Most monosaccharides are the producers of energy, however some are used for structural purposes, for example ribose and deoxyribose, both pentose sugars, are used to make RNA and DNA. Alpha and beta glucose and fructose are hexose sugars, which also have different functions in organisms. Glucose is used in respiration where it is broken down to release energy. It is also used to make polysaccharides and disaccharides. Fructose is the main part of nectar and is responsible for making fruits very sweet. Finally there is the triose sugar called glyceraldehyde, which is important in both respiration and photosynthesis.
Next there are the disaccharide sugars. Disaccharides are made when two monosaccharide molecules line up so that the OH groups are adjacent. This then undergoes a condensation reaction thus losing a molecule of water. The bond formed is called a glycosidic bond. The reaction can be reversed by adding water to the disaccharide, which would then be called a hydrolysis reaction. The general formula for disaccharides is: C H O .
Different disaccharide sugars have different functions and are made from hexose sugars. Take for example sucrose, which is formed from glucose and fructose; it is used in the sugar that we buy in shops because it is found in sugar beet and sugar cane. Its function is to be transported around plants and act as a storage compound in some plants such as onions. Another example of a disaccharide is lactose. This is formed from glucose and galactose and is found in milk. Its function is to act as a source of energy for suckling mammals. The final example for disaccharides is maltose, made from two glucose molecules. It is found in germinating seeds and its function is to act as an energy source for germinating plants.
Last but not least, there are the polysaccharides, which are non-sugars. They are made up of polymers formed by glycosidic bonding in a process that is similar to the formation of disaccharides. The most common polysaccharides known are starch, glycogen and cellulose, which are all polymers of glucose.
Starch acts as an important storage molecule in plants. It is well suited to this function because it is insoluble, compact, is easily hydrolysed and does not become involved in chemical reactions easy. Glycogen is an energy storage molecule in animals and is also found in the cytoplasm of bacterial cells. It is well suited to its function because it takes up very little space and is readily hydrolysed to glucose. Finally there is cellulose, which is an important structural component of plant cell walls. There are other polysaccharides, which are not as well known, however are still important. Here are just a few: callose (polymer of glucose found in plants), inulin (polymer of fructose found as a storage carbohydrate in some plants) etc.
All of these monosaccharides, disaccharides and polysaccharides are all of biological significance when talking about sugars. They all link under carbohydrates and each one has an important function, whether it be in animals or in plants.
Rosemarie Jeszke
