Classwork Series and Exercises {Chemistry- SS3}: Carbohydrates

Introduction

Carbohydrates (saccharides) – Molecules consist of carbon, hydrogen and oxygen atoms. A major food source and a key form of energy for most organisms. When combined together to form polymers, carbohydrates can function as long term food storage molecules, as protective membranes for organisms and cells, and as the main structural support for plants and constituents of many cells and their contents.

Two Types of Carbohydrates

There are two major types of carbohydrates in foods: simple and complex.

1. Simple carbohydrates: These are also called simple sugars. They are crystalline, soluble in water and have a sweet taste. Structurally, they can be further divided into monosaccharides e.g. glucose, and disaccharides e.g. sucrose
2. Complex carbohydrates: These are also called polysaccharides. They are non-crystalline, insoluble and tasteless substances, e.g. starch and cellulose. Starches include grain products, such as bread, crackers, pasta, and rice.

Monosaccharides

A monosaccharide or simple sugar has a formula that is some multiple of CH₂O. For instance, glucose (the most common monosaccharide) has a formula of C₆H₁₂O₆. This is the smallest possible sugar unit. Examples include glucose, galactose or fructose. Monosaccharides cannot be split into smaller units by the action of dilute acids. Monosaccharides are classified according to the number of carbon atoms they possess: trioses have three carbon atoms; tetroses, four; pentoses, five; hexoses, six; etc. Each of these is further divided into aldoses and ketoses, depending on whether the molecule contains an aldehyde group (–CHO) or a ketone group (–CO–). For example glucose, having six carbon atoms and an aldehyde group, is an aldohexose whereas fructose is a ketohexose. These aldehyde and ketone groups confer reducing properties on monosaccharides: they can be oxidized to yield sugar acids.

When we talk about blood sugar we are referring to glucose in the blood; glucose is a major source of energy for a cell. In human nutrition, galactose can be found most readily in milk and dairy products, while fructose is found mostly in vegetables and fruit.

Disaccharides

Disaccharides are sugars or carbohydrates made by linking two monosaccharides. Two monosaccharides joined together by a glycosidic linkage. Disaccharides are also oligosaccharides. An oligosaccharide consists of a small number of monosaccaharide units (from about two to ten) joined together. This is a list of some disaccharides, including the monosaccharides they are made from and foods containing them. Sucrose, maltose, and lactose are the most familiar disaccharides, but there are others.

1. Sucrose (saccharose)
   glucose + fructose
   Sucrose is table sugar. It is purified from sugar cane or sugar beets.

2.   Maltose

glucose + glucose
Maltose is a sugar found in some cereals and candies. It is a product of starch digestions and may be purified from barley and other grains.

Polysaccharides

Polysaccharides can be composed of hundreds to thousands of monosaccharides combined together. These monosaccharides are joined together through dehydration synthesis. Polysaccharides have several functions including structural support and storage. When all the monosaccharides in a polysaccharide are the same type the polysaccharide is called a homopolysaccharide, but when more than one type of monosaccharide is present they are called heteropolysaccharides.

Examples include storage polysaccharides such as starch and glycogen, and structural polysaccharides such as cellulose and chitin.

Polysaccharides have a general formula of C_x(H₂O)_y where x is usually a large number between 200 and 2500.

Starch

Starch is an important polysaccharide which occurs naturally in green plants. Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. The commercial sources being maize, wheat barley, yam, rice, potatoes and cereals.

Preparation of Starch

The raw material to be used is peeled cassava tubers which should be washed and ground into pulp. Water is then added to the pulp to extract starch. It forms suspension and this can stay for sometime before the water above is decanted and starch residue allowed to dry.

Physical Properties of Starch

1. Starch is a white odourless, tasteless powder with the formula (C6H10O5)n
2. It is insoluble in cold water but soluble in hot water forming a viscous solution which sets into a jelly on cooling

Chemical Properties of Starch

1. Starch gives the familiar characteristics deep blue colour with iodine solution
2. Hot dilute acids hydrolyse starch into maltose and glucose
3. It does not reduce Fehling’s solution
4. It decomposes on heating in the presence of the enzyme diastase to form maltose sugar.

Test for Starch

Add a few drops of iodine to some boiled starch. A dark blue colouration which disappears on cooling results.

Uses of Starch

1. It is used for stiffening linen
2. It is used to produce ethanol and glucose
3. It is used mainly as food

Cellulose

Cellulose is the highest of the polysaccharides. It is the main component of plant cell walls and plant fibres. The principal industrial sources are cotton and wood each of which contains about 50% of cellulose. Other sources of cellulose for textile purposes are floxi china grass, hemp and jute.

Physical Properties of Cellulose

1. It forms transparent fibres when it is pure
2. It is insoluble in water and in most organic solvents

Chemical Properties of Cellulose

1. Cellulose can be completely hydrolysed to glucose by hot acids
2. Hydrolysis of cellulose can also be carried out readily by the enzyme cellulose which is produced by micro organisms present in the digestive system of termites and herbivorous animals.

Uses of Cellulose

1. It is used in the manufacture of explosives, surface coatings, paper, textiles and ropes
2. In the manufacture of gum, cotton and explosives

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