2. The need for food
Organisms require food for one or more of the following
purpose:
1. To provide energy for the vital activities of the body.
2. To synthesize new protoplasm, i.e. for growth and repair of
worn out parts of the body and for reproduction.
3. To maintain health, i.e. prevent deficiency diseases.
3. The need for energy (continued)
• The energy released by food can neither be created nor
destroyed but it can be converted from one form to another.
• For example, muscle cells convert the energy into kinetic
energy as the muscles in the body contract to move the
skeleton and some energy is lost as heat energy.
4. Nutrients in food
Nutrients are chemical substances in food which nourish the body, i.e. provide energy
and materials needed by the body. The nutrients in the food we eat may be classified as
follows:
1. Carbohydrate
2. Fats
3. Proteins
4. Vitamins
5. Dietary fibre
6. Water
7. Mineral salts
Carbohydrates, fats, proteins, vitamins and dietary fibre are called organic nutrients
because they are obtained from living organisms. They are compounds of carbon.
While water and mineral salts are inorganic nutrients.
5. Carbohydrate
Definition of carbohydrate: Carbohydrates are organic compounds
made up of the elements carbon, hydrogen and oxygen. The hydrogen
and oxygen atoms are present in the same ratio as in water, i.e. 2:1.
There are three main groups of carbohydrates.
6. Carbohydrate (continued)
• Glucose is one of the simplest carbohydrates, has the formula C₆H₁₂O₆.
• The generalized formula for carbohydrates is CnH2mOm.
• For glucose, n=6 and m=6. But in sucrose, n = 12 and m = 11 and hence
its formula is C₁₂H₂₂O₁₁.
Condensation is a chemical reaction whereby two simple molecules are
joined together to form a larger molecule with the removal of one
molecule of water.
(a)Sucrose (cane sugar, beet and carrots): Glucose + Fructose = Sucrose.
b) Lactose (milk sugar): Glucose + Galactose = Lactose.
c) Maltose (malted cereals and sprouting grains): Glucose + Glucose =
Maltose
7. Carbohydrate (continued)
Functions of carbohydrate:
1. As a source of energy.
2. To form supporting structures, e.g. cellulose cell walls in plants.
3. To be converted to other organic compounds such as amino acids
and fats.
4. For formation of nucleic acids (e.g. DNA).
5. To synthesize lubricants, e.g. mucus which consists of a
carbohydrate and a protein.
6. To produce the nectar in some flowers. The sugary nectar attracts
feeding insects and enables cross pollination to occur.
8. Hydrolysis
A reaction whereby a water molecule is added on to split up a complex
molecule into its component units is known as hydrolysis or a hydrolytic
reaction.
9. Types of polysaccharides
A polysaccharide is made up of many monosaccharide molecules joined together.
The process of condensing many similar molecules to form a large molecule is
called polymerization. E.g. Starch, cellulose and glycogen.
Starch: Starch is made up of a very large number of glucose molecules (200
glucose units) condensed together.
We can partially hydrolyze starch by using a suitable enzyme, amylase, which
digests the starch to the sugar maltose. Maltose can be converted to glucose by
the enzyme maltase or by heating with acid.
10. Types of polysaccharide (continued)
b) Glycogen: Glycogen is sometimes referred to as an “animal starch”. It is a storage
form of carbohydrate in animals and also in fungi. In mammals, it is stored mainly
in the liver and in the muscles.
Glycogen and starch are suitable as storage materials because:
1. They are insoluble in water so they do not change the osmotic pressure in the
cells. (osmotic pressure is the pressure required to stop osmosis)
2. They are large molecules which are unable to diffuse through the cell
membranes.
3. They can easily be hydrolysed to glucose when needed.
4. Their molecules have compact shapes so they occupy less space than that
occupied by all their constituent glucose molecules.
11. Types of polysaccharide (continued)
c) Cellulose:
• Cellulose is the carbohydrate which forms the cell walls of plants. It consists of
glucose units linked together.
• Cellulose is inert and very few organisms can digest it. Man can not digest
cellulose, but it forms the bulk of undigested matter.
• This fibre is important to the proper functioning of the large intestine and also
helps in preventing constipation.
12. Fats
Fats are organic compounds made up of elements carbon, hydrogen and oxygen
but, unlike carbohydrates, they contain much less oxygen in proportion to
hydrogen.
When fats are hydrolyzed, they yield fatty acids and glycerol.
13. Fats (continued)
Fats are complex organic substances containing carbon, hydrogen and oxygen.
Functions of fat:
1. As an efficient source and storage of energy.
2. As an insulating material.
3. As a solvent for fat-soluble vitamins.
4. As a constituent of protoplasm.
5. As a means to restrict water loss from the surface of the skin.
Foods rich in fats include butter, cheese, fatty meat, olives, many nuts, and seeds
of castor oil, palm oil and many leguminous plants.
The types of fats: Animal fats are saturated fats and can contain cholesterol.
Cholesterol may get deposited on the wall inside of arteries. This may lead to heart
attack. Vegetable fats are unsaturated fats. They do not cause heart diseases. In
fact, they should be substituted for animal fats as often as possible.
14. Proteins
Proteins are very complex organic substances containing carbon, hydrogen, oxygen
and nitrogen. Sulphur and phosphorus are also often present.
Amino acids is the basic unit of proteins.
An amino acids possess an amino group (-NH₂), an acidic group (-COOH) and a
hydrocarbon chain (R).
The bond formed between two amino acids is strong and is known as a peptide
bond. If three or more amino acids are linked up by peptide bonds, a polypeptide
or peptone is formed.
Proteins Polypeptides Amino Acids
15. Proteins (continued)
Effects of protein deficiency:
In human beings, the average adult needs 50 to 100g of proteins a day. The
deficiency of proteins in the diet of children may lead to a protein deficiency
disease called kwashiorkor.
Function of Proteins:
1. Essential for the synthesis of protoplasm, for growth and repair of worn-
out body parts.
2. Used for the synthesis of enzymes and some hormones.
3. Used in the formation of antibodies and some hormones.
4. A source of energy.
16. DNA (deoxyribonucleic acid)
Nucleic acids are essential for life. They form the genetic material of all living
organisms. Nucleic acids are made up of units called nucleotides. These are
arranged to form extremely long molecules known as polynucleotides.
A nucleotide has three components:
i) A pentose (5 carbon containing) sugar
ii) A base
iii) Phosphoric acid
17. Water
Water: Essential constituent of protoplasm. 70% of body weight is water. We
require about 1.7L to 2L of water daily to replace water lost through the skin
during sweating, as urine from the kidneys, in faeces, and by evaporation from
lungs during breathing.
Functions:
• Require for all the chemical reactions.
• Helps to regulate the body temperature and cools down the body.
• As a transporting agent for digested food substances, excretory products,
hormones.
• Needed for digestion and photosynthesis.
• An essential component of lubricant fluid in joints, digestive juice and blood.
• Needed for hydrolytic reaction.
18. Vitamins
Vitamins are grouped into two categories:
• Fat-soluble vitamins are stored in the body's
fatty tissue. The four fat-soluble vitamins are
vitamins A, D, E, and K.
• There are nine water-soluble vitamins. They
are not stored in the body. Any leftover water-
soluble vitamins leave the body through the
urine. E.g. vitamin B and C.
19. Vitamins
Vitamins
Sources
Functions
Deficiency Disease
Vitamin C
Citrus fruits ( orange,
lemon), guava, tomato,
banana.
For healthy skin and
gums,
to heal the wound
quickly.
Scurvy- swollen bleeding
gums and loosening of
teeth.
Internal bleeding.
Vitamin D
Milk, butter, egg , fish.
UV rays of the sun can
convert a natural
substance to Vit D.
For strong bones
and teeth.
Rickets- poor teeth and
bone formation in which
bones become soft and
may bend.
20. Minerals:
Minerals are inorganic elements which have no energy value but are required for health.
Minerals Sources Functions Deficiency Disease
Calcium ( 1 gram)
Milk, egg, cheese,
cereals, soya beans,
spinach.
For strong bones and
teeth.
For blood clotting.
For normal functioning
of muscles.
Rickets- poor teeth and
bone formation in which
bones become soft and
may bend.
Iron (0.02 gram)
Liver, red meat, egg,
bread, flour, dark
green vegetable.
For the formation of
haemoglobin in RBC.
Anaemia- less
haemoglobin thus less
RBC, person becomes
pale and tired.
21. Dietary fibre or
Roughage
A type of carbohydrate (cellulose) that cannot be
digested by our body because we have no
enzyme that could digest cellulose, that means
that roughages enter the body from the mouth,
go through the digestive system, and out through
the anus unchanged.
Functions:
• Undigested food helps to clean bowels by
forming forces.
• It prevents constipation.
• Prevent colon cancer
• Help weight-control and reduce the chance of
obesity
• Sources: cereals, fruits, green leafy vegetables,
dried peas, nuts, lentils, grains etc
22. Balanced diet
A balanced diet is a diet that contains different kinds of foods in certain
quantities and proportions so that the requirement for calories, proteins,
minerals, vitamins and alternative nutrients is adequate. A balanced diet
should offer around 60-70% of total calories from carbohydrates, 10-12% from
proteins and 20-25% of total calories from fat.
