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Food is a complex energy-rich organic matter which living organism feed on to obtain nutrients and substances necessary for life. Food substances are classified to six groups

(i) Carbohydrate     (ii) Protein       (iii) Fat and oils    (iv)  Mineral salts  (v) Vitamins     (vi) Water and roughages.


These are made up of carbon, hydrogen and oxygen. They have general formula (CxH2O)y. carbohydrate consists of simple sugar, starches, cellulose and glycogen. They are grouped into simple sugar or monosaccharide, double sugar/disaccharides and poly saccharide. The common sources include yam, cassava, potatoes, bread, cereals e.g. rice, maize e.t.c.

Simple sugar or Monosaccharide

These are the simplest sugar which consists of one molecule of simple sugars. They have general formula C6H10O6. Examples are glucose, fructose and ribose (C5H10O5) or galactose

Disaccharide/Reducing sugar

These are sugars which consist of two molecules of simple sugar which formed by condensation with the general formula C12H22O11. Examples are sucrose, maltose and lactose

Sucrose: It is a non-reducing sugar which is formed from a molecule of glucose and a molecule of glucose is the main source

Maltose: This is obtained from the condensation of two molecules of simple sugar. It is reducing sugar

Lactose (milk-sugar): It is a reducing sugar. It is obtained from the condensation of a molecule of glucose and a molecule of galactose


These are complex carbohydrates. Examples are starch, cellulose, chitin and inulin.

  1. Starch: It has the formula C6H10O5)n where n represent a large number. It is formed from the condensation of numerous molecules of simple sugar. Examples of the sources of starch are yam, cereals, cassava and bread
  2. Cellulose: This is composed of several condensed unit of monosaccharides. It makes the cell wall of plant source of cellulose include whole meal bread, cereals, fresh fruit and vegetables.

iii.       Glycogen (Animal starch): This is the form which animals store their carbohydrate usually in the muscle or liver

       Importance of Carbohydrate

  1. It provides energy required by animals for their daily activities
  2. It provides heat during it oxidation used in maintain the body temperature

iii.       It forms certain body part of arthropods (exoskeleton)

  1. The mucus which is an important lubricant in the body is formed carbohydrate



Proteins are complex molecules and are made of smaller unit called amino acids. Proteins have to be digested to amino acids before they are absorbed in the body of animal. The breakdown of proteins during digestion takes place in the following

Protein                   Peptone                   Polypeptide                   Amino acid

Protein is composed of carbon, hydrogen, oxygen, phosphorus and sulphur

Sources of protein include both animal source and plant source

The animal source includes milk, egg, fish, cheese, meat and chicken while the Plant sources are beans, groundnut, soya beans and melon.

Importance of Protein

  1. Protein is needed for growth of young ones
  2. It is used for the repair of worn out tissues or cells

iii.       It aids reproduction

  1. It is used for the production of enzymes
  2. It is needed for the production of hormone
  3. It is needed for body building57


Fat and oils are also called lipids. They consist of only carbon, hydrogen and oxygen. However, the amount of oxygen in each lipid is very little e.g. tristearin, is C57H10O6. Fats are solid lipid at room temperature. Plant sources of oil include groundnut, palm oil, soya beans oil; coconut oil and melon oil white animal sources of fat include butter, fish or cod oil.

Fat and oil are hydrolysed during digestion to fatty acid and glycerol

Importance of Fat and Oil

  1. Fat and oil provides more energy to animal than carbohydrate
  2. Fat supplies essential fatty acid to animal

iii.       Fat and oil act as solvent for fat soluble vitamins

  1. They help in the maintenance of body temperature
  2. They act as insulator to animal which help them to conserve heat


Mineral salts are food substances that are required in traces for vital body process. They are taken in their ionic forms. Animal takes in their elements mainly by feeding on plants or their products except a few of them such as sodium chloride (table salt). Lack of mineral salts will result in nutritional deficiency. These mineral elements or salts include phosphorus, calcium, iodine, manganese, fluorine, copper and cobalt.

CALCIUM Milk, Cheese, Egg and Fish (i) Bone and teeth formation and development

(ii) Needed for blood clotting

(iii) Normal Functioning of the heart, nervous system and muscle

(i) Rickets

(ii) Osteomalacia

(iii) Tooth decay

PHOSPHORUS Milk, cheese, egg, fish and wheat (i) For strong development of teeth and bone

(ii) It forms part of DNA and RNA

(iii) Needed for respiration

i)   Rickets

(ii) Osteomalacia

(iii) Tooth decay

MAGNESSIUM Green, vegetables, milk, meat (i) For muscle contraction

(ii) Needed for utilization of iron

(iii) Needed for teeth and bone

Nervous disorder
POTASSIUM Fruits and other Natural food (i) Needed for functioning of the muscles

(ii) For transmission of impulses in nerves

It leads to muscle paralysis
SULPHUR Beans, fish, meat & egg Constituent of proteins, amino acids and vitamin B Poor growth
SODIUM & CHLORINE Table salt, fish fruit (i) Transmission of impulses

(ii) Maintenance of osmotic balance of the cell


(i) Dehydration

(ii) Muscle cramp

IRON Eggs, liver, kidneys, beans, vegetables (i) Formation of haemoglobin in red blood cell Anaemia


IODINE Sea foods (i) Required by the thyroid gland to make thyroxine


MARGANESE Egg, Milk, Meat (i) Required for normal growth

(ii) Acts as co-factors in some enzymatic reactions

COPPER Green vegetables, eggs, milk, meat (i) It catalyses the use of iron

(ii) For proper respiration in some aerobic organisms




Vitamins are organic food substances which are needed in small quantities or traces for body normal growth and healthy development in man and other animals. Absence or insufficiency of vitamin supply in diet may lead to deficiency diseases

Groups of Vitamins

(i)         Fat Soluble Vitamins: These are vitamins that are soluble only in fat e.g. vitamins A,D, E and K

(ii)        Water Soluble Vitamins: These are vitamins that are soluble in water e.g. vitamins B-complex and vitamin C. some members of vitamin B2(Riboflavin),  B3 (Niacin), b5 (Pantothenic) B6 (Pyridoxine), B12, (cyanocobalamine), Folic acid e.t.c.



Liver, eggs, fish, milk, palm oil, fresh vegetables (i) Required for normal growth of cells and skin

(ii) For proper vision of the eye

(i) Night blindness

(ii) Reduced resistance to diseases

VITAMIN B1 Yeast, unpolished rice, milk, beans, palm wine (i) Required for normal growth

(ii) Formation of co-enzymes involved in cellular respiration

(i) Slow growth

(ii)  Dermatitis

VITAMIN B2 Yeast, soya beans, egg, milk, green vegetables (i) Required for growth healthy skin and proper functioning of the eyes

(ii) Formation or co-enzymes involved in cellular respiration

(i) Slow growth

(ii) Dermatitis

VITAMIN B3 Yeast, beans, milk, palm wine, yam, vegetables Needed for cellular respiration Pellagra ( a skin disease and digestive problem
VITAMIN B12 Kidney, liver, fish, milk Formation of red blood cells Pernicious anaemia

(Ascorbic acid)

Fresh fruits e.g. Orange and green vegetables (i) Aids wound healing

(ii) Helps to resist infection

Scurvy (characteristic by bleeding gum, poor healing of wood an low resistance of infection)


Fish, milk, egg, liver, form in the skin by light Needed for strong bone and teeth formation and development Ricket, osteomalacia
(Ergo sterol)
Green vegetable, butter, liver Promotion of fertility in animals Reproductive failure e.g. sterility and pre mature abortion


Fresh green vegetable/cabbage spinach Aids blood clothing Haemorrhage



Water is composed of two elements hydrogen and oxygen. Source of water available to animals include metabolic water from food, drinking water from rivers, rain, pond e.t.c.

Importance of Water

(i)         It is required for metabolic activities in the body

(ii)        Water is necessary for digestion of food

(iii)       It can be used for maintenance of body temperature

(iv)       It is served as a medium of transportation of nutrients

(v)        It helps in excretion of metabolic waste products from the body e.g. urine

(vi)       It is the basis of body secretion from endocrine gland

(vii)      It helps in the maintenance of the osmotic content of the body



This is a diet containing the correct proportion or the right amount of all six food substances required by an organism. The balanced diet must contain the six food classes such as carbohydrate, proteins, fats and oil, minerals, vitamins and water


These are organic catalyst which are complex protein substances that are manufactured by living cells. They accelerate metabolic reactions without changing their composition in the process. Enzymes are produced by both plants and animals. Enzymes may be named according to the process in which they are involved processes like photosynthesis, respiration and digestion and enzymatic in nature.

   Characteristics of Enzymes

(i)         Enzymes are specific in their actions

(ii)        Only small quantity of an enzyme is required to catalyse a reaction

(iii)       Enzymes have a specific temperature range above or below which they work become inactive or denatured. They work best at about 37c

(iv)       Enzymes do not lose their chemical composition at the end of a reaction

(v)        Enzymes are affected by the acidity and alkalinity (PH) of a medium. An enzyme which is active in an acidic medium e.g. pepsin become inactive in alkaline medium and vice versa.

(vi)       Enzymes are usually involved in reversible reaction

(vii)      Enzymes are produced by glands of the system that require that activities e.g. digestive         enzymes are produced by various gland of digestive system

(viii)     Substance called inhibitor can stop the activities of enzymes

(ix)       The activities of enzymes can be enhanced when they are joined to a co-enzymes e.g inorganic subsistence such as phosphorus.


  • PROTEASES– These are protein digesting enzymes. They are present in the stomach e.g Renin and Pepsin and also in the duodenum (Trypsin) and ileum (Erepsin). They all digest protein and break them into smaller unit.
  • AMYLASE- They are enzymes which digest starches and sugars and convert them to glucose. Ptyalin or Salivary amylase is produced by salivary gland in the mouth. It can converts starch to maltose.

Pancreatic amylase is produced in the pancreas. It converts starch to maltose, sucrose and lactose. It also converts these double sugars to their final products. Maltose to glucose, sucrose to fructose and glucose and lactose to galactose and glucose.

  • LIPASES– These are enzymes which convert and oils to fatty acid and glycerol. They are produced in the pancreas and ileum.

Digestive enzymes and functions

Enzymes Sources Location Substrate acted upon Effect/product


Ptyalin Salivary gland Mouth Starch Partial hydrolysis of starch to maltose
Renin Gastric gland Stomach Proteins It coagulate or curdle milk
Pepsin Gastric gland Stomach Proteins It convert solid protein to peptones
Pancreatic Lipase Pancreas Duodenum Fat and oils It converts fat and oils to fatty acid and glycerol
Amylase Pancreas Duodenum Starch It converts starch to maltose
Trypsin Pancreas Duodenum Proteins or peptones It converts proteins or peptones to polypeptide
Erepsin Succus entricus Small intestine Poly peptide It converts polypeptide to amino acid


1a.            Define enzyme

  1.             State 5 characteristics of enzymes


See also



Organization of Life




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