Basic elements of life

The macromolecules of life are large organic compounds that are made up of many smaller compounds. Examples are carbohydrate, fats, proteins and nucleic acids.

1. Carbohydrates

Carbohydrates consist of carbon, hydrogen and oxygen (CHO) and include polysaccharides, disaccharides and monosaccharides. The polysaccharides include starch, cellulose and glycogen. Each polysaccharide is composed of many molecules of monosaccharides.

Starch is present in plants, plant grains and seeds. It can be broken down by smaller compounds by the action of the enzymes present in the saliva and the pancreatic juice.

Glycogen is a form of sugar that is stored in the liver, muscles and brain. When the blood sugar concentration falls below normal, the liver converts glycogen to glucose. Similarly, if the blood sugar is above normal, the liver converts the excess into glycogen.

Cellulose is a component of plant cell walls. It is a hard plant fibre for which mammals have no enzyme that can digest it. Herbivores have certain bacteria in their digestive tracts that can digest cellulose. In return, the bacteria receive nutrients and protection from the herbivores.

Disaccharides are sugars that are made from a combination of two monosaccharides with the release of a molecule of water.

C₆H₁₂O₆ + C₆H₁₂O₆ ® C₁₂H₂₂O₁₁ + H₂O

monosaccharide monosaccharide disaccharide water

A disaccharide can be broken into monosaccharides by the action of an enzyme or dilute acid.

C₁₂H₂₂O₁₁ + H₂O ® 2C₆H₁₂O₆

Maltose dilute acid glucose.

Monosaccharides are also known as reducing sugars because they reduce cupric acid in Fehling’s solution to cuprous oxide.

Tests for Sugar

(a) Starch

Mix cassava flour in water in a test-tube. Add a few drops of iodine solution. The colour turns dark blue.

(b )Glucose

Benedict’s test

Make a small solution of glucose in a test-tube. Add a soluble solution of copper sulphate. Heat the mixture. The heated solution turns into a brick red precipitate.

OR

To a solution of glucose, add equal amounts of Fehling’s solution A and B. Warm the solution. A red precipitate is formed.

© Disaccharides

Take a solution of sucrose in water. Add a few drops of dilute HCl and boil. The acid hydrolyses sucrose to a monosaccharide. Then add a few drops of NaOH solution to neutralise the acid. Add equal quantities of Fehling’s solution A and B. Warm the solution gently. A red precipitate is formed.

2. Fats

Fats, like carbohydrates, are made up of carbon, hydrogen and oxygen (CHO). They are insoluble in water and form soaps when boiled with caustic soda.

Fats contain a lot of water. The camel hump and the chick yolk sac contain fat that acts as a source of water. Fats are the major source of heat energy as well as insulation material for animals living in cold climates.

Animal fats such as butter and lard are called saturated fats because they are solid at room temperature. Plant fats remain liquid at room temperature. They are called unsaturated fats. Some plant-derived fats, like peanut butter and margarine are hardened by the addition of hydrogen in a process called hydrogenation.

Test for fats

1. Mix 2 ml of water with 2 ml of groundnut oil in a test-tube and add a few drops of Sudan 111 reagent. Shake. A red layer upper layer over the water. The red layer consists of fat globules that are less dense than the water. Sudan 111 stains fat but leaves the water unstained.

2. Add 2 ml of water to 2 ml of absolute ethanol in a test-tube and shake vigorously. Add 4 ml of cold water to the solution and shake the emulsion. Droplets of fat that shine in light are formed.

3. Proteins

Proteins are made up of carbon, hydrogen, oxygen, nitrogen, and several other elements, such as phosphorus and sulphur. Proteins are essential constituents of the tissues of living things. They are necessary for the growth and repair of the body. They form the structural building blocks of the body and play an important function as enzymes.

Test for proteins

Biuret reaction

Add a few drops of 1% CUSO₄ to a solution of egg white. Then carefully add a few drops of 40% NaOH. Mauve colour is formed.

Millon’s reagent

Mash a piece of lean meat in a mortar and transfer it a test-tube. Add a few drops of Millon’s reagent and 1 ml of water. Boil. A brick red colour forms.

Study Questions