 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