Analysing the Classes of Food


1. Food is collection of chemicals taken into an organism for the following purposes:

(a) It provides energy for cellular activities.

(b) It provides raw materials for growth and repair of worn-out tissues.

(c) It keeps the organism healthy.


2. The process of making or obtaining food is called nutrition.


3. There are seven classes of food:

(a) Carbohydrates         (e) Minerals

(b) Proteins                   (f) Fibre

(c) Fats                         (g) Water

(d) Vitamins



1. Carbohydrates are food substances consisting of carbon, hydrogen and oxygen in their molecules. The H : O ratio in carbohydrate molecules is 2 : 1.


2. Carbohydrates include sugars, starch, glycogen and cellulose.


3. Sugars are crystalline compounds that taste sweet and are water-soluble.


4. Examples of sugars are glucose (grape sugar), sucrose (cane sugar), lactose (milk sugar) and maltose (malt sugar).


5. Starch is stored in plant cells. Rice, potatoes and tapioca contain plenty of starch.


6. Glycogen is also known as 'animal starch' because it is stored in animal cells. Glycogen is stored mainly in the liver and muscles.


7. Cellulose froms the greater part of the cell walls of plants.


8. Starch, glycogen and cellulose are all made up of glucose molecules.


9. Starch and glycogen can be readily converted to glucose when needed.


10. Importance of carbohydrates:

(a) Main source of energy for cell activities

(b) Form supporting structures, such as cell walls in plants

(c) Can be converted to protein and fats



1. Proteins are food substances that contain carbon, hydrogen, oxygen and nitrogen. Sulphur and phosphorus are present too.


2. Every protein molecule is made up of basic units called amino acids.


3. Meat, fish and egg are animal proteins while beans, nut and seeds are plant proteins.


4. Importance of proteins:

(a) Needed for growth and repair of worn-out or damaged cells

(b) Used for the synthesis of enzymes and some hormones

(c) Important component of antibodies which help the body to fight diseases

(d) A source of energy


5. Insufficient protein in a child's diet may lead to a disease called kwashiorkor.



1. Fats consist of carbon, hydrogen and oxygen. Unlike carbohydrates, however, fats contain less oxygen in proportion to hydrogen.


2. Fats which are liquid at room temperature are called oils. Examples are palm oil and coconut oil.


3. Each fat molecule consists of a glycerol molecule and three fatty acid molecules.


4. Food rich in fats include butter, margarine, cooking oil and fatty meats.


5. Importance of fats:

(a) An efficient source and storage of energy. Each gram of fat provides about twice the amount of energy supplied by carbohydrates or proteins. Excess fats are stored as fatty tissue under the skin and around internal organs such as the kidneys and the heart to protect the organs from physical injury. 

(b) Act as solvent for fat-soluble vitamins A, D, E and K

(c) An important component of cell membrane

(d) Fats under the skin also act as an insulator to reduce heat loss from the body.


6. A diet rich in fats, however, increases the risk of heart disease.



1. Vitamins are organic compounds that are required in very small amounts for maintaining good health.


2. Vitamins are not energy providing foods.


3. Vitamins are divided into water-soluble vitamins and fat-soluble vitamins.


4. The water-soluble vitamins are vitamins B and C.


5. The fat-soluble vitamins include vitamins A, D, E and K.


6. Diseases caused by a lack of vitamins in our body are called vitamin deficiency diseases.


7. We should eat a varied diet to obtain all the different types of vitamins that our body needs.




Deficiency disease


Tomatoes, carrots, fish liver oil, green vegetables, egg yolk, liver

- For night vision or vision in dim light

- Healthy skin

- Night blindness

- Skin infections


Whole grain bread, cereals, eggs, milk, green vegetables


- Healthy skin

- Healthy nervous system

- Development of red blood cells

- Release of energy from carbohydrates 

- Beri-beri (paralysis)

- Dermatitis (skin disorder)

- Pellagra (mental disorder and skin disease)

- Anaemia (deficiency in red blood cells)


Fresh fruits, green vegetables

 - Healthy tissues

- Healing of wounds

- Resistance to diseases

- Scurvy (bleeding gums, internal bleeding in muscles and skin)


Liver, fish liver oil, egg yolk, formed in skin by sunlight

- Promotes absorption of calcium and phosphorus

- Formation of healthy teeth and bones

-  Rickets (bowed legs and knock knees)

- Osteomalacia (softening of bones)


Whole grain, wheat germ, vegetable oil - May be needed for reproduction - Sterility (failure to reproduce)


Green vegetables, tomatoes, egg yolk, made by bacteria in the large intestine

- Essential for bloof clotting

-Prolonged bleeding



1. Minerals are inorganic chemical elements that are needed in small amounts.


2. Minerals are required to regulate body processes.


3. A lack of minerals causes deficiency diseases.




Deficiency disease

Calcium Milk, cheese, eggs, anchovies, yogurt, green vegetables

- Building healthy bones and teeth

- Clotting of blood

- Normal functioning of muscles and nerves

- Rickets

- Delayed clotting

- Muscular spasms

Sodium Table salt, salty fish, salty eggs, cheese

- Normal functioning of nerves

- Maintenance of tissue fluids

- Muscular cramps


Iron Liver, kidneys, eggs, green vegetables

- Needed for the formation of red blood cells

- Activates certain enzymes

- Anaemia


Iodine Iodised salt, sea fish, shellfish

- Needed to make hormone of the thyroid gland

- Control body growth

- Goitre (swelling of the thyroid gland in the neck)


Phosphorus Milk, whole grain bread, cereals, meat

- Formation of healthy bones and teeth

- Proper functioning of muscles

- Formation of chemical substances that store energy

- Rickets

- Weakness


Potassium Meat, milk, fish, mushrooms, bananas

- Normal functioning of nerves

- Maintenance of tissue fluids

- Regulation of heartbeat

- Weak muscles

- Paralysis



Dietary fibre (Roughage)

1. Dietary fibre consists mainly of cellulose that is indigestible because we do not have enzymes to digest it.


2. Fruits and vegetables are rich in dietary fibre.


3. Importance of dietary fibre:

(i) Provides bulk to the intestinal contents

(ii) Stimulates peristalsis (rhythmic muscular contractions passing along the digestive tract)


4. Lack of dietary fibre in the diet leads to constipation (failure to pass motions).



1. Water is very essential for life because 60% to 70% of our body weight is water.


2. We can survive for several weeks without food but would die in a few days without water.


3. Importance of water:

(a) Acts as a solvent in which chemicals dissolve

(b) Acts as a transporting agent for digested food, excretory products, hormones and antibodies

(c) Takes part in biochemical reactions (for example: digestion)

(d) A major component of blood, body fluids, digestive juices and urine

(e) Regulates body temperature by evaporation of sweat from the skin


4. We require at least 1.5 litres of water daily to replace water lost in sweat, urine, faeces and expired air.


Food tests

1. We can carry out tests to find out substances contained in food.


2. We use different tests to test for different food substances.


3. Table below compares carbohydrates, proteins and fats.






 C, H, O

 Ratio of H : O is 2 : 1

 C, H, O, N often S   and P

 C, H, O

 Ratio of H : O is   more than 2 : 1

Basic units


 Amino acids

 Fatty acids and   glycerol


 - Supply energy, 17   kJ per gram

 - Provide structural   support, e.g.   cellulose in cell wall

 - Supply energy, 18   kJ per gram

 - From cells,   enzymes, antibodies   and hormones

 - Supply energy 39   kJ per gram

 - Provide heat   insulation,   waterproofing and   buoyancy

Chemical tests

 Benedict’s test for   glucose. A brick-red   precipitate indicates   presence of glucose.

 Iodine test for   starch. A blue-black   colour indicates the   presence of starch.

 Millon’s test for   proteins. A red   coagulation indicates   the presence of   proteins.

 Emulsion test for   fats.

 A milky solution   indicates the   presence of fats.

Examples of food

 Rice, bread and   potato

 Meat, fish and eggs

 Cooking oil, butter   and margarine