1. 2 Carbohydrates

• The elements found in carbohydrates are carbon (C), hydrogen (H), and oxygen (O). They have twice as many hydrogen atoms as oxygen atoms.
• The disaccharide maltose is formed in a condensation reaction from two alpha glucoses.
• The disaccharide lactose is formed in a condensation reaction from alpha glucose and galactose.
• The disaccharide sucrose is formed in a condensation reaction from alpha glucose and fructose.
• Glycogen is a polysaccharide of alpha glucose. It is a heavily branched structure which helps it to hydrolyse more easily so it can release glucose for respiration quicker. The bonds in glycogen are alpha-1,4 and alpha-1,6 glycosidic bonds.
• Cellulose is a polysaccharide of beta glucose. It has a linear chain with beta-1,4 glycosidic bonds between the monomers. This makes cellulose very strong but difficult to break down into its constituent parts.
• Monosaccharides are simple sugars that cannot be broken down further into simpler molecules. There are three types of monosaccharides: trioses, pentoses, and hexoses.
• Starch is made up of amylose and amylopectin. Amylose is an unbranched polymer of alpha glucose joined by alpha-1,4 glycosidic bonds. Amylopectin is also a polymer of alpha glucose but some of the chains are branched due to alpha-1,6 glycosidic bonds. Starch is stored in plants as a source of energy.
• Polysaccharides have many repeating units of monosaccharides linked together by glycosidic bonds. They include starch (amylose and amylopectin), glycogen, and cellulose.
• Testing for reducing sugars:

  -Add Benedict's solution to the sample and heat

  -A colour change from blue to green/yellow/orange/red precipitate indicates the presence of reducing sugar.

  -If the sample remains blue, there is no reducing sugar present. Test for non-reducing sugar next.
• Testing for non-reducing sugar:

  -Boil a fresh sample with HCl for 5 mins to hydrolyse glycosidic bonds.

  -Neutralise the solution by adding sodium hydrogen carbonate.

  -Add Benedict's solution to the sample.

  -A colour change from blue to green/yellow/orange/red precipitate indicates the presence of a non-reducing sugar.
• Testing for starch:

  -Add 2 drops of iodine solution to your solution

  -A blue/black colour indicates the presence of starch whilst a yellow/brown colour indicates the absence of starch.
• Sucrose is a non-reducing sugar.
• A colorimeter produces quantitative results whereas the Benedict's test is qualitative.