4.2 - Carbohydrates

Organic compound is a large class of chemical compound that contains carbon atoms. One or more carbon atoms are covalently bonded to atoms of other elements which can be hydrogen, nitrogen or oxygen atoms. The four major categories – carbohydrates, proteins, lipids and nucleic acids are found in all living things. They are macromolecules made up of smaller molecules called monomers.

Figure 3 - Carbohydrates such as rice, bread, grains and potatoes are a staple food to us humans. They provide energy needed for daily activities. The ratio of hydrogen to oxygen atoms is usually 2:1. The chemical formula of carbohydrate is (CH₂O) ₓ.

Types of Carbohydrates

There are three types of carbohydrates – monosaccharides, disaccharides and polysaccharides. Monosaccharides are the monomers that make up the carbohydrates. They are the simplest form of sugar and the basic units of all forms of carbohydrates. Many monosaccharides can combine together to form disaccharides or polysaccharides. Generally, they are crystalline solids at room temperature, taste sweet and soluble in water. The most commonly known examples are glucose, fructose and galactose.

• Glucose , also called dextrose, belongs to aldose family. It can be found in fruit, vegetables, rice and white bread. It is used by nearly all living organisms.
• Fructose is the sweetest carbohydrate. It can be found honey, corn syrup and naturally in nectar from flowers.
• Galactose is found in milk and dairy products made from milk. It is rarely existed in monosaccharide form (normally found as disaccharides)

Figure 4 - Monosaccharides have 6-carbon ring structure.

All monosaccharides are reducing sugars due to the presence of an aldehyde group. Due to this property, the presence of monosaccharides can be tested by using Benedict's solution. Blue copper (II) sulphate can be reduced to form brick-red precipitate copper (I) oxide.

Figure 5 - Benedict's solution is blue in color because of the presence of copper (II) ions. Reducing sugars reduce Cu²⁺ ions to Cu⁺ ions.

Disaccharides are sugars formed when two monosaccharides are joined together by glycosidic bonds. The process is called condensation. In the process, water molecule is produced as by-product. The backward process is called hydrolysis where water molecule is needed to break the glycosidic bonds. The examples of disaccharides include maltose, sucrose and lactose.

• Glucose + glucose ‹―› Maltose + water
• Glucose + fructose ‹―› Sucrose + water
• Glucose + galactose ‹―› Lactose + water

Maltose can be found in grains. Sucrose is found naturally in fruits and vegetables. Lactose can be found in milk, yoghurt and cheese. Lactose and maltose are reducing sugars and therefore can turn Benedict's solution to brick-red precipitate. Meanwhile, sucrose is a non-reducing sugar and hence the Benedict's solution remains blue.

Figure 6 - Sucrose is formed by combining 1 glucose molecule with 1 fructose molecule via condensation.

Figure 7 - Maltose is formed by combining 2 glucose molecules via condensation.

Figure 8 - Lactose is formed by combining 1 glucose molecule with 1 galactose molecule via condensation.

Polysaccharides are complex carbohydrates formed when many monosaccharides are joined together via condensation process. They have the following chemical properties:

• I nsoluble in water because of their large molecular sizes.
• Not sweet in taste
• Do not form crystals
• Not osmotically active in cells

The examples of polysaccharides are starch, cellulose and glycogen. They can be hydrolyzed under acidic conditions, high temperature and enzymatic actions.

Figure 9 - Starch (left) is made up of amylose and amylopectin. Plants store carbohydrates in the form of starch. Glycogen (middle) is a multibranched polysaccharide. Glycogen is more highly branched compared to amylopectin. Animals store carbohydrates in the form of glycogen. Cellulose (right) is made up of linear chains of glucose units.