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Organisms need food because food contains nutrients in the form of carbohydrates, fats, proteins, vitamins, minerals, water and fibre, which provides energy and materials necessary for
  • vital activities to stay alive
  • growth, development and repair of body tissues
  • maintenance of good health
  • movement


  • organic compounds made up of carbon, hydrogen, oxygen
  • ratio of hydrogen to oxygen present is 2 : 1
  • can be divided into
    • monosaccharides
    • disaccharides
    • polysaccharides

Needed for

  • source of energy for the vital activities of the body
  • synthesis of cellulose cell wall in plants
  • constituents of nucleic acid (DNA, RNA)
  • constituents of lubricants (mucus)
  • synthesis of other organic compounds eg amino acids and fats
  • sugar in nectar attract insects for pollination

Test for carbohydrates

  • a reducing sugar gives a red precipitate when heated with Benedict's solution
  • starch gives a blue-black color with iodine

Classification of carbohydrates

a. Sugar

i. simple
  • glucose, fructose, galactose (all reducing sugars)
  • monosaccharides are carbohydrates which cannot be broken down into simpler forms

ii. complex
  • maltose, lactose, sucrose (all reducing sugars except sucrose)
  • disaccharides are created when two monosaccharides are linked

 common disaccharides
 made up of
 maltoseglucose + glucose
 germinating seeds
 lactoseglucose + galactose
glucose + fructose
 sugar cane

b. Polysaccharides

i. cellulose (structural polysaccharide)
  • large molecule
  • most abundant organic compound on earth
  • consists of glucose molecules linked together to form long bundles of fibrils in the cell walls of all plant cells
  • it is the presence of these long fibrils, laid in criss-cross mesh that gives the cell wall its strength.
  • forms cell walls in plants
  • mammals unable to digest cellulose but herbivores have bacteria in their intestines to digest it because mammals have no enzymes to break the molecular linkages in cellulose
  • in other mammals, forms fibre in diet - their presence in our food helps in peristalsis

ii. starch (storage polysaccharide)
  • large molecule made of glucose units linked together
  • stored as starch granules in specialised structures known as plastids in plant cells
  • roots and seeds are examples of plant parts which store large amounts of starch
  • gives a blue-black color with iodine solution
  • serves as the main storage form of carbohydrate in green plants

iii. glycogen

  • large molecule made of glucose units linked together
  • remains a brown color with iodine
  • serves as the main storage form of carbohydrate in animals and fungi
  • stored in the liver and skeletal muscles of vertebrate animals
  • readily broken down into glucose

Breaking down and building up reactions which nutrients undergo in the body

  • hydrolysis is a chemical reaction whereby water molecules are needed to split up a complex molecule into simpler molecules
  • condensation is a chemical reaction whereby simple molecules are linked together to form a bigger molecule with water being produced


  • organic compounds made up of carbon, hydrogen, oxygen but unlike carbohydrates, they contain much less oxygen in proportion to hydrogen
  • fats are hydrolysed to form fatty acids and glycerol

Fats are needed

  • as source of energy
  • as insulating material to prevent excessive heat loss from skin of mammals
  • as solvent for fat-soluble vitamins and some hormones
  • as constituent of protoplasmic membranes

Test for fats

  • when the ethanol emulsion test is performed on fats, a cloudy white emulsion is formed


  • organic compounds made of carbon, hydrogen, oxygen, and nitrogen.
  • sulphur and phosphorus often present
  • proteins consist of amino acids linked together by peptide bonds
  • the different arrangement of the amino acids will determine the properties of the protein
  • the three-dimensional shape allows it to recognize and bind to specific molecules and thus perform a specific function in the cell.
  • if a protein loses its normal shape, it will also lose its ability to function
  • a change in temperature or pH of the surroundings can cause the protein to be unravelled or denatured
  • when proteins are hydrolysed they are first converted to polypeptides or peptones and then to amino acids

Proteins are needed

  • for synthesis of new protoplasm - hence for growth and repair of worn-out parts of body
  • for synthesis of enzymes and some hormones
  • as source of energy
  • for synthesis of antibodies

Protein test

  • when tested with Biuret reagent will give a violet color

Energy value of food

  • 1g of carbohydrate = 16kJ
  • 1g of protein = 17kJ
  • 1g of fat = 38kJ


  • organic compounds needed by mammalian body to maintain health and prevent deficiency diseases

Vitamin C (ascorbic acid)

  • needed for formation of intercellular substance and maintaining healthy epithelium
  • deficiency causes scurvy

Vitamin D (calciferol)

  • needed for absorption of calcium and phosphorus
  • deficiency causes rickets in children


  • essential constituent of protoplasm
  • as a medium for chemical reactions to occur
  • as a solvent for enzymes, food substances, products of excretion etc so that they can be transported from one part of the body to another
  • helps regulate body temperature
    • Water can absorb heat. enabling water in the intercellular spaces, cytoplasm and blood to absorb heat produced by the metabolic activities of the cells. As the blood is moved from the region of heat production, heat is removed.
  • Energy is absorbed to convert liquid water into water vapour (latent heat of vaporization) so evaporation of sweat is a major means of removing heat from the body and thus reduce body temperature.
  • reactant in hydrolysis and in the photosynthetic process


  • calcium: needed for formation of bones and teeth
  • iron: structural component of haemoglobin. Deficiency results in nutritional anaemia

A balance diet

  • contains the right amounts of carbohydrates, fats, proteins, vitamins, minerals, water and roughage to meet the daily requirements of the body

MCQ Questions

1. Which statement correctly describes one of the roles of water in the human body?
A. as a source of energy
B. as a solvent to dissolve some substances
C. as material for the build-up of muscles
D. as an insulating layer for the body

2. Which form of carbohydrate is usually stored in the human body?
A. glucose
B. glycogen
C. glycerol
D. starch

3. The diagram shows the formula of a molecule of an organic compound.
What kind of organic compound is this molecule most likely to be?
A. Glucose
B. Fatty acid
C. Amino acid
D. Vitamin A

4. Food tests are carried out on four solutions. Which solution contains only glucose and
proteins?(✓ indicates a positive reaction; ✗ indicates no change observed.)

MCQ Answers

1. B
2. B
3. C
4. C

Structured Questions Worked Solutions

1. David is given a slice of cucumber and a slice of tomato. He is curious to find out if the food samples contain reducing sugars, proteins or fats. Outline the experiments that he should conduct to test the {ood samples for reducing sugars, proteins and fats.


1. Cut the cucumber into small pieces. Divide the cucumber pieces into three portions.
2. Add 1 cm3 of distilled water to the portions. Mix well and decant.
3. Do the following tests:
  • Reducing sugar test Benedict's test + observations
  • Protein test - Biuret test+ observations
  • Fat test ethanol emulsion test + observations
4. Repeat for tomato.

2. The table below refers to three organic molecules. Complete the table.

  Carbohydrates Proteins Fats
 Elements present in
the molecule
 Basic units   
Type of reaction to
convert complex form to simpler form
 Two functions of the
molecules in a living


  Carbohydrates Proteins Fats
 Elements present in
the molecule
 C, H, O C, H, O, N
maybe P and S
 C, H, O
 Basic units Simple sugars Amino acids Fatty acids and
 Type of reaction to
convert complex form  to simpler form
 Hydrolysis Hydrolysis Hydrolysis
 Two functions of the
molecules in a living
 Any two of the
  • Energy source
  • Production of mucus
  • Constituent of cellulose cell wall
 Any two of the
  • Energy source
  • Production of mucus
  • Constituent of cellulose cell wall
 Any two of the
  • Energy source
  • Solvent for fat-soluble vitamins
  • Insulating layer

3. The diagram shows an amino acid molecule
a. What is R?
b. Based on the diagram shown above, draw two amino acids linked together.
c. Name the type of reaction by which amino acids are linked together.
d. Name the product formed when many amino acids are linked together.


a. A variable component
c.  Condensation
d. Polypeptide