Carbohydrates

They are substances which contain carbon, hydrogen and oxygen with the general formula of $(\text{CHO})_n$ where $n$ is a real number.

1. They are either simple sugars or compound sugars.
2. The compound sugars are formed by condensation of simple sugar molecules.
3. They are hydrates of carbon from the proportion of hydrogen and oxygen in water.
4. The basic carbohydrate unit is thus a sugar which is the derivative of a polyhydrosol alcohol.
5. Alcohol is the paraffin compound with hydrogen atom replaced by the univalent hydroxyl (OH) group.
6. Paraffin is aliphatic or chain compounds of carbon and hydrogen in which the carbon atoms are linked by single bonds to adjacent atoms, (see Example above). The carbohydrate contains several hydroxyl groups.
7. Some contain aldehyde (-CHO) group and others contain ketone group (-CO-)

Examples:

1. Glucose: is a pentahydroxyl alcohol with the aldehyde group.
2. Fructose: is the pentahydroxyl alcohol with ketone group.

Complex sugars are built from the basic sugar units called monosaccharides through the process of condensation polymerization.

Many sugars are reducing sugars and others are non-reducing sugars but give rise to reducing sugars on hydrolysis with enzymes or mineral acid (mostly dilute HCl).

NB: Carbohydrates are called reducing sugars because they act as reducing agents supplying electrons from their functional groups i.e. the aldehyde and ketone groups which can reduce the Cu²⁺ ions to Cu⁺ ions which appear orange or yellow ppt (precipitate).

The true carbohydrates are saccharides with a combination of sugar units. These are divided into three main classes:

1. monosaccharides - with a single sugar unit
2. disaccharides — with two sugar units.
3. polysaccharides — with many sugar units.

The carbohydrates are further classified into simple and complex which is mainly based on their chemical structure and degree of polymerization.

Simple carbohydrates have one or two sugar molecules. They are abundantly found in milk products, beer, fruits, refined sugars, candies, etc.

Have general formula $C_nH_{2n}O_n$

1. All are reducing sugars

They are classified according to the number of carbon atoms e.g.

1. Trioses have 3 carbon atoms
2. Tetraoses have 4 carbon atoms
3. Pentoses have 5 carbon atoms
4. Hexoses have 6 carbon atoms
5. Heptoses have 7 carbon atoms

Glucose is an example of a carbohydrate monomer or monosaccharide. Other examples of monosaccharides include mannose, galactose, fructose, etc. The structural organization of monosaccharides is as follows:

Monosaccharides may be further classified depending on the number of carbon atoms:

1. Trioses ($C_3H_6O_3$): These have three carbon atoms per molecule. Example: Glyceraldehyde
2. Tetroses ($C_4H_8O_4$): These monosaccharides have four carbon atoms per molecule. Example: Erythrose.

Similarly, we have-

1. Pentoses,
2. Hexoses, and
3. Heptoses

Two monosaccharides combine to form a disaccharide. Examples of carbohydrates having two monomers include: Sucrose, Lactose, Maltose, etc.

Carbohydrates formed by the condensation of more than one monomer are called polysaccharides. By this convention, trioses, pentoses, hexoses are all monosaccharides.

Complex carbohydrates have two or more sugar molecules, hence they are referred to as starchy foods. In complex carbohydrates, molecules are digested and converted slowly compared to simple carbohydrates. They are abundantly found in lentils, beans, peanuts, potatoes, peas, corn, whole-grain bread, cereals, etc.

Polysaccharides are complex carbohydrates formed by the polymerization of a large number of monomers. Examples of polysaccharides include starch, glycogen, cellulose, etc.

1. Starch is composed of two components- amylose and amylopectin. Amylose forms the linear chain and amylopectin is a much-branched chain.
2. Glycogen is called animal starch. It has a structure similar to starch, but has more extensive branching.
3. Cellulose is a structural carbohydrate and is the main structural component of the plant cell wall. It is a fibrous polysaccharide with high tensile strength. In contrast to starch and glycogen, cellulose forms a linear polymer.