oxygen from it and transferring electrons to it. For example, a reducing sugar will reduce the Copper II ion within Benedict’s solution to Copper I oxide ( Cu - →Cu ). Reducing sugars should, in theory, be capable of both dissolving in water and reducing the substance around it. This is because the hydrogen in the aldehyde group will have a slight positive charge owing to the polarity of the double covalent bond between the carbon and oxygen atoms. This will create hydrogen bonds between the polar sugars and the partially negatively charged oxygen atoms of water molecules, allowing the sugar to dissolve. The sugar should then go on to reduce the substance around it as it oxidises to become a carboxylic acid, gaining an oxygen atom and thereby reducing the substance around it of that oxygen atom. This may only be done if the carbonyl (aldehyde) group is free to undergo this reaction.
Aims
- To determine which of the sugars Fructose, Glucose, Maltose, Starch and Sucrose, dissolve.
- To determine which of the sugars contain starch.
- To determine which of the sugars are reducing agents.
Method
Refer to method sheet.
Changes to the method sheet include:
Include starch as a test for reducing sugar.
Results
Discussion
The results of the experiments were as expected, as fructose, glucose and maltose all dissolve in water, test negative for starch and are reducing agents. All monosaccharides and most disaccharides are reducing agents, and so it is no surprise that glucose, fructose and maltose resulted consequently. Starch is a polysaccharide, whereas all of the other sugars tested are single or double sugars, and therefore it is also predictable that starch would not be present in any sugar other than starch itself, which the results reflect.
Starch, however, did not fit the trend of the ability to be both soluble and a reducing sugar. This is because of it’s size, and there is so much starch in the cell that if it were soluble, it would cause cells to take up too much water through osmosis, thus upsetting a sensitive balance of water within the cell. It is, however, a reducing sugar as it breaks down into glucose monomers, which have free carbonyl groups to react with the copper in the benedict’s solution.
Sucrose is the other obvious fluctuation in the results obtained from the experiment. This is basically because of the way in which sucrose is formed in comparison to the other disaccharide tested, maltose. Below is a diagram of a maltose molecule, formed by a 1α 4 glycosidic linkage. Highlighted in red is the exposed carbonyl or aldehyde group with can be used for reduction.
This free aldehyde group is essential for reduction to occur. In maltose this is simple, as it is in glucose, fructose and starch, as the carbonyl group is free. However, sucrose does not have an exposed aldehyde group, and so cannot reduce its surroundings. Below is a structural diagram, and from this linkage of glucose and fructose, it can be seen that an aldehyde group is not free.
To test whether sucrose is indeed a non-reducing sugar, it was put through hydrolysis by the addition of hydrochloric acid. This split the disaccharide forming glucose and fructose, which in turn reduced the benedict’s solution as they are both monosaccharides. This was, therefore, the test for a non-reducing sugar.
Conclusion
From the results obtained and their explanation in the structure of the saccharide, it may be concluded that all monosaccharides and most disaccharides will be both soluble in water and reducing agents. However, there are exceptions to these generalisations, the main being that starch does not dissolve in water because of its sheer size, and that sucrose is a non-reducing sugar because of the glycosidic bond between its monomers of glucose and fructose, meaning that the necessarily exposed carbonyl group is not so.
References
