Sugar molecules are found in nearly all eukaryotic and prokaryotic cells. They provide energy in the form of chemical energy which cells use in metabolic reactions Energy is released from sugar molecules as ATP energy in the process called respiration Animals get sugar from eating food containing sugar or starch Plants make sugar molecules from water and carbon dioxide in photosynthesis
There are different types of sugar. Glucose, fructose and galactose are made of a single sugar unit (or monosaccharide). They all have the same chemical formula.(they are isomers) C 6 H 12 O 6 They are called hexose sugars because they contain 6 carbon atoms. How many carbon atoms would a triose sugar contain? And a pentose sugar? They are relatively small molecules, larger than water, carbon dioxide and oxygen but smaller than proteins or starch.
The molecular structure of α glucose shows a ring of 5 carbon atoms and one oxygen atom. The sixth carbon atom is above the ring attached to carbon 5 This type of glucose builds up to make larger molecules of starch and glycogen
This is a different form of glucose, can you spot the difference. It is called β glucose The hydroxyl group on carbon 1 is above the ring (and the hydrogen is below) This tiny difference changes the properties of the molecule. When a long chain of β glucose molecules form it makes cellulose not starch
Another group of sugar molecules are called disaccharides Each one is composed of two monosaccharides joined together by a glycosidic bond. Some examples of disaccharides are: maltose, sucrose and lactose
Maltose is made from two α glucose molecules A condensation reaction occurs between the hydroxyl groups on carbon 1 of one molecule and carbon 4 of the other. A water molecule is formed and removed leaving two glucose residues bonded together
A hydrolysis reaction occurs in the digestion of maltose An enzyme causes a water molecule to re- enter the glycosidic bond, separating the two α glucose molecules and providing the H and OH groups to form two separate glucose molecules.
Add Benedict’s reagent to a small quantity of the unknown solution in a test tube and heat in a water bath to 80 0 C. If a reducing sugar is present the Benedicts reagent will change colour from blue to a brick red precipitate.
First test a sample of the solution with Benedict’s reagent as before. If there is no change of colour then in a fresh test tube, take a second sample of the solution and boil in a water bath for about 2 minutes with a few drops of 1M hydrochloric acid. Cool slightly and add an excess of sodium hydroxide to neutralise the acid. Then add Benedict’s solution and heat again. A brick red precipitate should form
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