Form 1:
Nutrition in Plants and Animals

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Carbohydrates

Are compounds of carbon, hydrogen and oxygen in the ratio of 1:2:1.

They have a general formula (CH2O)n

Carbohydrates are grouped into three categories:

Monosaccharides

These are the simplest carbohydrates.

They include glucose, fructose and galactose.

Their general formula is C6H12O6.

Properties of Monosaccharides

They are sweet tasting

They readily dissolve in water

They are crystallisable

They are reducing sugars; monosaccharides reduce blue copper (II) sulphate inBenedict’s solution to red brown copper (I) oxide when heated.

Note:

Most fruits are sweet tasting because they contain a lot of monosaccharides.

Monosaccharide units can be combined to form complex carbohydrate molecules through a process known as condensation. Water molecules are produced in the process.

Functions of Monosaccharides

They are the chief respiratory substrate. They are broken down to release energy in the body.

They are condensed to form complex important carbohydrates.

Disaccharides

These are complex sugars formed by linking two monosaccharide units through condensation.

They have a general formula C12H22O11. The bond that holds two monosaccharide units is called glycosidic bond.

Examples of disaccharides include:

• Maltose-common in germinating seeds

• Sucrose-fruits and sugar cane. Sucrose is the form in which carbohydrates are transported in plants

• Lactose- found in milk

Monosaccharide units Disaccharides
Glucose+fructose Sucrose
Glucose+glucose Maltose
Glucose+galactose Lactose
Properties of Disaccharides

They are sweet tasting

They are crystallizable

They are water soluble

While they are non reducing sugars, some such as maltose is sugar reducing and is known as a complex reducing sugar.

They can be broken down into their constituent monosaccharide units through hydrolysis.

Hydrolysis is the process through which complex molecules are broken down in the presence of water molecules.

In living systems, hydrolysis is carried out by enzymes. However, in the laboratory,hydrolysis can be carried out by boiling the disaccharide in dilute acid such as hydrochloric acid.

Functions of Disaccharides

They are hydrolyzed into monosaccharides and respired on to yield energy

They are the form in which carbohydrates are transported in plants due to their soluble and inert nature.

Polysaccharides

These are formed through linking of numerous monosacchride units through condensation.

Their general formula is (C6H10O5)n where n is a very large number.

Properties of polysaccharides

They are non sweet

They do not dissolve in water

They are non crystalline

They are non-reducing sugars

Examples of polysaccharides

• Starch- Made by linking numerous glucose molecules. It is a form in which carbohydrates are stored in plants.

• Glycogen- Is a storage carbohydrate in liver and muscles of animals. It is broken down to glucose in animals when blood glucose falls.

• Cellulose- This is a structural polysaccharide in plants. It is a component of the cell wall

• Chitin- A structural carbohydrate found in cell wall of fungi and arthropod exoskeletons

Functions of polysaccharides

They are storage carbohydrates; their insolubility and inertness makes them ideal for storing carbohydrates.

They are structural carbohydrates e.g. cellulose forms the plant cell walls.

They can be hydrolyzed into monosacharides and be broken down to release energy.

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