Carbohydrates and Lipids

Note: This is based on the 2016 syllabus provided by CIE.

Carbohydrates and lipids have important roles in the provision and storage of energy and for a variety of other functions such as providing barriers around cells: the phospholipid bilayer of all cell membranes and the cellulose cell walls of plant cells. 

1. Describe the ring forms of alpha-glucose and beta-glucose

• The -OH group lies below the plane on the first carbon atom in an alpha-glucose but it lies above in a beta-glucose

2. Describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose.

• A disaccharide is formed when two monosaccharides are joined together by condensation where a water molecule is removed to form a glycosidic bond
• The disaccharide maltose is formed when two glucose molecules join together, sucrose is formed when a glucose molecule joins with a fructose molecule and lactose is formed when glucose is joined with galactose
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• As seen in the diagrams above, sucrose is formed when a glucose molecule forms a glycosidic bond with a fructose molecule
• The -OH group on the glucose bonds with the -H group from the -OH in fructose to form a water molecule, H2O
• A glycosidic bond is formed when that water molecule is removed
• The glycosidic bond formed is 1, 4 linkage
• A polysaccharide is a polymer where its subunits are monosaccharides and many monosaccharides are joined together by glycosidic bonds
• Some polysaccharides include starch, cellulose and glycogen
• The formation of the glycosidic bond follows the same process as in the formation of a disaccharide

3. Describe the breakage of glycosidic bonds in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test

• The breakage of glycosidic bonds is done by a hydrolysis reaction which includes an addition of a water molecule
• Polysaccharides can be broken down to disaccharides and to their monosaccharides
• Disaccharides can be broken down to monosaccharides
• An example of dissacharide being broken down is sucrose, where its monomers are a glucose molecule and a fructose molecule
• Sucrose is a non-reducing sugar so if tested with Benedict's reagent, there will be no change in colour
• However, if sucrose is broken down into its monomers which are glucose and fructose which are both reducing sugars, there will be a positive result when tested with Benedict's test
• Therefore, there is a need to perform a non-reducing sugar test on sucrose
• Sucrose is hydrolysed with dilute hydrochloric acid for a few minutes to hydrolyse the glycosidic bonds 
• Then the solution is neutralised by adding solid sodium hydrogen carbonate until it stops fizzing
• The new solution is tested with Benedict's test which will give a brick-red precipitate, showing that reducing sugars are present

4. Describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate…