All animals feed on ready made food and are holozoic in their mode of nutrition. The main sources of food for animals are plants, plant products and other animals. Food is used to provide energy for life activities and to provide new materials for growth. It is also needed for repairing and replacing of worn out tissues.
Classes of food
The major classes of food substances are carbohydrates, protein and fats. Those regarded as welfare food substances are vitamins, mineral salts and water because they are essential for the well being of an individual.
1. Carbohydrates: Carbohydrates are made up of carbon, hydrogen and oxygen. Some examples of carbohydrates are sugars, starch, and cellulose. Carbohydrates are the major sources of energy for the body. They are also used to build up certain parts of the body like the exoskeleton of insects (arthropods).
Types of carbohydrates
a. Monosaccharaides: These are simple reducing sugars or six carbon sugars with the formula (C6H12O6). Examples are glucose, fructose and ribose.
b. Disaccharides: These consist of two monosaccharide units in each molecule. Examples are sucrose, maltose and fructose. They are non-reducing sugars with molecular formula of C12H22O11.
Lactose ………………………………….. Glucose+ Galactose
c. Polysaccharides: These are complex carbohydrates with many monosaccharide units. The formula is (C 6 H 10 0 5 ) n . Examples are glycogen (animal starch) and cellulose found in plant cell walls.
Sources of carbohydrates
Cassava, yam, rice, millet, wheat, potato, maize.
(i) They provide energy and warmth to the body.
(ii) They are useful in synthesis of fats and oils.
2. Proteins: These are complex organic compounds made up of units of amino acids. They are made up of carbon, hydrogen and nitrogen. Sulphur and phosphorus may be present.
Milk, meat, egg, beans, groundnut and fish.
They are required for:
(i) growth and repair of damaged tissues.
(ii) providing energy and heat, when there is shortage of carbohydrates and fats.
(iii) manufacturing of enzymes and hormones.
3. Fats and oils: These are called lipids and are made up of fatty acids and glycerol. They consist of carbon, hydrogen and oxygen. Fats contain saturated organic acids while oils contain unsaturated organic acids.
Butter, beef, groundnut oil, pork, palm oil.
(i) They provide more energy than carbohydrates.
(ii) They serve as heat insulators in mammals.
(iii) They serve as storage of energy in the time of need.
4. Vitamins: These are complex organic compounds required in small quantities for proper and healthy functioning of the body. Vitamins are either fat soluble or water soluble and a diet deficient in a vitamin causes deficiency diseases (see Table 3).
Fresh vegetables, oranges, eggs, liver, milk and oil.
5. Mineral salts: These are needed in minute quantities for growth, protection and are also used for regulation of metabolic processes in the body. (See Table 4).
Liver, eggs, green vegetables, beans, egg yolk.
6. Water: Water is an inorganic liquid made up of hydrogen and oxygen in the ratio 2:1. Water constitutes about 75% of the human body. Water has the following functions:
(i) It provides the medium for all chemical reactions to occur in living organisms.
(ii) It is a medium for transporting food and other substances.
(iii) Water is necessary for the removal of excretory waste products like urea from the body.
(iv) It forms a major part of protoplasm.
(v) The digestion and absorption of food substances require the presence of water.
This is a complete diet that contains adequate proportions of the different classes of food which can provide the body with all its needs. It must have the right amount of carbohydrates, proteins, fats and oils, vitamins, mineral salts, water and roughage. If a child is fed with a meal rich in carbohydrate and deficient in protein, he suffers from kwashiorkor.
The symptoms are swollen limbs, cracking of the skin, weak liver and protruded abdomen. A balanced diet provides the body with energy. It is needed for normal growth and repairs of worn out tissues. It offers resistance to the body.
1. Test for carbohydrates
a. To test for starch: Make a solution of starch suspension, e.g., pap in a test-tube. Add iodine solution to it. Blue-black colour is observed, indicating the presence of starch.
b. To test for a simple reducing sugar
(i) Fehling’s test: Mix equal amounts of Fehling’s solution and glucose in a test-tube and place in a boiling water bath, formation of orange-red precipitate indicates the presence of simple reducing sugars.
(ii) Benedict’s test: Add Benedict’s solution to glucose in a test tube and place this in a boiling water bath. A brick red precipitate indicates the presence of simple sugars.
(c) To test for complex sugar: Add dilute hydrochloric acid to the solution of a complex sugar in a test tube. This hydrolyses the complex sugar to simple sugars. To neutralise the excess acid, add dilute caustic soda to the above mixture in the test-tube. Then add equal amount of Fehling’s solution and place the test tube in a boiling water bath. An orange red precipitate indicates the presence of simple sugars.
2. Test for proteins
a. Millon’s test: Add few drops of trillion’s reagent to a protein food substance, e.g., egg albumen. A white precipitate which turns red on heating confirms the presence of proteins.
b. Biuret’s test: Add few drops of sodium hydroxide (NaOH) and copper sulphate (CuSO4) to any protein source and shake the mixture. A purple or mauve colour confirms the presence of protein.
c. Xanthoproteic test: To a protein solution in a test, add concentrated nitric acid (HNO3). A white precipitate forms which turns yellow on heating. Add a few drops of ammonia solution. On cooling the precipitate, it turns orange.
3. Tests for fats and oils
a. Translucent paper test: Put a drop of oil, e.g., groundnut oil on a piece of paper. The translucent spot indicates the presence of fat and oil.
b. Sudan II test: Add a few drops of Sudan II solution to some oil or mashed food containing fats or oils. A red colouration is observed, which turns into a black precipitate on boiling. This confirms the presence of fats and oils.
c. Emulsion test: Add ethanol to fatty substances and allow to settle. Pour the ethanol on water, a white or milky emulsion indicates the presence of fat.
d. Osmic acid test: Add osmic acid to fats and shake. Fat droplets turn black, confirming the presence of fats and oils.
Enzymes are organic catalysts secreted by a special group of cells in a living body and are responsible for controlling all the metabolic reactions taking place in a living organism.
Classes of enzymes
Digestive enzymes are classified according to the types of food on which they act. The main groups of digestive enzymes and the type of food on which they act are:
Enzymes may also be classified according to the processes in which they are involved. Examples:
Characteristics of enzymes
(i) Enzymes are soluble.
(ii) They are biological catalysts which are protein in nature.
(iii) They are specific in their actions, e.g., amylases act only on carbohydrates, proteases act on proteins.
(iv) Enzymes are sensitive to temperature, they function best at a definite optimum temperature (35 degree C to 40 degree C). The optimum temperature for enzymes to work best is 37 degree C.
(v) Most enzymes bring about reversible reactions.
(vi) Some enzymes need co-enzymes to activate them.
(vii) Enzymes are sensitive to the degree of acidity and alkalinity of the surrounding medium or environment. Some require alkaline medium, while others require acidic medium.
(viii) They speed up chemical reactions in the bodies of living organisms but remain unchanged at the end of the reaction.
(ix) Enzymes can be inactivated by inhibitors or poison, e.g., cyanide and mercury.