Nutrients
Summary
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
Carbohydrates
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
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
Fats
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
Proteins
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
Vitamins
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
Water
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
Minerals
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.
Solution
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.
Solution
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.
Solution
a. A variable component
b.
c. Condensation
d. Polypeptide