Carbohydrates are important in cells as a useable energy source and form important parts of the cell wall in plants. Common carbohydrates are sugars and starch. The basic structure of all carbohydrates is the same. They are all made up of carbon and hydrogen.

Monosaccharides & Disaccharide's

Specification Reference

"Know the difference between monosaccharides, disaccharides and polysaccharides"
Monosaccharides are sometimes referred to as simple sugars. They have the general formula (CH2O)n where n can be any number but is usually low.

Disaccharides are made up of two monosaccharides joined together by a glycosidic bond in a condensation reaction. These are sometimes referred to as the double sugars and have the general formula (C6H10O5)n

Hexose, Glucose & Pentose Ribose Structure

Specification Reference

"Know the structure of the hexose glucose (alpha & beta) and the pentose ribose"
Glucose comes in two different forms known as alpha-glucose and beta-glucose. These two isomers are caused by the different arrangements of the atoms on the side chains of the molecule. The change is only very subtle but gives the molecules very different properties. For the exam it is important to know the exact structures of the molecules and the differences between each.

Formation & Properties of Disaccharides and Polysaccharides

Specification Reference

"Understand how monosaccharides (glucose, fructose, galactose) join to form the disaccharides (sucrose, lactose, maltose) and the polysaccharides (starch formed from amylose and amylopectin; glycogen) through condensation reactions forming glycosidic bonds and how these can be split through hydrolysis reactions."
Disaccharides are formed when two monosaccharides join together in a condensation reaction to form a glycosidic bond. A condensation reaction involves the elimination of one molecule of H2O.
  • The use of numbers shows us which carbon atoms are involved in the glycosidic bond. eg: 1,4-glycosidic bond.
  • The following table shows us which monosaccharides join together to form each disaccharide:
  • Monosaccharide Disaccharide
    Sucrose a-glucose + fructose
    Lactose a-glucose + b-galactose
    Maltose a-glucose + a-glucose
    These are polymers containing many monosaccharides joined together by glycosidic bonds through condensation reactions. The structure of polysaccharides give them the following properties which make them ideal as storage molecules within the cell.
    • They can form compact molecules which take up very little space.
    • They are physically and chemically inactive, so they do not interfere with other functions of the cell.
    • They have little solubility in water so have no effect on water potential and cause no osmotic movements.

    The glycosidic bonds in the polysaccharide can be broken down to release monosaccharide units for cellular respiration. The bond is split through a process known as hydrolysis (water is added to the bond).

    Carbohydrates as Energy Stores
    Starch is an important energy store in plants. The sugars produced by photosynthesis are rapidly converted into starch which is an insoluble and compact material that can be easily broken down. All starch is made up of alpha-glucose but comes in two different forms. These different forms are caused by different carbon atoms being used in each glycosidic bond.
    • Amylose is the first type of starch. This is an unbranched polymer which forms a straight helix shape as the chain lengthens. These chains are made up purely of 1,4-glycosidic bonds.
    • Amylopectin is the second type of starch. This is a branched molecule made up of 1,4-glycosidic bonds and some 1,6-glycosidic bonds. It is the 1,6 bonds that cause the branching in the molecule which results in chains being more easily removed which is especially useful when energy is required quickly.

    This combination of straight and branched molecules means that starch is a good source of energy for athletes. Amylose provides long release energy whilst amylopectin provides shorter release energy.
    • Glycogen is another type of storage molecule. Chemically it is very similar to amylopectin; it is made up alpha glucose only and is very compact. However, glycogen molecules have more 1,6-glycosidic bonds giving the molecule more side branches. As a result it is a molecule which can be broken down very quickly making it suitable for use in metabolically active organisms such as animals.

    Carbohydrates in Plants
    Polysaccharides are also very important for plants. They provide the main energy source in plants and are also key structural materials.

    Cellulose is an important structural material in plants. It is found in the cell wall and provides the cell with strength, protection and support.

    The molecule is made up of beta glucose molecule in which every other molecule is inverted to allow bonding to take place in a straight and orderly fashion. The link of the beta glucose in this way means that the hydroxyl groups stick out both sides of the molecule.

    This means that hydrogen bonds can form between the partially charged hydrogen and hydroxyl atoms. This is known as a crosslink and hold chains firmly together.
    • Many of these hydrogen bonds form, making cellulose a material with considerable strength. Cellulose does not spiral or coil, instead it remains as a long straight chain giving rise to the above properties and functions.

    Structure & Function

    Specification Reference

    "Understand how the structure of glucose, starch, glycogen and cellulose relates to their function."
    • Glucose stores energy, can form compact molecules, is chemically inactive and is not soluble in water so causes no osmotic movements. This makes it suitable as a storage molecule in cells.
    • Starch is made up of amylose and amylopectin meaning that it can provide both short release and long release energy.
    • Glycogen is made up of mostly 1,6-glycosidic bonds meaning it has a highly branched structure. This allows molecules to be easily removed for energy production making it a suitable storage molecule for metabolically active organisms such as animals.
    • Cellulose is made up of beta glucose and every alternate molecule is inverted. This allows bonding to take place in straight lines and therefore allows hydrogen bonds to form resulting in a strong and rigid material suitable for use in the cell wall.