Carbohydrates
Carbohydrates are the most common compounds we encounter in daily life. The doors we use in our houses, furniture in the form of wood, most of our food, clothes made from cotton and linen are carbohydrates.
- Carbohydrates are the most abundant organic compounds in nature, although they are scarce in living organisms.
- Carbohydrates are the main energy and nutrition source for all living organisms including human beings.
- Some of the more common carbohydrates in our daily life are;
- cellulose which exists in plant tissue and is a raw material for paper,
- starch which is an energy resource for plants,
- glycogen which is used as an energy resource in animals and humans,
- Glucose and fructose which exist in fruit and honey
- Lactose, maltose and sucrose.
- Carbohydrates are mostly produced by green plants. In the leaf of a green plant carbon dioxide from the air and water, taken in from the roots of the plant, are converted into glucose by the catalysts chlorophyll and sunlight. This process is known as photosynthesis.
6C02(g) + 6H20(l) à C6H1206(s)+602(g)
As can be seen from the reaction, plants take carbon dioxide from the air and convert it into oxygen. Living organisms can then use this oxygen to break down carbohydrates to produce energy in the process of respiration.
C6H1206(s) + 602(g) à6C02(g) + 6H20(l) + Energy
Structure of Carbohydrates:
The general formula of the most carbohydrates is Cn(H2O)m.
Formaldehyde (CH2O) is the simplest carbohydrate.
Carbohydrates contain the functional groups; alcohol –OH, aldehyde 
and ketone 
- Glucose:
Molecular formula of glucose is C6H12O6. As its structural formula reveals, there are two functional groups in structure of glucose: carbonyl (-COH) and hydroxy (-OH) group. So it shows both aldehyde and alcohol properties.
Physical properties: Glucose is a colorless crystal with a sweet taste. As it contains hydrogen bonds, glucose is highly soluble in water. It is found in fruits, grapes and honey. Glucose is the main energy source of the cells.
Chemical Properties: Glucose contains the functional groups of both alcohols and aldehydes. So it undergoes the same reactions with alcohols and aldehydes.
1) Complete Oxidation: In cells, glucose is oxidized to carbon dioxide, CO2.
C6H12O6+6O2à6CO2+6H2O ΔH = - 2870 kj/mol
In the absence of oxygen, fermentation takes place:
C6H12O6à2C2H5OH+2CO2
2) Oxidation to acid: Glucose reacts with silver (I) oxide to give glucanic acid.
CH2OH(CHOH)4COH+Ag2OàCH2OH(COH)4COOH+2Ag
To check presence of glucose copper hydroxide can also be used. Solid copper hydroxide disappears in solution of glucose.
- Sucrose:
The most important commercial disaccharide is sucrose, ordinary table sugar. It is obtained from sugar cane and sugar beets. Its molecular formula is C12H22O11.
Physical properties: Sucrose is a white crystal having a sweet taste. It is very soluble in water. It melts at 186oC. It exists in the structure of the plants. 16-20% of sugar beet and 14-26% of sugar cane is sucrose.
In production of table sugar, first sugar cane and sugar beets are divided into small pieces and then they are treated with hot water. Being very soluble in water, sucrose particles dissolve in water. To get rid of the impurities in the solution, carbon dioxide and copper hydroxide are used. Then pure sucrose is obtained by evaporation of water from the solution. Refined sugar is obtained by repeating the dissolution and crystallization processes.
Chemical Properties:
1) Sucrose is formed from combustion of one glucose and one fructose with loss of 1 water molecule.
C6H12O6+C6H12O6àC12H22O11+H2O
2) Sucrose can easily be hydrolyzed in presence of acid catalyst.
C12H22O11+H2OàC6H12O6+C6H12O6
- Starch:
Starch is the most important polysaccharide and is formed by combining glucose molecules with a glycosidic linkage. Corn, potatoes, rice and the roots of plants and seeds are the main sources of starch. Starch is an essential nutritional resource. One molecule of starch consists of thousands of glucose molecules. Starch is insoluble in water.
When starch is heated with a dilute acid, it undergoes a hydrolysis reaction resulting in glucose as the final product.
(C6H5O5)n 
(C6H5O5)n 
(C6H5O5)n
Starch dextrin maltose
(C6H5O5)n
glucose